Project Gutenberg's The Preparation of Plantation Rubber, by Sidney Morgan
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Title: The Preparation of Plantation Rubber
Author: Sidney Morgan
Contributor: Henry P. Stevens
Release Date: March 7, 2011 [EBook #35510]
Language: English
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| TRANSCRIBER'S NOTES: |
| |
| * Minor typographical and lay-out errors have been corrected. |
| * Inconsistencies in spelling (e.g. hyphenated vs. non-hyphenated |
| words) have not been corrected. |
| * Italics are represented by underscores as in _text_. |
| * The original book uses a V symbol to describe V-shaped cuts. These |
| V symbols are represented as [V]. |
| * Changes made to original text: |
| * Table of Contents: |
| * "Early collection of latex transport" changed to "Early |
| collection of latex--Transport". |
| * "Roof brick built houses" changed to "Roof--Brick built |
| houses". |
| * The order of the sections under Chapters XII and XXII has been |
| changed slightly to reflect the order of the sections in the |
| text. |
| * "Tephrosia candiad" changed to "Tephrosia candida". |
| * "Archiev" changed to "Archief". |
| * "about 1/2 square" changed to "about 1/2 inch square". |
| * "asbesto-slate" changed to "asbestos-slate" as elsewhere. |
| * "Formaline" changed to "Formalin" as elsewhere. |
| * Footnotes moved to under the paragraph they belong to. |
| * Page 141: "The lengths of crepe were weighed carefully at 8 a.m. |
| and 4 a.m." should probably read "The lengths of crepe were weighed|
| carefully at 8 a.m. and 4 p.m." |
+----------------------------------------------------------------------+
THE PREPARATION OF PLANTATION
RUBBER
THE PREPARATION
OF
PLANTATION RUBBER
BY
SIDNEY MORGAN, A.R.C.S.
VISITING AGENT FOR ESTATES IN THE EAST; FORMERLY SENIOR SCIENTIFIC OFFICER
AND NOW HONORARY ADVISER TO THE RUBBER GROWERS' ASSOCIATION
IN MALAYA
WITH A PREFACE AND A CHAPTER ON VULCANIZATION
BY
HENRY P. STEVENS, M.A. (OXON.,) PH.D., F.I.C.
CONSULTING CHEMIST TO THE RUBBER GROWERS' ASSOCIATION IN LONDON
CONSTABLE & CO. LTD.
LONDON : BOMBAY : SYDNEY
1922
PRINTED IN GREAT BRITAIN BY
BILLING AND SONS, LTD., GUILDFORD AND ESHER
PREFACE
Mr. Sidney Morgan's work on Plantation Rubber in the East is so well known
that he hardly needs introduction.
An earlier book, published in 1914, by the Rubber Growers' Association,
entitled "The Preparation of Plantation Rubber," was well received and
widely read. This book dealt in a very practical manner with problems with
which the industry had to contend. A second edition was subsequently
published. Both editions are now out of print. The present opportunity was
therefore taken to revise the original work, with the result that it has
been enlarged and practically rewritten. The information given is brought
up-to-date, and covers the whole process of production, commencing with the
planting of the tree, passing on to the collection, coagulation, and curing
of the rubber, and concluding with the packing for export. In the course of
his work for the Association, Mr. Morgan carried out a great deal of
industrial research in rubber production, including lengthy experiments on
tapping, the use of different coagulants and different conditions of
coagulation, and also on varying modes of rolling, drying, and smoking
rubber. He also went very fully into the types of construction and details
of the machinery and buildings employed on estates.
Much of this valuable work has escaped notice, owing to its having been
published in reports with limited circulation. Also a great deal of
information was supplied to planters in a quiet and unobtrusive fashion, in
interviews, visits to estates, and on other similar occasions. The
knowledge and experience thus accumulated has been embodied in the present
volume. The subject-matter should interest not only those actually engaged
in rubber planting, but those otherwise directly or indirectly connected
with the industry, such as importers, brokers, and particularly the rubber
manufacturers in this country and in America. My experience has been that
manufacturers as a whole have but a vague idea as to the methods employed
in the preparation of plantation rubber, and this work provides them with
the opportunity of obtaining an insight into the actual operations on the
estates. It is most desirable that a closer bond should unite the
plantation and manufacturing rubber industries. Such a result is best
promoted by a better understanding of the problems with which each is
confronted. Perhaps I may go so far as to suggest that some leading
scientific officer in the employment of one of the large manufacturing
concerns may take in hand a book which will give the planters the
equivalent of information in regard to the manufacturing industry which the
planters are now offering to the manufacturers.
The photographs in the earlier part of the book will give the layman some
conception of the enormous amount of labour that must be expended in the
opening up, planting, trenching, and weeding the plantations which have
replaced the virgin jungle. The authors are indebted for most of these
photographs to Mr. H. Sutcliffe, one of the mycologists of the Rubber
Growers' Association. The pictures of spotless coagulating tanks and tiled
verandahs regularly hosed down will indicate the cleanliness necessary for
the preparation of the beautifully clean sheet and crepe rubber which
became available with the advent of plantation rubber. These results are
largely due to the work of Sidney Morgan and his colleagues, on whom the
planters have relied for technical guidance and advice.
As regards my own contribution this is confined to a general outline of the
subject. I have, therefore, omitted reference to a number of matters which
would have been dealt with in detail had space permitted. The information
given is based on researches on vulcanisation carried out for the Rubber
Growers' Association by the writer over a period of nine or ten years. It
was not found practicable to give detailed references in all cases. The
reports on which the conclusions are based will, however, be found among
the regular quarterly reports made by the writer for the Association up to
June, 1919. Subsequent reports have been published in the Monthly Bulletin
of the Rubber Growers' Association. We are indebted to the Association for
permission to publish details from these reports, and also for the use made
of numerous earlier reports published both in London and in the East.
CONTENTS
PART I
_FIELD OPERATIONS_
PAGE
CHAPTER I
PLANTING
Seeds--Seed selection--Strain improvement by bad propagation--
Nurseries--Stumps--Seed at stake--Basket plants--Preparation
of land--Danger of disease--Clean clearing--Loss of top-soil--
Silt-trenches on slopes 1
CHAPTER II
FIELD MAINTENANCE
Clean weeding--Selective weeding--Loss of top-soil--Grass
ridges--Lallang eradication--_Mimosa gigantea_ (_M.
invisa_)--Green cover-plants--Connection between weeding,
soil conservation, and soil improvement 13
CHAPTER III
THINNING OF AREAS
Original planting per acre--Ultimate stand per acre--Close-
planting _versus_ wide-planting--When to commence
thinning operations--How to select in preliminary rounds--
Later selections based on yields of individuals--Yields per
tree, present and future--Trees per acre 19
CHAPTER IV
TAPPING SYSTEMS
Former methods--Former systems--Tendency to reduce number of
tapping cuts and frequency of tapping--Period allowed for
bark-renewal--Modern systems--Superimposed cuts--Single cuts,
etc.--Tapping experiments--R.G.A. experiment--Alternate-daily
_versus_ daily tapping 28
CHAPTER V
TAPPING AND COLLECTING
Tapping knives--Personal equation in use of knives--Choice of
latex cups--Cleaning of cups--Water in cups--Premature
(spontaneous) coagulation--Prevention of spontaneous
coagulation--The use of anti-coagulants in the field--
Collecting pails--Payment by result--Methods for calculation
of yields per coolie--Tree-scrap, oxidation of--Prevention of
oxidation--Bark-shavings--Collection and storage of shavings--
Treatment of shavings--Collection of earth-scrap 38
CHAPTER VI
TRANSPORT OF LATEX AND COAGULUM
Percentages of "first" latex and other grades--Early
collection of latex--Transport, nature of--Light railways--
Motor-lorries--Bullock-carts--Care of transport vessels--Use
of an anti-coagulant during transport--Transport by coolie--
Coagulation centres (stations)--Transport of coagulum 59
PART II
_FACTORY OPERATIONS_
CHAPTER VII
PRELIMINARY TREATMENT OF LATEX
Reception at store--Receptacles--Jars--Tanks--Necessity for
close supervision--Need for utmost cleanliness--Straining of
latex--Strainers--Facilitation of straining--Bulking of latex
--Standardised dilution of latex--Facilities for receiving and
handling latex--Reception verandahs--Receiving vessels--Types
of installations 65
CHAPTER VIII
COAGULATION
Choice of coagulant--Strength of acid solution--Making stock
solution--Quantity for use--Quantities under modern
requirements--Care in mixing--Method of mixing with latex--Use
of sodium bisulphite as an anti-oxidant--Quantities for use--
Formulæ--Abuse of the chemical--Residual traces in the dry
rubber--Use of sodium sulphite as an anti-coagulant,
quantities for use--Formulæ--Use of Formalin as anti-coagulant
--Formulæ for use 74
CHAPTER IX
PREPARATION OF SHEET RUBBER
Pale (air-dried) sheets--Uniformity of product--Pans _versus_
tanks--The ideal tank--Modern installations--Care of tanks--
Standardised dilution of latex--Variation in dimensions and
density of coagulum--Standardising instruments--Method of
using--Skimming latex--Style of sheets--Standard sheets--
Rolling and marking--When to work the coagulum--Hand-rolling--
Power smooth-rolling--Marking rolls--Preparation for smoke-
curing--Caution against accumulation of wet sheets--Hot-water
treatment--Dripping in the open air--When to place in smoke-
house 89
CHAPTER X
PREPARATION OF CREPE RUBBER
First consideration, fine pale crepe--Standardised dilution of
latex--Coagulation and coagulant--Quantities of coagulant--
Colour of rubber--Sodium bisulphite (use of)--Evaluation and
deterioration of the bisulphite and sulphite of sodium--To
distinguish between these two chemicals--Care of sodium
bisulphite--Mixing solution with latex--Former methods of
making pale rubber--Working the coagulum--Lower grades of
crepe--Naturally coagulated lump--Skimmings and washings--
Tree-scrap--Bark-shavings--Earth-scrap--Fibrous matter in low-
grade rubbers--Scrap-washers--Compound crepes--Increased care
with lower grades--Block rubber from crepe--Smoked crepe
_versus_ sheet clippings 110
CHAPTER XI
DRYING OF RUBBER
Air-drying of crepes--Artificial driers for crepes--Vacuum
drying--Hot-air driers--Michie-Golledge system--Rate of air-
drying--When drying takes place--Increase in weight of drying
crepe--Differences in weight--Aids to normal drying--Smoke-
curing of sheet rubber--Instruments for recording temperature
--Temperatures of smoke-house--Period of drying--Fuels for
smoking--Sun-drying of sheet rubber--Artificial driers for
sheet rubber 132
CHAPTER XII
SORTING, GRADING, AND PACKING
Reducing number of grades--Reduction carried too far--R.G.A.
recommendations--Care in sorting--Choice of packing cases--
Bags--Bales--Folding of crepe--Mechanical folders--Care in
assembling--Methods of packing--Weight of contents--Short
weights 150
PART III
_MACHINERY AND BUILDINGS_
CHAPTER XIII
MACHINES
Quality of metal in rolls--Nature of roll-bearings--Brass
liners--Liners of alloy or of cast-iron--Adequacy of machines
--Arrangement of battery--Speed of machines--Gear ratios--
Grooving of rolls--Heating of rolls--Sheeting machines--
Lubrication--Trays--Position of battery--Drainage of battery--
Access to back of machines--Engines--Power 159
CHAPTER XIV
FACTORIES
General construction--Plenty of light--Floors--Drainage of--
How many storeys--Verandahs--Tanks, situation of--Designs and
lay-out--Drains--Water supply 172
CHAPTER XV
OTHER BUILDINGS
Drying-houses for crepe rubber--How many storeys--Ventilation
--Windows--Effect of light--Effect of direct sun-rays--Hot-air
houses--Smoke-houses--Various types--Ordinary smoke-houses--
General ventilation--Windows--Racks of supports--Floors--
Furnaces in general--Pit-fires--Pot-fires--Iron stoves--
Horizontal drum-furnaces--Rate of combustion--Brick stoves--
Pataling type of--Consumption of fuel--Floor of furnace room--
Roof--Brick built houses--"Third Mile" type--Jackson cabinet--
Devon type--Detailed description of--Barker patent design 178
CHAPTER XVI
OTHER BUILDINGS (_continued_), AND SITUATION OF BUILDINGS
Sorting-room--Packing room--Store rooms--Storage of rubber--
Need for special accommodation--Floor of store room--Local
conditions--Temperature and humidity--Incidence of moulds--
Effect upon smoked sheets--Tool-sheds and stores--Situation of
buildings--Position with respect to points of the compass--
Choosing a factory site--Centralisation--Decentralisation 211
PART IV
_THE FINISHED RUBBER_
CHAPTER XVII
DEFECTS IN CREPE RUBBERS
General style of finish--Dirty edges--Iron-stains--Rust-stains
--Oil-marks--Trays--Dirt--Holes--Greenish and tacky streaks--
Not due to oil _per se_--Tackiness and copper--Cotton and
other fibre--Bark and grit--Sand--Oxidation streaks--Yellow
streaks--Bisulphite streaks--Spot disease--Cause of--Influence
of rate of drying--Percentage of moisture--Humidity of
atmosphere--Prevention of disease--Infection by contact--
Outbreak of dormant spores--Rules to be observed--Surface
moulds or mildew--Tackiness in general--Full discussion of--
Experimental reproduction--Lack of uniformity in colour--
Defects in block rubber 223
CHAPTER XVIII
DEFECTS IN SHEET RUBBER
Defective coagulation--Coloured surface blotches--General
darkening of surface--Soft coagulum--Spongy underface--Tearing
--"Pitting" of surface--Thick ends or edges--Mis-shapen sheets
--Thick patches--Torn sheets--"Dog-ears"--Creases--Greasiness
of surface before smoking--Surface blemishes--Uneven
appearance--Variation due to oxidation--Colour when dry--
Surface gloss--Dull surface--Moist glaze and greasiness--
Virgin spots--Surface moulds or mildew--Black streaks or spots
--White or grey streaks--Rust--Theories on formation of--
Prevention of--Two methods--Other views on causation--Bubbles
--Causes of formation--In the field--In the factory--Blisters
--"Spot" disease in sheet rubber--Support marks--Stickiness--
Surface pattern--Sheet clippings--Other infrequent defects--
Dirt--Ash--Bark--Splinters 249
PART V
_GENERAL_
CHAPTER XIX
CHOICE OF COAGULANT
Acetic acid in general use--Is a coagulant necessary?--Acetic
acid--Formic acid--Citric acid--Tartaric acid--Oxalic acid--
Sulphuric acid--Hydrochloric and nitric acids--Hydrofluoric
acid--Alum--Pyroligneous acid--Smoked water--Chinese vinegar--
Sulphurous acid--Sugars--Various salts--Proprietary compounds
--Carbonic acid gas--Alcohol--Vegetable extracts 278
CHAPTER XX
SPECIAL METHODS OF PREPARATION
Da Costa process--Byrne curing process--Freezing process--
Wickham process--Derry process--Spontaneous coagulation--
Definition of--Discussion of types--Ærobic--Anærobic--
Organisms--Maude-Crosse patent--Method of operation--
Accelerating action of sugars--Accelerating action of soluble
calcium salts--Ilcken-Down process--Slab rubber 290
PART VI
_VULCANISATION_
CHAPTER XXI
INTRODUCTORY DEALING WITH TREATMENT AND VULCANISATION
Wild rubber contrasted with plantation rubber--Milling and
mixing--Preparation for vulcanising--Vulcanising 301
CHAPTER XXII
TESTING OF PLANTATION RUBBER
Tests on raw rubber--Breaking strain--Behaviour of rubber
during milling, etc.--Preparation for testing--Tests on
vulcanised rubber--Choice of a formula--Physical tests 309
CHAPTER XXIII
THE PROPERTIES OF RUBBER
Raw rubber--Physical tests--Vulcanised rubber--"Inner
qualities" of raw rubber--Defects of crepe and sheet--
Variation in physical properties--Rate of cure--Influence of
various factors in raw rubber on rate of cure--Other types of
plantation rubber--Fine para 313
INDEX 327
LIST OF ILLUSTRATIONS
PAGE
SEEDS, SHOWING VARIABLE SIZE, SHAPE, AND MARKING 2
FELLING LIGHT (SECONDARY) JUNGLE 3
SEEDLING, SHOWING ROOT-SYSTEM WITH SEED STILL ATTACHED 4
NEW CLEARING 5
TYPICAL YOUNG CLEARING, AGED ABOUT THREE YEARS, PLANTED ON
VIRGIN SOIL. ORIGINAL JUNGLE TIMBER SLOWLY ROTTING 6
LIGHT JUNGLE 7
DENSE JUNGLE 8
CLEARING READY FOR PLANTING 9
NEW CLEARING: SLOPES "HOLED" FOR PLANTING; FLAT AREA BEING
DRAINED 11
TYPICAL YOUNG CLEARING, WITH TIMBER 15
TYPICAL YOUNG CLEARING, WITH TIMBER 17
TYPICAL YOUNG PLANTED AREA 20
ANOTHER EXAMPLE OF A RECENTLY PLANTED AREA 21
WIDELY PLANTED YOUNG AREA, JUST READY TO BE BROUGHT INTO
TAPPING 24
FIELD OF OLD RUBBER TREES IN WHICH THINNING HAD BEEN DELAYED
TOO LONG 25
TWO CUTS ON A QUARTER CIRCUMFERENCE, ON AN OLD TREE 31
THE SINGLE CUT ON A QUARTER CIRCUMFERENCE, ON AN OLD TREE AND
ON RENEWED BARK 33
SINGLE CUT ON HALF CIRCUMFERENCE (HALF-SPIRAL) 35
A [V]-CUT ON HALF THE CIRCUMFERENCE 37
SINGLE CUT ON TWO-FIFTHS OF CIRCUMFERENCE 41
EFFECTS UPON RENEWED BARK OF PREVIOUS TAPPING 44
ANOTHER EXAMPLE SHOWING THE EFFECTS OF PREVIOUS TAPPING 45
1. SHOWING EFFECT OF "WINTERING" 48
2. NEW GROWTH OF YOUNG LEAF ON SAME TREE 49
EFFECTS OF DISEASE--"MOULDY ROT" 50
EFFECTS OF DISEASE--"MOULDY ROT" 51
EFFECTS OF DISEASE--"MOULDY ROT" 52
EFFECTS OF DISEASE--"MOULDY ROT" 53
RAISED VERANDAH FOR RECEPTION OF LATEX; LIKEWISE EQUIPPED WITH
FACILITIES FOR CALCULATING INDIVIDUAL DAILY "YIELD PER
COOLIE" BY SAMPLING OF LATEX 66
END-SECTION SKETCH OF VERANDAH, ETC., SHOWING A GOOD METHOD
FOR RECEIVING LATEX AND FILLING TANK 70
RAISED VERANDAH FOR RECEPTION AND HANDLING OF LATEX 71
ANOTHER SET OF DILUTION TANKS ON RAISED VERANDAH 72
TWO VIEWS OF DILUTION AND MIXING TANKS 81
UNIT MODERN COAGULATING TANK (TWO VIEWS) 91
ANOTHER BATTERY OF TANKS, WITH DILUTION TANKS, RAISED, ON THE
RIGHT 92
CLOSER VIEW OF FOREGOING 93
ANOTHER BATTERY OF TANKS, WITHOUT DILUTION TANKS OR MEANS OF
GRAVITATING LATEX 95
A SHEETING TANK CONTAINING COAGULUM FOR CREPE PREPARATION 96
A "BATTERY" OF SHEETING TANKS (PATALING ESTATE). DILUTION
TANKS, RAISED, ON THE LEFT 97
THE OLD METHOD OF "DRIPPING" FRESHLY ROLLED SHEETS WITHIN THE
FACTORY 108
THE NEWER METHOD OF HANGING IN THE OPEN AIR 109
THREE GRADES OF CREPE RUBBER 111
A WASHING SHED 112
DRYING GRAPH. PALE CREPE (THIN) 140
A SHIPMENT OF RUBBER, PACKED AND READY FOR TRANSPORT 155
ON ITS ROAD TO THE RAILWAY: BULLOCK-CART TRANSPORT 157
A BATTERY OF MACHINES 165
"THIRD MILE" TYPE; HORIZONTAL DRUM 190
"THIRD MILE" TYPE OF FURNACE, USED IN CONJUNCTION WITH "THIRD
MILE" SMOKE-HOUSE 190
SIDE SECTIONAL ELEVATION (PATALING TYPE OF FURNACE) 193
PATALING TYPE OF FURNACE 193
LARGE SMOKE-HOUSE OF ORDINARY CONSTRUCTION, WITH SHIELDED
VENTILATORS PERMANENTLY OPEN 194
BRICK AND CEMENT SUPERSTRUCTURE OF FURNACE INSIDE THE
BUILDING, BUT FED FROM OUTSIDE 195
GENERAL VIEW OF SHELTERS COVERING APPROACHES TO FURNACES 196
NEAR VIEW OF SHELTER 197
"THIRD MILE" TYPE OF SMOKE-HOUSE 199
GENERAL VIEW OF DOUBLE "DEVON" TYPE OF SMOKE-HOUSE 201
GENERAL VIEW OF DOUBLE "DEVON" SMOKE-HOUSE AND FACTORY
BUILDINGS 202
VIEW OF PLATFORM OF "DEVON" SMOKE-HOUSE; DOORS OF COMPARTMENTS
OPEN, AND ONE RACK PARTIALLY WITHDRAWN 203
DOUBLE "DEVON" SMOKE-HOUSE OF BRICK, WITH ROOF OF CHINESE
TILES, SHOWING LOADING PLATFORMS WITH RACKS WITHDRAWN FROM
SMOKING CHAMBERS 204
SIDE-VIEW OF PRECEDING PHOTOGRAPH, SHOWING EXTERNAL
ARRANGEMENT FOR STOKING FURNACES 205
FRONT VIEW OF DOUBLE "DEVON" TYPE OF SMOKE-HOUSE 206
SIDE-VIEW OF DOUBLE "DEVON" TYPE OF SMOKE-HOUSE 207
THE NEW "BARKER" TYPE OF SMOKE-HOUSE: A SMALL UNIT 210
SUGGESTED ARRANGEMENT OF BUILDING 218
THREE SPECIMENS OF FINE PALE CREPE SUFFERING FROM "SPOT"
DISEASE 237
THE PREPARATION OF PLANTATION RUBBER
PART I
FIELD OPERATIONS
CHAPTER I
_PLANTING_
To criticise the methods of the pioneer planters of _Hevea Brasiliensis_
presents no difficulty in the light of present comparative knowledge, and
to be "wise after the event" is a failing which is not confined to those
interested in modern planting methods. Looking at the matter broadly,
however, it must be acknowledged that the pioneers, wrong though they may
have been on some points, did remarkably well, considering that there
existed no real knowledge on the subject and that the methods employed were
perforce of an empirical nature. Although we know a little more concerning
the scientific aspects of rubber planting, the sum total of that knowledge
does not justify any drastic criticism of the methods employed by our
predecessors. In fact, although we may be of opinion that on general lines
there is little now to be learned regarding the planting of _Hevea
Brasiliensis_, our present knowledge does not preclude the possibility that
future investigations may bring against us charges similar to those
sometimes levelled at the earlier planters.
The main theme of the present volume is that of the preparation of rubber
for the market. Hence it is not proposed to deal in detail with the work
attaching to the opening and development of rubber estates. For this the
reader is referred to the literature dealing specifically with rubber
planting. Certain points in connection with planting may advantageously be
treated in a general way according to modern knowledge, and of these it is
proposed to discuss a few in the following pages.
[Illustration: SEEDS, SHOWING VARIABLE SIZE, SHAPE, AND MARKING.]
SEEDS.--The view is now generally held that many areas were planted from
seed which was not collected in a discriminate manner; and that probably
the comparatively low yields obtained on areas of some estates may be due
to the employment of seed from a poor strain. To be able to decide whether
such explanation fits the case demands a full knowledge of all the possible
factors governing the question of yields. It may, or may not, be a fact
that seed from a poor strain is wholly or partially accountable for low
yields; but whatever the degree in which the seed influences the result, it
is an axiom that to obtain the best results in all planting industries a
most judicious selection of seed should be made. In short, seed obtained
from good-yielding specimens by selective treatment will eventually produce
progeny of good-yielding strain.
[Illustration: FELLING LIGHT (SECONDARY) JUNGLE.]
The recognition of these principles as applied to the planting of _H.
Brasiliensis_ has focussed recent attention upon the desirability of
planting nurseries with seeds obtained from those trees which are known to
be good producers of latex of normal consistency. It does not follow that
the tree of most rapid growth and development is necessarily the best
yielder; such is often not the case. In the matter of selection, therefore,
one has to take other standards than that of size; and the issue is
narrowed chiefly to a consideration of the yields of latex given by
individual trees. It has been found by various experimenters that there is
no necessity to proceed to such a refinement as the determination of the
actual weight of rubber yielded. The dry rubber content of latices from the
same trees is found to be so comparatively regular, allowing for climatic
changes, that it is sufficient for the purposes of selection to measure the
volumes of latex yielded by individual trees.
[Illustration: SEEDLING, SHOWING ROOT-SYSTEM WITH SEED STILL ATTACHED.]
Unfortunately the industry is so young that the question of seed selection
yet awaits study. The task presents certain practical difficulties, and
would be by no means so easy to control as in the case of seed selection
from other plants. It will be obvious that several generations of trees
raised from selected seed would have to be under observation before any
sound deductions could be made from statistics obtained in the course of
the work. Thus the problem of seed-selection as it concerns the
establishment of a high-yielding strain would involve many years of
observation on the part of a trained man. Unfortunately neither the man nor
the facilities for such experimental work exist at the present moment in
the Federated Malay States. On the scientific side the industry is
incommensurably staffed, and most of the workers' time is occupied with
routine work connected with estate practice.
[Illustration: NEW CLEARING.
In the middle distance, felled trees awaiting burning; in the foreground, a
flat and wet area with main drainage outlined.
(_By courtesy of the manager of Membakut Estate, British North Borneo._)]
[Illustration: TYPICAL YOUNG CLEARING, AGED ABOUT THREE YEARS, PLANTED ON
VIRGIN SOIL. ORIGINAL JUNGLE TIMBER SLOWLY ROTTING.]
SELECTION.--It is possible, however, that the question of strain
improvement will be solved in another manner than that of successive
breeding from the seeds of high-yielding trees. Such investigatory work is
now occupying the attention of scientific organisations in the East, and
credit is due to the stations in Java which have begun experimental work in
this direction. In brief, the scheme may be outlined as follows. Trees
known to be uniformly good yielders are kept under observation, and the
seeds gathered carefully. These seeds are germinated in a special nursery,
and the best-grown seedlings are selected for further operations. At a
certain stage a bud is taken from a high-yielding parent tree and grafted
upon the stem of the seedling. When this has "struck" the original head of
the seedling is removed. This ensures that one has in the seedling both the
stem and future branch system of the same strain as the parent
high-yielding trees. It is possible to go a step farther, and by certain
processes induce a new root system to grow above the existing roots, which
are then removed. One is then able to guarantee that the roots, stem, and
branches will be of the original high-yielding strain. An objection
sometimes made against the third operation of inducing a new root system is
that the original tap-root is removed and that the subsequent system
consists only of laterals. Against this argument may be quoted the observed
fact that in actual development any one of the laterals may under such
circumstances function eventually as a tap-root.
[Illustration: LIGHT JUNGLE.]
On the whole, this system of propagation receives the approval of
investigators, and removes the objections which may be advanced against the
development of a scheme entirely founded upon successive breedings from
selected seed. The course of the investigations, also, are thereby
shortened considerably. Care must be exercised in the work of obtaining and
grafting the buds, but it has now been proved that by exercising reasonable
precautions which are not beyond the intelligence and ability of
subordinates, an extremely high percentage of success can be attained.
[Illustration: DENSE JUNGLE.]
Until such time as this process becomes practicable the inception of a
planted area must follow the lines usually adopted.
NURSERIES.--The usual practice is to obtain seeds from some estate which
has a reputation for good yields and for exercising care in the gathering
and shipping of seeds. The seed is planted in specially prepared beds, and
the percentage of germination noted for future reference. The plants should
be tended carefully, and close observation made for the detection of
disease or pests. It is not uncommon to find that owing to lack of care in
the preparation of the seed-bed, the young plants are attacked by disease.
[Illustration: CLEARING READY FOR PLANTING.
Surface timber removed, but stumps remaining.]
STUMPS.--At a stage, varying according to the requirements of the estate,
when the plants are from twelve to eighteen months old, they are lifted
from the earth. The roots and head are cut off, and the "stump" is ready
for immediate planting in the field. Naturally any appreciable delay in
planting, or unfavourable weather conditions, will militate against the
chances of successful "striking"; and it is not uncommon to find that a
certain number of "supplies" will be necessary.
SEED AT STAKE.--A method sometimes adopted is to put out seed in the field,
in prepared holes which indicate the exact position of the future trees.
Usually three seeds are placed in each hole, and if two or three germinate,
the plant having the healthiest appearance is retained, and the others
removed. The possible objections to this method of planting are obvious to
those acquainted with field conditions, but in actual practice planting
seed "at stake" has often proved highly successful. Naturally the results
obtained must depend upon the selection of good seeds, the care exercised
in the preparation of the "holes," weather conditions, and the
discrimination exercised in the selection of the plants to be
retained--apart from such disabilities as the depredations of rats and
other pests.
BASKET PLANTS.--Yet another and perhaps the most popular method at present
is the germination and growth of seedlings in baskets specially constructed
for the purpose. These plants are kept under observation until of the
required age and growth. They are then conveyed to the field, and the
baskets are planted in prepared holes. The baskets, being of vegetable
material, are liable to be attacked by various diseases while in the
nursery or after planting. It is considered advisable, therefore, to treat
them by dipping into some disinfectant such as tar, or a mixture of tar and
one of the common proprietary disinfectants. Otherwise a disease may be
conveyed from the basket to the seedling.
PREPARATION FOR PLANTING.--There can be no other opinion than that ideally
all land required for planting should be perfectly clear of timber of every
description. After felling and burning, under ordinary conditions a certain
amount of clearing is effected, but in actual practice this amounts to
comparatively little. Big logs and stumps are left because the cost of
clean clearing is judged to be prohibitive and non-economic. Surface timber
is gradually cleared in the course of development, and usually large stumps
are the last to be tackled. The objection to this procedure is really not
strong, but unfortunately an important point is generally overlooked.
Granted that most of the dreaded diseases travel beneath the surface of the
ground by means of buried timber, it is plain that as far as stumps are
concerned, the chief source of danger lies in the existence of the roots.
If these were carefully exposed and removed, the isolated stumps would then
not be such potential infection points. It follows from this argument that
the importance of removing buried timber cannot be too strongly insisted
upon. It is not uncommon to find that some years after the opening of an
estate, and after surface timber has been removed, a large number of trees
are affected with _Fomes lignosus_ (formerly known as _Fomes semitostus_).
Such cases are directly attributable to the existence of buried timber, and
no local treatment will be successful unless the whole of the area is dug
over carefully, and all pieces of timber removed.
[Illustration: NEW CLEARING; SLOPES "HOLED" FOR PLANTING; FLAT AREA BEING
DRAINED.
(_By courtesy of manager, Membakut Estate, British North Borneo._)]
SILT CATCHMENT TRENCHES.--Granted the ultimate necessity of clean clearing,
it becomes necessary to take some precautions to prevent loss of soil by
"wash" in young areas planted on sloping land. An argument often used in
extenuation of the practice of allowing large surface timber to remain
until it becomes rotten is that it is an aid in preventing loss of soil by
wash. Its removal necessitates the institution of some method of preventing
"wash." The establishment of terraces on steep slopes tends to the
achievement of the desired result, but this method is not extended to more
moderate slopes where loss by wash is still considerable. It is the opinion
of the writers and others that the general case calls for the institution
of silt catchment trenches, which, as the name denotes, fulfil the duty of
catching any surface soil and of retaining rainwater. These trenches are
usually laid out on contour, and do not exceed a length of 20 feet. They
are usually from 18 inches to 2 feet wide and deep, and are so arranged on
the slope that they occupy overlapping positions. The actual number of
trenches required will depend upon the angle of slope; the steeper the
slope the greater the number required--_i.e._, the shorter will be the
length of slope between any two trenches. Given a clean area, it is obvious
that the momentum acquired by running water (and hence the amount of soil
removed) on any one slope will depend upon the distance travelled. It is
advisable, therefore, to place a larger proportion of the trenches on the
upper part of the slope than on the lower, so as to guard against the
breaking down of the trench system under an abnormal downpour of rain.
On land thus prepared the writer has seen areas successfully planted,
which, under ordinary conditions, were condemned as being too steep for
planting. It is true that these trenches necessitate continual upkeep until
the soil becomes well shaded by trees, but the actual amount of work
demanded in cleaning and maintaining the trenches will depend largely upon
the thoroughness with which the original work was planned and executed.
Whatever may be the weaknesses exposed as a result of providing an
insufficient number of trenches of inadequate dimensions, there can be no
question that they are a necessity.
CHAPTER II
_FIELD MAINTENANCE_
CLEAN WEEDING.--Intimately connected with the growth and development of the
rubber tree one has to consider the conditions under which it is allowed to
mature. The argument has been used that, since the habitat of _Hevea
Brasiliensis_ is in the jungle, we should be proceeding against nature by
introducing conditions unlike those under which the "wild" rubber tree
grows. It is difficult to treat such an argument seriously, as by quoting
parallel instances in arboriculture it could be shown that growth,
development, and yields are improved by cultivation of "wild" plants.
It needs small experience with rubber-tree plantations to be convinced of
the necessity for dealing with other growths, which would otherwise soon
surround and overshadow young rubber trees.
Apart from checking and preventing woody undergrowths it is considered
advisable to keep the ground more or less free from light vegetable
growths, which are roughly grouped under the heading of "weeds."
Naturally, if these weeds are allowed to flourish and seed, their eventual
eradication may be a matter of extreme difficulty and expense. It is the
aim, therefore, of properly conducted estates generally to institute such a
system of work that the weeding-gangs cover the whole estate at regular
intervals; and, as a general rule, it may be accepted that the shorter the
interval between successive visits by the gang to any particular area, the
easier it is to keep weeds in check, and the cheaper the work will
eventually be done. This procedure defines roughly what is implied by the
term "clean weeding," and it is the policy adopted by most estates.
Strict adherence to this practice in rubber cultivation has been inculcated
by the older school of planters who obtained their experience in the
cultivation of other crops such as tea, coffee, tobacco, etc.
In latter years the wisdom of scrupulous clean weeding under all conditions
has been questioned; and there can be no doubt that under certain special
conditions a continuation of the policy of clean weeding is calculated to
produce, in course of time, more harm than benefit. As an instance, the
case might be cited of steep slopes on poor land. Continual clean weeding
on such areas will lead eventually to a great loss of the surface soil,
unless some precautions are adopted for catching and retaining the fine
silt particles. It is to be noted that such a type of soil and slope, when
the shade is appreciable, often produces no weeds heavier in growth than a
very light grass. It is urged that the necessity for strict clean weeding
on such soils does not exist, and, in fact, that it would be an injurious
policy. Such arguments appear to be well founded in experience, and the
writers are in thorough agreement that such special cases deserve special
consideration. Rigid adherence to a policy of clean weeding, without regard
to special conditions, would be most inadvisable.
Nevertheless, such exceptional cases do not detract from the wisdom of
clean weeding in general. Every planter of experience realises how easily
fields become infested with weeds if the regular work is suspended or
delayed. It is probably quite true that the harm due to the presence of
some weeds on an occasion is negligible; but apart from this debatable
point, there is the solid fact that if once an area is allowed to become
weedy it may soon demand a much greater expenditure to bring it back to
normal condition than if it had been regularly weeded. This is common
experience, and for that reason alone a general policy of clean weeding is
thoroughly sound; especially if combined with some system of
silt-retention.
GRASS SQUARES.--On some estates the practice of clean weeding is undertaken
in combination with a system of silt-retention, which depends upon the
development and maintenance of ridges. These are built up from the débris
of weeding in the form of hollow squares. Grass is allowed to sprout and
grow in these ridges, and when it attains a certain height it is trimmed
down so as to keep it within bounds. The soil within the hollow square is
clean weeded; and it is maintained that loss of soil by wash is avoided.
Under certain conditions there is a great deal to be said in favour of the
method, but in the opinion of the writers it should be regarded only as a
method of expediency. It is not to be preferred to the more thorough
practice of soil-retention by means of silt-trenches, although the latter
method may be slightly more expensive in the end.
[Illustration: TYPICAL YOUNG CLEARING, WITH TIMBER.
Planted "rubber-stump" in foreground.]
"LALLANG" ERADICATION.--The greatest bugbear of the planter in connection
with weeding is the incidence of lallang. Many proposals have been put
forward at various times for the complete eradication of this pest; but at
present, under ordinary circumstances, there would seem to be no better
method than by heavy and deep digging, followed by regular attention. The
method is acknowledged to be expensive, but any half-hearted measure
otherwise taken will eventually prove to be even more costly.
One has to differentiate, of course, between the incidence of lallang
attributable to negligence on the estate itself, and the occasional
outbreaks near boundaries, due to seeds having been wind-borne from patches
of lallang outside the boundaries But, in general, it would be safe to
remark that the appearance of lallang could be taken as evidence of a
failure to cover the area at sufficiently short intervals.
As already intimated, the usual method of eradication of areas of lallang
is by thorough digging, and the exposure of the strong root system to the
sun. As a matter of interest it may be noted that recently some success has
been obtained by another method[1] on areas which one may have in view for
planting at some future date.
[1] "Eradication of Lallang," W. P. Handover, _The Planter_, Vol. I., No.
1, August, 1920.
It consists in the employment of _Mimosa gigantea_, which eventually
smothers the growth of lallang.
The seeds are sown broadcast, in drills, or in pockets, amongst the
lallang. In the course of about three months it overtops the grass and
proceeds to travel. At this stage the whole mass is pressed down, and the
pressing is repeated at regular intervals. Under favourable conditions, in
about twelve months, an impenetrable mat has been formed, which gradually
forms a good mulch. When it is desired to remove the Mimosa, the mass
(pressed down) is cut and rolled up like a carpet. Cleared in this manner,
the area then needs regular weeding, in order to check the development of
any stray lallang shoots. In actual practice it was found that the cost of
this method was approximately two-thirds that of the usual digging method.
GREEN COVER PLANTS.--Some years ago it was quite common to find green
cover-plants employed on estates with the primary idea of minimising
weeding costs. With most of these it was found later that their value was
not real, and that they harboured diseases, and pests. Moreover, when they
were removed, it was often found that an abundant crop of lallang and
weeds resulted.
There can be no question that certain plants can be employed with
advantage, not only in the control of weeds, but also by reason of benefit
to the soil in which they are established. These plants are leguminous, and
their use is restricted almost entirely to young areas, inasmuch as they
will not continue to grow when shade becomes marked. Of those best known in
modern practice might be mentioned _Tephrosia candida_ (Boga bean),
_Centrosema Plumerii_, and _Dolichos Hoseii_ (Sarawak bean).
[Illustration: TYPICAL YOUNG CLEARING, WITH TIMBER.
Young rubber plants in foreground. Two of these are easily distinguishable,
both with small crowns of leaves.]
It is wrong to imagine, however, that the establishment of such leguminous
cover-plants obviates weeding. So far is this from being the case, that in
practice it is found that the weeding "rounds" must be conducted at first
with the same regularity as in ordinary working, but that naturally there
is much less work to be done.
As the plants develop, they can be pruned or dug into the soil, as the case
may be. The addition of the green material to the soil, either by digging
or by burying in open trenches, is calculated to cause improvement in the
condition of the soil. There may thus be a close connection between
weeding, soil conservation, and soil improvement.
CHAPTER III
_THINNING OF AREAS_
On this subject there is unanimity regarding the necessity for the
operation. Divergence of opinion exists only as to a matter of degree.
On the one hand there is the school of planters who would advocate the
advisability of planting up to, say, 200 trees per acre, with subsequent
thinning out by selection. At the other extreme there is the opinion that
we should plant only a few more trees per acre than it is intended
eventually to maintain, the argument being that by this method the growth
and development of individual trees will be so much greater than in close
planting that the necessity for drastic thinning out will not arise.
Unfortunately for the latter school, a very important point is
overlooked--viz., that size and general development are not criteria of
yielding capacity. It might thus follow that a stand of ninety well-grown
trees per acre might give very disappointing yields per acre. In a few
instances this has been noted with 30 by 30 feet planting, but it is
doubtful whether the factor influencing such results has been appreciated.
The apostles of close-planting have this in their favour: that if the trees
to be removed are selected on proper lines, it is possible to have all
remaining trees of comparatively high-yielding strain. This is a very sound
argument, but its practicability is limited very largely by the question of
early growth and development. It would seem the sane course in any event
not to plant more trees per acre than may grow normally, and without branch
or root interference up to the fifth year (the normal first year of
tapping).
Before this stage has been reached, stunted or deformed trees will have
been noted and removed, so that in the first year of tapping thinning
proper can be commenced. In the past this has been effected wholly by
selection of trees according to their general appearance and situation; but
it is now safe to predict that future operations will be based upon sounder
and more scientific lines.
Trees will be selected for removal according to their individual yields, a
standard which we have been advocating for years without much practical
success. In Java and Sumatra much good work has been done in this
direction, and recently a commencement has been made in the F.M.S.
[Illustration: TYPICAL YOUNG PLANTED AREA.
Heavy original jungle timber.]
It is within the daily observation of all planters that certain trees
regularly give greater yields than others, and that such trees are not to
be distinguished by size or general development. Moreover, with slight
variations, it has been found that a good yielding tree is consistently a
good yielder, and the converse holds true.
If, therefore, measurements of individual yields are taken at intervals,
and the results recorded during the first year of tapping of an area, an
excellent guide is obtained for the first round of thinning. It is found
in actual practice that five, or even three, readings during the year are
sufficient to give the indication required. It is not essential that
simultaneous readings should be taken over a large area; in fact, such a
step is really impracticable at first. The simplest method is to employ
either--
(_a_) A small uniform vessel in which the latex is measured by means of a
thin slip of bamboo upon which graduations are marked.
(_b_) A glass measure graduated regularly.
[Illustration: ANOTHER EXAMPLE OF A RECENTLY PLANTED AREA.]
In both cases it is immaterial what units are represented by the
graduations--whether cubic centimetres, quarter ounces, half-ounces, or
ounces, as long as the unit is not too large. It is preferable to employ a
fairly small unit, so that in taking readings from young trees a wider
range may be obtained between poor yields and good yields. In the case of
older trees a larger unit may be taken.
The first stage in the operations is to number all trees in the field to
be tested, and to prepare a rough register, with three or five vacant
columns opposite each tree number.
It is not advisable to commence the record of yields until the panel of
bark has been under tapping for a month or two. It is found that an
intelligent coolie can be taught the method of measuring and rough
recording. The latter is accomplished by means of marks made upon the
virgin bark of the tree above the tapped area. The marks may be made with a
tapping knife, by means of paint, or with a lead pencil. The simplest form
of record consists in putting one mark for each graduation of reading.
In practice it is found that, commencing about an hour after the first tree
has been tapped (in the case of young trees) and following the course taken
by the tapper, the measurer of yields is able to do about 2 full tasks (650
to 750 trees) per diem. Each day progress is made through the field.
Obviously on such a small scale and utilising only one measuring coolie the
comparison is restricted very much; but in any case this is immaterial as,
owing to the personal equation of the tapper, comparison strictly should be
limited and internal--_i.e._, it should really be confined to one task only
at a time. In this way the worst trees in any task are indicated.
The keeping of the records may be entrusted to a field clerk, but is better
placed in the hands of a European. The register is taken into the field and
the rough records found on the trees are noted in the columns against the
tree number.
Most planters are aware in a general way of the disparity between the
yields of individual trees, but they would probably be surprised if they
undertook the institution of such records.
The following figures must not be taken as typical. They represent the
average results from several tasks in a young field from which all
ill-grown and deformed trees had been removed. It is immaterial what the
units represent, as they are purely arbitrary and were selected for the
purpose of obtaining a fairly wide range.
Any trees which failed to yield sufficient latex to reach the first mark
were registered at zero. The following percentages were obtained:
Zero 3 per cent.
Above mark 1 6 "
" " 2 16 "
" " 3 42 "
" " 4 12 "
" " 5 14 "
" " 6 6 "
" " 7 0 "
" " 8 1 "
" " 9 0 "
---
100 "
It may be remarked that, judging by ordinary standards, it was impossible
to discriminate between good yielders and others, and if thinning were to
be done on the usual lines it is quite possible that some of the best
yielding trees would be removed.
Taking the mark No. 5 as the datum line, it will be noted that 79 per cent.
of the trees come below and 21 per cent. above. In the latter proportion
the majority lie close to the datum line. It will be seen that there are
outstanding yielders even amongst these young trees, and that it would be
possible to mark about 10 per cent. of the stand per acre at once for
removal in the first round of thinning.
In the case of old trees it is possible that one would encounter greater
extremes of yields than those shown in the foregoing table, especially if a
certain amount of thinning had been done previously on empirical lines.
Sufficient has been written to show that the only reasonable basis for
selection of trees in thinning is that of yields; and it is obvious that if
the method be adopted the future yield per acre of any area is bound to be
in excess of the same area as thinned on rule-of-thumb lines.
YIELDS PER TREE.--A great feature is made in estate reports of the figure
showing the average yield per tree per annum. Assuming an area to be
yielding at the average high rate of 540 lbs. per acre per annum, with an
average stand of ninety trees per acre, the yield per tree per annum
averaged over all trees is 6 lbs. Keeping in mind the test-figures on a
previous page, it will be obvious that some of these trees may have given
very much more than 6 lbs. during the year, and some less. In view of
present information it would not be surprising to find that a few might
have been yielding upwards of 15 lbs. per annum. Unfortunately this
information is only to be obtained by individual tests, and under normal
estate conditions the facts escape notice. Cases are known in which
out-standing individual trees have been known to yield at the rate of 25
lbs. and more per annum.
[Illustration: WIDELY PLANTED YOUNG AREA, JUST READY TO BE BROUGHT INTO
TAPPING.]
[Illustration: FIELD OF OLD RUBBER TREES IN WHICH THINNING HAD BEEN DELAYED
TOO LONG.
Note height and comparative lack of girth.]
FUTURE YIELDS PER TREE.--It has been shown that by selective methods based
on yields, poor trees can be eliminated. Whether by a process of
seed-selection or by means of propagation based on bud-grafting and
marcotting, it needs no great stretch of imagination to forecast future
conditions under which trees may be bred which will be capable eventually
of giving an average yield of 25 lbs. per annum over any given area.
Yields of 1,000 lbs. per acre per annum should be obtained easily.
TREES PER ACRE.--This brings us to the question as to how many trees one
should leave to the acre after thinning operations. Figures have been given
by various authorities, but it appears to the writer at the present time to
be impossible to lay down a general rule. So much depends upon conditions.
In certain cases where the soil is admittedly poor, the average growth
below normal, and thinning has been postponed too long, the writer has been
forced to the conclusion that it would be most inadvisable, and
commercially unsound, to reduce the stand of trees below 120 per acre. In
such instances the average yield per tree equalled only 3 lbs. per annum,
and although the trees were upwards of nine or ten years old the crowns
were small and sparse. It is doubtful whether such trees will ever exhibit
any further development, and to thin them further would probably lead only
to a diminution in the crop per acre.
Under normal conditions of growth an arbitrary figure of eighty trees per
acre has been selected as a standard by many estates. In these cases it
would probably be correct to state that thinning was undertaken on almost
purely empirical lines--_i.e._, that trees were not selected by tests of
individual yields. As far as such a method retained the apparently most
vigorous trees it was successful; but in view of what has been written it
might explain some of the disappointing results which have followed upon
such a system of thinning.
It will be clear that any decision regarding the number of trees to be
retained must be derived from a study of the detailed results of individual
tests. If the large majority of the trees appear to be fairly uniform in
yields the first thinning must be confined to comparatively few trees.
Where there is, on the other hand, a good percentage of high-yielding trees
the final stand per acre may be appreciably less. Unless and until such
information is available, one cannot give any definite opinion as to the
requisite number of trees to be retained per acre.
Similarly, intelligence must be displayed in deciding which of several
uniformly-yielding trees should be removed. In the average sense of this
consideration one must pay no attention to symmetry of spacing, but when
dealing with trees of fairly uniform yields one needs to study the
characteristic development of the trees individually, in order to retain
those which would appear to be most favourably situated with regard to
surrounding trees.
CHAPTER IV
_TAPPING SYSTEMS_
Broadly there are only two methods employed in obtaining the latex from
_Hevea Brasiliensis_. The first is that employed in South America, where
incisions are made by means of a light axe. The other is the system of
excision, or paring, of the bark practised on plantations in the East.
In the early days of the plantation industry, the South American method
seems to have been employed, and the writer has knowledge of trees on one
of our best-known estates in Malaya which still exhibit the outward and
visible signs of that method. At a comparatively early stage, however, the
method of excision was introduced. Curiously enough there appears to be no
record of its inception or of the individual who was responsible for the
substitution of this method. We have been so accustomed to regard it as one
of the ordinary facts of estate procedure, that this point seems to have
escaped notice and enquiry.
As a variant of these two main methods, a slight vogue was for a short
while obtained by the operation known as "pricking." This was generally
combined with excision of bark, and was then known as the "paring and
pricking" method; but the simple operation of pricking alone had its
adherents, and various forms of instruments were designed to achieve the
object. As a means for obtaining a flow of latex, pricking may have been
effective, but the general difficulties attaching to the collection of the
latex was such as to put the method out of favour.
In the employment of "paring and pricking," a thin shaving of bark was
excised on one occasion. At the next tapping no bark was excised, but a
pricking instrument was used along the previously cut surface. It was not
proved that any advantage was gained by this method, which was more
commonly employed in Ceylon than elsewhere, and it would be surprising to
find it in use at the present day.
In the ordinary way the method of excision is practised in such a manner
that the "cut" gradually descends to the base of the tree.
Planters with original views, and of an enquiring nature, often query the
common practice; and it has been suggested that "as the latex descends by
the force of gravity," one's paring should be done in an upward direction,
thus obtaining a greater pressure of latex--and hence a greater flow. It
will be obvious that it would be no simple matter to collect effectively
the latex thus obtained from the under edge of a sloping cut, but apart
from this the argument would appear to be founded upon what is now accepted
to be a fallacy--viz., that the latex _per se_ is manufactured in the
leaves and gravitates down the tree.
FORMER SYSTEMS OF TAPPING.--To hark back ten years in the plantation rubber
industry is equivalent to delving into history, since development has been
so rapid. It was then thought necessary to place upon the trees a number of
simultaneous cuts which the modern planter would judge to be inconceivably
excessive. Were it not for evidence in the shape of photographs extant, it
would be difficult to convince a young planter that such systems were
employed.
It was not uncommon for trees to have from six to ten cuts, sometimes all
placed on one half of the tree in a herring-bone fashion, and sometimes
divided into two portions, each of which tapped the opposite quarter panel
of the tree's circumference. Such superimposed cuts were spaced from 1 foot
to 18 inches apart.
On other occasions, a spiral cut was employed, commencing at a height of,
say, 5 feet, and gradually descending to the cup at the base of the tree.
Later systems varied from several cuts on a half-circumference, or on a
quarter of the tree, tapped either daily, or on alternate days, to cases in
which one-third or one-fifth of the tree was employed. Also popular were
the systems of the [V] and half-spiral cuts on half the circumference.
It did not take long to be recognised that with all these systems demanding
a number of simultaneous parings from the same panel of bark, the rate of
excision was so heavy that the period available for the renewal of bark was
insufficient for continuous tapping.
As a result most of the systems specified have fallen into desuetude, and
the tendency has since been to reduce the number of cuts, or the
periodicity of tapping, so as to allow for increasing periods of bark
renewal.
In the earlier days, a period of four years was thought to be an extremely
generous allowance, whereas six years is now becoming recognised as a
minimum necessity. Eight years is not regarded as extravagant, while with
older bark on some estates periods of ten and twelve years have to be
allowed for full renewal. Even so no finality has been reached, and no
general rule can be laid down. Local conditions of planting and growth
exercise great influence, and the writers have in mind instances in which a
period of eight years has proved to be insufficient even for a first
renewal after the excision of virgin bark.
In the main the most popular systems of tapping are:
(_a_) One cut on a quarter of the tree, tapped daily.
(_b_) One cut on a third of the tree, tapped daily.
(_c_) One cut on half the circumference, tapped on alternate days.
(_d_) A [V] cut on half the circumference, tapped on alternate days.
Variants and extremes are:
(1) One cut on a quarter, tapped on alternate days.
(2) One cut on a half, tapped daily.
Superficially viewed the latter is four times as strenuous as the former,
and the relative position seems to be inexplicable. It may be explained
that as a rule the former system is practised on old trees with poorly
renewed bark, in order to allow for adequate bark renewal; and the latter
is employed in opening young trees just brought into tapping, when the rate
of bark renewal is at a maximum.
[Illustration: TWO CUTS ON A QUARTER CIRCUMFERENCE, ON AN OLD TREE.]
A few estates in this country still continue to tap trees by means of two
superimposed cuts on a quarter of the tree. This was a very popular system
some four or five years ago, but it has come to be recognised by practical
experience that any system employing superimposed cuts leads to a high
consumption of bark without proportionate increase in yield. For instance,
if one compares the system of two cuts on a quarter tapped daily with a
similar system employing only one cut, one finds that the major quantity of
latex is yielded by the lower cut, and that the single-cut system which
excises approximately half the amount of bark gives about 80 per cent. of
the yield obtained by the tapping of two superimposed cuts.
Of experiments to test the relative values of different systems of tapping
there have been many. Most of them suffered from the initial handicap that
they dealt with systems which were then popular. In order to obtain any
valid result they had to be undertaken over a long period. Meantime there
was a progressive movement in actual estate practice towards a greater
conservatism in bark removal, and hence the experiments as originally
planned lost value.
Moreover, in Malaya it was difficult for experimenters to obtain practical
support in the form of areas of trees suitable for experiment. As a result
experiments were often confined to small blocks of trees, and a small
number of blocks, from which any conclusions derived were subject to
considerable errors of experiment. Often comparisons were made between only
two blocks, and no allowance was made for varying factors, such as initial
differences in yielding capacities of the trees, soil conditions, or the
personal equation of the tappers. As a general rule, therefore, the results
were vitiated to a very appreciable extent.
All these factors were later taken into consideration in an experiment
undertaken on behalf of the Rubber Growers' Association. In this instance
unique facilities were provided by the London Asiatic Rubber Company on
their property at Semenyih Estate, and it is only fitting that the company
should receive the recognition which its enterprise deserves.
It would have been a great advantage to have included in that experiment
other features which have since come into prominence, but the original
scope of the experiment had to be confined to the point of comparing yields
obtained in making comparative tests based on one system of tapping with
different frequencies. Such data were required as a check upon a Ceylon
tapping experiment which had attracted much attention. In that experiment
trees were tapped at intervals ranging from one day to seven days; and it
was concluded that after a period of three and a half years trees tapped
with greater intervals gave yields equalling or exceeding those obtained
from trees tapped with shorter intervals.
[Illustration: THE SINGLE CUT ON A QUARTER CIRCUMFERENCE, ON AN OLD TREE
AND ON RENEWED BARK.]
In the Semenyih experiment the system chosen was that which had the
greatest contemporary vogue--viz., two superimposed cuts on a quarter of
the tree. The various blocks were tapped respectively every day, every
second day, and every third day.
It was found that the conclusions drawn from the Ceylon experiment were not
confirmed. After a period of three and a half years' continuous tapping
neither the alternate-day system nor the third-day system gave results in
any way approximating to the yield of the daily system.
The actual average yields from these systems over the whole period were in
the order of--
_Daily._ _Two Days._ _Three Days._
100 per cent. 60 per cent. 45 per cent.;
and throughout the course of the experiment neither of the other sections
showed any appreciable improvement in position relative to the daily
section.
In actual yields "per tapping" over the whole period the alternate-day and
the third-day divisions showed advantages of 20 and 35 per cent.
respectively over the daily portion.
At the beginning of the second year of experiment another section of blocks
was opened with a single cut on a quarter, tapped daily. This enabled
direct comparison between the values of one cut and two cuts on a quarter
in daily tappings and between a daily single cut and two cuts tapped
alternate daily.
It appeared that the daily single cut yielded over the period of experiment
80 per cent. of that obtained by tapping two cuts daily; and that in the
comparison between two cuts tapped alternate-daily and a single cut tapped
daily the latter had an advantage of about 40 per cent. in yield.
This result has been used by advocates of daily tapping generally, but it
does not constitute a fair argument, inasmuch as the single cut was tapped
twice as often, and its position was always relatively low on the hole of
the tree. It has been shown in the comparison between the daily single cut
and the two cuts daily that the influence on yields of the superimposed cut
is relatively small. A fairer comparison would have been obtained if the
two cuts tapped alternate-daily had been either amalgamated to form one
long cut on half the tree or to form a [V] on half the tree, thus placing
the cuts in the opposing sections on the same level. With the knowledge
that the yield obtained from cuts is _always greater per tapping_ by using
the alternate-daily system, it would appear to be plain that the one long
cut on half the tree would at least equal the yield of the single short cut
tapped daily on a quarter tree.
[Illustration: SINGLE CUT ON HALF CIRCUMFERENCE (HALF SPIRAL).
_Note._--In this particular instance the cut is changed to the opposite
half of the tree every half-year.]
Unfortunately no opportunity has been afforded up to the present of
definitely proving this point by prolonged experiment under strict
conditions. It is true that the view is held strongly in some quarters as a
result of the experience of managers, chiefly on their own estates, that
alternate-daily tapping generally gives better yields than daily tapping.
In a number of instances this view is probably correct, and the writers are
in agreement; but it is necessary to clear away some misconceptions which
confuse the issue. In the main there are two schools, one of which plumps
for alternate-daily tapping, while the other adheres strongly to daily
excision. Great confusion exists, inasmuch as in many instances the
disciples of these schools are really discussing different matters. In the
case of managers who argue for alternate-daily tapping their experience is
gained, with very few exceptions, from systems in which the excision covers
half the circumference of the tree; whereas in almost all cases daily
tapping is confined to a single cut on a quarter of the girth. Bearing on
such a comparison there are, as far as the writers are aware, no reliable
published experimental results. To compare the results obtained from one
system practised on one estate with the results of the other system
established on another estate is not strictly permissible, as we know that
conditions generally may vary to an enormous degree.
The controversy has raged, however, to such an extent that many who are not
directly engaged in estate practice have obtained confused impressions. For
instance, it appears to be the belief in some quarters that alternate-daily
tapping, when applied to a single cut on a quarter of the tree, will yield
more than an exactly similar cut tapped daily. In support of such a
statement there does not appear to be any confirmation under normal
conditions; although such a result might be obtained in the case of old
trees which have been heavily over-tapped in the past, and on which the
rate of bark renewal has been appreciably retarded. It might also be the
case eventually when trees with the opposing frequencies have been tapped
for a period extending into many years; but it is the opinion of the
writers that under normal conditions such a result would be extremely
doubtful.
When we come, however, to a comparison of daily tapping on a single cut on
a quarter with double the length of that cut on half the circumference, at
the same height, tapped alternate-daily--whether in the form of one long
cut or in the form of a [V]--we arrive at a contrast which gives a clear
issue. As already stated, facts and figures of reliable experiment are
wanting; but it is the opinion and experience of the writers that the
alternate-daily system at least suffers no disadvantage on the point of
yields, and in other respects, such as conservation of labour and costs, is
superior to the daily system.
[Illustration: A [V]-CUT ON HALF THE CIRCUMFERENCE.]
CHAPTER V
_TAPPING AND COLLECTING_
TAPPING KNIVES.--The choice of a tapping knife is a subject upon which
there is much divergence of opinion. This must be so because no known knife
has such apparent outstanding superior features or claims as would enable
one to settle the point. Moreover, the personal factor is so large that, as
far as the knives in common use are concerned, it appears to exert the
greatest influence. The possibility of obtaining the ideal knife, which
will go to sufficient depth into barks of varying thickness to yield the
maximum quantity of latex without wounding, is quite as remote at the
present time as it was some years ago. Meanwhile the search for that ideal
knife continues, and occasionally one learns of the alleged merits of some
new instrument which, it is said, fulfils all requirements. It is only to
be regretted, both for the sake of the inventor and for the expectant
buyers, that the claims always fail in some one or more particulars.
In Malaya probably the number of different types of tapping knives may
amount to a half-dozen, but those most commonly in use are:
(1) The gouge--straight or bent.
(2) The ordinary farrier's knife.
(3) Modifications of the farrier's knife, such as the "Jebong."
Argument on the respective merits of knives is popular, and discussion
seems endless. It is claimed for the bent gouge that it is superior to the
straight instrument, because, the leverage being downwards on the handle,
the tendency is to lift the cutting edge upwards and out of the bark,
whereas with a straight gouge the tendency is to push the knife downwards
into the bark. It is claimed, therefore, that the average shavings taken
off by the bent gouge should be thinner than those obtained by the use of
the straight instrument.
For similar reasons it is asserted that the "Jebong" and other
modifications are superior to the original form of the farrier's knife.
These points are generally accepted without great argument, but when
comparisons are made between the gouge and the farrier's knife (with its
modifications) the opinions of planters are so varied and conflicting as to
be almost irreconcilable. Two opinions based on experience with both types
of knives are often wholly contradictory.
There can be no doubt that the likes and dislikes of operative coolies have
a considerable influence in determining the measure of success obtained
with any one knife. Should coolies have been accustomed to the use of a
particular form of instrument they become quite expert, and any proposed
change creates in the minds of coolies a prejudice which is considerable in
effect on the quality of the handicraft. Such prejudice may be overcome in
course of time, but in the interval not a little damage may have been done
in the shape of tapping wounds. So considerable is this question of
personal favour that even on estates where a standard pattern of knife is
issued coolies often modify that knife slightly on their own accord. Such
alteration is ignored by the superintendents as long as the quality of the
tapper's work is maintained at a high standard.
Naturally there is a limit to such leniency, and this limit is soon reached
in the case of knives having adjustable parts controlled by screws, or nuts
and bolts, etc. Some knives of this description really merit a much wider
use than is afforded them at present; but in view of the potential damage
which might be done as a result of adjustments made by the coolies these
knives do not become popular.
It is not proposed here to enter into a description of even recent
instruments for which strong claims are being made by their inventors or
vendors. If they possess the merits attributed to them they will soon find
favour, as managers are always keen on studying the points of any new knife
which will lead to a conservation of bark and a reduction in the number of
wounds. On the whole, it may be advanced that the best general results are
obtained by the adoption of a simple non-adjustable knife and the retention
of its use.
THE CHOICE OF LATEX CUPS.--It has come to be recognised that the maximum
possible cleanliness is essential in all details of estate work, and the
younger generation of planters could scarcely be aware that a few years ago
it was deemed sufficient to use coco-nut shells for the reception of latex
on individual trees.
Terne-plate cups ousted the coco-nut shell, and they had the merit of being
cheap. The interior coating of tin did not last long if the cups were
properly cleaned. The iron being exposed, with a minutely roughened
surface, each microscopic projection served as a point around which latex
coagulated. Scrapping the film of interior rubber became more and more
difficult, and often the cups were burnt in order to get rid of the
accumulation of rubber. The last state of such cups was worse than the
preceding one. On some estates fairly successful attempts were made to keep
these cups clean by making the coolies bring them into the store each day.
Terne-plate cups are not now in common use.
Aluminium cups have their advocates, but much the same argument applies to
the difficulty of keeping them clean as was used in the foregoing
paragraph. On many estates, however, they are used with success, the usual
method of treatment being to make the coolies bring them into the store and
clean them there. Owing to the comparative lightness of the material such a
scheme is more feasible than was the case with terne-plate cups.
The cups now most in general use are either of glass or white-ware, and
probably those of glass are the most extensively employed. There are many
details to be studied in the choice between these two types of
cups--_e.g._, percentage of breakage in transport and in the field, price
when breakage is taken into account, etc.; but these apart the glass cups
have one advantage--namely, the ability of the superintendents to see
whether the cups have been properly cleaned. In the case of white-ware cups
this means an inspection and handling of individual cups, whereas in the
case of glass the point is settled by visual examination at a comparative
distance.
[Illustration: SINGLE CUT ON TWO-FIFTHS OF CIRCUMFERENCE.
The opening cut covers two-fifths. Subsequent cuts occupy one-fifth of
circumference.]
Glass cups are made in two patterns, one having a flat bottom and the other
a conical base. The latter is convenient for use when wire supports are
employed, the cup fitting into a loop placed beneath the spout. Used on the
ground its shape is an obvious disadvantage, as, unless a hole is scooped
for its reception, it has to be propped up with sticks or stones. Often a
touch is sufficient to upset the balance, and latex is lost.
The flat-bottomed cup, on the other hand, may be used with success equally
on a wire support or on the ground. It is sometimes said that owing to its
shape the ease of cleaning, as compared with the half-spherical cup, is
diminished, and that if the cups when not in use are kept inverted upon
sticks placed near the foot of the tree the breakage is apt to be high.
This latter objection is being rapidly removed as the practice of using
these sticks is losing vogue for various reasons, and wire cup-holders will
be in general use as soon as the cost of material becomes normal.
There are on the market, and in fairly wide use, cups of Chinese and
Japanese manufacture. These generally consist of brown earthenware with an
interior glass finish. These are cheap in comparison with glass and
white-ware cups, but it is a pity that the glass does not extend over the
whole of the cup. The outer surface has a tendency to collect rubber and
dirt. On some few estates small china bowls or saucers are still used and
are quite satisfactory, except for the favour with which they are regarded
by natives on the outskirts of the estates.
CLEANING CUPS.--The question of cup-cleaning would appear to be a very
simple one; but in practice it is quite a source of worry to managers,
especially where a mixed labour force is employed. Tamil coolies can be
made to clean their cups in the day's task and at odd times. Chinese
coolies, more often than not, either refuse to give the necessary attention
or else demand extra pay for the work.
The method of cup-cleaning employed more popularly within recent years was
that of daily washing. The tapper carried two buckets, one for receiving
the latex and the other containing water. Pouring the latex in the bucket
the coolie then added a little water to the cup and added these rinsings to
the latex collected. The cup was next washed hastily in the bucket of water
and replaced. By the time the coolie has emptied and washed some 200 cups
(about half the task generally) the water has the consistency of dilute
latex, and the wet cup when replaced becomes coated with a thin film of
rubber. If the latex is always collected in one direction it will be clear
that, while the cups at one end of the task are comparatively clean, those
at the other end have the chance of being correspondingly dirty.
Controversy has raged respecting this question of cup-washing, and many
estates have abandoned it as a daily practice. Coolies have not to carry an
extra bucket of water. The contents of the cups are poured into the
latex-bucket, and the bulk of the latex film remaining is also removed by
the aid of a finger. The cup is then replaced, a thin skin of rubber
forming on the interior surface. As a general rule this is easily removed
on the next occasion, except perhaps in dry weather. It is the custom on
most estates employing this practice to have all cups receive special
attention at regular intervals.
There are certain economic factors entering into the difference of opinion
regarding the two broad methods employed. In some cases--_e.g._, on old
areas--it would be practically impossible to follow the older method of
daily cup-washing, as the tappers have to employ two buckets for the
collection of the latex. The employment of special coolies for cup-washing
would be necessitated, such as may be seen sometimes on estates working
Chinese "squatter" labour--where the man taps, a child assists in
collecting, and another child, or the mother, washes the cups. It may be
pointed out that in such instances the helpers are not paid by the estate.
Their services merely mean a saving in time which is spent in the
squatter's garden, and perhaps the permission to the tapper to work a
larger number of trees than would be allotted ordinarily to a task.
Again, on some estates, the tappers, while not being required to carry a
bucket of water for cup-washing, are given an increased number of trees to
tap. Furthermore, on hilly areas under tapping, it is often manifestly
unfair to expect the tapper to be able to carry two buckets during
collection, when the slope is such, as to make the manipulation of even one
bucket a matter of difficulty.
It will be seen, therefore, that there is no clear issue for argument
concerning the two methods, and that the point must be decided on the
economic factors peculiar to each estate or district.
[Illustration: EFFECTS UPON RENEWED BARK OF PREVIOUS TAPPING.
Note uneven surface and callosities.]
[Illustration: ANOTHER EXAMPLE SHOWING THE EFFECTS OF PREVIOUS TAPPING.]
WATER IN CUPS.--Much discussion used to take place regarding the necessity
or otherwise for placing a small quantity of water in the cups when
tapping. It was recognised that the permission to use water (with the idea
of preventing coagulation) led to much abuse, apart from the question as to
the utility of the method. Dirty water was often used, although clean water
may have been placed in the buckets when coolies left the muster-ground.
The small quantity of water often exceeded the actual yield of pure latex
by hundreds per cent., with the result that on arrival at the factory the
diluted latex was below the standard desirable for the preparation of a
good sheet-rubber.
PREMATURE COAGULATION.--Other opinion to the contrary it is now generally
acknowledged that the possibility of premature coagulation in the cup or
bucket is at least not diminished by the addition of even clean water. The
use of water often obtained from estate drains clearly led to increased
trouble. The extent to which such premature coagulation takes place varies
greatly under the influence of many factors--_e.g._:
(_a_) Cleanliness of cups and spouts (the latter an important item
often overlooked, and involving the presence of certain organisms
which effect coagulation).
(_b_) Climatic conditions.
(_c_) Rate and volume of flow of latex.
(_d_) Size of tappers' tasks (involving the length of interval between
tapping, and the collection of latex).
(_e_) Distance to be traversed between the site of the task and the
store.
(_f_) Care in collecting, to exclude extraneous matter.
(_g_) Nature of transport; agitation of the latex to be reduced to a
minimum.
(_h_) Nature of the soil, and situation of the estate.
The last mentioned factor is of great importance. As a general rule it is
noted that premature coagulation is less marked on estates situated on
comparatively hilly land. The greatest effect is remarked on estates
situated on the flat lands of the coastal area where peaty soils are a
feature. On many such estates, in spite of the observance of all ordinary
precautions, it is not possible to receive the latex at the factory without
a large percentage of prematurely coagulated rubber being found in the
transport vessels.
ANTI-COAGULANTS.--For this reason on these (and other) estates, the use of
small quantities of anti-coagulants is common. The effect of these is to
keep the latex liquid and thus render possible the preparation of a higher
percentage of first-grade rubber than would be otherwise obtained.
Among the better known agents which have such an effect upon latex,
formalin and sodium sulphite (not bisulphite) are the chief. The latter is
the more popular as it is slightly cheaper and much more stable. As now
used, it is in the form of an easily soluble powder (anhydrous sodium
sulphite). The ordinary crystalline form of sodium sulphite as used in
photography is not recommended, on account of its comparative lack of power
and its poor keeping qualities.
It will be obvious that, given two equal quantities of different latices,
different amounts of an anti-coagulant may be required to produce the same
effect. Hence it should be remembered that a formula which suits the needs
of one field or one estate will not necessarily prove suitable in the case
of another field or estate. Unless this point is appreciated trouble may
ensue. On some estates it has been the custom to give equal quantities of
sodium sulphite solution to all coolies irrespective of the ages of the
trees in the fields to be tapped. Thus it happened that the latex from one
field was found to have insufficient anti-coagulant present, while that
from another field could only be coagulated by the addition of an excess of
acid. In this matter the experience of the preliminary trials should have
caused some discrimination to be exercised as to the quantities of solution
to be issued in each field or division. It has been found sometimes that a
moist glossiness in the smoked sheet could be attributed to the use of an
excess of sodium sulphite. Traces of the salt remained in the rubber, and
as the substance is hygroscopic, moisture was being absorbed from the air,
to cause a surface deposit which often returned even after the sheets were
surface-washed and re-dried.
If sodium sulphite is to be used in the field, the following formula, which
is in wide use, may serve as a basis for trials.
_Formula for Use of Sodium Sulphite in the Field._
(_a_) Dissolve anhydrous sodium sulphite in water at the rate of 1
pound to 3 gallons.
(_b_) Of this solution each coolie is given about 3/4 pint. This is
usually sufficient for a task of 350 trees. The solution is used by
shaking a few drops into the cup or, diluted with an equal volume of
water, it is run down the main channel when the latex flows.
[Illustration: 1. SHOWING EFFECT OF "WINTERING."]
On some estates it is found either unnecessary or impracticable to use the
solution in this manner. Instead the anti-coagulant is placed in the bottom
of the bucket prior to the commencement of collection. The solution is made
as in (_a_) above, and roughly half an ordinary latex-cupful is placed in
each bucket.
[Illustration: 2. NEW GROWTH OF YOUNG LEAF ON SAME TREE.]
COLLECTING PAILS.--All vessels intended for the transport of latex should
have a smooth and curved interior, so that cleansing may be easy.
Preferably the interior and exterior surfaces should be glazed, but it is
often found that the enamel chips easily, and that the handles are too
frail in construction. The shoulder-pieces, to which the handles are
joined, are often too lightly attached to the bucket. Something stouter in
the shape of enamelled ware is required, without an appreciable increase in
weight. Until such a utensil is available, the heavily galvanised and
brass-bound milk-pails used on some estates are as good as anything at
present in vogue, providing they are kept scrupulously clean.
[Illustration: EFFECTS OF DISEASE--"MOULDY ROT."
(_a_) Note on right hand the panel next in order for tapping; a hopeless
position.]
[Illustration: EFFECTS OF DISEASE--"MOULDY ROT."
(_b_) The present cut badly infected; above there is no renewal of bark.]
The collecting pails should be kept under cover, when not in use, either at
the muster grounds or at the factory. On some estates coolies are allowed
to take them to their quarters, where they are used for various purposes.
Curious effects of this practice have sometimes been noticed. As an
example might be quoted an instance in which premature coagulation was
found to take place to a surprising degree. It was discovered eventually
that the coolies (Javanese in this case) were in the habit of utilising the
buckets for the preparation of their food. A liquid extract of a popular
fruit was often made. This extract was very markedly acid in character,
and as the buckets were not afterwards thoroughly cleansed, the latex of
the following day suffered.
[Illustration: EFFECTS OF DISEASE--"MOULDY ROT."
(_c_) As in (_b_); another tree.]
Preferably all buckets should have a lid of slightly funnel shape. This is
inverted during collection, and thus prevents much dirt falling into the
latex.
[Illustration: EFFECTS OF DISEASE--"MOULDY ROT."
(_d_) At close quarters. Note wounds due, apparently, to bad tapping, but
really caused by the disease.]
PAYMENT BY RESULT.--The arguments for and against the institution of this
practice are many. In actual result there can be no question that a higher
yield is obtained by the adoption of a scheme under which the coolie is
either given a bonus based on result or is paid at a definite rate per
pound. It is fully recognised, both by advocates and opponents of payment
by result, that the personal equation of the tapper is a very important
factor. A good skilled tapper will always obtain a higher yield than an
ordinary individual from the same task of trees, and without any more
injury to the trees. It is argued, therefore, that such an operative should
be given the benefit of his skill. Apart from this, it is claimed that even
the average tapper does not do his best work if he knows that he will get
his daily wage, no matter what his yield may be, as long as he does not
injure the trees by wounding. It is claimed that this sense of security
leads to shallow tapping which, while it has an agreeable appearance, does
not produce the available amount of rubber.
On the other hand, it is advanced in opposition that under a scheme of
payment by result the tappers' only consideration is the matter of
obtaining rubber, and that considerable damage in the form of wounds is
done by over-deep tapping. That there is a great deal of truth in these
statements is not to be doubted. Much, of course, depends upon the amount
and quality of the supervision possible, and upon the standard demanded. It
is a notable fact, however, that on estates which first introduced the
system some years ago the quality of the tapping compares favourably with
that of average estates, and in a few instances within the experience of
the writer the tapping is of a high standard. Possibly these are
exceptional instances, and there can be no doubt that the opposition of
many managers of considerable experience is founded upon the deterioration
in the standard of tapping which often follows the institution of payment
of tappers by result.
It will be recognised by planters that apart from the personal factor in
tapping, the worker might be so unfortunate as to be placed in an area from
which the yield is naturally low, either by reason of its youth or from
other natural causes. Obviously such individuals are entitled to special
consideration in respect of the rate per pound paid for the rubber
obtained. Again, on very hilly land it may be not humanly possible for a
worker to tap the usual number of trees. Hence to place him on a parity
with other tappers, as far as wage-earning capacity is concerned, a higher
rate than ordinary must be given. It will be plain, therefore, that on any
one estate it is generally impossible to set a standard rate per pound for
payment by result; the rate may vary, for example, from, say, 3 cents per
pound in old and high-yielding tasks to 12 cents or more per pound on young
areas of the same estate.
Naturally the actual rates paid will primarily depend upon the average
yield per tree or yield per acre, and the lower the average yield the
higher the rates to be paid per pound. Thus, on low-yielding properties
where the natural conditions render a high yield impossible the rate per
pound may reach a figure of 22 cents (approximately 6d.).
The methods of arriving at the yield of rubber brought in by individual
tappers vary, but broadly they fall into two classes:
(_a_) That in which the volume of latex is ascertained (either by
measuring or by weighing), a sample is drawn, and the final
calculation made from the weight of the more or less dry sample.
(_b_) That in which, after noting the volume, the calculation is based
upon a reading of the dry rubber content of the latex, obtained by
means of an instrument such as the "Metrolac," or any other instrument
working on the same principle.
Quite a number of estates which have not adopted the full system of payment
by result yet employed some such method of checking the yields of
individual coolies, as the observed results act as a great deterrent
against various malpractices, such as neglecting to tap trees, adulteration
of the latex, etc.
TREE-SCRAP.--The thin film of latex which coagulates naturally upon the
surface of the tapping cut after the latex has ceased to flow is known as
"tree-scrap." Normally it is collected on all estates, but the method of
collection varies according to the class of labour employed. On most
estates, where the labour is Tamil or Javanese, it is supposed to be
removed as fully as possible before the tapping cut is reopened. The narrow
strips are then placed in a bag or basket carried by the tapper. Chinese
tappers usually decline to follow this practice of first peeling off the
scrap, and remove it by the operation of tapping, with the result that the
scrap when brought into the store has adhering to it various shavings of
bark. Unless these can be thoroughly cleaned off the scrap cannot truly be
classed as "tree-scrap."
OXIDATION OF TREE-SCRAP.--It is often noted that some scrap is dark in
colour, and in this condition it is generally spoken of as "oxidised"
scrap. The oxidation is probably due to an enzyme, and also to the presence
of chemical substances of a phenolic nature. In the course of laboratory
experiments with normal latex, it was found possible to reproduce this
darkening due to oxidation by the addition of very small quantities of
various phenols used in general chemical processes, and the rapidity with
which the darkening was effected depended upon the quantity of the phenol
added. If this rapidly oxidising latex be mixed with normal latex, it would
seem that the whole bulk of the latex is affected by this tendency to rapid
oxidation. It is observed that this condition under which any tree may
yield rapidly oxidising latex is not a permanent one.
CARE OF TREE-SCRAP.--As these scraps eventually give a grade of rubber
which compares well with other and better-looking grades care should be
exercised in collection and treatment so that its quality is not impaired
in any way.
TO PREVENT OXIDATION.--As a rule the scraps are picked over, and heavily
oxidised pieces are sorted out; otherwise the crepe rubber prepared
exhibits black streaks. The scraps should not be allowed to remain in the
sun (which induces "tackiness"), and if they have to be kept over night
they may be placed in a weak solution (1 per cent.) of sodium bisulphite to
arrest oxidation. It should be recognised that such a solution will not
"bleach" already darkened scrap-rubber, and the nature of its action is
only anti-oxidant.
BARK SHAVINGS.--In the matter of collecting bark-shavings much depends upon
the organisation and nature of the labour force. Probably, on the majority
of estates bark-shavings are collected systematically, but on quite a
number considerable laxity in this respect has been noted. This may arise
from lack of adequate supervision or from the peculiar systems of working
which seem to pertain to Chinese labour. Granted that the trees are well
"scrapped," and that the percentage of rubber obtained from shavings under
such circumstances would be extremely small (say 2 per cent. by weight on
the total output), it does not need much calculation to see that annually
the loss of rubber to the estate must be considerable. It would also seem
to follow that, if the adult labour declines to pick up bark-shavings
carefully, it might pay to employ children for the purpose. Or, as is done
in some places, the adult labour might find it advantageous to collect
bark-shavings at low rates per pound.
It is a well-known fact that if bark-shavings be allowed to accumulate in a
heap for any but a short period, a fermentative and heating action is set
up. The heat developed in these piles of shavings is so considerable that
it is impossible to keep the hand in a heap for more than a second or two.
Should this be allowed to persist, as would happen in the case of a
breakdown of engine or machines, it usually results in the final crepe
rubber becoming tacky when approaching dryness.
To avoid this heating effect it is necessary to have spare jars or proper
tanks in which the shavings may be soaked in water. In this condition
bark-shavings may be kept for many days.
For the same reason (_i.e._, the heating effect and consequent tackiness)
the custom followed on some estates of allowing coolies to keep
bark-shavings in their "lines" until they have accumulated a fair quantity
cannot be commended, quite apart from the possibility of actual loss by
theft, which is thus rendered easy.
It will be clear that where the trees are scrapped efficiently before
tapping, the amount of rubber to be obtained from the treatment of pure dry
shavings would be almost nil, and would scarcely repay the cost of
collection and working. In actual practice, however, it is not possible to
guarantee that the shavings are free from some scrap-rubber. Shavings
brought in by Tamils and Javanese carry only a small amount of rubber,
whereas where Chinese tappers are employed the yield of rubber may be as
high as 35 to 40 per cent. upon the total weight of the material treated.
Few estates now are not equipped with "scrap-washers"--machines specially
designed for removing the bark from the rubber--and if they function
efficiently the resulting crepe should be free from bark-particles.
COLLECTION OF EARTH-SCRAP.--This, the lowest grade of rubber, is found at
the base of the tree. Theoretically, if proper precautions are observed,
the amount should be comparatively small, but in actual practice it may be
very appreciable. The usual contributory causes are:
(_a_) Failure to replace cups beneath the spouts of trees which
continue to drip latex after collection.
(_b_) Collection of latex at too early a stage.
(_c_) Failure on the part of the tapper to ensure the flow of latex,
by means of the spout, into the cup.
(_d_) Flowing of latex over the edge of the cut before it reaches the
vertical channel.
(_e_) "Wash-cuts" on wet days, when the volume of rainwater down the
tree is sufficient to wash the latex out of the cup.
The amount of earth-scrap collected on any estate will depend, all other
things being equal, upon the labour expended in its collection. Certainly
on well-organised estates, having ample labour, the amounts collected are
huge in comparison with other estates. The ground at the base of the tree
below the latex-spout is systematically turned over with pointed sticks and
large clots of rubber are often picked up. Here, again, it is advised that
the collected earth-scrap should not be allowed to remain in heaps upon the
floor of the factory. It should be placed in suitable tanks containing
water, and quite a considerable portion of the cleansing work is thus taken
from the machines.
CHAPTER VI
_TRANSPORT OF LATEX AND COAGULUM_
PERCENTAGE OF FIRST LATEX AND OTHER GRADES.--One of the problems
confronting any manager is the question of the percentage of first-grade
rubber calculated upon the whole output. Inquiries are constantly being
received for advice as to what the various percentages of each grade of
rubber should be. This is a question to which no definite list of figures
can apply. There are so many little factors influencing the result. Some
estates are not particularly careful in collecting tree-scrap. Hence quite
a quantity of tree-scrap finds its way into the crepe made from
bark-shavings. On the other hand, bark-shavings are not collected
systematically on some estates, and the total output is thereby diminished.
In consequence the first-grade rubber shows a higher percentage than it
would otherwise. Again, if the earth-rubber is not regularly collected the
percentages of the best grades are higher than they should be. In comparing
the percentages of each grade of rubber from any two estates, therefore,
one should have all the information possible as to the various working
details of the estates. Without wishing to lay down any definite
proportions which can be applied to all estates it might be said that,
taking averages over a large number of estates, the percentages to be aimed
at are:
First-grade latex 75 per cent. to 80 per cent.
Other grades 20 " " 25 "
For these figures one promises that all lower grades are collected and
accounted for carefully and regularly. The distribution of the lower grades
will depend upon the field practices of the particular estate, but the
following list might be given for an estate keeping all lower grades
distinctly separate:
First-grade latex 75 per cent.
Cup-washings }
Coagulated lump, etc. } 10 "
Tree-scrap 9 "
Bark-shavings 4 "
Earth rubber 2 "
---
100 "
Emphasis is again laid on the statement that these figures must not be
accepted as a standard. Nevertheless, they may prove of some service to
managers in giving an idea of what the general line of percentages may be.
There are special circumstances, such as distance of transport and the
nature of the land, which at present would render the attainment of more
than 75 per cent. first-grade rubber impossible on some estates. Still the
fact remains that if the percentage is low through distance of transport,
etc., some method will have to be discovered by means of which the
difficulty maybe overcome. On a few estates the percentage of first-grade
rubber obtained sometimes reaches 85, but these results are rather out of
the ordinary. An estate which collects all lower grades properly is doing
well if the percentage of first-grade rubber is 75 or over.
EARLY COLLECTION.--As already noted in the preceding chapter, one of the
factors influencing premature coagulation is that of the interval elapsing
between the commencement of tapping and the collection of latex. It will be
seen that this ordinarily would depend, in turn, upon such considerations
as the size of the tappers' tasks, the spacing of the trees, and the
natural conformation of the land over which the tappers have to perform
their tasks. In general it need only be remarked that every possible
consideration should be given to this question, and that any delay should
be avoided.
TRANSPORT.--Wherever possible it is endeavoured to convey latex from field
to factory by man-power. Tamil coolies, as a rule, place the bucket on the
head; Chinese and Javanese coolies like to use a balanced carrying-pole.
Where distance renders these methods too costly in time and labour, it is
usual to have field centres where the latex is collected and dispatched to
the factory generally (_a_) by means of vessels conveyed on light railways;
(_b_) in large cans placed on motor-lorries; (_c_) in cylindrical
galvanised drums supported on two wheels and drawn by bullocks. There may
be variants, but these are the chief means of transport in bulk over a
distance.
Where possible, the best system is that employing a trolley-line, as great
agitation of the latex is avoided, and the time in transit is much reduced.
The usual method of transport by bullock power is slow, and as estate roads
(and even Government roads) are often below the standard expected in this
country, the jolting undergone by the latex is, to say the least, not
calculated to afford a high yield of first-grade rubber. The late Mr. F. W.
F. Day advocated the use of a circular perforated wooden grid, to be
floated on the latex, in order to moderate the wave effect produced by
jolting.
Whatever the means of bulk-transport employed, it should be the care of
those in charge to see that vessels are not allowed to remain in the sun
longer than is necessary. Even during the journey they should be shaded in
the best possible manner.
These large transport vessels usually receive what is really only
perfunctory attention in the matter of cleaning. They should receive the
same care as would be exercised in dealing with milk cans in other
countries. Ordinary sluicing with water is not sufficient, and if they
cannot be sterilised by means of boiling water, they should be treated,
after ordinary washing, with a 5 per cent. solution of sodium bisulphite
every day.
ANTI-COAGULANT FOR TRANSPORT.--When anti-coagulants are not used in the
cups or buckets, it is advisable to use them in the bulk-transport vessels.
Either formalin or sodium sulphite is of service, but the great objection
advanced against the former is its loss due to evaporation while the carts
are going to the fields or waiting at the centres. For this reason sodium
sulphite is now generally employed.
_Formula for Use of Sodium Sulphite in Transport._
(_a_) Dissolve 1 pound of powder in 3 gallons of water.
(_b_) Of this solution, place half a gallon in the vessel for every 30
to 40 gallons of latex.
TRANSPORT BY COOLIE.--As already pointed out, the extent to which man-power
can be used in transport of latex is generally limited. On small estates it
is an easy matter for coolies to carry the latex to the factory, but on
larger estates many difficulties may arise, which may also militate against
the successful use of other means of transport. It is not uncommon to find,
therefore, that a policy of decentralisation has been adopted.
COAGULATION CENTRES.--Divisions of the estate have their own small stations
at which latex is received and coagulated. In this way it is possible to
receive latex without much delay, and with benefit to the resultant rubber,
especially if prepared in sheet form. Much controversy has arisen regarding
these decentralised establishments, but the fact remains that on large
estates, which are efficiently controlled, the scheme has been highly
successful from all points of view. On the other hand, it is alleged that
this method of working increases costs, and often gives an unsatisfactory
quality of rubber. Concerning the latter point it seems to be reasonable to
expect that the European in charge of any division should be conversant
with the method of preparation required, and should be capable of seeing
that no mistakes are made. Given uniform equipment in all stations, and
uniform rules for treatment of the latex, there does not appear to be any
valid reason why the product of one station should be inferior to that of
the others. Neither is it so in the case of several estates which might be
quoted.
In the matter of costs of working the writer has had to investigate several
cases regarding which there was dissatisfaction. In some instances it was
found that the stations had not been placed advantageously with respect to
a water-supply; and instead of one or two coolies pumping for an hour or
two, a larger number had to be employed for hours in the carriage of water
from the nearest available source. This meant that, as the coolies were on
daily wage, the force appeared to be much bigger than that usually
required. In other cases there were too many store coolies, when often the
place of some could have been taken for the necessary period by tappers
arriving early from the nearer fields. Sometimes costs were increased by
reason of the use of an excess of chemicals, owing to the lack of uniform
rules throughout the several stations. In spite of all that has been
written, and the verbal instructions that have been given, it was not
uncommon to find unstable chemicals such as sodium bisulphite exposed to
the moist air. In this way not only was there waste of material, but also
the probability of inferior rubber being made.
TRANSPORT OF COAGULUM.--On the whole if it is a question between the
transport of latex and the transport of coagulum, the writer would always
favour the latter, for reasons which have possibly been made clear in the
preceding paragraphs. In effect, it should be recognised that the less
handling and transport the latex receives the better the general result.
If proper precautions are taken, the transport of coagulum intended for the
preparation of crepe should present no difficulty, and should have no
injurious effect upon the quality of the resultant rubber. It is only too
common, nevertheless, to note defects, in the finished crepes, which can
only be attributed to a failure to observe reasonable care in the transport
of the coagulum. For example, it has been observed that a mass of coagulum
from a coagulation station has been conveyed on the floor of a
bullock-cart, or motor-lorry, previously used in the carriage of other
materials. Unless the boards have been most scrupulously cleansed, the
coagulum is found to be contaminated, often to a marked degree. Again,
although the cart may be clean, it may have to travel some distance on
roads carrying a fair amount of motor traffic. Even should the cart have a
canopy, road-dust is often whirled through the open sides of the cart; and
in the districts where red laterite roads are common, the stain of such
dust often persists in the finished crepe. It scarcely need be remarked
that coagulum should be transported in closed wooden boxes or in galvanised
iron drums fitted with lids; and that preferably sufficient water should
be present in these receptacles to allow the coagulum to float. All such
containers should receive the same scrupulous attention as the vessels
employed in the transport of latex.
The successful transport of coagulum for sheet-making is fraught with much
greater disabilities, and it is usual to note on most estates that the
resulting sheets from out-stations are always inferior, in final result, to
those coagulated and prepared at the central factory. If the flat pieces of
coagulum are placed in piles of any height it is common to find, on arrival
at the factory, that much adhesion has been caused. There is great
difficulty in separating the pieces, and often the successful operation is
impossible. It is usual to hand-roll the coagulum before transport, but it
is often found that by the time the rubber reaches the factory it has
become too hard for subsequent good results.
One of the strong arguments in favour of the establishment of divisional
stations is to be found in the preceding paragraph. Sheet-making, as it
necessitates the employment of only light machines suitable for hand-power,
is a feasible proposition in a field station. There is no reason for sheets
made thus to be in any way inferior to those made at a central factory; in
fact, they are often better, as the latex has the chance of being treated
when comparatively fresh.
If it is found necessary to transport sheet-coagulum, every possible
precaution should be taken against piling the pieces.
After hand-rolling some estates bring the rubber from the field-stations to
the central factory in drums of water, others in shallow boxes containing
not more than half a dozen sheets in a pile. A method proposed long ago,
but not in practice, was to have a number of shallow trays subdivided so
that each compartment held one sheet only. If these trays were properly
made and carefully fitted there appeared to be no reason why they should
not form sliding parts of a large box, in which squeezing and adhesion of
the pieces of coagulum would be avoided. Naturally any such device would
increase appreciably the weight to be transported, and on this ground would
not find popular favour except where motor-power is used for road
transport.
PART II
FACTORY OPERATIONS
CHAPTER VII
_PRELIMINARY TREATMENT OF LATEX_
RECEPTION OF LATEX AT THE STORE.--Bearing in mind the remarks in Chapter
VI. on the conditions under which latex is transported, it follows that
nothing but the very best and most suitable vessels should be used in the
store. A point to which adequate attention is not given in many factories
might be mentioned here. Considering the importance attached to colour in
the dry rubber by brokers and consumers, and knowing how extremely trivial
are the causes which may mar the colour, it is rather surprising that
better provision is not made for the reception and handling of latex in
factories. Too often the receiving vessels are placed on the floor of the
store close to the entrance. Coolies bringing in latex cannot avoid
bringing with them quite a considerable amount of dirt. Presuming that a
hose-pipe has been installed, and that the floor is constantly being
sluiced down with water, no great harm will result. But would it not be
ever so much better if the dirt were kept out? In how many factories is
provision made for this? Such an arrangement is not difficult to make, and
is already in practice on a few estates. A verandah is built outside the
wall of the factory and all latex is received there. In another place open
chutes are provided which terminate in the straining sieves. The coolie
thus stands on the verandah where he removes coagulated lump and impurities
from the latex, which is then poured down the chute, passing through the
sieve into large coagulating jars or tanks.
Too often it would appear, from the writers' observation, there is a lack
of adequate supervision on the arrival of latex at the store. Much can be
learned from an inspection of the coolies' buckets, and the cause of small
defects in the finished rubber can often be thus traced. Leaves, stems,
bark-shavings, and dirt appear in the buckets, and it is a source of
constant surprise to imagine how even unintelligent coolies can allow such
things to happen. These objects are removed before or during straining, but
still they ought not to be there in the first place, and the fact that such
a state of things exists is evidence of neglect on the part of the coolies
or lack of supervision. Efforts are made in a large number of cases to cope
with these troubles, but on some estates things are allowed to proceed in
the same slipshod way, and too much responsibility is thrown on the
straining process.
[Illustration: RAISED VERANDAH FOR RECEPTION OF LATEX; LIKEWISE EQUIPPED
WITH FACILITIES FOR CALCULATING INDIVIDUAL DAILY "YIELD PER COOLIE" BY
SAMPLING OF LATEX.]
It is suggested that it should be the business of a European to supervise
the reception of latex every day. This is at present quite impossible on
some estates, but it does not alter the fact that this supervision should
be provided, and is extremely necessary.
It is surprising how the point is overlooked in many factories--not that
they are in a dirty state, but they fall short of being classed as clean
factories for want of the little that makes the difference. Possibly those
in charge do not believe that all this fuss need be made, but the writers
can assure them, from a practical knowledge of a very large number of
factories, that cleanliness does pay.
It might not be credited to Tamil coolies, but yet it is probably true,
that the moral effect of working under the cleanest and best conditions has
an influence upon the store coolies, and that their work is better in
consequence. Everything which will tend to simplify the cleansing of the
factory should therefore be installed. Hose-pipes, glazed tiles, clean
floors, plenty of light and air are not fads or fancies, but considerable
factors in determining the final quality of the rubber. There is
considerable truth in the suggestion that the coagulating room and machine
room should be as "spick and span" as a modern home dairy.
STRAINING OF LATEX.--This is a most necessary process, and one which
usually entails much trouble and time which one could wish avoided. It will
be admitted that the trouble could be reduced greatly if the regulation of
field processes could be made more stringent. In spite of knowledge that
impurities must not be allowed to enter the cups, coolies will ignore the
rule that the cup must not be placed in position until the bark shaving has
been cut. The result is that pieces of bark fall into the cups, and coolies
are generally too careless or too hurried to remove them.
Again, when cups are placed on the ground, it is easy to see that dirt may
adhere to them. In the collection of latex some of this dirt may fall into
the bucket. Since the introduction of cup-holders on many estates the
trouble from this source has decreased considerably, but, nevertheless, it
may be taken for granted that even under the best of conditions all latex
requires straining.
The best type of strainer has yet to be evolved. Usually it consists in
principle of a piece of fine brass mesh contained in some form of holder.
Theoretically such a strainer should work well, but in actual practice
nearly all strainers are a source of continual worry. Undiluted latex, as
received at the factory, is of a rich consistency, containing very fine
particles of dirt and often minute particles of prematurely coagulated
rubber. The latter soon clog a fine mesh strainer, while the former may
pass through. When the flow through the strainer becomes slow, the coolie
in charge generally rubs the top surface of the sieve with a piece of
coagulum, thus forcing material through the mesh. He then rubs the
under-surface, with the result that undesirable matter falls into the
strained latex. In theory it seems a simple matter to have a number of
sieves ready so that a clean one may be substituted for a clogged one,
which should be cleansed at once with water. In practice the factory coolie
will probably only carry out instructions when the eye of the
superintendent is alert. As a result of the rubbing and consequent strain,
the brass mesh usually breaks away from its support and the fracture may
not be detected for some time, during which irreparable damage may have
been done to the resultant rubber.
In view of the presence of the fine particles of dirt, to which allusion
has been made, fine sieving of the latex appears to be essential,
especially when sheet-rubber is to be prepared. The fine sieves are
generally of the type known as "60 mesh," and they do not usually give
thoroughly satisfactory results even when the gauze is supported and
strengthened by means of cross-wires placed underneath. The general fault
with these strainers is that a sufficiently wide "selvage" is not allowed
in the clamped edges of the gauze, or that the edges of the support are so
sharp and abrupt that the strands of the gauze are soon severed by the
strain imposed in vigorous cleaning.
Many estates use two strainers; the first a more robust one containing "30
mesh" gauze, and the second the fine "60 mesh." Even this device does not
bring about the desired immunity from trouble. Relief could be obtained if
the latex were always in a more freely fluid form. Estates employing
anti-coagulants in the field benefit in this respect. Other estates,
although finally using the finest of mesh, experience far less trouble than
most estates by reason of a difference in method of working. This can be
explained by an outline of the system adopted on a particular estate:
(_a_) On arrival of the rich latex at the store, all visible
coagulated lumps and other extraneous matter are removed by the
tapper.
(_b_) Each tapper's latex is diluted with a quantity of water.
(_c_) The diluted latex passes through two sieves, one above the
other. The top sieve is of stout perforated zinc sheet, with 10
circular holes to the inch. This removes all large particles. The
lower is of "30 mesh" brass gauze, and practically no rubbing is
required. The latex is now in glazed-tile tanks, in which it is
further diluted to the required standard by means of a recording
instrument.
(_d_) The latex flows by means of a chute into the coagulating tanks,
passing through a large "60 mesh" sieve.
It is not guaranteed that this method will furnish a complete absence of
very fine particles of dirt in sheet rubber, as the human element enters so
largely into the question; but it can be stated that no complaints have
been received on the point of "specks of dirt" since this system was
inaugurated.
On the same estate fine sieving in the preparation of pale crepe has been
abandoned as an unnecessary refinement. The two coarse sieves mentioned
above are employed only, and it is to be acknowledged that the results
justify the procedure.
BULKING OF LATEX.--Not long ago advanced estates used to combine all latex
before coagulation, in order to obtain uniformity of product. Previously it
had been the custom to deal only with comparatively small separate volumes
of latex, with obviously great disadvantage.
Since the introduction of instruments such as the "Metrolac," by means of
which any volume and all volumes of latex may be reduced to a common
standard of dry rubber content, the necessity for "bulking" has passed. It
is not now necessary to keep latex standing, perhaps for two hours,
awaiting the arrival of other latex from distant fields.
STANDARDISATION OF LATEX.--In modern practice, as already pointed out, it
is possible now to handle any volume of latex with a view to its reduction
to any required standard of dilution for the purpose of obtaining a
uniform product. For the reception and subsequent handling of the latex
various schemes have been devised, and they are usually planned in
connection with coagulating tanks used in the preparation chiefly of sheet
rubber.
[Illustration: END-SECTION SKETCH OF VERANDAH, ETC., SHOWING A GOOD METHOD
FOR RECEIVING LATEX AND FILLING TANK.
T, Sheet coagulation tank; C, cylinder for reception and dilution of latex;
GG, gutter; PP, raised platform on verandah; SS, steps leading to platform;
W, dwarf wall; EE, expanded metal partition; OO, open.]
In the successful working of a tank it is necessary, in order to obtain the
best results, to standardise all latex. This cannot be effected properly in
the tank itself, and hence it is the practice to dilute each lot of latex
to standard before it is run into the tank. In the ordinary way this would
entail a great deal of labour in handling the diluted latex. To obviate
this, the scheme outlined in the accompanying sketch has been suggested on
several occasions and in various quarters. Such a scheme or modification of
it has been put into successful practice on several estates. Although the
drawing was made some considerable time ago when estates were not then
prepared to go so far in this direction, subsequent modifications show only
minor differences which, while leaving the original principle intact,
testify to a fertility of resource in adapting the idea to existing
circumstances and buildings. The drawing is _in toto_ almost a replica of
the original installation now in successful use on the Kinrara Estate of
the Ledbury Rubber Company. On this company's Ledbury Estate likewise a
similar system is employed, except that the reception verandah is part of a
natural formation and needed no such direct raising. Several other estates
have now adopted the scheme, which has been proved to be of practical
value. The writers make no claim to originality in the idea, which might
have occurred to many independently on the introduction of coagulating
tanks.
[Illustration: RAISED VERANDAH FOR RECEPTION AND HANDLING OF LATEX.]
VERANDAH.--In reproducing the drawing it is believed that the sketch will
convey practically all the information required. It may be explained that
the coolies are allowed to enter only the outer part of the verandah. The
buckets are handed across the low wall into the care of factory coolies,
who strain the latex through gauze sieves into the latex cylinders.
LATEX RECEPTION VESSELS.--These cylinders may be similar to the tanks
commonly used for transport of latex from distant fields to the factory. An
80-gallon cylinder is easily mounted by its trunnions on a suitable iron
framework which is superimposed on a skeleton truck.
[Illustration: ANOTHER SET OF DILUTION TANKS ON RAISED VERANDAH.]
The latex is diluted down to standard in the cylinders, the truck is moved
opposite the compartment to be filled, and a light movable gutter is placed
beneath the vent of the outlet pipe. This pipe is fixed in the bottom of
the cylinder, and is provided with a large stop-cock which is operated by a
spanner key. The stop-cock should be of the simplest type, capable of being
taken apart and assembled in a minute or so. The orifices should be large
enough for a coolie to insert at least two or three fingers so as to
facilitate cleaning, and the pipe should have no right-angle bends.
On the inside of the cylinder a scale of gallons may be painted, so that
one may possess a knowledge of the quantities run into, or required for the
completion of, any compartment.
A SCREW PLUG UNSATISFACTORY.--It may be of benefit to managers who
contemplate such an installation to know that the adoption of a stop-cock
in the vent pipe of the cylinder is the outcome of experience. In one
instance the vent pipe as designed was fitted with a screw plug at the end.
Unfortunately with this arrangement the flow could not be regulated, and
owing to the "head" of the latex it dashed violently down the gutter,
struck the bottom of the coagulating tank, and thence was scattered over
the factory.
ANOTHER INSTALLATION.--In another type of installation, in place of the
vessels travelling upon a raised verandah platform, the standardised latex
is conveyed to the coagulating tanks by means of drums supported by hooks
to a chain-block and pulley which travels on an overhead gantry. This
method is practicable, but may be regarded as less satisfactory in general
working than the verandah method of treatment.
A MODERN INSTALLATION.--In the most recent scheme for dealing with the
reception of latex, its standardisation, and conveyance to the coagulating
tank, the main principle of the first system outlined is retained; but the
receptacles are not mobile. Glazed-tile tanks are employed, the capacity of
each being approximately equivalent to that of each unit coagulating tank.
The accompanying illustrations show the general arrangement and some
details of the system of reception tanks employed on the well-known
Pataling Estate.
CHAPTER VIII
_COAGULATION_
Whether it is necessary to employ any coagulant, or whether latex should be
allowed to coagulate naturally, will not be discussed at this stage.
Neither will mention be made of any patent processes of coagulation which
employ other than acid mediums. These subjects will be treated in a
subsequent section of the book.
CHOICE OF COAGULANTS.--It is not proposed here to enter into a discussion
as to the merits of the dozens of known coagulants. Suffice it to state
that acetic acid, although the oldest general coagulant, still remains the
best and safest at the present time. There is a deal to be said in favour
of the use of another organic acid, formic acid. It is equally as safe as
acetic acid, and quite efficacious; the only drawback is that, taking all
things into consideration, it is very slightly more expensive. Acetic acid,
therefore, will always be implied in this chapter when the word "acid" is
used.
STRENGTH OF ACID SOLUTION.--In the old days it was the rule rather than the
exception to find pure, undiluted acid used in coagulation. In many cases
no harm resulted, for the simple reason that, owing to the large proportion
of water in the latex, the acid was thereby very much diluted. The estates
had to thank the over-dilution of the latex for the non-injury of the
resulting rubber.
Some estates make up a stock solution of 1 part acid to 20 of water, and
use this with success because of the fair amount of added water present in
the latex.
It must be understood that what is being referred to now is not the
absolute quantity necessary for coagulation, but the proportions--_i.e._,
the respective volumes of acid and water in the solution of acid made up
every day. That the strength of the acid solution, as well as the quantity
used, has an effect upon coagulation can be easily demonstrated in the
following way:
Take separate and equal lots of the same latex, and to each add the same
quantity of pure acid, but in each case diluted with varying quantities of
water. It will be found that coagulation is quickest where pure acid is
employed, and slowest where the acid is most dilute. It will also be found
that, providing the quantity of acid employed was sufficient for
coagulation, the best and most uniform coagulation is obtained from the use
of the most dilute acid, within limits. It will often be found that where
pure acid has been employed coagulation is local--_i.e._, we have lumpy
coagulation, and often a very milky remaining liquor. This is due to the
fact that, as coagulation is immediate upon the spot which is first touched
by the pure acid, a deal of the acid is enclosed within the rubber at that
spot, and hence other portions of the latex are deprived of acid. It is in
such cases that most air-bubbles are enclosed.
As the dilution of the acid solution is increased the mixing is more
thorough and uniform. Coagulation is slower, and air-bubbles can escape to
the surface.
METHOD OF MAKING STOCK SOLUTION.--Experiments have been repeatedly made in
the laboratory with acid solutions of varying dilution, from pure acid down
to 1 part of acid in 500 parts of water. While it has been found that a 1
in 5 solution can be used where the latex is very dilute (say, 1 part of
latex to 5 parts of water), and a 1 in 20 solution may be used in fairly
dilute latex (for crepe-making), it is undoubtedly a fact that for latex as
generally "standardised" on estates a much more dilute solution of acid
should be used--_e.g._, 1 in 100, or even 1 in 200, of water. It must be
borne in mind that the quantity of acid necessary for coagulation is not
changed, but merely the dilution. Let us take a concrete case to illustrate
the point:
On an estate at present the stock solution is made up by diluting 1
pint of acid with 20 pints of water, and 1 gallon of this is necessary
to coagulate 50 gallons of pure latex.
It is desired to use a stock solution of 1 pint of acid to 100 pints
of water. Evidently, therefore, 5 gallons of this stock solution
contain only the same quantity of pure acid as 1 gallon of the old
solution contained, and it will be necessary to add 5 gallons for
every 50 gallons of pure latex. Thus:
1 to 20; 1 gallon necessary for 50 gallons pure latex.
1 to 100; 5 gallons necessary for 50 gallons pure latex.
It may be pointed out that the quantities worked out in the foregoing
examples are not absolutely and mathematically correct, but they are quite
close enough for all practical purposes.
It may be advanced by someone that if a dilute solution of acid, such as 1
in 100, is used the bulk of this stock solution (5 gallons to 50 gallons of
latex) is very great, and might be injurious to the quality of the
resulting rubber. A moment's consideration will show that, after all, the
volume of acid solution is only one-tenth that of the volume of latex. This
can have no effect upon the quality of the rubber. Even dilution of the
pure latex with half its bulk of water in the factory will have no effect
upon the quality of the resulting rubber. It is to be remembered that,
except in cases where the proportion of added water to latex is absurdly
large, the main argument against putting water into the latex-cups is
against the possible poor quality of the water rather than against the
actual small quantity theoretically added. It is acknowledged that, where
the water to be put into the cups can be guaranteed to be of good quality,
no great objection would be raised against placing the smallest possible
quantity of such water in the cups. But how many estates have such good
water easily available to the coolies, and how many estates can be sure
that only that smallest possible quantity would be used? It is a notorious
fact that, even on estates where the quantity of water used was supposed to
be a minimum, the proportion of water to latex in some cups often exceeded
even three or four to one. In any case it may be stated as an elementary
truism that the absence of water is more to be desired than water of
doubtful quality.
QUANTITY OF ACID.--As a result of repeated experimental work it has been
found that, for pure average latex, the quantity of acid necessary for
complete coagulation, reckoned in parts of pure acid to parts of latex, is:
1 part pure acid; 1,000 parts average latex.
Where the latex is rather richer than average (above 30 per cent. dry
rubber) probably a little more acid would be required, and similarly if the
dry rubber content is lower the quantity of acid must be less.
It used to be a common belief that the more dilute the latex the greater
the quantity of acid necessary, but this would only apply to cases of
extreme dilution of latex.
As a matter of fact, up to certain limits of added water, the reverse is
actually the case--_i.e._, the more water in the latex the less acid must
be added, assuming that for pure latex the proportion of pure acid to latex
is taken as 1 part to 1,000 parts. This was found to be the case up to
dilutions of three or four times the volume of latex. To take concrete
examples which will perhaps make the truth more clear:
Assuming we commence by making up our stock solution of acid by adding
100 parts of water to 1 part of pure acid, this gives us a mixture of
1 to 100. For 1 gallon of pure latex it would be necessary to add
one-tenth of its volume of the above mixture--_i.e._, 16 ozs.
Suppose we take a gallon of pure latex and add a gallon of water, we
now have 2 gallons of so-called latex. But we still have only 1 gallon
of real latex present in the diluted latex, and it is only necessary
to add sufficient acid to coagulate this gallon--_i.e._, 16 ozs.
Further, if 1 gallon of latex be diluted with 2, 3, or even 4 gallons
of water it is still only necessary to add 16 ozs. of the acid
mixture.
At dilutions beyond this limit, however, it is necessary to add a
little more acid to obtain complete coagulation.
In the process of preparing sheet rubber it is very necessary to see that
the minimum quantity of acid is used, otherwise visible defects are caused.
But in coagulating latex intended for preparing crepe, where the rubber
undergoes protracted washing on the machines, the presence of a slight
excess of acid in coagulation is not calculated to cause any deterioration
in the quality of the rubber. Advantage must not be taken of this statement
to argue that more than a slight excess may be used without injury to the
rubber, as it can be shown that the use of a large excess of acid results
in an inferior rubber.
QUANTITIES NECESSARY FOR MODERN REQUIREMENTS.--It may be commended to the
notice of the beginner that any further experimental work as to the
quantity of acetic acid necessary for complete coagulation would only
involve a waste of time and energy.
The general figure given in a preceding paragraph (1 part pure acid to
1,000 parts of latex) may be accepted as the rough basis for working. In
modern practice, however, undiluted latex is usually diluted to a standard
which may vary on different estates from 1-1/4 lbs. to 1-1/2 lbs. dry
rubber per gallon.
Latices of these strengths can be coagulated at a ratio of 1 part pure acid
to 1,200 parts of standardised latex; and this quantity need not be
exceeded, except in cases where an appreciable amount of some
anti-coagulant is present in the latex. The proportion may then be raised
to 1 in 1,000.
If considered advisable the acid may be used in a 1/2 per cent. solution
for sheet preparation; but in any case it is advised for the sake of
uniformity that a 1 per cent. solution should be employed in the
preparation of both sheet rubber and crepe rubber. In most modern
factories, measuring vessels of various capacities are to be found, and it
is always more satisfactory to have the solution made up in approximately
correct strength at the rate of 1 oz. of pure acid to 5 pints of water.
Often, however, on some estates European supervision of this work is not
possible, and the preparation of the acid solution has to be left in the
hands of a (more or less) skilled coolie. It is thus necessary to find some
less fine, but still approximately correct, method of procedure. In the
East the kerosene tin is in universal favour for the carriage of water, and
there is no reason why it should not be utilised as a standard measure for
preparing the dilute acid solution, _providing it is not allowed to become
rusty_. The capacity of the tin is 4 gallons (640 fluid ozs.), so that a
one-hundredth part would be approximately 6-1/2 ozs. It is suggested that
this quantity should be measured out by means of a glass graduated vessel,
and then that an aluminium cup should be cut down so as to hold the exact
quantity.
This would reduce the making of a solution, sufficiently approximate to 1
per cent. strength for all practical purposes, into a simple operation of
mixing pure acid and water in the ratio of one cupful of acid to 1 kerosene
tin of water.
The actual quantity of solution required for the coagulation of any volume
of standardised latex can be calculated easily from the ratio 1:1,200. As
the strength of solution is 1:100 it will be seen that the quantity to be
taken is _always one-twelfth_ that of the volume of latex--_e.g._:
(_a_) If the latex tank holds 90 gallons of standardised latex, 7-1/2
gallons of dilute acid solution are required.
(_b_) A tank containing 120 gallons of latex would need 10 gallons of
the 1 per cent. acid solution.
It is assumed that all estates, not only in the preparation of sheet
rubber, but also in the making of crepe rubber, always employ the system of
standardising latex in order to obtain uniformity. They are ill-advised if
they do not follow this practice; but in case average undiluted latex is
treated in coagulation, the quantity of acetic acid to be used should be
calculated from the ratio 1:1,000.
If the acid solution is to be employed in 1 per cent. strength, _one-tenth_
of the volume of latex to be treated will indicate the required quantity of
solution necessary for complete coagulation unless anti-coagulants have
been used, when the quantity must be increased as experience directs. It
will be recognised, of course, that undiluted latex may only be used in any
case for the preparation of crepe rubber; or in some exceptional case, such
as the special preparation of "slab" rubber.
CARE IN MIXING.--It is essential that the mixture of dilute acid and latex
should be thoroughly intimate. This can only be attained by careful
manipulation, especially in the case of sheet preparation. Where crepe
rubber is to be made it may be permissible to employ a solution stronger
than 1 per cent., but it is not advised. The acid should be poured into
the latex while stirring, and the agitation should continue for such a
period as to ensure thorough mixing in all parts.
It will be appreciated that in the preparation of sheet rubber this period
may not be unduly prolonged, otherwise the latex will have begun to
coagulate before skimming and the placing of the partitions in their
respective slots can be effected. Furthermore, while in the preliminary
treatment for crepe rubber, the formation of enclosed bubbles and surface
froth is immaterial. For sheet preparation it is essential that the
stirring shall be done so carefully as to try to avoid internal bubbles and
to reduce surface froth to a minimum. For crepe-making a perforated board,
with handle attached at right angles to the face of the board, may be used;
but in shallow sheet-coagulating tanks, broad paddles (which may or may not
be perforated) give good results as long as there is a sufficient number
used to cover the area of the tank in reasonable time. Obviously also,
where the area of any tank or compartment is of any appreciable size, the
dilute acid solution should be poured in from various points so as to
obtain a good even distribution. In some cases the acid is distributed from
a sprinkling can, but this is a refinement which experience shows to be
unnecessary. In actual practice, working on a tank measuring 12 ft. by 4
ft., no difficulty is found if coolies pour in acid solution from four
points. The degree of success depends entirely upon experience and
efficient supervision. This remark applies equally to the use of various
devices, such as rakes with broad teeth, used as stirring implements. There
is room for display of ingenuity in this direction, and it is found often
that, while they are used successfully on one estate, they may be condemned
on another.
[Illustration: TWO VIEWS OF DILUTION AND MIXING TANKS.
Below, on the right, coagulating tanks. At the far end strainers. Each
dilution tank is of equal capacity to the corresponding coagulating tank.]
USE OF SODIUM BISULPHITE.--Some few years ago a demand for pale crepe
rubbers sprang up, and this demand has been maintained. The total quantity
of pale rubber put on the market previously could only have amounted to
very little, and that little was obtained by luck and various tricks in
manipulation. It must be premised that if coagulation is allowed to take
place, either naturally or with the aid of acetic acid, the resulting
rubber will almost inevitably oxidise on the surface, except in the cases
of very dilute or young latices. Even supposing that this darkening of the
surface does not take place in the wet stage, it is often found that a
rubber expected to dry to a pale colour does not fulfil expectations, and a
dull neutral shade results. This darkening of crepe rubber may be
attributed to a slow process of oxidation, which continues until the rubber
is dry. From these remarks it will be seen that the process of oxidation is
a natural one, and that any pale rubber formerly shipped was the outcome of
circumstances outside the control of the estate, except in such cases where
boiling of the coagulum, etc., was resorted to. The fact that one rubber
happened to be a shade darker than another was absolutely no criterion as
to the value of the rubber, but apparently the market thought, and still
thinks, otherwise, although the actual necessities of manufacturers for a
pale crepe to be employed in special processes must be comparatively small.
The prevention of this natural oxidation was a problem which exercised the
minds of all responsible for the preparation of pale rubbers, and much time
and thought were expended upon it. Various theories were propounded, and
the chief conclusion arrived at was that the darkening of rubber was to be
prevented by excluding all the light possible from the drying houses. To
this end windows were to be kept shut, or else they were provided with
ruby-coloured glass, which incidentally kept out the air. In spite of these
precautions, little success attended the expenditure of so much energy and
thought. It was absolutely necessary that some chemical agent should be
discovered which would make the preparation of pale crepe possible for any
estate. This chemical would have to fulfil several requirements before it
could become popular:
1. It must be a simple substance capable of being easily handled.
2. It must be very soluble, so that solutions could easily be made up
by inexpert workers.
3. It must be cheap.
4. It must be quite innocent of any harmful effect upon the quality of
the rubber.
After months of investigation into the properties of other chemicals the
writers decided that the only one which satisfactorily answered all
requirements was sodium bisulphite. The writers make no pretension to any
claim of having discovered the properties of this substance, which was a
common chemical, and widely known. Even its action on latex was suspected
before they engaged upon the work. These matters are only mentioned because
the credit, if any, should be given to the laboratories of the Rubber
Growers' Association.
As soon as it began to be known on the market that sodium bisulphite was
being used in the preparation of pale crepe, a great outcry was made, and
estates were warned that no more rubber prepared in this way would be
accepted. It was said that the chemical would destroy the "nerve" of the
rubber,[2] and it was definitely stated that rubber prepared with this
chemical was brittle. It must be remembered that brokers had some
legitimate excuse in raising objections to the introduction of new and
strange chemicals for preparing rubber, as they were quite without means of
judging whether the rubber had suffered harm or not. Still, on the other
hand, private tests had been made in conjunction with Messrs. Beadle and
Stevens for fully eight months before the name of the chemical was
mentioned in reports, and they had decided from the results of
vulcanisation tests that the chemical was quite innocuous. Then, and only
then, did we consider it advisable to recommend the use of sodium
bisulphite in general estate practice. Owing to the initial prejudice
against rubber prepared with sodium bisulphite, the results of our
preliminary work were published by permission of the Rubber Growers'
Association.[3] The original instructions to estates regarding the proper
employment of this chemical were given in the private reports issued by the
Rubber Growers' Association in 1911. At the present time it is probably
accurate to state that it is now used by all estates preparing fine crepes.
Representatives of manufacturers have sometimes given us to understand
that the question of paleness of colour in such rubber is of no such
importance as is impressed upon us as producers. While we are prepared to
believe, we can only plead that from our point of view the supply arises
from the demand. Such are the conditions governing the sale of rubber that,
irrespective of the requirements of the ultimate user, we have to market
rubber which is valued almost completely upon its appearance at the time of
sale.
[2] Williams, International Rubber and Allied Congress, London, 1914.
[3] "The Employment of Sodium Bisulphite in the Preparation of Plantation
Rubber," Beadle, Stevens, and Morgan, _India-rubber Journal_, August 2,
1913.
As long as such conditions prevail estates must continue to adopt any
device of proved harmlessness, in order to obtain the best possible price
for their product, and not because we desire to continue a practice which
some assure us to be unnecessary, and which, moreover, adds somewhat to the
cost of production.
QUANTITIES OF SODIUM BISULPHITE.--It must be premised that, although sodium
bisulphite is employed on some few estates in the preparation of sheet
rubber, we do not advise the practice. It is unnecessary, and may lead to
some little trouble and delay in drying. In any case, sodium sulphite gives
the results desired for sheet rubber (see following). It must be
understood, therefore, that we are concerned here, in the case of sodium
bisulphite, with its employment in the preparation of fine pale crepe only.
As the dry rubber contents of latices vary with the age of the trees, the
general health of the trees, the seasons and general climatic conditions,
the relative strain imposed by depletion of reserves through tapping, etc.,
it will be clear that the effect produced by a definite quantity of sodium
bisulphite in any given volume of latex will also vary--_i.e._, the effect
depends upon the potential amount of rubber present. A dilute latex needs
less sodium bisulphite than a richer latex to produce the same effect in
colour.[4]
[4] Incidentally there are certain occasions, as in the opening of areas of
bark rested for long periods, when the latex is of a rich yellow colour.
Sodium bisulphite will not "bleach" this colour, and it is well to remark
again at this stage that the action of the chemical is only to avoid or
arrest oxidation (darkening).
Hence it follows that if in any factory uniform quantities of the solution
are used for any given volume of undiluted latices from different areas of
the estate, the effect upon the dry rubbers will vary. This explains why
some estates obtain different shades of rubber in their fine pale crepes.
The remedy obviously is to reduce the variation in latices by diluting them
all to a standard rubber content as is done in sheet preparation. One is
thus assured that the prescribed quantities of sodium bisulphite will meet
requirements in every case, and that waste will be avoided.
Working with a standard of 1-1/2 lbs. dry rubber per gallon the following
formula should serve as a _maximum_:
_Formula for Use of Sodium Bisulphite._
(_a_) Dissolve sodium bisulphite in water at the rate of 1 lb. to 10
gallons.
(_b_) Of this solution use 1 gallon to every 10 gallons of latex.
MAKING A SOLUTION.--The making of a solution of the chemical would seem to
be a simple matter, but to judge by the ill-effects sometimes observed in
the dry rubber the simplicity of the operation appears to have been
overrated. Great care must be exercised in preparing the solution, and the
work should not be left to the few minutes preceding its actual
requirement; such has been found to be the case in several factories, so
that it is not surprising if the rubber is defective.
The powder should be added gradually to water with thorough stirring, which
should be continued for five minutes at least. Even then there may often be
seen at the bottom undissolved particles, sand, and other impurity. It is
necessary, therefore, in such cases to decant the solution through a piece
of cotton cloth before using. No solid particles should be allowed to enter
the latex.
ABUSE OF SODIUM BISULPHITE.--It is now generally recognised that the abuse
of sodium bisulphite, in the form of an excess, leads mainly to delay in
the period of drying and the production of an overpale rubber.[5] It is
probable that few estates, if any, now experience trouble due to this
non-observance of the rules and quantities laid down for use.
[5] "The Preparation of Plantation Rubber," Morgan, 1913, p. 74.
RESIDUAL TRACES OF SODIUM BISULPHITE.--The prolongation of the drying
period was attributed to the fact that traces of substances caused by the
decomposition of sodium bisulphite remained in the rubber if the rubber
were not sufficiently worked and washed on the rolls. These traces must
have been very minute, but they were sufficient to retard the progress of
drying. That much depended on the care exercised in washing is evident from
the fact that samples prepared with varying quantities of the chemical show
varying results on extraction. These samples were tested for the presence
of sulphates. Of the series tested that sample prepared with bisulphite in
the proportion of 1 part to 600 parts latex showed only a trace of sulphate
present; while the one prepared 1:2,400 gave an equal quantity.
Intermediate samples contained no trace of sulphate. On the whole,
therefore, the presence of sulphate in crepe rubber is adventitious, and
properly washed crepe prepared with moderate quantities of bisulphite may
be taken as free from any residual quantities. Meanwhile there cannot
possibly be any doubt of the advantages gained by the use of sodium
bisulphite, and it would not be very wide of the mark if the statement were
made that, in the event of this chemical being discarded, most contracts
for pale crepe could not be fulfilled.
SODIUM SULPHITE.--It would not be amiss to insist upon the point that while
the nature of sodium _bisulphite_, as employed in the preparation of
rubber, is anti-oxidant, sodium sulphite is employed chiefly for its
anti-coagulant property. It is not used, therefore, in the making of crepe
rubber, but is of service in the preparation of sheet rubber, where the aim
is to keep the latex in good fluid condition as long as is necessary, and
to retard coagulation slightly so that enclosed bubbles of gas or air may
escape. Formulæ have been given for its use in the field when required. On
some estates this practice is not found necessary, but a quantity of
solution is always placed in the bottom of the reception vessels prior to
the straining of latex into them. Only a small quantity is used, and as a
working basis the following formula may be adopted:
_Sodium Sulphite: For Use in the Factory._
(_a_) Dissolve 2 ozs. of anhydrous sodium sulphite in a gallon of
water.
(_b_) The gallon of solution, placed in the bottom of the reception
jar or tank, is sufficient for the treatment of 40 gallons of
standardised latex (1-1/2 lbs. dry rubber per gallon).
The warning previously given regarding the necessity for thoroughness in
the preparation of solutions is here reiterated. Stirring should be
thorough, say for five minutes, and if there is any sediment or undissolved
matter the solution should be strained through cloth before using.
Where uniform jars or tanks are in use, the majority of which will contain
uniform quantities of latex daily, the practice of using the chemical can
be made almost fool-proof even in the hands of coolies. A calculation is
made of the quantity of powder required for each vessel daily. The
necessary number of lots is weighed out each morning and each placed in an
envelope. The process is thus simplified by the fact that the contents of
an envelope, neither more nor less, are required for each unit reception
vessel. Even the weighing can be done by a coolie if he is given a
counterpoise (of lead, for example) equivalent to the required weight.
It will not be found necessary to do any vigorous stirring of the solution
with the latex, as the latter is strained into the solution and the
continued addition of successive quantities is sufficient to give a good
mixture.
USE OF FORMALIN.--Few estates now use formalin (formaldehyde) as an
anti-coagulant. It must be acknowledged that when not abused there are
points in favour of its employment in preference to sodium sulphite, but
these are outbalanced by certain disadvantages. The argument may be stated
thus:
_Points for_: (1) If made up freshly it is an effective
anti-coagulant.
(2) Formalin being the solution of a gas in water, there is no
residual substance left in the rubber to delay drying.
(3) Its use gives a bright clear rubber.
_Points against_: (1) Its cost at all times is greater than that of
sodium sulphite.
(2) If the jar is not sealed there is loss by evaporation, thus
increasing the cost.
(3) Its effect upon the rubber is uncertain. Even in normal quantity
it is said to cause "brittleness" or "shortness."
Certain few estates, however, have continued its use, and no trouble is
claimed to ensue. The following formula is stated to give satisfactory
results in the preparation of sheet rubber, when applied as in the
preceding paragraphs bearing on the employment of sodium sulphite:
_Formula for Use of Formalin (Formaldehyde)._
(_a_) 1 pint of formalin is diluted with 5 gallons of water.
(_b_) Of this solution 1 gallon is required for 50 gallons of
standardised latex.
In noting this formula the writer gives no recommendation regarding its
use. Whatever may be the actual facts regarding the effect of formalin upon
the vulcanisation of rubber, when used in minimum proportions, there can be
no question concerning its injurious effect if used in excess. Beyond this
the factors of cost and loss militate against its wider employment.
CHAPTER IX
_PREPARATION OF SHEET RUBBER_
PALE SHEET.--The first form in which plantation rubber was prepared was as
"biscuits" or sheets. This form remained in favour for some years. The
first biscuits or sheets were rather dark in colour owing to the natural
oxidation which followed. Then it was discovered that by diluting the latex
the degree of oxidation was diminished, and later it was found that if the
soft coagulum were placed in almost boiling water for a short time the
resulting rubber was pale. Thus there arose gradually a demand for pale
sheet. With our present knowledge we are in a position to state that the
pale biscuits were not in any way superior to the darker ones, and they
were in most cases actually inferior.
It was found also as time progressed that sheet rubber, on air-drying,
became covered with external surface moulds, and that, more often than not,
the smell of the drying rubber was the reverse of pleasant. Even when dry
the sheets had to be continually brushed free from moulds, and by the time
the rubber reached the market it was again usually mouldy. Such are, even
now, the handicaps under which those who prepare pale sheets have to
labour. Few, however, are the estates making pale sheets, and they are
confined almost entirely to native holdings.
To those accustomed only to the preparation of crepe rubber, where
coagulation can be effected in large batches, the preparation of sheet
rubber always seems to demand much more labour. As a matter of fact,
although the preliminary operations certainly do demand more care and
labour than in crepe-making, there are compensating advantages in the
machining stage. For the preparation of sheet of the highest quality on any
but the largest scale, elaborate installations of machinery are quite
superfluous, as equal results can be obtained with pairs of rolls worked by
hand.
UNIFORMITY OF PRODUCT.--There will be no need to enter again into a
discussion of the preliminary operations of receiving and straining latex
for sheet-making. They have been fully dealt with in Chapter VII. It used
to be the general custom to mix the acid and latex in each individual dish,
and in some small or non-progressive factories that is still the procedure.
Quite apart from the question of labour entailed, the process is quite
unnecessary. Even if comparatively small volumes of latex are handled,
standardisation by dilution should be the rule, and the acid solution
should be added to the bulk. It is possible to stir in the acid and to
ladle out uniform quantities in each pan or dish from a bulk volume of up
to 40 gallons if the organisation is efficient.
On any but a small scale the labour entailed in the handling and cleaning
of pans is excessive, and shallow tanks are now employed on most estates.
The reception and standardisation of latex by dilution has already been
discussed in Chapter VII. The combination of this practice with the
employment of shallow coagulating tanks has simplified working and reduced
the cost of labour. It is not intended to enter into any lengthy discussion
relative to the merits of sheets made in pans as against those made in
tanks. It is granted that it is possible to make a "pan" sheet superior in
appearance to the general average of "tank" sheets; but from an economic
standpoint the introduction of the use of tanks into all but the smallest
factories is only a matter of time, if the demand for this class of rubber
persists.
THE IDEAL TANK.--Even the most modern installations of sheet-coagulating
tanks must be regarded as merely temporary devices, as, given facilities,
the room for improvement is so wide.
The first tanks made erred in being too large, and as the result of
experience the size of units has now been reduced to a maximum of 12 feet
by 4 feet by 1 foot deep.
[Illustration: UNIT MODERN COAGULATING TANK (TWO VIEWS).
Construction of brick and cement with lining of glazed tiles. Note slots
incorporated in side tiles. Partition boards in piles in the background.]
Tanks are at present constructed either of hard timber or of brick and
cement faced with glazed tiles; both types have inherent drawbacks. The
wooden tanks are difficult to keep clean and in "sweet" condition. The
glazed tiles, unless extremely well laid, allow the acid serum (from which
the rubber is removed) to percolate between the interstices. Thus "pockets"
of liquid collect beneath the tiles, and in process of the decomposition of
certain constituents dissolved in the serum evil-smelling gases are set
free.
[Illustration: ANOTHER BATTERY OF TANKS, WITH DILUTION TANKS, RAISED, ON
THE RIGHT.
Note drainage cocks, chute, and sieve in position.]
It should not be a matter of difficulty for manufacturers to make sheets of
thick glass sufficiently large to form the bed-plate and side-pieces
necessary in the lining of a tank. If such adjuncts could be secured, the
disabilities indicated above would be perhaps wholly removed. Unless there
is a demand from estates, however, it is idle to expect a supply to be
forthcoming.
An even greater improvement would take the form of unit tanks cast in
glazed white-ware with the necessary slots incorporated in the sides. At
present no known firm makes such tanks of sufficient size. A unit could
measure (internally) 6 feet by 4 feet by 1 foot deep, with slots 1-1/2
inches apart, and 3/8 inch in width. The tanks might be reinforced with
iron bars, so that they could either be used alone or embedded in the usual
brick structure. The junctions of the bed-plate and side-pieces could be
finely rounded so as to facilitate cleaning, and at one end a draining-hole
could be made, say, 1 inch in diameter.
[Illustration: CLOSER VIEW OF FOREGOING.
Note partitions in position and coagulum being removed.]
Meantime both the hard-wood tanks and those of glazed tiles find their
particular applications. The former is generally employed in smaller
factories, or where future large increases of crop preclude the present
installation of a fixed system. The latter find use in large factories, or
where no new areas remain to come into bearing.
MODERN INSTALLATION.--As an example of a modern installation of coagulating
tanks, we can do no better than offer reproductions of the system now in
use on Pataling Estate.
A warning must be given against employing all tanks of stone-ware or cement
unless well glazed. Almost without exception, irrespective of the material
used in the construction of coagulating tanks, wooden partitions are
employed. In the few exceptional cases the partitions are either of glass
or of aluminium. The former would appear to be the ideal substance, were it
not for initial cost and loss by breakage. These disabilities may possibly
be overcome in course of time.
CARE OF TANKS.--The use of aluminium would have been wider had it not been
for lack of supplies and the question of cost during the War. A novel
method of employing aluminium partitions was introduced in the factory of
Tremelbye Estate. There were no slots in the sides of the glazed-tile
tanks, but the necessary slots were very ingeniously created by means of
aluminium "distance-pieces," the two long edges of which were turned at
right angles to the face of each piece to a depth of about 1/4 inch. The
ends of the thin aluminium partition moved in the slot thus formed between
two adjacent "distance-pieces." The friction between the surfaces was
sufficient to allow all the partitions, when in position, to be raised well
above the floor of the tank, so that a uniform level of latex was obtained.
Slight hand-pressure only was then required to push the partitions down.
Naturally the cleansing of glass or aluminium partitions presents no
difficulty, but in the case of wood failure to ensure thorough cleanliness
leads to possible defects in the finished dry rubber. Provided the wood
could be made waterproof, no trouble would ensue, and hence various
measures have been tried with that object in view. When new the boards have
been surface-waxed or varnished, and the treatment has been repeated on
occasions. But in course of time the surface film of waterproof material
has disappeared, partially or wholly, and the trouble recurs. When
partitions become sodden with serum, the surfaces are liable to be coated
with a slime, consisting largely of organic growths which have an effect
upon the latex, causing "pitting" on the surface of the coagulum and
enclosed bubbles within.
[Illustration: ANOTHER BATTERY OF TANKS, WITHOUT DILUTION TANKS OR MEANS OF
GRAVITATING LATEX.]
It is recommended, therefore, that wooden tanks, after ordinary cleansing
daily, should be swabbed out with a 5 per cent. solution of sodium
bisulphite. Wooden partitions should receive the same treatment, and once a
week at least (or every day if possible) they should be placed in the sun
for an hour or two, care being taken that both sides of a partition are
exposed in turn. Before being placed in the latex, all wooden partitions
should be made wet on the surfaces.
Some years ago the writers had made a partition of vulcanite, which
apparently would have proved of great service but for the initial cost. The
advent of the War put the matter out of the question, but it is possible
now that such a material would be worthy of extended trial. Except in the
matter of cost, it would appear to have advantages over any substance yet
tried; and if it were possible for estates to supply their own lower grade
rubbers direct, the cost might be reduced considerably.
[Illustration: A SHEETING TANK CONTAINING COAGULUM FOR CREPE PREPARATION.
Behind wall in background are the tanks in which latex is standardized.
Note vent, to the left, through which latex flows and wooden "stopper" on
edge of tank.]
STANDARD LATEX.--Enough has been written (see Chapter VII.) to familiarise
the reader with the use of this term for the description of latex diluted
daily to a level of dry rubber content. Whatever may be the practice
elsewhere, it is now fairly general on estates in Malaya to reduce all
latices to a uniform "strength" for the preparation of sheet rubber. It is
claimed that only in this manner can uniformity of product be achieved.
The selection of a standard has been the outcome of general experience. It
has been found that if too high a standard is taken difficulties arise,
such as (1) unsatisfactory and uneven coagulation, (2) too thick a coagulum
for easy working in general, (3) too extended a period of drying and
smoke-curing, and hence too dark a colour in the finished rubber.
[Illustration: A "BATTERY" OF SHEETING TANKS (PATALING ESTATE). DILUTION
TANKS, RAISED, ON THE LEFT.]
On the other hand, too low a standard also brings trouble in its train. The
coagulum is too porous, will not stand handling, and the resultant sheet is
too thin unless an abnormal thickness of coagulum is prepared. Furthermore,
over-dilution means an increase in the number of tanks required for any
original volume of latex. This involves an increase in floor area, and
perhaps in the size of the building. The soft sheets, when rolled, may
spread to such a width as to cause the edges to be squeezed under the
cheek-blocks of the machines, etc.
For all practical purposes, whether sheets are prepared in pans or in
tanks, it has been found that the optimum results are obtained by the
adoption of a standard approximating and not exceeding 1-1/2 lbs. dry
rubber per gallon. Primarily this standard has a direct connection and
interdependence with the distance between the partitions (or between the
slots) in coagulating tanks. The distance found most practicable is 1-1/2
inches. This thickness of coagulum, when prepared from latex not exceeding
a standard of 1-1/2 lbs. dry rubber per gallon, is found to yield a very
satisfactory sheet in all respects.
It will be seen that we have two possible main factors of variation:
(_a_) Distance between partitions, causing visible differences in
thickness of coagulum.
(_b_) Dry rubber content of latex, causing differences in the density
(_e.g._, hardness or softness) of the coagulum.
The effect of variation in (_a_) will be clear. Even when latex of a
standard of 1-1/2 lbs. per gallon is employed the resulting sheet may be
either too thin or too thick.
Similarly, as already argued, the use of too low or too high a standard of
dilution (when the factor of distance between partitions is not allowed to
vary) is capable of causing much difficulty. While this is correct,
broadly, it is found in the experience of some estates that their
requirements are satisfied by a slightly lower standard than 1-1/2 lbs. per
gallon. Thus it is not uncommon to note the adoption of a standard
equivalent to 1 lb. 4 ozs. or 1 lb. 6 ozs. dry rubber per gallon.
Experience dictates, however, that for the recognised standard measurements
of modern tanks the practical limits of satisfactory density of latex lie
between 1-1/4 lbs. and 1-1/2 lbs. per gallon.
STANDARDISING INSTRUMENTS.--For standardising latex by dilution all that is
required is an instrument which will preserve a perpendicular position
while floating in latex, will be sufficiently sensitive to indicate fairly
small differences in density of latex, and has one mark on its aerial
portion accurately indicating a density corresponding to the required
standard. On scientific grounds it can be demonstrated that such an
instrument as employed in common practice would not be strictly
accurate.[6] It is not proposed, in this section of the book, to discuss
such considerations.
[6] De Vries, "Archief voor de Rubbercultuur."
Instruments of this nature are represented by the "Metrolac" (originating
from the Rubber Growers' Association) and other similar recorders. They
generally consist of a submersible bulb with a projecting stem which is
graduated. The "Metrolac" differs from others in that the bulb is of
torpedo form (thus reducing "skin friction"), and the graduations on the
stem indicate actual weight of dry rubber per gallon instead of the
ordinary specific gravity figures.
Theoretical considerations to the contrary, it is found in actual practice
in Malaya and Ceylon that, although such instruments are naturally delicate
and require careful manipulation, they are of considerable practical value
and satisfy a definite requirement. Until an instrument of greater accuracy
and equal simplicity can be discovered all estates should regard the
possession of a few "Metrolacs" as essential.
The nature of their construction and the average conditions under which
they are used (and abused) make it impossible to rely upon their accuracy
indefinitely or for any long period. It is always recommended, therefore,
that there should be at least two instruments available, one of which may
be in daily use, while the other is kept in safe custody and only employed,
say, once a week for purposes of checking the accuracy or degree of
inaccuracy of the other. This can be done with reasonable approximity by
placing both instruments in a tall vessel containing well-mixed and diluted
latex. Instruments showing a marked degree of inaccuracy should not be
preserved; but in cases of necessity "Metrolacs" from estates belonging to
company members of the Rubber Growers' Association may be sent to the
laboratories for repair and adjustment.[7]
[7] This applies to the gilt brass instruments. As the result of experiment
the Rubber Growers' Association are now introducing glass instruments.
These are necessarily more fragile, but while unbroken can be relied on to
give correct readings.
Where field coagulating stations have been instituted on estates, it is
strictly necessary that instruments should be provided in all cases; and it
should be a rule that these are tested and corrected weekly by a standard
instrument employed for that purpose only. This need was well recognised by
many estates when, during the War and the consequent shortage of supply of
"Metrolacs," a demand arose which was met in some degree by crude
instruments of local manufacture, such as that commonly known as the
"Castlefield bobber," contrived and made by the enterprising manager of the
estate of that name. The demand for the more accurate instruments can now
be met.
METHODS OF USING THE INSTRUMENTS.--The "Metrolac" was devised and
introduced by the writers on behalf of the Rubber Growers' Association, and
directions for its use were given. Tables were prepared by means of which
simple calculations for the dilution of any given latex could be made.
These did not find an extended application, inasmuch as in the majority of
cases native workers only were in charge of the processes of rubber
preparation. In point of fact, such calculations are not strictly
necessary, as the operation of standardising the latex can be done quite
simply and skilfully by a trained native. Latex as it reaches the store in
average weather from any particular division or field does not vary greatly
in density. The trained coolie or foreman, basing his practice on
experience, adds to the latex a quantity of water, and then makes a first
test with the standardising instrument. Several additions of water (with
thorough stirring) may have to be made before a test indicates that the
correct density has been obtained, but it is surprising how quickly a
skilled worker will arrive at the desired standard. Extreme or absolute
accuracy is not insisted upon or desired, as avoidable delay is to be
deprecated, and the result in any case is sufficiently exact for practical
purposes.
SKIMMING.--During the gravitation of the latex from the reception vessels
(in which the standardising of the latex is effected) to the coagulating
tanks, much surface froth is usually caused. This is best removed by means
of a thin board of a width slightly less than the breadth of the tank. The
skimmings are sometimes placed in pans and subsequently made into a second
grade of sheet rubber, or they receive treatment with a small proportion of
sodium bisulphite and eventually appear as fine pale crepe. The practice
varies usually according to the form in which the general No. 1 grade is
prepared.
On some estates a great deal of the frothing is avoided by placing in
position at the receiving end of the tank a perforated partition. This
partition may be made of wood, or of stout zinc (or aluminium) carrying ten
circular holes to the inch. Through this the latex percolates, while the
froth is retained on a small area. The froth is removed prior to the
addition of the acid. After stirring in the acid solution most estates
again skim the surface of the latex; but if the stirring has been performed
properly there should be little froth. This, when it collapses, in any case
will appear only on the upper edge of the strip of coagulum, and after
rolling should not be visible. It would appear, therefore, that the second
skimming is not necessary.
STYLE OF SHEET.--Within the last few years the custom of making plain
sheet--_i.e._, sheet having a plain surface--has gradually given place to
the preparation of ribbed sheet--_i.e._, sheet having a pattern marked on
the surface. It would probably be correct to say that plain (smooth) sheet
is now only prepared by natives or by some estates just come into bearing.
Even in the latter case there is no reason why smooth sheet should be made,
as hand machines are sold which will do all the work required. It will be
evident to anyone acquainted with rubber preparation that in the matter of
actual quality of rubber the question of smoothness or a pattern can have
no bearing on the result. One advantage claimed for ribbed sheet which may
entirely justify the preference exhibited by consumers, relates to the
question of packing. When rubber arrives at home it is frequently found to
be in an almost solid block, due to the pressure of the sheets superimposed
in the case. The smoother the surfaces of the rubber in contact the greater
the adhesion and the denser will be the mass, and consequently the greater
the difficulty in separating individual pieces. Under such circumstances it
is plain that the difficulty is diminished if the sheets have a raised
pattern on them. It is noted also that the liability to mildew-growth is
greater the smoother the surfaces of the rubber.
On these grounds the "marking" of sheet rubber is to be commended. These
reasons apart, it is really astonishing the difference made in the
appearance of the sheets by impressing upon them a ribbed pattern, and it
is highly probable that the market value of the rubber is slightly
increased. It is not our duty to attempt to reason why this simple
operation should increase the market value of sheet rubber; it is
sufficient to recognise that it is so, and that money may be thrown away by
neglecting to cater for the taste of the market. Of the patterns impressed
upon sheet rubber there is a variety, but the general style is that known
as the "spirally close-cut ribbing."
STANDARD SHEET.--Leaving for the present the question of pattern of mark,
one cannot do better by way of introduction than to reproduce the
instructions[8] given generally to estates.
[8] "Handbook on Preparation of Rubber," Rubber Growers' Association, May,
1917, p. 28.
ROLLING AND MARKING OF SHEET RUBBER.--Working with standard latex it is
found that strips of coagulum 1-1/2 inches in thickness require little
rolling to produce sheets of desirable thickness.
(1) The sheets or strips are first given a preliminary rolling with a
heavy hand-roller made of hard wood. The roller is passed once in one
direction, and once in the reverse direction.
(2) The coagulum is then passed through smooth machines twice, once
with the rolls fairly open, and once with a narrower space. It is not
found advisable to close the smooth rolls so tightly that the rubber
is made too hard.
(3) The sheets or strips are then passed once through a pair of
marking rollers. Various types of patterns are used, but the one which
appears to give the most satisfactory results is that known as the
"close-cut spiral." This produces the semblance of a small diamond
pattern on the rubber. The surface of the sheet is raised in
well-defined ridges, which appear to present the maximum drying
surface exposed to the atmosphere of the smoke-house. Thus, not only
is the appearance of the sheet rendered attractive, but also the
period of drying is reduced. Starting with standard latex and
following the procedure here described for rolling and marking, sheets
should be ready for packing in ten or eleven days. If the period is
longer, it is possible that the design or structure of the smoke-house
is at fault.
WHEN TO WORK THE COAGULUM.--Before proceeding to discuss other points the
question remains to be settled as to how long it may be necessary or
advisable to allow the coagulum to remain in the serum before rolling it.
For reasons of practical economy in factory working, it is usual to allow
sheet rubber to remain over night, and the coagulum receives attention
early next morning. During the interval (averaging about eighteen hours),
the coagulum consolidates, leaving an almost clear serum if the correct
quantity of acid has been added to the latex. Any but the very slightest
trace of milkiness in the serum indicates an insufficiency of coagulant. If
the serum is always definitely clear, there may be grounds for believing
that an excess is being used. If the quantity of coagulant has been
calculated to an average nicety, the serum should be just dubiously free
from milkiness.
The firmness gained by the coagulum on standing in the serum overnight
should enable it to be handled next morning without any marked stretching,
and in some estates the rubber is put direct through the first pair of
smooth rolls without a preliminary consolidation by means of hand-rolling.
Some estates prefer to handle the coagulum while rather softer, as it is
claimed:
(_a_) That the coagulum is easier to work, and sheets of improved
appearance can be made.
(_b_) That there is greater freedom from "bubbles."
(_c_) That the incidence of "rust" is lessened.
These claims are substantiated in practice; but in the case of the
third, it only holds provided that the rubber can be finished and
placed in the smoke-house almost as soon as the last sheet has been
machined.
In such cases all latex must reach the store comparatively early in the
day--_e.g._, before noon. Three hours is allowed for coagulation, and the
working of the rubber is then commenced. As a general rule this means that
the operations of rolling and marking must be completed, a short interval
given for dripping, weighing must be done, and the rubber placed in the
smoke-house before night falls (as a rule about 6.30 p.m.).
Unless factories dealing with a large crop are well equipped with
artificial light, such a course is not open to them; in any case it remains
true that night work should be avoided if possible. If, however, it can be
arranged without increasing the cost of production, there would appear to
be no objection to the early working of the coagulum as described above.
HAND-ROLLING.--As already indicated, some few estates do not give the
strips of coagulum any preliminary hand-rolling, as the rubber is
considered to be sufficiently firm to be handled into the first machine.
On most estates hand-rolling is found necessary, owing to the tendency of
the long strips to stretch unduly, giving badly shaped sheets. This
hand-rolling should be done carefully, and is best effected on a specially
constructed table. This consists essentially of an inch-thick hard-wood
plank about 2 inches wider, and 4 or 5 feet longer, than the strip of
coagulum. Along the edges of the plank, and at right angles to its upper
flat surface, may be fastened strips of wood about 1/2 inch square in
section, thus forming a shallow tray open at either end. These strips serve
two purposes:
(_a_) As the wooden roller is wider than the plank, they prevent the
coagulum being rolled too thin and too firm.
(_b_) They prevent the coagulum being squashed too wide, and tend to
keep the edges straight.
To avoid "thick ends" it is sometimes considered advisable to insert, at
either end of the rolling table, shallow wedges about 6 inches long, of the
same width as the table (between the edge-strips), and with the sharp end
of the wedge pointing in the direction of the length of the table. The ends
of the coagulum are drawn up and finished on these inclined planes.
These points may appear to be extreme refinements, but as long as rubber is
valued on such grounds we must endeavour to meet the system imposed upon
us.
SMOOTH-ROLLING.--It is advised that, after hand-rolling, the coagulum
should be passed through at least two machines having smooth-rolls. On some
estates three such machines are employed. The purpose of this procedure is
to reduce the thickness of the coagulum gradually. The same could be
effected, of course, on one machine; but obviously the distance between the
rolls would have to be readjusted at each operation and for each piece of
coagulum. Apart from the time thus wasted, there is the certainty, in view
of the rough adjustment of the machines, that the chances of obtaining
uniformly thick sheets would be slight.
The machines should be arranged as a battery, with the marking rolls at one
end, so that the operations are consecutive. It is erroneous to imagine
that heavy machines (such as those used in crepe preparation) are required.
Light machinery only is necessary for sheet-making; but any available heavy
smooth-roll machines in a crepeing battery may serve admirably for the
purpose.
MARKING.--Heavy machines are unnecessary for the purpose of putting a
pattern on sheet rubber. If the rubber has been properly prepared a light
pair of rolls is capable of exerting sufficient pressure to cause a good
upstanding pattern.
Rolls are cut in various designs: some with "diamond" grooves on both
rolls; some with grooves of varying width and depth encircling the
circumference of the rolls, thus creating a "stripe" effect on the rubber;
and some with diagonally-cut spiral grooves placed closely together. The
last has the greatest vogue at present, while the first has almost gone out
of favour. An objection lodged against the second design is that the edges
of the grooves sometimes cut through the rubber, so that the dried sheet
divides in strips. It would appear in such instances that either the
coagulum was too thin and soft, or that the grooves had been cut too deeply
and sharply. In any case the choice of a design is an arbitrary matter, and
should depend upon the effect produced on the rate of drying and the
general appearance.
The popular "close-cut spiral" roll is machined with varying measurements,
but the usual design has grooves 1/8 inch wide by 1/8 inch deep and 3/16
inch apart.
Many estates have a particular "brand" cut in the middle of the rolls for
purposes of identification. If this is done it is advised that the main
grooving of the rolls be carried into the "branding" strip; otherwise grip
will be lacking on this portion, and a certain amount of "cockling" of the
sheets will result.
Sheets are often seen in which the potential effect of the grooving is
reduced to a comparatively flat pattern in place of the desired ridges. The
fault is generally attributed to the shortcomings of the marking rolls.
While it is true that the grooving often deteriorates by friction-wear when
the rolls are running "free," experience generally decides that the
deficiency in the appearance of the rubber should be attributed to faulty
previous preparation rather than to the marking rolls. Sets of rolls have
been changed often without justification or an improved result. It would
always be well to be certain first that the trouble did not emanate from
the fact that the coagulum had been previously rolled so thin and hard that
the rubber could not be squeezed so as to fill the grooves. This has been
found to be a common fault, and the general effect is to delay drying in
spite of the thinness of the rubber.
Again, the trouble may have been due to an incorrect standardisation of the
latex, generally in the direction of too heavy a density (too rich a latex)
being employed. The original thickness of the coagulum would be normal, but
owing to the abnormal rubber-content the effect of passing through the
smooth rolls would be the production of a strip thicker and firmer than
ordinary. If this firmness is appreciable the resistance of the rubber to
the squeezing action of the marking rolls will result in a flat
pattern--_i.e._, the grooves cannot be filled, and the ridges are low.
It is advised that all rolls used in the preparation of sheet rubber should
be at least 18 inches wide, in order to avoid the appearance of thickened
edges which delay drying.
Working with the correct standard of dilution of latex, and following the
procedure indicated in the foregoing paragraphs, the dry sheet should not
exceed an average thickness (over ridges and depressions) of 1/8 inch.
PREPARATION FOR SMOKE-CURING.--It used to be the custom to allow sheet
rubber to air-dry first for periods varying from one to several days.
Naturally moulds were soon formed, and when the sheets were quite
smoke-cured a mass of the dead moulds could be seen, if not over the whole
sheet, at least in the corners of each diamond mark. It has been
demonstrated in practice that there is no advantage in allowing sheets to
air-dry partially before smoking. In fact, to obtain the greatest benefit
from smoke-curing, sheet rubber should be placed in the smoke-house as soon
as possible. The same effect of mould-growth may be noted if the wet sheets
are placed in a smoke-house insufficiently heated.
Other defects may arise which can be traced to faulty treatment of the
marked coagulum prior to hanging in the smoke-house and subsequent to
rolling. These will be enlarged upon in a subsequent section of the book,
and at present it will suffice to indicate the procedure which experience
directs as likely to give the best results.
When the lengths of coagulum leave the marking machine they are usually
laid in piles containing two dozen or more strips. The piles are then cut
into the required lengths, the exact length generally being determined by
the available perpendicular distance between the supports in the
smoke-house. It is necessary to remark that the piles of sheets should not
be allowed to accumulate, but should be dealt with in subsequent treatment
progressively. If for some reason this is not possible, then all piles of
sheets should be turned on edge so as to assist the draining away of the
serum or "mother-liquor," which continues to ooze from the rubber for some
time after the squeezing in the machines.
Where hot water is available the freshly cut sheets should be passed into
it as soon as possible, and given a thoroughly good swilling. The caution
must be given that the hot water should be changed very frequently and, if
possible, after every batch, say, of a hundred sheets.
The sheets should then be carried immediately to racks on which they are
hung to drip. Generally these racks are situated under cover, but there is
no reason why they should not be placed in the open air without cover or
shade. From continued experience of this practice over a period of years it
is found advantageous and to be preferred to the usual method of allowing
sheets to drip under cover.
While the sheets are fresh and loaded with internal moisture, the effect of
sun-heat upon the surface, when exposed for, say, two hours, is nil; and
the safety of the process can be guaranteed, provided the stated limit is
not exceeded to an appreciable extent.
[Illustration: THE OLD METHOD OF "DRIPPING" FRESHLY ROLLED SHEETS WITHIN
THE FACTORY.]
After dripping for an hour or so, the sheets should be placed in the
smoke-house. If it is a bright sunny day, no extra precautions need be
taken; but on cool, dull days it would be advisable to light the fires
earlier than usual, consistent with the work required to be done in the
house--_e.g._, in the removal of dry rubber. There would appear to be no
reason why the dry sheets should not be first removed, so that on dull or
wet days smoking can be commenced as soon as the wet rubber has been hung.
On a few estates where the smoke-houses are worked continuously, except for
a few hours in the morning, a portion of the building is separated by a
partition for the reception of the wet rubber. The sheets are taken
directly from the marking rolls and placed in the chamber, beneath which a
fire is started. The sheets thus drip in a warm and smoke-laden atmosphere
until next morning, when they are weighed and removed to the smoke-house
proper. It is claimed that freedom from "rust" is thus obtained.
It will be clear that in the treatment of the rubber preparatory to smoking
the whole process should be continuous, and delay should be avoided if the
best results are to be obtained.
[Illustration: THE NEWER METHOD OF HANGING IN THE OPEN AIR.]
SMOKING OF RUBBER.--The assumption may have been noted above that the sheet
is to be smoked. As far as our knowledge extends, none but small native
estates now prepare sheet rubber of any other type, with the exception of
certain patent processes. Air-dried sheets are generally made on
small-holdings, and are bought in the market chiefly for the purpose of
macerating and making into blanket crepe. We have no intention, therefore,
of discussing the possibilities or qualities of air-dried sheets, as the
output of sheet-rubber from our estates is always in smoked form. The
drying (or, properly, smoking) stage will be discussed in Chapter XI.
CHAPTER X
_PREPARATION OF CREPE RUBBER_
NO. 1, OR FINE PALE CREPE.--Considering first the preparation of the
highest grade, fine pale crepe, it must be stated that the difficulties
attached to the process are generally not sufficiently appreciated. In this
pale rubber minor blemishes are so plainly apparent that their importance
is highly exaggerated, and what would worthily escape notice in smoked
rubber assumes disproportionate prominence in pale crepes. The very fact
that such a delicate material as colourless coagulum has to be manipulated
in coarse iron rollers, with the attendant oil and grease worries, should
be sufficient to deter one from criticising too harshly the occasional
lapses of an estate struggling to give of its best to the market. At the
same time there can be no doubt that if precautions are taken to attend to
all likely sources of contamination, defects in pale crepe may be avoided
to a wonderful extent; and on some estates the observance of elementary
rules enables the preparation of the finest pale crepe to be made almost
mechanically.
STANDARDISATION OF LATEX.--The question of the standardisation of latex has
been dealt with in a general way in Chapter VII., and the reader is now
familiar with the trend of the argument in its favour. It will be
recognised that the necessity for standardisation exists to the same degree
in the correct preparation of pale crepe as in the case of smoked sheet.
Unless the dry rubber content is invariable, and the quantities of
chemicals fixed, the colour of the crepe may vary appreciably.
It may be pointed out that it is not _essential_ to adopt the same standard
of dilution as for sheet preparation. Given that latices from all fields or
divisions are fairly uniform, and of high rubber content, the standard may
be taken at a figure equivalent, for example, to 2 lbs., or 2-1/2 lbs., or
even 3 lbs. per gallon. It is wise, nevertheless, to take a lower standard
for several reasons. For instance:
(_a_) The average dry rubber content varies with climatic conditions,
position of the cut on the tree, general health of the tree, etc. On a
rainy day the dry rubber content may be lowered too greatly by
adventitious circumstances.
(_b_) Recording instruments often fail to give even approximately
correct readings in rich latex. Errors may thus be made easily.
(_c_) A fairly soft coagulum means easier working on the machines,
less labour, and proportionately cheaper costs.
[Illustration: THREE GRADES OF CREPE RUBBER.
Left to right: fine pale crepe; second quality pale crepe; compound crepe.]
It is advised, therefore, that for general purposes the same standard as
that found suitable for sheet rubber should be taken--viz., 1-1/2 lbs. dry
rubber per gallon. At all events the standard should not exceed 2 lbs. per
gallon.
COAGULATION AND COAGULANT.--Coagulation may be undertaken without any
special arrangement of tanks, and is usually effected in the ordinary
"Shanghai" glazed earthenware jars containing about 45 gallons. Given
reasonable care, and a fairly fool-proof system of calculation for the
quantities of chemicals required, no difficulty need be experienced.
[Illustration: A WASHING SHED.
Sheets are soaked in hot water in tanks in the background, and then
scrubbed under a spray of cold water.]
On a larger scale it is advised that proper reception tanks, in which
standardisation can be effected, should be installed. Where both sheet
rubber and fine crepe are being prepared, the whole system of
sheet-coagulating tanks may be employed with considerable advantage, even
to the insertion of the partitions.
If ordinary jars are used, and the coagulum is left until the following
morning, the mass of rubber has to be cut up into pieces of a size suitable
for the machines. The knives or saws are sometimes rusty, and the colour of
the coagulum is affected. The coolies often feed into the machines lumps
which are too large, with the result that portions are thrust under the
cheek-blocks and become stained.
When a sheet-coagulating tank is used all labour of cutting the coagulum is
obviated. The long strips are handled and fed into the rolls easily. It may
be seen, likewise, that actual work is thus saved in machining.
QUANTITY OF COAGULANT.--For a general discussion on the coagulant and
quantities employed, the reader is referred to Chapter VIII. It is there
recommended that for latex standardised to a level of 1-1/2 lbs. per
gallon, the proportion of pure acetic acid should be in the ratio of
1:1,200. Directions are there given for the making of the solution, and the
calculation of the quantity required for any given volume of latex.
It is pointed out that for average _undiluted_ latex the basis of
calculation, for quantities of acetic acid required, should be taken on the
ratio 1:1,000. Where latex exceeds a dry rubber content of 3 lbs. per
gallon, it may be necessary to increase the quantity of acid to 1:800.
If a standard of 2 lbs. per gallon is adopted, the formula given for the
1-1/2 lbs. standard will not give full satisfaction, and the quantity of
acid solution must be increased slightly in order to obtain complete
coagulation. Assuming that the original solution is prepared in 1 per cent.
strength, the following difference would be noted:
(_a_) One part pure acetic acid to 100 parts water (theoretically 99
parts).
(_b_) _1-1/2 lbs. per gallon._ | _2 lbs. per gallon._
|
Of the above solution use 1 gallon | Of the above solution use 1
to every 12 gallons of standardised| gallon to every 10 or 11
latex. | gallons of standardised latex.
It is not possible to lay down an exact figure governing all cases, as so
much depends upon the treatment undergone by the latex before it reaches
the store.
Some estates continue to use solutions of greater strength, generally 5 per
cent., in crepe preparation. While such solutions may be effectively
stirred in when the latex is dilute, it is advised that for intimate
mixture the solution need not be stronger than 1 per cent.
In estimating the quantities of acetic acid required much depends upon the
interval which is to elapse between the addition of acid and the time of
working of the coagulum. If the rubber is to remain until next morning, the
average formulæ will be found suitable; but if it is required to work the
coagulum with an interval of less than three hours, an excess of acid must
be employed. The excess need be comparatively small, unless the interval is
much reduced. For instance, it is the practice on some few estates to begin
the machining of the coagulum about half an hour after coagulation
commences; in which case it is usual to add from a quarter to a half of the
normal quantity in excess. It need scarcely be pointed out that unless this
procedure is strictly unavoidable it should be discouraged on account of
the waste of coagulant involved. Incidentally, the use of strong solutions
of acid under such circumstances may lead to increased deterioration of the
rolls.
COLOUR OF FINE CREPE.--We are sometimes assured that manufacturers do not
pay the attention to the question of colour which sale conditions would
lead one to believe. As far as we are concerned, and as long as there is no
direct traffic between producer and consumer, it must be recognised that in
the vast majority of cases we are forced to concern ourselves only with the
standards set up in the markets. This, in spite of the knowledge that, all
other things being equal, the arbitrary distinctions in colour afford no
indication of the intrinsic value of the rubber. Under present
circumstances it is plain that if paleness is demanded it has to be
supplied.
Probably without exception all estates employ sodium bisulphite as the
agent for the prevention of that darkening (oxidation) which is natural in
drying rubber.
SODIUM BISULPHITE.--A formula for use of this chemical is given in Chapter
VIII., and is applicable to latex standardised to 1-1/2 lbs. dry rubber per
gallon. If a higher standard is chosen the quantity calculated as in (_b_)
of that formula may be increased slightly, and the exact requirements found
by experience. The caution must again be given that the employment of an
excess of sodium bisulphite will lead to the production of an over-pale
rubber, and a prolongation of the drying period. If thick crepes are made,
an excess of the chemical is sometimes made visible by a greyish powder
deposited on the edges of the strips of dry rubber.
It must be emphasised that the formula in Chapter VIII. indicates the
_maximum_ quantities required for use with standard latex. Many estates
will find it expedient to use less of the chemical; and if it is found that
the desired result is not obtained from normal proportions, attention
should be directed to the points discussed in the following paragraph.
EVALUATION AND DETERIORATION OF SODIUM BISULPHITE AND SODIUM
SULPHITE.--Sodium bisulphite and sodium sulphite are both bought for our
purpose in the form of a fine crystalline powder, and on analysis good
specimens should contain over 90 per cent. pure substance, when packed in
well-sealed vessels.
It has often happened that shippers or local sellers, by inadvertence, have
supplied the one chemical in place of the other--to the detriment of the
rubber and the discomfiture of managers of estates. The error, as a rule,
has not been detected for some time, and then perhaps only as a result of
complaints or enquiries reaching the laboratories. To the layman, and
certainly to the native who usually has charge of these substances, it is
not a simple matter to distinguish between them without special knowledge.
There are certain elementary tests, however, which can be applied on all
estates serving to make the distinction, but affording no information
regarding the actual quality of the chemicals. They are given in a
comparative form on page 116. Samples of doubtful specimens may be sent to
the laboratories for analysis, but the bulk of the chemical should not be
used.
During the War some very poor shipments were received, and much trouble was
caused. Under normal conditions there can be no question that it is to the
interests of chemical manufacturers to supply the best article; and it is
anticipated that in future there should be no difficulty in procuring
shipments of a high degree of purity.
_Sodium Bisulphite._ | _Sodium Sulphite._
|
1. If in good condition it | 1. It has no perceptible
has a powerful odour of | odour.
sulphur dioxide.[9] |
|
2. In solution it should turn | 2. In solution it should turn
a blue litmus-paper red. | a red litmus-paper blue.
|
3. It exhibits a marked tendency | 3. The tendency to "cake"
to "cake" if the | is less marked than in
drum is allowed to | the case of the bisulphite.
remain open. |
[9] High-grade sodium bisulphite has very little odour, but by the time it
reaches the estate, and as a result of short exposure to the moist
atmosphere of the tropics, a little decomposition sets in and a strong
odour of sulphur dioxide gas is noticeable.
It will be evident that, as sodium bisulphite under normal conditions gives
off a gas when exposed to the atmosphere, it deteriorates in quality
continuously. It is the potential presence of this gas which makes the
powder effective as an anti-oxidant and disinfectant. It is within the
experience of all accustomed to the handling of this chemical, that in
addition to the loss of gas, the powder cakes into a hard mass on exposure.
If only the top layer is caked, the remainder may be in fair condition; but
no caked portions should be used, as they cannot be of good quality. They
may, however, be used for the treatment of scrap rubber, to be discussed
later.
CARE OF SODIUM BISULPHITE.--The ready tendency of sodium bisulphite to
deteriorate on exposure should give sufficient indication regarding its
treatment in storage. It should be bought only in drums (or other air-tight
containers), and should be stored in a dry place. No drum should be opened
until required, and the common practice of keeping an open drum on the
floor of the factory should be avoided.
Drums are of two sizes, generally containing 1/4 or 1/2 cwt. respectively.
It will be obvious that, although the prime cost may be cheaper with the
larger quantity, it would always be preferable to secure the smaller
drums, as the loss on exposure will be less.
Most commonly the 56 lb. drum is purchased. It should not be difficult to
calculate the period during which the contents will be consumed, on the
basis of a maximum of 1 lb. per 100 gallons of latex. A 56 lb. drum,
assuming no loss or waste, should be sufficient to treat _at least_ 5,600
gallons of latex (say, 8,500 lbs. of rubber)--if the bisulphite is of
first-class quality, and the use is applied only to the preparation of fine
pale crepe.
Where the quantity used per diem is small, it is advised that precautions
should be taken to preserve the quality of the chemical when a drum is
opened. It might be of advantage to place the contents of the drum in
smaller sealed tins, or to have made a special container (with a closely
fitting lid) into which the powder can be placed as soon as the drum has
been opened.
MIXING SOLUTION WITH LATEX.--Emphasis has been laid, in Chapter VIII., upon
the necessity for care in the preparation of the solution. Equal regard
must be given to the mixture of the solution with the latex.
On a few estates it used to be the practice to add the powder to the
solution of acid, with stirring. Obviously this led at least to a great
loss of efficiency, owing to the rapid escape of the gas which was evolved.
The solution of sodium bisulphite should be poured into the latex in as
uniform a distribution as possible. The mixture of solution and latex
should be thoroughly stirred, and if only natives are in charge a minimum
period of five minutes should be prescribed before the addition of the
coagulant. A thorough stirring should again follow the advent of the acid.
If these elementary rules are not observed faithfully, the deficiency will
most probably be manifested in the dry rubber in the shape of streaks of
varying shades of colour.
Finally it may be insisted upon that deteriorated sodium bisulphite should
not be used. In order to obtain an effect double the quantity may be
required, and the residual salts left in the rubber on evaporation of the
moisture will be responsible for prolonged drying, surface deposits, and
possibly "spot disease."
FORMER METHODS OF MAKING PALE RUBBER.--Merely as a matter of historic
interest it may be mentioned that previous to the introduction of sodium
bisulphite pale crepes were made in comparatively small quantity by various
devices, among which the following might be quoted:
(_a_) Use of excessive quantities of strong acetic acid.
(_b_) Extreme dilution of latex in conjunction with excessive
quantities of acid.
(_c_) Extreme dilution in conjunction with steaming and excess of
acid.
(_d_) Extreme dilution of latex in conjunction with excess of acid and
subsequent heating of the coagulum in hot water.
(_e_) The use of excess of a mineral acid such as sulphuric acid.
(_f_) The skimmings and very dilute latex, coagulated with excess of
acid.
WORKING THE COAGULUM.--Description of the details of necessary machinery
for crepe-making is relegated to Section III. of this book. Here we shall
treat only of the matter in general.
In the preparation of crepe rubber heavy machinery is necessary, and ample
engine-power must be available. The machines should comprise three types:
(_a_) With rolls cut in such fashion, and run at such different
speeds, as to have a macerating effect upon the coagulum. Such
machines or rolls will be referred to as "macerators."
(_b_) Intermediate rolls, grooved in varying designs and geared
differentially. These reduce the thick rough crepe obtained from the
macerators into a form suitable for passing to the rolls described in
(_c_). They are sometimes called "crepers," but as this term may be
applied equally to the macerating rolls, they will be termed the
"intermediate" rolls.
(_c_) Smooth rolls usually run at approximately even speeds and, as
their name denotes, devoid of any grooving. They are called "smooth"
rolls or "finishers."
Without such equipment it is not possible to prepare the grade which is
known as "fine pale crepe." In the common acceptation of this term crepe of
No. 1 quality generally connotes fineness and paleness with a thin crepe
which has a good, smooth, and fairly well-knit texture.
It is, of course, possible to make a thick pale crepe, using only the
macerators and intermediates, but the "finish" will be that typical of the
particular grooving of the intermediate rolls. For the preparation of crepe
ordinarily, the possession of smooth rolls is a _sine qua non_.
For reasons which will be explained more fully in the chapter dealing with
the defects of crepe rubber, the practice of preparing thick crepes direct
from the coagulum is now very uncommon. Thick crepes are generally made by
reworking dry rubber, either in the form of thin crepes or from air-dried
sheets. The market for the latter in Malaya is confined almost entirely to
Singapore, where factories buy native rubber and re-work it into thick
crepe.
The bulk of the output of No. 1 crepe from estates is in the form of thin
"fine pale crepe." The artificial standard set up by buyers and brokers
necessitates this thin crepe being of even texture and fairly free from
small holes ("looseness"). What difference the small holes are to make in
the vulcanising properties of the rubber is beyond our knowledge; but such
being the standard, it must be attained if the full price is to be
obtained.
In order to secure the desired effect the coagulum must be passed
consecutively through the three types of rolls, and undergoes a varying
degree of working in each.
Given the necessary equipment of machines, it is possible to make a good
specimen of thin pale crepe if the coagulum passes through all the rolls a
total of twelve times (or even less in exceptional cases). There is no
intention of suggesting that this is possible on all estates. Clearly the
number of times the rubber passes through the rolls will depend upon the
total efficiency of the machines. This in turn involves such factors as
(_a_) the size of the rolls, (_b_) the number of machines of each type,
(_c_) the gearing of the pinions, (_d_) the speed of the drive, etc. Again,
much depends upon the nature of the coagulum worked. A fairly soft coagulum
will offer less resistance, and conversely a dense coagulum will require
more machining.
It has been shown by the writers in previous publications that over-working
of the coagulum has an effect on the vulcanisation of the rubber; and this
has been confirmed by others.[10] Apart from this point, it should be
recognised that over-working, beyond that necessary to produce a thin crepe
of even texture, is to be deprecated, on the ground of economy, in working.
[10] Bulletin No. 27, Department of Agriculture F.M.S., April, 1918,
"Preparation and Vulcanisation of Plantation Para Rubber," Eaton, Grantham
and Day.
Owing to the existing differences in equipment and speed of drive, etc.,
the regular practice of any one estate may be unsuitable for another. It
remains, therefore, a matter of study for each estate to discover the
minimum number of times which rubber should pass through the machines,
consistent with the factors indicated above. In any case it may be assumed
that if any factory cannot prepare a good crepe by passing the rubber, say,
twelve times through the rolls, there is some deficiency in the machines,
or of speed; the coagulum may be too hard, or the rolls may be badly worn.
LOWER GRADES OF CREPE RUBBER.--Even a few years ago it was plain that the
lower grades of crepe (_i.e._, all grades lower than first latex rubber)
were not sufficiently appreciated in the market. There was often a marked
difference in price between a first-grade crepe and crepe made from
naturally coagulated lump. This arose chiefly from lack of knowledge. It
has since been recognised in some measure that no reason exists for such a
wide difference in price, and more recently the margin between even the
first-grade rubber and the lowest grade of scrap rubber has been a
gradually diminishing one. Providing sufficient care is exercised in the
preparation of the lower grades, one would expect to see but very small
difference in prices between any two grades. It is true that adequate
attention has been given to the preparation of the scrap grades only in
comparatively recent years, and it is acknowledged that when high prices
were ruling for first-grade rubbers sufficient attention was not generally
given to the subject of the preparation of the lower grades.
NATURALLY COAGULATED LUMP RUBBER.--The grade of rubber made from the
naturally coagulated lump which forms in buckets and carts is usually of a
mixed colour, due to the fact that the lumps oxidise very quickly. When
they are allowed to remain overnight before being machined, it can be
imagined that the colour of the dry crepe would be very dark, or would
contain very dark streaks. Such is ordinarily the case, unless special
precautions are taken.
Providing that the coagulated lump is free from bark, leaves, and
leaf-stems, and certain other precautions taken, the difference in price
between coagulated-lump crepes and first-grade crepes should be very
slight. Too often, however, not sufficient supervision is given to the
coagulated-lump rubber, and it is common to see it come into the factory
containing leaves and bark. These should be picked out before the latex is
strained, but obviously it would be better to ensure that they did not
enter the buckets in the first place.
It would seem reasonable to suppose that if some means could be employed
for preventing or checking the surface oxidation of naturally-coagulated
lump rubber, there would be a corresponding improvement in the colour of
the dry crepe. That such a method is practicable has been demonstrated on
many estates. The lump when lifted out of the latex is allowed to drain for
a few minutes, and is then (without squeezing) placed in a dilute solution
of sodium bisulphite. A 1 per cent. solution is sufficiently powerful. It
is not to be thought for a moment that by the use of sodium bisulphite any
previous oxidation will be counteracted; all that is claimed for the
treatment is that any further surface oxidation will be checked, and the
rubber may be allowed to remain until the next day, for working, if it is
so desired. It will probably be found that quite a quantity of latex has
been expressed from the lumps by contraction, and acid may be added to
obtain the rubber from this. On other estates the lump rubber is worked on
the machines as it is received, and the resulting crepe is submerged in a
weak solution of sodium bisulphite over-night. It is then rinsed in water
and hung to drip before weighing and placing in the drying house. Under
certain conditions some of the lump rubber darkens rapidly during transport
to the store, and any such oxidised portions must be rejected if a uniform
colour is to be expected in the crepe.
Following the procedure indicated above, some estates find it possible to
prepare from naturally coagulated lump rubber a crepe which can be classed
as No. 1 grade.
SKIMMINGS AND WASHINGS.--The skimmings of tanks, as already shown, may be
prepared sometimes as a second quality of smoked sheet; but generally they
are amalgamated with the rinsings of cups and buckets, treated with sodium
bisulphite and acid, and made into crepe form.
The cup-washings, as they arrive at the store, represent a very dilute
latex, the rubber from which is generally of a greyish colour.
Bucket-washings should yield a good type of pale rubber if they are
obtained properly. To obtain the maximum quantity of good rubber the
buckets should first be rinsed. A gang should be taken, a small quantity
(say a quart) of water poured into the first bucket, and this dilute latex
used progressively in all the buckets of that gang of tappers. The result
is a fair latex which can be added to the bulk of No. 1 latex, provided it
is free from dirt. Where sheet rubber is being prepared, carefully strained
cup-washings or bucket-washings may be utilised in the dilution of the
latex to the required standard, thus increasing slightly the percentage of
first-grade rubber.
TREE-SCRAP.--As tree-scrap is a naturally coagulated rubber, it should be
expected to show up well in quality. This is usually the case; but from
what has been said of the effect of sun-heat it will be understood that if
trees are not regularly "scrapped," there is a danger that the crepes may
be found to contain tacky streaks due to the inclusion of old scrap which
has been sun-baked. In hot dry weather, on widely planted areas tapped on
alternate days, it has been noticed that scrap remaining for two days often
exhibits a resinous appearance, and feels sticky to the touch.
If tree-scrap is to be made as a separate grade, as used to be the general
custom, care should be taken to see that it is free from bark and dirt. On
some estates where scrap-rubber is paid for per pound collected, it is
usually the rule to insist that scrap shall be washed free from dirt and
picked free of bark. This course is to be commended, but might probably
prove impracticable to the majority of estates. Theoretically, of course,
the operation of machining should rid the scrap of all traces of bark; but
in practice it does not do so.
Some proportion of the tree-scrap is usually found to be heavily oxidised,
and naturally if a crepe of uniform colour is to be obtained these dark
scraps must be rejected, otherwise dark streaks will be formed. Coolies
should be instructed to sort out the dark pieces before arriving at the
store.
BARK-SHAVINGS.--It has been intimated in a previous section that the method
of obtaining and collecting bark-shavings varies with the type of labour
employed.
Where the scrap is removed from the edge of the bark on each occasion
before tapping, the amount of rubber to be extracted from the dry shavings
is very small--so small, in fact, that when the price of rubber is low, it
is doubtful whether it pays to collect and work the material.
On the other hand, where trees are not "scrapped" before tapping, the
bark-shavings and tree-scrap are collected together, and the amount of
rubber derived from the mixture may be 30 to 40 per cent. upon the gross
weight--depending chiefly upon the quality of the tapping (_i.e._, in this
case, the thickness of the paring). Another factor influencing this figure
would be the effect of using an anti-coagulant on the cuts.
Bark-shavings entail such wear upon the ordinary machines during working,
especially if fairly free from rubber, that unless the factory is equipped
with a special "scrap-washer" it is advised that this material should be
sent for working to a factory having the necessary equipment. Whenever
possible, bark-shavings should receive treatment on the day of collection.
It used to be quite common to see heaps of bark-shavings accumulating on
the floor of a factory, and generating excessive heat. Yet these heaps were
allowed to stand about for a day or days. Is it any wonder then that
tackiness was found to develop when the rubber was dry? It is here
definitely laid down that no heaps of bark-shavings should be accumulated
even for half a day. Tanks should be provided in which the shavings should
be submerged in water.
EARTH-SCRAP.--Of all grades of crepe this is the one most liable to become
tacky in transit. This tackiness to a large extent cannot be avoided, as
old pieces of earth-scrap may be brought in amongst the bulk. Probably
these old pieces have been exposed to the sun for days, and have become
quite resinous. It would be practically impossible to go through all
earth-scrap in order to find these odd pieces, but unless this were done
one could not guarantee that the earth-rubber would always be free from
tackiness. The difficulty does not appear, however, on estates where
earth-rubber is collected systematically at very frequent intervals.
FIBROUS MATTER IN LOW-GRADE RUBBERS.--It is sometimes found in this and
other lower grade rubbers that pieces of cloth or cotton-waste are
concealed. Coolies may have used them for cleaning cups, or the store
coolies may have been at fault. Earth-scraps especially should be examined,
before working, for such extraneous matter.
SCRAP-WASHERS.--These are heavy machines specially devised for the
treatment of lower grade rubbers. In these the raw rubber is well
masticated and freed from impurities, if the machine functions efficiently.
There are several types of these machines, all of which are efficient. That
best known is the "Universal" washer, made by Joseph Baker, Sons, and
Perkins, Ltd. (formerly Perkins Engineering Company). Coming into local
favour during the War, the "U.E." scrap-washer, made by the United
Engineering Company (Singapore), gives very good service. The "C.C.C."
washer, made by the Colombo Commercial Company, is suitable for the
purposes of an average estate. There are others, less well known. Most of
these machines are made in varying sizes to meet the requirements of small,
medium, or large estates; and if funds are available, a scrap-washer should
be regarded as an essential item in the machinery of any estate employing
engine power.
The rate of output of scrap-washers will depend mainly upon the speed at
which they are driven, and when ordering the equipment it would be
advisable to state the ordinary speed of the back-shaft, length of drive,
etc. It does not follow that the larger the rate of output, the greater is
the efficiency of the washer. The point is not as to what quantity of
rubber can be taken out per hour, but what quantity is actually freed from
impurities.
It is advisable for the superintendent to obtain a thorough knowledge of
the general construction and principles of the particular scrap-washer
employed. In the past it was not uncommon to find superintendents innocent
of the fact that a certain type of washer possessed movable parts upon
which the efficiency of the cleansing largely depended. It was often found
that these parts, which were intended to be removed and cleaned at
intervals, had become firmly fixed and could not be removed for inspection.
It must be recognised also that the machines are liable to considerable
damage if extraneous substances are allowed to enter--for example,
tapping-knives, stones, pieces of iron, spouts, etc., which are sometimes
present in the loose scraps of rubber or shavings, owing to the
carelessness of coolies. Under the best regulated-system, such accidents
occasionally occur, but a great deal of trouble could be avoided by having
it understood that each charge must be sorted over before entering the
washer.
Again a deal of extra work, and much wear and tear, is caused by the
_abuse_ of the scrap-washer--_e.g._, in the cleansing of earth-scrap. As
this reaches the factory it often contains a quantity of internal or
adhering earth. Before entering the washer a good proportion of the
external soil could be removed if the scraps were thrown into a tank and
given a thorough soaking and stirring. In a similar manner dry
bark-shavings, which have been allowed to accumulate, could be softened.
In the actual working of scrap-washers instructions are generally given by
the makers. These sometimes advise the introduction of warm water (or of
steam into the cold water supply) for an interval during the working of
each charge. Where possible, such instructions should be followed, as by
this means the individual pieces of rubber are massed together, in the
final stage, into a "sausage" form which is easy to transport and to
manipulate in the ordinary crepeing battery.
COMPOUND CREPES.--The attitude of both buyers and sellers with regard to
the types of lower grade rubbers appears to be changing. In the past, from
any one estate there might be obtained as many as six grades of crepe below
No. I. These comprised:
(1) A pale rubber (often streaked) obtained from coagulation of cup
washings and bucket rinsings.
(2) A pale rubber (often streaked) obtained by coagulation of the
skimmings from the surface of the No. 1 latex.
(3) A streaked and dull rubber prepared from naturally-coagulated
clots found in cups, buckets, and latex carts.
(4) A streaked rubber prepared from scrap which had coagulated upon
the face of the cut bark.
(5) A brownish and streaked rubber made by maceration of bark-shavings
to which pieces of tree-scrap adhered.
(6) A dark rubber, often tacky, prepared from scrap found in or on the
ground near the base of the trees. As it is often a matter of weeks
between any two regular collections, it is easy to understand why the
dry rubber was more liable to be "tacky" than any other grade of
crepe.
It will have been evident to all who have acquaintance with these grades,
as shipped from many different estates, that the diversity in the various
shipments must have been rather bewildering. There appeared to be a
regrettable lack of uniformity, even in the appearance of, say, a bark
scrap rubber from any two estates. When, in addition to these variations,
the further complication of condition of cleanliness is introduced, one may
realise the difficulty attaching to the evaluation of these rubbers as they
appeared upon the market.
Although the foregoing paragraph is written in the past tense, it should be
pointed out that within certain limits the trouble continues to exist with
respect to the output of a great number of estates.
In the case of many, it has been realised that the manufacturer does not
want to buy a large number of "parcels," all differing in some respect. It
is probably correct to state that what a manufacturer requires is a big
"parcel" uniform in appearance and treatment, even though the colour may
not be so light as that of many smaller lots. This statement is modified
with the proviso that the rubber, no matter what its colour or appearance
may be, must be free from dirt, grit, and bark.
The difficulty of making a uniform product from several types of lower
grade rubbers has been successfully solved on several estates by the
preparation of a "compound" crepe composed of a mixture of the best lower
grades in approximately definite proportions daily. Naturally the shade of
colour of this compound crepe will depend largely upon the types of rubber
employed, but as a rule it is somewhat darker than the highest of the types
employed in the mixture. To the writers this seems immaterial as long as
the manufacturer is offered a larger and more uniform lot which can be
given uniform treatment in vulcanisation processes. Neither would it appear
that the seller suffers any monetary loss. In point of fact it will be
found probably that the reverse is the case. For instance, supposing it
were decided to mix for a compound crepe--
(_a_) Naturally coagulated lump rubber.
(_b_) Tree-scrap.
(_c_) Bark-shavings scrap.
The product would be darker in colour than (_a_) and slightly better than
(_b_). Let it be granted that there might be a monetary loss on (_a_), it
is probable that there would be a slight gain in comparison with the usual
prices obtained for (_b_) and (_c_). Now, as a general rule, the actual
percentage of crop made into (_b_) is appreciably less than that made into
(_c_) and still less than (_b_) and (_c_) together. Apparently, therefore,
there would be a margin of profit on the whole by making a compound crepe.
It may be pointed out, on the other hand, that there might be expended on
the manufacture of this crepe more time and labour, but as against this the
labour of sorting and grading would be simplified.
Unfortunately this process is not open to estates which do not possess a
scrap-washer. _It is essential that the rubber should be free from grit,
sand, and bark particles._ In the absence of a scrap-washer for the
cleansing of the bark-shavings, it would be futile to attempt to make a
compound crepe containing that type of rubber, as one would run the risk of
spoiling the whole. It seems certain that in course of time a scrap-washer
will be considered as necessary a piece of machinery as an ordinary
crepeing machine in the factories of estates having sufficient means. Until
that time the preparation of compound crepes must be the privilege only of
well-equipped estates, unless other estates can send their lower grade
rubbers for treatment in a scrap-washer to their more fortunate neighbours.
In previous publications a diminution in the number of grades of crepe
rubber has been advocated, and it is gratifying to find that in many cases
the amending grades suggested have been improved upon. Many estates now
make only three grades of crepe--viz.:
(_a_) No. 1. From latex coagulated in the store.
(_b_) No. 2. Compound.
(_c_) No. 3. Earth-rubber.
It will be seen that the compound crepe includes all types between fine
pale crepe and earth-rubber. Naturally one could not safely recommend the
inclusion of earth-rubber in any compound crepe, as the risk of possible
"tackiness" in the whole would be serious. In the case of the bark-shavings
rubber to be incorporated, it is first cleaned alone in the scrap-washer.
Then all types are mixed together again in the scrap-washer in proportions
ruled by the experience of the usual average percentages of each grade of
the crop.
Besides the estates having only three grades, there are others which make
four--viz.:
(_a_) No. 1. From latex coagulated in the store.
(_b_) No. 2. Compound, from cup washings, etc., skimmings, and
naturally coagulated lump.
(_c_) No. 3. Compound, from tree-scrap and bark-shavings rubber.
(_d_) No. 4. Earth-scrap.
Other variations are possible, but their number is limited, and they
all conduce to simplification of working, and a supply to the market
of rubber having greater uniformity.
NEED FOR INCREASED CARE WITH LOWER GRADE RUBBER.--In the ordinary procedure
of estate-working there appears to be an undesirable variety in the style
of lower grade crepes. On some estates an examination of these rubbers
would appear to suggest that there need be no expenditure of care in the
preparation or the form in which it is made. This is a great mistake. With
the exception of the lowest grade (earth-rubber), it would not be unfair to
state that the quality of the rubbers on testing should be very little
inferior to the No. 1 product. Often, as in the case of naturally
coagulated rubbers, they are superior in some respects to ordinary fine
pale crepe. Doubtless manufacturers are aware of these facts, but what
course is open to them if they find the rubber spoiled for their purpose by
the presence of particles of sand, grit, or bark? The possible injury
caused by these ingredients cannot be insisted upon too strongly, and it
must be evident that great care should be exercised in the preparation of
the lower grades of crepe.
As to the particular form of the lower grade crepe rubber, one may apply
the remarks made under the section dealing with the best grades. It is
common to find thin crepes, medium crepes, and "blanket" crepes. More often
than otherwise, the medium and thicker crepes are prepared direct in those
forms. It follows that they are liable to attacks of "spot" disease, which,
however, is not easily visible in the lowest grades, owing to the dark
colour of the rubber. Furthermore, it is not possible to cleanse the rubber
so thoroughly if thick crepes are made.
BLOCK RUBBER.--Few estates now prepare block rubber, which is essentially
crepe rubber pressed into blocks. In the ordinary process the fresh
coagulum is lightly rolled into thin crepe, which is then vacuum-dried.
There are slight variations in the subsequent procedure. Sometimes the
rubber as it comes from the vacuum drier is merely allowed to remain on
racks overnight before blocking. In other instances, the sticky rubber from
the vacuum drier is passed once or twice through wet, smooth rolls and hung
to dry for some days. The dry crepe is then folded into the pressing box or
cut to suit the size of the box. Pressure is applied for some time, and
finally the rubber is taken out in one homogeneous mass. Naturally the
appearance of the block depends upon the quality of the parent crepe. Some
block rubber is made up thick; other is made in slabs about 3 inches or 4
inches in thickness. With the latter, it should be possible, when held up
to the light, to see the shape of a hand held between it and the source of
light.
It is possible that an erroneous idea of the strength of block rubber has
been formed. It should only be necessary to point out that essentially
block rubber is merely pressed crepe rubber. It is inconceivable that the
mere action of pressing layers of crepe together would increase the
physical quality of the rubber.
The advantages which block rubber possesses are the compactness of the
output, its ease of packing, and a saving in freight; but there is the
disadvantage, from the consumer's point of view, that extra labour is
involved in the preparatory work of cutting up the blocks.
SMOKED CREPE AND SHEET CLIPPINGS.--There appears to be no certain demand
for any grade of smoked crepe, and probably all which is put into the
market is really comprised of (1) clippings obtained from the ends of
sheets, (2) sheets which have been malformed in machining, or (3) sheets
showing the presence of many "bubbles."
As to the first class it might be explained that through defective rolling,
thick ends or edges may be caused. These show signs of contained moisture
when the bulk of the sheet is perfectly dry, and as undue delay would
otherwise result these moist strips are trimmed and either returned to the
smoke-house, or machined to form crepe.
Similarly a torn or otherwise badly formed sheet, when brought from the
smoke-house, may be made into crepe, rather than it should prejudice the
selling price of the bulk under ruling conditions.
In the same manner, although "bubbles" have no influence upon the quality
of the rubber on vulcanisation, sheets thus affected are generally made
into crepe.
It cannot possibly be argued that rubber of this description would be in
any way inferior to the best smoked sheet for manufacturing purposes, but
owing to the prevailing system of evaluation for market purposes, it is
necessary to resort to the expedients indicated above.
On some estates the rubber specified in the three classes mentioned is not
made into crepe, but cut up into small pieces and shipped as "sheet
clippings" or "sheet trimmings"--a procedure which would appear to be
justified by a steady demand. In point of fact, the buyers are really
obtaining a first-class article (except in superficial appearance) at a
reduced price.
CHAPTER XI
_DRYING OF RUBBER_
AIR-DRYING OF CREPE.--It is still the prevailing custom to air-dry crepe
rubbers. A few estates, it is true, have artificial driers installed, and
in some necessary cases others will be erected. But in the majority of
cases where money has been expended in building air-drying sheds, as long
as it is only possible to ship rubber regularly air-drying is likely to
remain in favour.
The great drawback to air-drying is that one is so dependent upon the
weather conditions. In favourable weather the rubber dries well, but in a
long period of wet weather rubber may accumulate at an alarming rate, and
the accommodation is sometimes severely taxed. Of course, the rate of
drying under the best conditions is mainly dependent on the thickness of
the crepe, and every endeavour should be made to maintain a thin style of
preparation. If this precaution is not taken, the rubber is liable to
recurrent attacks of "spot" disease, and one's troubles are very much
augmented. This is a disability to which rubber treated in artificial
driers is not liable. Still, air-dried rubber can be made equal, if not
superior, in appearance to pale rubbers prepared by other processes.
For the lowest grades of crepe air-drying is always likely to remain the
only method, as it would be extremely unsafe to submit them to heat.
It is noted in ordinary practice that the rate of drying on different
estates, for the same type of rubber, may vary widely. Naturally the
construction of the house has a great effect, and this subject will receive
attention in a subsequent chapter.
Similarly the position of the drying-shed exerts an important influence,
and the erection of the building in low-lying surroundings is always
calculated to prolong the drying period appreciably. Incidentally this
means that the building must be larger than a normal rate of drying would
otherwise demand.
The combination of a poor type of drying-house, a low-lying situation, and
a prolonged wet season, might render it advisable to abandon the air-drying
of high grade crepes in favour of artificial drying.
ARTIFICIAL DRIERS FOR CREPE.--It is more common to find artificial driers
in use in Ceylon than in Malaya, possibly because these driers have been in
use in Ceylon for other products. Some time ago the question of installing
artificial driers received the serious attention of a number of estates in
this country, chiefly on account of the incidence of fungoid and bacterial
diseases in crepe rubber. The simple treatment for the prevention of these
diseases is to get the rubber dry in the shortest possible space of time.
In most cases it is found sufficient to roll crepe thin for air-drying in
order to prevent the appearance of coloured spots. It is found, however,
that some drying-houses are so badly planned and constructed, that quick
drying under even the best of conditions is a practical impossibility.
Cases have been known in which the disease may disappear almost entirely
during a period of freedom from rain, only to recur as soon as wet weather
sets in again. There can be no doubt that, on the whole, the number of
cases of "spot" disease is on the decline; but equally it is certain that a
very few estates will always be liable to outbreaks as long as drying is
attempted in existing houses. For these reasons it is a poor policy to
temporise, and the only sound policy in extreme cases would be to give up
ordinary air-drying in favour of some method of artificial drying. As
regards the majority of estates preparing pale crepe for various reasons,
it is not expected that any will instal artificial driers. Money has been
expended in elaborate buildings which certainly do the work for which they
were designed. As long, therefore, as the accommodation is sufficient, and
regular shipments are the rule, it is expected that ordinary air-drying
will still remain the general practice.
Of the better-known artificial driers, there are only three which merit
serious consideration in these pages. They are the vacuum driers, the
Colombo Commercial Company's hot-air drier, and the Michie-Golledge
process.
VACUUM DRIERS.--The vacuum drier is so well known that only a brief
description need be given. It consists of a chamber heated by steam pipes
and capable of having the contained air and moisture withdrawn by a pump.
This description sounds very simple, and in practice the operation of
vacuum drying is really a simple one, and can well be entrusted to an
intelligent coolie under efficient supervision. Indicators are fitted which
show the vacuum pressure and the pressure of steam in the heating pipes
which travel underneath horizontal slabs upon which trays may be placed.
Still, in spite of the apparent simplicity of the process, there would
appear to be a number of little details which, if overlooked, prove to be
factors influencing the result to a considerable degree. Thus it is not
uncommon to find complaints that the rubber is not dry when packed. The
writers have seen rubber taken from a vacuum drier still containing a
visible quantity of moisture. One would have imagined that continuous
working of the drier would give the experience necessary to obtain dry
rubber, but, apparently, such is not the case in a number of instances.
Elaborate instructions are given by the makers, but often they are more
honoured in the breach than in the observance. Really, there are only two
points to bear in mind:
(1) That the rubber must be fairly thin.
(2) That the temperature be not allowed to rise too high. Some makers
advise 140° F. as a maximum, but no harm results from a temperature of
150° to 160° as long as the interval is not prolonged.[11]
[11] These figures refer to temperatures recorded by thermometers placed in
the folds of the rubber.
These two points presume that the vacuum drier is true to its name, and
that one can obtain a maximum steady pressure. The machines are so well
made now that no drier should be taken over from those responsible for its
erection unless it can show a vacuum pressure of 28 inches within fifteen
minutes of starting the pump; and with the pump stopped, there should not
be a greater fall in pressure than 1 inch within ten minutes after stopping
the pump.
One of the most frequent sources of error is the control of steam pressure
which is responsible for the temperature of the drier. It is quite
unnecessary and unwise to maintain any steam pressure once the drying is
well under way. All that is necessary is to heat the chamber well, with a
steam pressure of 5 lbs., before inserting the rubber. As soon as the
maximum vacuum pressure has been obtained, steam should be shut off from
the heating pipes, and it will be found that the temperature is well
maintained throughout the operation with a rise of ten to twenty degrees at
the end. If the drier is working at a vacuum pressure of 28 inches, and if
the crepe has been prepared thin enough, the rubber should be quite dry
within two hours. Should the operation have to be extended to two and a
half hours at 28 inches vacuum pressure, it is a sign that the crepe is too
thick. On such occasions it is often noticed that these thicker crepes are
not thoroughly dry, having moist spots enclosed in them. On re-rolling,
these moist patches become easily visible, and are a source of great
annoyance, inasmuch as they take quite a long time to dry out.
As mentioned before, the crepe for vacuum drying should be thin. There is
no necessity to give it a superfine finish, and the presence of small holes
is quite permissible, as they disappear on subsequent re-rolling. The thin
crepe may be folded loosely to the length (or breadth) of the tray several
times, but in no other way can the drier be expected to perform its work
satisfactorily. A case was noted in which thin crepe was excellently
prepared, and four or five layers were rolled together for vacuum drying.
Naturally this mode of procedure does not give the drier a fair chance, and
it would be ridiculous to judge vacuum drying on the results. After two and
a half hours at a temperature of 145° F. the rubber appeared to be only
about three parts dry, and the subsequent air-drying extended well into a
fortnight.
It is the common practice to screw up the door of the chamber as tightly as
possible. As a rule it is found in course of time that the obtainable
maximum vacuum pressure decreases. This may be attributed solely to the
forcible screwing up of the door. Around the inside edges of the door are
strips of rubber compound, the function of which is to form a tight joint.
Should the door be screwed up too tightly, these strips become deformed in
course of time, and slight leaks occur. It should be pointed out that it is
only necessary to screw up the door at the beginning of the operation. When
the vacuum has been obtained, the screw pressure may be released, as the
atmospheric pressure outside the chamber is more than ample to keep the
door in a close fitting position. This will be obvious from the fact that
the difference in pressure between the inside and the outside of the door
amounts to, say, 28 inches atmospheric pressure--_i.e._, nearly 14 lbs. per
square foot. By slackening the screw handles, therefore, as soon as the
indicator shows the maximum working vacuum pressure, the life of the door
joints may be prolonged, and the drier will remain efficient for a longer
time.
A careful consideration of the question of temperature leads one to the
conclusion that the practice of placing a thermometer through the roof of
the chamber does not enable one to determine the temperature correctly. In
the same way a thermometer suspended behind the observation window cannot
indicate the temperature of the rubber, as in both of these positions the
thermometer must be influenced by radiation from the walls of the chamber.
The only position in which the correct temperature could be indicated is
between the folds of crepe. This can be arranged easily so as to enable one
to read the temperature from the observation window.
COLOMBO COMMERCIAL COMPANY'S DRIER.--The drier of the Colombo Commercial
Company consists in principle of a number of small chambers or units in
which crepe rubber is placed, and through which hot air is passed. As in
the case of vacuum drying, a great deal depends upon the preliminary
treatment of the rubber. If the crepe is not rolled thin enough drying will
be unduly prolonged, with a possibility that the rubber will become tacky.
The temperature usually obtained is about 150° F., and if the rubber is
thin the production of an installation of two chambers should be at the
rate of 1 lb. of dry rubber per minute. The usual period of drying is
under two hours. One advantage which this drier has over the vacuum drier
is that the chamber can be opened at any time for a short period to
withdraw or insert trays. The thin crepe is folded several times, as in the
case of vacuum-drying.
Figures obtained from the actual working of a drier in Ceylon are given
below:
-------------------------------------------------+-----------------------
CHAMBER 1.--TEMPERATURE 160°-180°F. |
-------------------------------------------------|CHAMBER 2.--TEMPERATURE
_NO. OF | _DRYING | _WEIGHT OF | _WEIGHT OF | 150°-165°F.
Tray._ | Period._ | Wet Rubber._ | Dry Rubber._ |
--------+----------+--------------+--------------+-----------------------
|Hrs. Mins.| Lbs. | Lbs. |
1 | 1 22 | 7-1/2 | 6 |Worked similarly
2 | 1 42 | 7-1/2 | 6 |to No. 1. Yielded
3 | 1 57 | 7-1/2 | 6 |in 2 hrs. 23 mins.
4 | 1 57 | 7-1/2 | 6 |70-3/4 lbs. dry rubber,
5 | 1 57 | 7 | 5-3/4 |from 87-1/2 lbs. wet
6 | 1 57 | 7-1/2 | 6 |rubber.
7 | 2 0 | 7-1/2 | 6 |
8 | 2 0 | 7-1/2 | 6 |
9 | 2 11 | 6-1/2 | 5 |
10 | 2 11 | 7-1/2 | 6 |
11 | 2 11 | 7-1/2 | 6 |
12 | 2 18 | 7-1/2 | 6 |
--------+----------+--------------+--------------+-----------------------
| | 88-1/2 | 70-3/4 |
--------+----------+--------------+--------------+-----------------------
It will be seen, therefore, that the drier had an output in 2 hrs. 23 mins.
of 141-1/2 lbs., which is at the rate of 1 lb. per minute approximately.
As the rubber leaves the driers it resembles vacuum-dried rubber in being
surface-sticky. This stickiness is only temporary, and is got rid of by
passing the crepe through wet rolls. Opinions differ as to when this
rolling should be given. On some estates the rubber is only allowed to cool
a little before passing through the rolls; on others it is given a day or
so before rolling. The methods of rolling also differ. In some factories
the rubber has been cut to lengths before drying, and these lengths are
merely rolled together by simple pressure. Other estates prefer to
re-macerate the crepe while still fairly warm and soft. It is probable that
little harm, if any, results from this re-maceration while the rubber is
soft, as it is more easily worked in this condition. The thick rubber is
then generally hung for a few days to air-dry before packing. As most of
the moisture taken up by the dry rubber is surface moisture, three or four
days is usually found ample for air-drying.
MICHIE-GOLLEDGE SYSTEM.--The Michie-Golledge system comprises a process of
preparation and drying. The latex is diluted with two, three, or four
volumes of water and coagulated with acid in a vessel which is rotated with
a churning motion. In this cylinder there are curved and fixed blades. The
revolving cylinder and its ribs force the latex against the curved blades
so as to cause an eddy in the middle of the machine. Here the rubber
coagulates and accumulates, the remaining liquor whirling round outside the
blades. It can be imagined that with such dilute latex, the coagulum is
very soft and spongy. This soft mass is passed through a machine which cuts
it into "worms" about 3/16 inch in section. These are placed upon wire
trays and dried by means of hot air. The "worms" when dry are re-macerated
and built up into medium and thick crepes. The colour of the rubber
prepared by this process is usually very good. When treated in a Colombo
drier the "worms" usually require about two hours to dry, so that crepe
rubber may be packed at latest on the fourth or fifth day, as in the case
of vacuum-dried rubber.
RATE OF AIR-DRYING OF CREPE RUBBER.--In spite of the facts that some
estates have been making thin pale crepes for years, and that so much has
been written concerning the preparation of this grade of rubber, one
occasionally meets with a case in which an estate seems to be unable to
prepare thin pale crepe, or if it does the period of drying is much longer
than obtains on most estates.
Again, when cases of infection by spot disease in fairly thin crepes are
submitted, it is usually found that the particular crepes are of that type
which, though fairly thin, show whitish spots of moisture when the bulk of
the rubber is nearly dry. This type of crepe is to be noted for the
excessive period of drying in comparison with other crepes of equal
thinness. It has been advanced elsewhere[12] that a factor of the most
considerable importance in the rate of drying of crepe rubber is the type
of drying-house and its situation. This accounts very largely for observed
differences in the rate of drying of thin crepes on different estates. Yet
even where two drying-houses may be of the same type, and the situations
may be comparable, one still observes that one thin crepe dries more
quickly than another. It has been remarked also that a thin crepe in one
old drying-house dries in a shorter period than a similar crepe in another
more modern house, although the methods of coagulation and preparation
exhibit no apparent diversity. In all these conflicting cases allowance is
made for the weather conditions, and the observed differences seem to be
inexplicable. It has always been the opinion of the writers that the actual
rolling of the rubber plays an important part in determining the rate of
drying of crepe, apart from the question of thinness; and it seemed
possible that this factor would account for the discrepancies noted above,
either partially or wholly.
[12] "Preparation of Plantation Rubber," Morgan, 1913, chapters xii. and
xiii.
With a view to determining to what degree the drying of crepe rubber was
hastened by the extent to which the rubber was rolled, experiments were
made. It was hoped, also, that some idea would be gained of the particular
stage in crepe rolling which had the greatest effect upon the rate of
drying. In preparing crepe in the estate in the ordinary way the coagulum
is passed through three sets of rollers, and the stages may be described
as:
(1) Rough rolling.
(2) Medium rolling.
(3) Smooth rolling.
In the first the coagulum is broken down by passing through the machines
until a thick rough crepe is formed. This passes to the intermediate
rollers, where it is worked down to a medium crepe. The rubber finally goes
to the smooth running at approximately even speeds. Passing through these a
number of times it emerges as a thin uniform crepe, free from "lumpiness"
and free from holes, which should dry in from ten to twelve days.
In the experiment the rubber was passed through the machines with varying
frequency, the number of times in each machine being progressively
increased, while the working on the other machines remained constant.
It was determined that the rate of drying was affected only by the extent
to which the crepe was worked in the smooth rolls. The less often the
rubber passed through these rolls, the slower the rate of drying. Beyond a
limit in the other direction, increased rolling did not reduce the period
of drying. It follows, therefore, that crepes which have a good thin finish
should dry in a minimum period.
[Illustration: DRYING GRAPH. PALE CREPE (THIN).]
WHEN DOES AIR-DRYING TAKE PLACE?--Experiments[13] were conducted with a
view to discovering, if possible, the rate at which crepe rubber dries, and
the extent of drying during the night under weather conditions such as
prevail ordinarily in Malaya. It is to be remembered that, during the day,
most drying-houses are fairly open and that the temperature ranges from
about 88° F. in the lower rooms to over 100° in the upper rooms (near the
roof) when the sun shines. At night, however, there is usually a decided
drop in the temperature, and unless it is a very clear night the air is
generally saturated with moisture. In addition the drying-house is closed
as thoroughly as possible, and we should expect the atmosphere of the house
to be laden with moisture from the wet and drying rubber. It would be a
just inference, therefore, that the rate of drying during the night would
be much less than the rate of drying during the day, and the results of
experiments confirm this very fully. One was hardly prepared, however, to
find that, under certain circumstances and at a certain stage, the amount
of drying is nil; not only so, but it was found under certain conditions
that the amount of drying which took place was negative--_i.e._, the rubber
weighed slightly more when taken out in the morning than it had weighed the
previous afternoon.
[13] Rubber Growers' Association, Malaya Local Report, No. 2, 1914.
CREPE MAY INCREASE IN WEIGHT.--As an instance of the kind of result
obtained a graph is here given of the rate of drying of a batch of pale
crepe. This was hung to dry in the top room of a drying-house in which
rubber ordinarily dries quickly. The rubber was hung in a good position,
with the bulk of output, near a window which was open for some time during
the day. In order to restrict the day interval of drying to the actual
period in which the sun was likely to be in evidence, the day was taken to
begin at 8 a.m. and end at 4 p.m., the night interval covering the
remaining sixteen hours. Thus the night interval was twice as long as the
period of day drying. The lengths of crepe were weighed carefully at 8 a.m.
and 4 a.m., and the average of the several weights was plotted in a graph.
The weights are placed vertically and the duration of drying horizontally.
It will be seen that the rubber was quite dry and fit for packing on the
sixth day, as far as could be judged in the usual way by casual inspection.
Peculiarly enough at this time it weighed slightly more than had been
registered on the fourth and fifth days, but the difference did not amount
to more than about 0·4 per cent. In examining the graph it should be borne
in mind that the steeper the slope of the curve downwards the quicker the
rate of drying, and that when the curve takes an upward direction there is
an addition of moisture instead of abstraction. It will be noted that when
drying takes place the slopes more nearly approximating the vertical
represent the extent of day drying, and that often the night drying is
represented either by a very flat curve or even by an upward curve which
shows the addition of moisture. A striking feature of the experiment is
shown by the rapidity with which drying takes place during the first few
days and the comparative slowness with which the remaining moisture is got
rid of. Thus, from the graph, it may be calculated that about 80 per cent.
of the total moisture content was lost in the first two days, and over 93
per cent. in two and a half days. Yet three days had to elapse before the
remaining 7 per cent. of total moisture was lost--_i.e._, before the rubber
was judged to be ready for packing. It will be seen that after this stage
had been reached the rubber alternately lost and gained in weight, with a
tendency to increase. This increase was attributed to the presence of
surface moisture after hanging overnight, when the rains had become
frequent. Some light is thus shed upon a subject which has puzzled both
shippers and receivers of crepe rubber.
DIFFERENCES IN WEIGHT.--It will be obvious that if rubber is allowed to
hang after becoming dry, and is taken down, packed, and weighed in the
early morning, it will weigh more than when it reaches a drier climate. The
loss in weight under such circumstances might amount to even 1 per cent. It
may seem to some an unnecessary refinement to introduce, but it would
appear from the graph that rubber should be packed for preference in the
afternoon if the weights are to be more nearly correct.
It is extremely singular to note how quickly the curve changes its slope
after the major portion of the moisture has evaporated, and it will be very
plain that in the last stages any decrease in weight during the day would
appear to be counterbalanced, or more than counterbalanced, by the addition
of moisture during the night. It may be pointed out, however, that this
increase in weight during the later stages of drying of pale crepe is
mainly, if not altogether, due to surface moisture. The chief point of
interest is the fact that in the case of thin pale crepe, quite 80 per
cent. of the total moisture content is lost during the first two or three
days, and that, owing to the negative influence of the night atmosphere,
the final drying is delayed. It will be understood that the foregoing
results applied to thin pale crepe. Thin lower-grade crepes appeared to dry
at more uniform rates, but the differences between the rates of drying at
night and during the day were similarly notable.
AIDS TO NORMAL AIR-DRYING.--These experiments were undertaken in a
drying-house, favourably situated for rapid drying, in which the average
period of drying for thin crepes is nine days. It is easy to imagine that
the condition of affairs as revealed would be much exaggerated in a
drying-house situated on low-lying ground and surrounded by trees. In
extreme cases of this nature the use of large fans and heating pipes has
been advocated. It is believed that in some cases these installations have
given satisfaction, but that in others the degree of improvement obtained
has not been in economic proportion to the outlay incurred.
SMOKE-CURING OF SHEET RUBBER.--It will have been evident that one of the
disadvantages of air-drying sheet is the incidence of moulds. Now it is
found that moulds should not develop in smoke-curing; and if they do, then
the smoke-curing has been insufficient or inefficient. The difference in
the drying period also is a strong argument in favour of smoke-curing, so
that all-round it is seen that there are many valuable advantages to be
gained by smoke-curing sheet in comparison with air-drying, and no
disadvantages.
The manipulation of the rubber, after it leaves the marking rolls and
preparatory to smoke-curing, has been discussed in Chapter IX. It is
sufficient only to allow adequate time for furnace water to drip from the
sheets before transferring them to the smoke-house. As it is the general
rule to roll sheet rubber in the morning, this arrangement fits in very
well. The furnaces of the smoke-house are usually extinguished as soon as
the sun is well risen, and the rest of the day is occupied in sorting dry
sheets, etc. Towards noon the day's wet sheets should have been admitted,
and smoking may be commenced as soon as the sun is well in the west--say,
at half-past four o'clock or earlier.
It used to be the custom on a few estates to smoke during the daytime and
to discontinue smoking at night. As the night-air in Malaya is usually
heavily laden with moisture, it will be plain that such a policy was a
topsy-turvy one. It is vastly more reasonable to smoke-cure at night;
usually the heat of the sun during the day is quite sufficient in itself to
promote the drying of rubber; but there is no reason why smoking should not
be carried on in the daytime in wet weather, should it be found expedient
to do so.
RECORDING INSTRUMENTS.--During the night the care of the smoke-houses is
usually in the hands of natives, except for occasional surprise visits from
a European superintendent. To all acquainted with the ways of the native it
must be plain that means must be provided for the checking of the
temperatures attained in the smoke-house. Ordinary thermometers are quite
unsuitable, and even thermometers registering maximum and minimum
temperatures are of little avail, inasmuch as they record only the degree
of heat attained at a particular moment, and do not indicate any period
during which a particular temperature was maintained.
It is evident that something more informative is required. There are many
types of suitable recording instruments or "pyrometers," some of which can
be electrically connected, so as to cause the ringing of a bell, placed in
the superintendent's office or house, on the attainment of a certain
temperature. The type best known in estate practice is that named the
"Thermograph," in which a pen traces a curve or graph on a plotted piece of
paper carried by a rotating cylinder which is actuated by clockwork. Such
instruments can be purchased through most of the local firms dealing in
estate supplies. From experience it can be asserted that, given intelligent
attention, these instruments yield very satisfactory results. The apparatus
should not be placed always in one position in the smoke-house, but should
be moved frequently so as to obtain information regarding the distribution
of heat.
TEMPERATURE OF SMOKE-CURING.--In the question of temperature of drying, it
is well to be as strict as possible; not that any great harm will result
from a rise of 10° above that recommended, but because the higher the
temperature recorded the larger the fires must have been, and consequently
the more real danger there was of the store becoming ignited. It has been
shown[14] that the temperature giving the maximum benefit of drying and
quality was found experimentally to be rather above the temperature usually
prescribed for smoke-houses, but in the experimental work there was no
danger from fire.
[14] "Preparation of Plantation Rubber," Morgan, 1913, chapter x.
The figure given in previous publications as a maximum working temperature
for smoke-houses was 110° F., but certainly the temperature may be as high
as 130° if it is considered safe to allow fires to be so arranged. One or
two estates are known to work at temperatures of 130° F. and over, in spite
of the recommendations of the writers. If those estates care to risk it
they may do so, with increased rapidity of drying; but no responsibility
can be taken for whatever may happen in smoke-houses where the temperature
is allowed to remain, as in one case, at 160° F. Naturally the range of
temperature is strictly limited by the properties of the substance to be
treated, and with a substance such as rubber it would be far better to err
on the side of caution than to risk damage to such a commodity, apart from
the consideration of the possible destruction of the building.
PERIOD OF DRYING.--Considerable differences are noted in the periods of
drying on various estates; but, as there is more than one factor
influencing the results, it is not easy at first to find why these
differences should exist. Really there are three factors:
(1) Relative thickness of rubber.
(2) Extent and quality of rolling.
(3) Temperature of drying.
It is presumed that the smoke-houses are identical in type and efficiency,
and that smoking is in force for the same length of time each day. There
need be no discussion of these points; the effect of each is so obvious.
The thinner the sheet, the quicker the rate of drying; the better the sheet
has been rolled, the shorter the period of drying; the higher the
temperature, the more rapid the drying.
It has been shown in Chapter IX. that the condition of the sheet after
rolling depends primarily upon the standard of dilution of the latex and
the original thickness of the coagulum. If these factors are correctly
controlled, the rolling should give a sheet which is fairly soft and
porous--_i.e._, it should not have been subjected to such pressure as to
make it both thin and hard. An average sheet of rubber which has been well
rolled should be smoke-dried at a temperature of 120° F. in about ten days.
If sheets take appreciably longer to dry, then the three foregoing factors
must be examined.
On the other hand, it is often found that thin sheets made from very dilute
latex dry so quickly that they are considered to be fully smoke-cured in
from five to seven days. It frequently happens in such cases, however, that
the smoking is insufficient, and by the time the rubber reaches home it has
begun to show signs of surface moulds. It is evident, therefore, from this
discussion that:
(1) If smoked sheet develops surface moulds within a short period
after smoking, the duration of curing has been insufficient, or the
quality of the smoking is at fault.
(2) The actual time taken to smoke-dry rubber may be insufficient to
smoke-cure it.
(3) The rate of drying of smoked sheet depends upon--
(_a_) The relative thickness of the rubber.
(_b_) The preliminary treatment of rolling.
(_c_) The temperature of the smoke-house, and
(_d_) The type of smoke-house used. This point will be treated
in a subsequent chapter.
FUELS FOR SMOKING.--The general idea formerly held was that the beneficial
effects of smoking were to be attributed to the constituents of the smoke,
and chiefly the creosotic substances. This is not now the opinion of the
writers, who attribute the effect largely to the temperature of drying and
constituents of the smoke other than creosotic substances. There can be no
doubt that the presence of creosotic bodies is responsible largely for the
absence of moulds and the existence of the typical odour, but it is
becoming increasingly known that the employment of substances rich in
creosote is not required or desirable.
Estates used to be put to considerable expense in the purchase of "bakau"
(a mangrove timber rich in creosote and creating much heat), under the idea
that it was the best material and almost indispensable. Most estates now
restrict themselves to the consumption of timber obtained from their own
areas. Thinning-out programmes are largely responsible for the supply, but
the local authorities are much concerned regarding future supplies; and
consideration has been given in some quarters to the question of the
development of quick-growing trees on estates with a view to safeguarding
the future. This seems to be desirable, as it is difficult to imagine that
the place of timber can be taken by any other material in the smoke-curing
of rubber. Unless some such precautions are taken it is not difficult to
predict that, in course of time, some estates will be able to continue the
preparation of smoked sheets only at considerable expense in obtaining
suitable fuel from a distance.
It is not true that _any kind of timber_ is suitable as a fuel to be used
in a smoke-house. All timbers are suitable, either alone or in mixture with
others, provided that the wood is not too green.
Naturally an absolutely dead and crumbling wood will smoulder, but does not
develop sufficient smoke. A green timber will give an acrid and moist
smoke, but demands the consumption of a certain amount of dry timber in
addition if it is to be used.
Rubber-tree prunings and sawn rubber trees obtained by thinning-out may be
used in mixture with dead wood, provided the logs are stacked to dry in the
sun for some weeks before use. If the timber is too green, steam is formed
as well as smoke, and the sheets of rubber may have a moist surface glaze.
SUN-DRYING SHEET RUBBER.--Among the first curious sights which impress the
visitor or newcomer to this country is the spectacle of sheet rubber
hanging in the sun on native holdings. From what one has learned of the
extraordinary care which must be exercised in all the processes of rubber
preparation, one fails to understand how such rubber reaches the market
without becoming tacky. That some of it does become slightly tacky is
certain, but on the whole native rubber, though crudely prepared, is
usually sound. The native idea of giving sheet rubber a preliminary drying
in the sun is to hasten the total period of drying. That the period is
curtailed would seem to be the case, but it is open to doubt, as the effect
of sun-drying, if unduly prolonged, is to create a thin surface film of dry
rubber which retards the drying of the rubber below the surface. Working
with wet crepe rubber, the writer found that, to all external appearances,
there was no effect upon the rubber when it was allowed to sun-dry for four
or five hours. With periods of from six to ten hours the crepe becomes
slightly sticky, chiefly on that portion across the support. When removed
to the air-drying house this tackiness developed further, and the rubber,
on the line of support, became so weak that it stretched and broke.
Reasoning by analogy, it would appear that no apparent harm would result to
sheet rubber from sun-drying for periods up to four or five hours. From
experience (see Chapter IX.), not the slightest ill-effect is found to
result from the short interval of preliminary drying or dripping practised
on many estates preparatory to smoke-curing.
ARTIFICIAL DRIERS FOR SHEET RUBBER.--It is understood that when vacuum
driers were first applied to the drying of rubber it was thought possible
to dry sheet rubber in this way. The practice was found to be impossible,
as the length of time required and the temperature were responsible for the
destruction of the form of the rubber; it became tacky and semi-liquid.
THE "CHULA" DRIER.--Although several suggestions of devices for
artificially drying sheet have been made, only one is known to be in use at
the present time. In the original form this was used for drying other
tropical products. It consists of a large iron chamber, in which are
several compartments divided by means of baffle-plates. At one end there is
a small furnace and, by means of a fan, smoke and hot air are drawn
through the compartments. Owing to the temperature attained (140° to 160°
F.) sheet rubber cannot be completely dried in the chambers, and is, as a
rule, only treated in this manner for one or two days. Drying is then
completed in an ordinary air-drying house. It is claimed that drying is
expedited, and that the rubber can be packed in ten days.
In the more recent modification, the smoke and hot air which leave the
Chula drier pass through a large room in which may be hung either sheet or
crepe rubber. It would seem that all sources of danger have not been
eliminated from the process, as on one estate a wooden room containing
rubber was ignited by a spark which passed through the drier.
Yet another form exists in which the furnace is outside the main building,
and in the ordinary course of working only heats a series of open pipes
through which air is drawn by a powerful fan. By means of a valve it is
possible to allow smoke from the furnace to pass into the room with the hot
air for the preparation of smoked rubber. The hot air or smoke is
distributed in the lower room by means of main and branch pipes, and passes
through an open floor to the room above. With such an arrangement it is
possible, therefore, to prepare either air-dried or smoke-cured rubber. If
the method could be successfully applied to the drying of crepe it would be
of great assistance on some estates. There would seem to be a difficulty in
working it for the drying of sheer rubber and crepe together, as the
temperature suitable for the one is excessive for the other. Given an
efficient control over the temperature of the hot air, the house should be
successful in the drying of crepe, provided the rubber is not hung in folds
of too great length. For smoke-curing sheet rubber the period is said to be
reduced by several days in comparison with the time occupied in an ordinary
smoke-house, but it is not clear that such a system would have any
advantage over a modern smoke-house, in types of which rubber can be fully
cured in periods ranging from five to ten days.
CHAPTER XII
_SORTING, GRADING, AND PACKING_
The question of standardising the output of our plantations is one which
has occupied attention for some years, with a not inconsiderable degree of
success.
Meanwhile opinion is growing in favour of proceeding along the line of
reducing the number of plantation grades to a minimum. At present some
confusion exists. Some estates make up tree-scrap and bark-shavings
together; one estate puts tree-scrap, earth-scrap, and bark-shavings into
one uniform crepe; other estates have three or more separate scrap
grades--_e.g._, lump-rubber and "washings," tree-scrap, earth-scrap, and
bark-shavings scrap. There is a movement on foot at present to try to
restrict plantation rubber to three grades:
CREPES--1. _First Quality Latex._--_I.e._, crepe made from the true
coagulum obtained from the regulated coagulation of strained latex. This is
a pale rubber, and may be prepared satisfactorily if the directions given
in preceding chapters are followed. Naturally there must be, in all
factories, some defective rubber of this grade. For various reasons the
crepe may be of inferior colour, or is slightly contaminated with dirt or
traces of oil and grease, etc. This defective rubber should be placed aside
most rigorously and plainly marked as "off-quality."
If a proper scheme of standardisation of latex and chemicals is followed,
there should not be any such variety in shades of colour, such as was
common in No. 1 crepe in the past.
Comparatively few estates in Malaya now prepare thick (or blanket) crepes
in the No. 1 grade, but in such cases the same rules must be applied as
govern the sorting of thin fine pale crepes.
2. _Compound Crepe, No. 1._--In this it is proposed to include
cup-coagulated lumps, coagulated lumps from transport vessels, skimmings,
bucket rinsings, cup-washings, and tree-scrap. It has been shown in Chapter
X. that strict care is necessary to eliminate all oxidised (dark) scraps.
These are relegated to a lower grade. The possession of a "scrap-washer" is
necessary if the best results are to be obtained.
On some estates the ingredients of this compound crepe, while fresh, are
placed in a common jar or tank to which a quantity of sodium bisulphite (1
per cent. solution) and acid are added. The resulting conglomerate mass is
cut up for working.
3. _Compound Crepe, No. 2._--This grade would include the remaining lower
grades--viz., bark-shavings, scrap, and earth-rubber scrap.
REDUCTION CARRIED TOO FAR.--However desirable it may be to diminish the
number of grades, it must be pointed out that diminution and simplification
are not necessarily synonymous terms in this matter. It is well known that
on estates where the earth-rubber is only brought in at lengthy intervals,
say of a week, the resulting crepe is sometimes very tacky. This is only
natural, and is due to the prolonged exposure to the sun's rays. With the
improved machinery now at our disposal, and with the increasing attention
which will be given to the lower grades in the future, it is possible to
prepare from average bark-shavings crepe free from bark, and of quite a
good colour. Where trees are not "scrapped" before tapping, there would
seem to be no objection to amalgamating the rubber obtained from the
bark-shavings with the No. 1 Compound crepe; and it would be a distinct
danger and possible loss if this good rubber were to be mixed with earth
rubber. The liability of the latter to become tacky is well recognised; and
if possible it should be maintained as a separate grade, in which it would
be permissible to mix only rubber obtained from actually dry shavings from
"scrapped" trees, or heavily-oxidised scraps which have been rejected from
other grades.
SHEETS.--Broadly there are no fine distinctions to be made at present in
the grading of smoked-sheet rubber; it is either No. 1, or if any
so-called defect is visible the sheets must be rejected and plainly marked
as "off-quality."
Clippings (trimmings) may either be made into crepe or shipped under their
own description.
RUBBER GROWERS' ASSOCIATION'S RECOMMENDATIONS.--Taking the foregoing
arguments into full consideration, it would seem that, strictly speaking,
the number of grades cannot be reduced to less than four at present without
producing some amount of confusion.
In its handbook,[15] the Rubber Growers' Association remarks:
[15] "Preparation of Plantation Rubber," 1917.
"The fewer grades the better, and regularity of each grade is most
important.
"The grading should be as follows:
"(No. 1) Fine crepe (or No. 1 sheet), made from the free or
liquid latex.
"(No. 2) Clean light brown crepe, made from lumps and skimmings.
"(No. 3) Scrap crepe, made from tree-scrap.
"(No. 4) Dark crepe, made from bark-shavings, earth rubber, and
the lower quality of scrap.
"Tacky rubber should be packed separately.
"_Compound Scrap Crepe._--Estates using scrap-washers should make a
compound crepe of grades Nos. 2 and 3, which will make one compound free
from bark and specks. All rubber intended for No. 4 should be most
thoroughly washed."
Concerning these recommendations the remarks in preceding paragraphs should
be studied.
CARE IN SORTING.--Whether dealing with smoked-sheet, pale crepe, or lower
grades, the strictest care is necessary in sorting and grading. This work
must of necessity be relegated to coolies, and they should be trained men.
Instructions must be definite, and doubtful specimens of rubber should
always be placed aside for the decision of the European superintendent. Any
pieces showing unmistakable signs of what are regarded as defects should be
stringently rejected. In the case of pale crepe, when the defect is
confined only to a small area it is permissible to cut out the affected
portion. Similarly there can be no objection, in the case of smoked sheets,
to an occasional sheet being treated in this manner. On the majority of
estates these rules are observed carefully, but some estates yet have to
learn that defective pieces of rubber may not be concealed in a bulk of
otherwise good quality. Samplers have often an uncanny knack of hitting
upon the defective specimens, and it is entirely the fault of the estate's
sorters if these pieces are submitted as being representative of the mass.
CHOICE OF CASES.--Consumers complain justly of the presence of chips,
splinters, and wood-dust. It will be evident, therefore, that whatever the
type of case employed the interior surfaces should be smooth, there should
be no cracks or gaps in the timber, and the cases should be cleaned out
before using. There remains great room for improvement in the means and
method of packing, and in spite of suggested alternatives we are at present
restricted to the use of wooden cases.
From comparisons of actual quality and fulfilment of the requirements
indicated above, there can be no question that cases made of three-ply
wood, such as the "Venesta," are in every respect superior to the ordinary
wooden cases of "Momi" type. The consideration of cost and available
supplies, of course, enters largely into the question, and three-ply cases
are not at present so largely employed as they deserve to be.
A new type of case was recently exhibited in Singapore. It emanates from
the U.S.A. and is made of a fibrous material, resembling in appearance a
very stout cardboard. The complete case when assembled consists really of
two boxes, one of which is inverted and slides down over the other. Packing
is completed by means of stout wire, which is strained by a simple ratchet
arrangement. It is claimed that from 225 to 250 lbs. of rubber can be
contained. Other claims made amount to the statement that the case is
practically indestructible under normal conditions of handling and
shipping. A demonstration given certainly appeared to substantiate the
statement fully. Rubber packed in cases of various and average type was
allowed to fall from a height of about twenty feet. In all instances the
wooden cases of every type were either smashed or badly burst, whereas the
fibre cases were merely dented. These cases are obtained in flat sections,
which, in assembling, are folded and clamped by means of copper rivets in
a special but simple machine. It was pointed out that objection might be
lodged against the use of copper for this purpose.
More recently there is announced a new packing case which is stated to be
made from low-grade rubbers, but information is rather vague.
BAGS.--There are in local use stout canvas bags which have the advantage of
being used many times, as long as they are waterproof and kept in good dry
condition. Their employment for the conveyance of smoked-sheets would
appear to be permitted, but crepe rubbers sent in them are often reported
upon as being "massed" at the edges, and hence difficult to "sample."
BALES.--Attempts to bale rubber for the market have been frequent, but no
success seems to have attended the efforts. In some quarters the failure
has been ascribed to prejudice on the part of buyers, but it is the opinion
of the writers that the objections to baling are, or could be,
well-founded. Massed rubber often cannot be inspected properly, and hence
is always open to suspicion that internally there may be unsuitable
portions.
There have been several schemes put forward for winding crepe rubber on
spindles so as to form a cylindrical package complete in itself. We have
seen the process, and certainly the method had much which appeared
commendable. Apart from other objections which might be raised, there is
always the one prominent objection mentioned in the preceding paragraph.
While baling of rubber is thus not likely to suit the general market, there
is no reason why, as in one or two instances, it should not be practised by
agreement between producer and consumer. It is believed that "slab" rubber
is shipped in bales from Sumatra to the U.S.A.
Quite recently a proposal has been put forward to revert to a simple form
of baling for ordinary plantation rubber. Under this scheme wooden cases
are discarded, the packing material being composed of scrap-grade crepe
rubber which, it is claimed, could be put to use by the manufacturer. An
obvious drawback would be evident if these bales happened to be exposed to
direct sunlight or a continuous high temperature. The tackiness which might
supervene would make the handling of such bales unpleasant, even if it did
not affect the internal rubber.
FOLDING FOR PACKING.--In the packing of smoked sheets it would appear to be
advisable to avoid, if possible, the folding of any pieces, as the
objection is made that such rubber is difficult to "sample" on arrival,
especially in cold weather. Sheets should be prepared or cut to such length
that they occupy the full superficial area of the box, either singly or
side by side.
[Illustration: A SHIPMENT OF RUBBER, PACKED AND READY FOR TRANSPORT.]
The same remark applies to the packing of crepe rubbers, except that here
we deal with units of folded rubber. Crepes are generally folded by hand,
and coolies usually work to a certain dimension by means of a standard
stick. The work is slow, but often gives employment, at a cheap rate of
pay, to women and weak coolies.
Several machines have been invented to replace this labour. The best of
these yet seen has a simple device by means of which the length of the fold
is adjustable to suit the size of any packing case. It is called the
"Senang" folder, and is made by the General Engineering Company (Radcliffe)
Ltd., Radcliffe, near Manchester.
CARE IN ASSEMBLING.--Whatever the type of case employed, great care must be
given to the assembling of parts and the final fastening. It is not
uncommon to find in the operation of putting on the "strapping" that nails
have been driven into the rubber. Extra bands of strapping are sometimes
advised, and where these bands pass over the sides (not edges) of the case
only specially short nails should be used.
All wood should be planed, and in cases other than three-ply should be of
stout wood, not less than 5/8 inch in thickness. All timber used should be
of uniform type and thickness.
METHODS OF PACKING.--The usual method of packing crepe is to fold the
lengths to some measure of the dimensions of the case. This is done in a
haphazard fashion on some estates, with the result that either space is
lost or the packing is badly arranged.
Some ingenuity can be displayed in the packing of sheet rubber in order to
avoid folding the sheets, which, besides increasing the difficulty of
sampling, leads to loss of space. Endeavours are being continually made on
estates to prepare sheet of such a size as to obtain the maximum benefit of
space both in smoke-house accommodation and in packing. A few estates
employ tanks of such calculated dimensions as will yield uniform sheets
which pack flat and fill the superficial area of the case.
In view of the contamination which sometimes characterises the employment
of wooden cases it is sometimes advised that the interior should be lined
with sheets, or pieces of crepe, the ends of which are later folded over
the top of the mass. In this manner it is stated that contamination is
confined only to the exterior of the contents of the case.
WEIGHT OF CONTENTS.--The dimensions of average cases are 19 inches by 19
inches by 24 inches, giving a capacity of 5 cubic feet.
In these it is possible to pack 150 lbs. of crepe rubber and 200 lbs. of
sheet rubber (about 5 per cent. more in cases of three-ply wood). It may
be noted that boxes arrive in better condition when fully packed. The
foregoing figures are not adhered to strictly. For example, some estates
find it expedient to ship rubber in actual ton lots, and for this purpose
pack only 140 lbs. of crepe per case, giving sixteen cases to the ton.
Other estates, using presses, pack more per case than the quantities noted
above. At present there does not appear to be any definite regularity in
practice.
[Illustration: ON ITS ROAD TO THE RAILWAY: BULLOCK-CART TRANSPORT.]
In all instances it should be the invariable rule that the rubber should be
weighed before packing, and that all cases should contain uniform nett
quantities of any particular type of rubber. Invoicing, etc., will thus be
greatly facilitated. If these practices are followed, and the rubber always
weighed on the same scales (assuming it to be perfectly dry when packed)
complaints of "short-weight" should be infrequent.
"SHORT" WEIGHTS.--In some cases the occurrence of "short" weights on
arrival at ports would appear to be inexplicable. It often happens that the
constituent parts of wooden cases have been in stock for a considerable
period. If for no other reason than that indicated below, all cases,
either before or after assembling, should be thoroughly dried in the sun.
"Short" weight could be accounted for to some degree by a lack of
observance of this elementary rule, as it is most probable that there would
be a perceptible difference in weight of the wooden case in a drier
atmosphere.
(_a_) If rubber is weighed in the box, and the average tare of the
case deducted from the gross weight (in order to obtain the nett
weight), any loss in the weight of the timber would appear as a
deficiency of rubber at the distant port.
(_b_) Whether the same effect would be produced eventually in the case
of rubber which is weighed before packing will depend upon the method
of weighing at the warehouse. If the rubber is weighed in the box, any
observed deficiency would be attributed to a loss of weight in the
rubber.
PART III
MACHINERY AND BUILDINGS
CHAPTER XIII
_MACHINES_
The number of manufacturers of machines for preparing rubber would seem to
be on the increase, and there can be little doubt that this competition
will result in a continued improvement in the design of machines. It cannot
be denied that there has been room for such improvement, and it is believed
that manufacturers will display judgment in putting only their best quality
into the work. While design and finish are very excellent in their way, it
is to be regretted that in a number of cases in the past the material of
rolls has been found to be of inferior quality. Generally, the complaint
seemed to be that the rolls were too soft, and that the "grinding" effect
was far too great. The damage to pale rubber in such cases is considerable,
as it is impossible to keep the rolls free from fine dark powder. The
effect is generally noticed more in the smooth rolls with which a finish is
put upon the crepe.
Cases have occurred frequently in which rolls have been returned, because
of the injury caused to pale rubber, and there can be little doubt that the
life of quite a large number of rolls is even now far too short in
comparison with the expense involved.
It is a moot point, however, in many instances how far the quality of the
rolls is actually responsible for the damage done to the rubber. In the
experience of the writers it is certain that complaints regarding the rolls
were unjustifiable, and that the injury had been caused by carelessness in
the "feeding" of the machine. Especially in the case of smooth finishing
rolls, it is clear that if the rolls are allowed to run idle for more than
the briefest possible interval grinding must take place.
The complaints apply not only to the rolls themselves, but also to the
brass linings for shaft-bearings. Cases are known in which a brass "liner"
was so worn within a few weeks as to be quite useless. If the matter ended
there it would not be so bad; but there is always the possibility of
particles of brass finding their way into trays, and so into the rubber.
The damage which ensues to the rubber is quite irreparable. This particular
defect arising from the presence of brass will be dealt with in a later
chapter. But here again it is necessary to point out that such wear on
brass liners may be caused by the standards (ends) of the rolls being
eccentric; and the case may be analogous to the placing of "new wine in old
bottles."
_En passant_ it may be remarked that in any case brass liners are not
strictly necessary. White-metal alloys are in use on rubber machines, and
cast-iron bearings have been employed satisfactorily for years.
It would be well for managers to remember, therefore, that when machines
have to be ordered, nothing but the best is good enough, and that the
difference between good machinery and passable machinery is probably
immensely greater in effect than any saving in expenditure would warrant.
ADEQUACY OF MACHINES.--In general, the factories which prepare sheet rubber
are usually equipped with adequate machinery. This arises from the fact
that machines are necessary for preparing all grades below the first, even
if they are not necessary for the making of sheet. Thus all the necessary
macerators and finishing machines are installed, but the major part of the
output is in sheet form. For the preparation of sheet, no heavy machinery
is required; all that is necessary are light machines for rolling the
sheets and expressing as much moisture as possible. To obtain a pattern on
the sheet, another light machine may be used. It may be imagined, then,
that the work of rolling sheet rubber by power machines is small, and that
a large quantity of rubber can be worked off in a comparatively short time.
It follows, therefore, that the preparation of the lower crepe grades can
be proceeded with at once, and that the whole work of the factory is
expedited.
The case of factories which have to prepare all first-grade rubber in crepe
form is quite different, especially when thin rubber has to be made. The
care which has to be exercised in preparing pale crepe rubber is very great
in comparison with what is demanded by sheet rubber. The rubber has to go
first through the uneven-speed macerators, from there to the intermediate
rollers, thence to the finishing rollers. Considerable ingenuity has to be
displayed in the arrangement of the machines, so that one section will not
work faster or slower than another. More often than not, the attempt to
arrive at such a desirable arrangement fails, owing to an insufficiency of
machines. Such a statement will probably read strangely to the uninitiated;
but an example will make it plain. A factory may have a battery of six
machines, one only of which is a finishing machine (smooth rolls). With
five macerators and intermediate machines working continuously, it will be
more than the work of one finishing pair of rolls to keep pace, especially
as so much more care has to be exercised in finishing than in rough
crepe-making. The obvious course to adopt is to substitute a pair of smooth
rolls, with suitable gear ratio, for a pair of macerators or
"intermediates."
If, however, the macerators and intermediates are already fully occupied
the whole of the time, any such change would be of small benefit. What is
really needed in this case is more machinery.
It might be pertinently asked what constitutes an adequate equipment of
machines for crepe-making. The writers cannot give a number, but have no
hesitation in stating that if a factory cannot complete its whole day's
work before dark, it is inadequately equipped. No work should be done after
dark, if possible, as it cannot receive the supervision which crepe-making
demands. To make comparison between the number of machines in any two
factories and their respective outputs is not sound argument, as the
out-turn of two similar machines will depend upon the speed at which the
rolls travel--_i.e._, the gearing between the machines and the engines.
Thus, while one machine will out-turn 40 lbs. of crepe per hour, another
may only have an output of 30 lbs., although the machines may be identical
in pattern. To make calculations based on a rate per hour for any known
make of machine, and to apply those calculations to the existing machinery
in any factory, in an attempt to judge whether there is a sufficient number
of machines, would be a mistake, unless one were also supplied with the
relative speeds at which the rolls work.
Finally, on the question of adequacy of machines, it must be pointed out
that an insufficient number of machines must result in a poor product,
since all rolls have to be used for all grades. Even with the greatest
possible care it happens that pale crepe is sometimes spoiled because it is
contaminated with foreign matter, resulting from the working of lower
grades on the same machines. This is one of the great arguments in another
direction for the installation of a scrap-washer.
In conclusion, the writers can only give their opinion that one must not
decide the question of adequacy by the number of existing machines, but by
the time taken each day in working off the rubber, providing one can be
satisfied that the best arrangement of the existing machines has been made.
IDEAL ARRANGEMENT.--As to what this best arrangement may be, guidance can
be obtained from the results of experience here given. It must be premised
that the output of any factory preparing fine pale crepe is limited by the
output of the smooth finishing rolls. Broadly, it will be recognised that
if there is any excess of capacity in a battery it should be found in the
smooth-roll machines. This sufficiency, or excess of capacity, may
sometimes be attained by an alteration in the gearing of the drive of the
rolls from the back-shaft, or by an addition to the number of machines. In
the former case, there are practicable limits of speed, beyond which the
second alternative measure must be adopted.
SPEED.--The usual speed at which the back-shaft travels ranges from 60 to
70 revolutions per minute. Taking first the macerating machines, the
intermediate gearing between the shaft and the rolls should give a driving
speed of about 20 revolutions per minute on the faster-travelling roll.
This is equivalent, with a 15-inch diameter roll, to a peripheral speed of
about 60 to 65 feet per minute.
The intermediate and smooth rolls can be arranged to travel more quickly,
but the maximum comfortable speed for proper feeding and control appears to
be about 25 revolutions per minute on even-speed rolls. In view of the fact
that the rubber at each successive machine becomes longer and thinner, it
will be seen that a smooth-roll machine could not cope with the output of a
macerator in the same period of time. If, therefore, the macerator is fully
occupied for the greater part of the time, an additional smooth-roll
machine must be installed, even though the existing one has been "speeded
up" to practicable limits.
For the information of the uninitiated it might be explained that in the
macerating and intermediate machines the cog-wheels driving the two rolls
are of different sizes (_i.e._, differentially geared), as opposed to the
smooth rolls on which the cog-wheels are usually of the same size (_i.e._,
even speed). The idea in the one case is to exert a "working" influence
upon the rubber while it is being washed by the stream of water coming from
above; in the smooth rolls a squeezing action only is effected.
To give an idea of the ratio of the speeds of the rolls in each machine in
a typical working battery, the following particulars may be noted:
GEAR RATIOS.--
_Machine._ _Differential Ratio._
1. Macerator 32-17
2. Intermediate (coarse grooved) 32-17
3. " (fine grooved) 30-19
4. Smooth (uneven speed) 30-19
5. " (finishing) 25-24
6. " ( " ) 25-24
It will be seen that the so-called "even-speed" smooth rolls run at
approximately the same rate.
It is advised that in all cases the gear wheels should be cut helically.
Those who have experience of the noise of some batteries after they are
slightly worn will appreciate such a remark.
GROOVING OF ROLLS.--Concerning the choice of grooving, there is divergence
of opinion, some managers preferring one type, which others reject in
favour of another type. Provided any particular type can be shown to be as
effective as required, no necessity for laying down hard-and-fast rules
seems to exist.
The following particulars serve to describe a battery well known to the
writers, and accustomed to produce the finest quality of thin pale crepe
and lower grades:
-----------------+-----------------------------------+-------------
| |_No. of Times
_Machine._ | _Grooving._ |Rubber passes
| | through._
-----------------+-----------------------------------+-------------
1. Macerator | Deep horizontal grooves; |
| square-cut, 5/16 inch × 5/16 inch |
| × 5/8 inch spaces | 3
2. Intermediate | Horizontal grooves; 3/16 inch |
| × 3/16 inch × 3/8 inch spaces | 2
3. " | Fine spiral grooves; 1/8 inch |
| × 1/8 inch × 1/4 inch spaces | 2
4. Geared smooth | Nil | 1
5. "Even" smooth | " | 1
6. " " | " | 1
-----------------+-----------------------------------+-------------
| Total | 10 times
-----------------+-----------------------------------+-------------
The actual rate of output of this installation is the capacity of the last
smooth machine. This is about 180 lbs. per hour, while the output of the
macerator is approximately double this amount. Thus the macerator only
works for about half the time. This applies also to the two intermediate
machines. After a study of the preliminary remarks, it would not be
difficult to suggest methods for improving the condition of affairs. It
would appear that, in order to obtain a uniform rate of working in such a
battery, the relative peripheral speeds of the several machines should
be--(1), (2), and (3) 100; (4) 150; (5) and (6) 200. The remarks on the
practical limits of speed should be borne in mind. In this case the smooth
rolls travelled at 23 revolutions per minute.
As already stated, it is not intended to lay down definitely that, _e.g._,
horizontal grooving alone should be cut on macerating rolls. Some estates
employ with satisfaction a deep square-cut spiral 1/4 inch by 1/4 inch by
1/4 inch or 1/2 inch spacing; others use a large diamond pattern. Similarly
various types of grooving are cut in the intermediate rolls.
[Illustration: A BATTERY OF MACHINES.
On the left, light marking rolls for sheet rubber; on the right, heavy
machines for crepe preparation. In the middle background, "scrap-washing"
machines outside the main building.]
It has been remarked in the chapter dealing with crepe preparation that
much depends upon the condition of the coagulum. There is no necessity, or
desirability, for having a standard higher than 2 lbs. dry rubber per
gallon, and it has been argued that it would be better to select a standard
of 1-1/2 lbs. The tougher the coagulum, the more the power required, and
the slower the rate of output of the leading machines.
In ordering machines for crepe-making, only large rolls should be
considered--_e.g._, rolls having a diameter of 12 inches to 18 inches and
from 15 inches to 18 inches face.
ROLLS RUNNING HOT OR "FREE."--If the rolls are found to become hot, work on
that machine should be stopped, and an examination made, otherwise there
is the possibility of the crepe becoming sticky and "tacky" when dry.
Although comparatively cold water may be flowing upon the rubber and the
rolls, little alleviation may be noticed, inasmuch as the source of heat
lies generally at the bearing ends of the rolls. This may be tested by
placing the hand on the top of the "standard" of the machine. The
development of the heat may be due to lack of lubrication, worn bearings,
or sometimes faulty setting-up of the machines.
Allusion has been made to the necessity for avoiding the running "free" of
rolls--_i.e._, in the absence of rubber. The grinding of the rolls, when
working close together, produces a fine powder, which causes a more or less
pronounced deposit on pale crepe. When the rolls have been in action for
some time and become slightly worn, this deposit may be confined only to
the edges of the rubber.
SHEETING MACHINES.--The foregoing paragraphs have dealt entirely with
machines for crepe preparation. Concerning machines for use in
sheet-making, the ground has been mainly covered in Chapter IX.
Where both crepe and sheet are made, it is permissible and advantageous to
use the heavy smooth rolls for the rolling of the sheets, and it is only
necessary to instal one or two light machines for placing a pattern on the
rubber.
Where a heavy battery does not exist, light machines with smooth rolls may
be employed satisfactorily. Even engine-power is not necessary for the
preparation of excellent sheets, but the output is limited where hand-power
only is employed. Estates are known on which upwards of 1,000 lbs of sheet
rubber are made daily with hand-power machinery in one station. Beyond this
figure, it is deemed advisable to instal a small engine, say of 7-9
horse-power. This is ample to drive a battery of three smooth-roll machines
and two markers, and yet have sufficient reserve to actuate a small pump
for the water supply.
LUBRICATION OF MACHINES.--It must always appear to those inexperienced in
engineering matters that existing methods for lubricating rubber machinery
are distinctly crude, when one considers the delicacy of the material to
be prepared. Many existing machines are still lubricated with oil, which
has to be administered in generous quantities. Generally, such machines
have been so designed that the excess of oil may find an easy passage into
the tray which receives the rubber. If not, it drops just outside the tray
to the floor, and is washed away in great gouts. Even where grease-cap
lubricators are fitted it is common to find that the excess can often be
transferred from the bearings to the trays and so to the rubber. One would
have expected from the attention which is being given to machinery for
rubber estates that some improvement in lubrication methods would have been
devised.
It is probable, however, that a great deal of the disabilities attaching to
present methods of lubrication might be obviated if closer attention were
given to the actual operation of the lubricators. Coolies should not be
allowed to handle them, and the responsibility should be placed upon a
foreman or the engine-driver.
TRAYS.--The most unsuitable and damage-causing part of the vast majority of
machines, without doubt, is the tray. On nearly all machines the tray is
wider than the effective portion of the rolls, so that any excess of
lubricant may drop into it. On others, not only is the tray wider than the
rolls, but its edge either is in contact with the shaft of a roll or just a
small distance away. The edge of the tray is thus favourably situated for
acting as a "wipe," and the lubricant is transferred to the inside of the
tray. Considering that the effective portion of rolls is about two-thirds
of their length, it must be unnecessary to have trays wider than the length
of the rolls. For the preparation of fine crepe trays are quite
superfluous, and their place can be taken by a narrow piece of board if
required. If the bed of the machines has been covered with glazed tiles,
even a piece of board is not necessary. Where trays have been removed from
the fine-crepe rolls on a number of estates, a marked decrease in the
number of spoiled pieces of rubber has resulted.
It must be recorded that the foregoing paragraph appeared in our 1913
publication. After a lapse of over seven years, the remarks remain as true
as when originally written. One of us is continually meeting with cases in
which the defects are plainly attributable to the cause indicated above,
and the fault often lies with the management of estates. On most machines
the trays are not fixtures, and could be removed if desired.
ARRANGEMENT OF MACHINES.--In considering the future arrangement of
machines, the first care should be to see that machines and windows are to
be found together.[16] Of all the factory operations, rolling of rubber
should be given the maximum light. At the same time it would not be
advisable always to choose a southern aspect, unless outside shades were
supplied. The best position for setting up machines, therefore, is along a
wall having a number of windows. This is extremely convenient also from the
view of power transmission, and gives the maximum free floor space to the
factory. In setting up machines, foresight must be displayed, otherwise one
may find, when future extensions are made, that the extra machines may
obstruct an entry or exit.
[16] Windows imply the existence of walls. Such is the conventional design
of factories. It may be pointed out that walls are not necessary. The roof
may be supported on pillars between which expanded metal of large size may
be placed. This fulfils all requirements and gives the maximum of light and
air. Many new factories have been erected to such a design.
For the actual erection of machines, no labour should be accepted without
European supervision. At present there are machines which are practically
useless owing to faulty workmanship, and on many machines bearings run hot
for no apparent or explicable reason. Whether the fault lies with the
turning of the rolls or the setting of the machine cannot be decided; but
at any rate too much care cannot be expended on the supervision of setting
up machines.
There is no reason why everything in a factory should not be made as easy
to clean as possible. For this desirable condition all machines should have
the beds faced with tiles. A word of caution should be given against using
marble slabs under the machines, as they would be eroded in time by the
slight amount of acid washed out of the rubber. There would be no such
objection against the use of white glazed tiles, if they are well set.
ACCESS TO BACK OF MACHINE.--In a few factories it has been noticed that the
drainage of water from the machines runs to the front of them. This means
that the coolies are put to unnecessary inconvenience and discomfort, and
they often suffer from sore feet. All water should drain to the back of the
machines. The necessity for seeing that these drains are kept clear might
then induce those in charge to examine the back of the machines. It is
often the case that, while the front of the rolls and tray are kept clean,
little attempt is made to cleanse those parts which are not visible or
accessible from the front. There should be no need to point out that any
labour expended in such "front-window" work is rendered useless by the
contamination from accumulations of old rubber and grease at the back of
the machines. In the course of visiting factories one of us has many times
seen great surprise exhibited by the manager or assistants on being shown
the state of affairs at the back of the machines. There should have been no
occasion for such surprise, for the back of the machines is quite as
accessible to them as to the visitor.
In conclusion it might be said that the manager needing advice as to the
best machines cannot go far wrong in purchasing any of the better-known
makes, such as Shaw's, Bridge's, Robinson's, Bertram's, Walker's, Carter's,
Iddon's, etc. This list does not include local manufacturers such as the
"United Engineers." It must not be imagined that their machines are not
recommended. As a matter of fact, their machines compare well with those
made at Home. It would be well to judge in the final decision upon--
1. Cost.
2. The experience of those already using the machines.
3. Simplicity of parts.
4. Lubrication system.
5. Mode of adjusting rolls.
6. Fitting of trays.
ENGINES.--It is not intended here to discuss particular makes of engines,
or even to attempt to lay down definite statements with regard to the type
of engine. Without a full knowledge of local circumstances, it is not
possible to recommend whether the engine shall be oil-driven, gas-driven,
or steam-driven.
Assuming a copious supply of very cheap timber, there could be no objection
to the employment of a steam-engine; but for most estates such a choice is
out of the question.
Again, in deciding between oil and gas, local economic factors must be
considered. Suction-gas plants are now made, in which a wonderful variety
of refuse can be consumed in the production of gas, whereas ordinarily
estates are restricted to the use of either charcoal or anthracite coal.
Both oil and gas driven engines are eminently suitable for the purpose of a
rubber factory, and the results obtained on different estates with either
are often discussed in favour of one or the other. The selection ultimately
narrows itself down to one of cost of running, in which availability of
supplies becomes an essential feature.
POWER.--No matter what type is selected, there should be made an ample
allowance for margin of power. The general experience of estates has been
that when the first portion of the estate comes into bearing, there is a
desire to avoid great outlay, which should really have been secured in the
original capital. The result has been that as later the estate expands, the
original power unit is found to be inadequate, and a larger engine has to
be purchased. In a short while the original engine is found to be
unsuitable even as a "stand-by," inasmuch as it is incapable of doing more
than a portion of the work required. This means eventually that another
large engine is required. Had sufficient margin of power been allowed
originally, only two engines would have been bought, as against the three
indicated above. Without going into finer details, it is usual to allow a
rate of 10 horse-power per heavy machine used for crepe preparation. In
actual practice, when a battery is working under full load, the power
demanded is about 6 horse-power per machine. Thus a 50 horse-power engine
running six machines and a scrap-washer is really running with only a small
margin of power, and if large pieces of hard coagulum are placed in the
washer or the macerator there may be a sudden stoppage. Assuming an average
estate commences with only three machines for crepe-making, on an expanding
programme, allowance of power should be made for six machines and a
scrap-washer, if the purchase of larger power units is to be avoided
later.
CHAPTER XIV
_FACTORIES_
GENERAL CONSTRUCTION.--On the question of general construction there is
little to be said, except that buildings are now being properly designed in
more permanent form than were some of the earlier buildings. On the whole
there is little fault to be found with factories in general, except in so
far as the output has outgrown the accommodation.
Most factories are now erected in iron, but there are a few which are built
of bricks. It should be premised that a factory in which rubber is to be
prepared should be as light and airy as possible. In this respect quite a
number of the older factories are lacking, and they seem to have been
designed to exclude as much air and light as possible. Under these
circumstances, the building is always dark, there is always an air of
dampness, dirt may accumulate, and there is usually a bad smell. Rubber
prepared under these conditions is always liable to be below the high
standard which should be attained, and the general tone of the factory is
depressing.
PLENTY OF LIGHT.--The old idea that light must be excluded is now known to
be erroneous; so that in designing a factory, provision should be made for
ample light and air. It should not be forgotten that in tropical climates,
iron buildings may become uncomfortably hot, as most of our older factories
are. Usually it will be found that the ventilation is imperfect. There is a
lack of window space, and the roof is imperfectly ventilated. The ridge of
the roof should be opened up by means of a "jack-roof," so that the warm
air rising naturally may escape at the highest point of the building. These
are defects which should be remedied in old buildings.
As a rule no rubber remains in the factory at night-time, except in the
form of coagulum, the loss of any of which would be noted with ease. The
conventional idea of enclosing the factory with walls of galvanised
sheeting, wood, or brick, is not strictly necessary. In modern buildings
these walls are replaced by large-mesh expanded metal, thus making the
machine-room perfectly light and plentifully ventilated. Under such
conditions, dirt cannot accumulate unseen, and the general tone of the work
is raised.
THE FLOOR.--The floor should be of thick concrete, and have a good surface
layer of cement. Preparations are now advertised for which claims are made
that their employment renders the surface of such floors waterproof and
dustproof. If these claims can be substantiated when the use is applied to
the floors of rubber factories, the employment of a preparation of this
nature should result in a considerable saving of expense and trouble.
Preferably the floor should not be flat, but should slope slightly from the
longitudinal middle of the building to the sides on either hand. If the
floor is level it usually results in accumulation of water, the cement
breaks in patches, and the factory always appears to be dirty.
POSITION OF MACHINES.--All machines should be arranged adjacent to and
parallel with one of the long sides of the building, and should be raised
about 6 inches above the floor, so that water may escape easily. Tanks for
the reception of latex, scrap rubber, etc., should be placed along the
opposite wall to the machines, and the intermediate length of the building
should be entirely free from fixtures. It was not uncommon in older
factories to find the engine situated in the middle of the floor, so that
what with the space occupied by the engine, and the space rendered
unavailable by the belt-drive, the real accommodation of the factory was
sadly diminished. In no modern factory should the engines be brought into
the main room. They should always be accommodated in a special compartment,
situated outside the wall, along the inside of which machines are placed.
In this way considerable floor space is left available, and the machines
may be worked by direct drive. Not only so; but if a suction-gas plant is
worked, there can then be no excuse for particles of coal or charcoal dust
being found in the factory.
POSITION OF ENGINES.--It scarcely need be pointed out that if the engines
are placed outside the wall which is opposite the machines, a long
belt-drive would be necessitated, and that the presence of the belt would
prevent the use of end doors. It is presumed in these arguments that two
engines are to be installed. One can hardly imagine a modern factory in
full working being equipped with only one engine, which might possibly have
an excess of power necessary to drive all the machines. In the case of
breakdown, which sometimes happens in the best supervised factories, it
would be small consolation to know that this excess of power was present
theoretically.
HOW MANY STOREYS.--There can be no doubt that, taking all things into
consideration, the best type of factory is that consisting only of one
floor. The factory should be quite separate from all other buildings, and
if attempts are made to conserve ground space by putting a drying-room over
the factory, much trouble will ensue, especially if pale crepes are to be
made. In the first place, the factory is made very much darker, and hence
more difficult to keep clean; secondly, the ventilation of the factory is
seriously interfered with; and thirdly, it is manifestly prejudicing the
drying of rubber to place it directly over a room which is always more or
less awash with water. At night such a building would reek with a
moisture-laden atmosphere, and little drying could be expected to take
place in that interval. From actual experience it has been shown that
rubber hung to dry in such a room, situated over a damp factory, is very
liable to attacks of "spot" diseases, since the presence of perpetual
moisture is favourable to the development of these diseases. If a
double-storey building has to be worked, it will be readily seen that no
first-grade rubber should be allowed to dry in it. The accommodation over
the factory may be restricted to the purpose of receiving lower grade
rubber which is not so liable to "spot" diseases, and possibly does not
take so long to dry as first-grade rubbers of equal thickness. It is
evident, therefore, that the erection of double-storey factories is false
economy, as separate drying-houses have to be built eventually. This
conclusion does not apply with the same force to factories worked in
conjunction with smoke-houses for preparing sheet rubber, but,
nevertheless, such a factory should not have another floor above the
work-room.
VERANDAHS.--One of the worst features in many factories is the necessity
for coolies to bring latex into the factory. As already mentioned, the
floors of factories are usually running with water (or should be), and it
can be imagined that the passage to and fro of scores of coolies must bring
in a great quantity of dirt. Not only so; the very presence of the coolies
is a hindrance to the efficient working of the factory, and considerable
floor-space and time are wasted.
This feature in factory working is all the more annoying because the
necessity for it could so easily be obviated. All that is necessary is the
erection of a wide, open verandah outside the wall of the factory. Here all
latex could be received and strained, scrap-rubbers could be received and
passed through an opening into tanks placed in convenient position. Water
could be laid on in this verandah so that coolies might wash their buckets,
and the whole verandah might be enclosed only with expanded metal so as to
avoid interference with the lighting of the factory. In this way it would
be quite unnecessary for any field coolie to enter the factory proper, and
this would facilitate cleanliness. Such an arrangement has been discussed
by the writers many times during the last few years, but the number of
estates which have made such provision is still in the minority, and the
same slipshod and dirt-making procession of coolies continues to walk
through the factories, and the same piles of bark-shavings and scrap-rubber
continue to accumulate and ferment in a few instances.
An indication of types of verandahs is given in Chapters VII. and IX. These
are not intended to be representative of a universal design, but may be
suggestive in the planning of others according to local conditions.
SITUATION OF TANKS.--It will be noted that these verandahs are raised from
the ground-level to a height of about 3 feet in order that latex may be
gravitated, with a slight fall, into the coagulating tanks which are within
the factory. There exists a real necessity for this practice, inasmuch as
otherwise to obtain gravitation of latex (which is quicker and cheaper
than handling) the coagulating tanks would have to be either placed on the
floor or sunk beneath the level. The risk of contamination of latex or
coagulum under such circumstances would be appreciable. Apart from this, it
is advisable to have the coagulating tanks raised to a height of between 2
and 3 feet, to secure the advantage of ease of working in the processes of
coagulation and the handling of coagulum--a not inconsiderable factor.
In some modern designs it is proposed to place the coagulating tanks in a
separate building. This would seem to be an unnecessary refinement in a new
building, if observance is given to the suggestions made in previous
paragraphs.
DESIGNS AND "LAY-OUT."--In a previous publication[17] comment was made upon
grievous errors in designs prepared by those inexperienced in the
requirements of the tropics. There is little ground now for complaint, and
local engineering firms are fully capable of advising upon, and
constructing, suitable buildings.
[17] "Preparation of Plantation Rubber," Morgan, 1913.
In considering the first installation of a factory and equipment one always
has to weigh the question of prime cost against the probability of future
expansion of crop. If it should be decided at first merely to cater for
contemporary requirements, the fullest consideration should be given in
discussing design of building and lay-out of machinery to the
practicability of later extension. The site should be large enough for the
eventual group of buildings, the building should be easily capable of
extension with the least cost, and the same forethought should govern the
lay-out of the machinery.
DRAINS.--Lastly, there is the question of drains. Generally speaking, all
factories are well provided with drains, and the only difficulty is that of
getting an adequate fall for efficient drainage. But there is a certain
amount of laxity exhibited in the matter of providing sieves in drains. To
anyone acquainted with factory working, it must be apparent that quite a
lot of small pieces of rubber are washed into the drains. This rubber
should be collected at intervals during the day; but in many instances
that collected is only a fraction of what escapes. Wherever possible the
drainings of a factory should be carried as far as is practicable from the
buildings by means of cement drains. Too often these are short, and lead
into earthen drains. Even if no pieces of rubber are present, the serum
from the coagulum is subject to decomposition, the effluvium from which is
objectionable.
WATER SUPPLY.--It is essential that a good supply of water should be
available. This should be distributed by pipes all round the building, so
that a hose may be used in every part for the thorough cleansing of the
factory at intervals during the hours of working.
Summing up, it might be said that a good factory, therefore, should have
the following features:
1. Plenty of windows, or walls of expanded metal.
2. A jack-roof in the ridge, and hence a good system of ventilation.
3. Engines in compartments outside the walls of the factory.
4. Machines close to and parallel with the wall outside of which the
engines are placed.
5. Latex tanks and other fixtures along the wall opposite the
machines.
6. A long middle free space, at either end of which a large double
door should be placed in the end walls.
7. A good concrete and cement floor sloping slightly from the middle
towards each long wall.
8. An abundant water supply, and several lengths of hose.
9. The building should be of only one floor, and have ample head room.
10. There should be an outside, open verandah upon which latex may be
received, etc.; preferably outside the wall which is opposite to the
machines.
11. The system of drainage should be thorough, and the drains should
be adequately screened, so that all particles of rubber may be
collected.
CHAPTER XV
_OTHER BUILDINGS_
DRYING-HOUSES FOR CREPE.--It has already been shown in the previous chapter
that one type of drying-houses--viz., that over a factory--stands
condemned, except for the drying of low-grade rubbers. Generally speaking,
a great advance has been made in the design of crepe drying-houses during
recent years, and it has been possible even to improve older ones so as to
bring them into line with the more modern buildings. Houses for drying
crepe rubber may be of one floor, two floors, or even three floors.
Doubtless those built with three floors were designed with a view to
economising the available site for factory buildings, and as long as the
ventilation is good there can be no very great objection to them. It might
be pointed out, however, that even with the best of ventilation the air
passing successively through three layers of rubber must be fairly
saturated with moisture by the time it leaves the building. The effect of
this upon the rate of drying in the uppermost chamber will not be so marked
as it will be in the middle floor, as the temperature of the top floor must
be many degrees higher than that of the other two rooms. It would be
expected, therefore, that the rate of drying in the middle storey would be
slower than that in either of the other two.
In houses of two floors this objection would not have to be met, and
drying-houses of this type are successful and common.
HOW MANY STOREYS?--Again nothing could be urged against a building of two
or three storeys in which the ground floor was occupied as a packing-room,
except that, by negligence in not allowing wet crepe a preliminary dripping
period, water might fall upon the packed rubber below.
As a matter of experience, such a house is, taking all into consideration,
the cheapest and most suitable type for any estate with an increase in
output. Even at the outset there should be a separate room in which sorting
and packing is undertaken. This is conveniently the lower room of a
drying-house. The only stipulation to be made for a house with two storeys
is that the floor of the upper room should be of an open pattern, so that
the air may circulate right through the building. This is usually and very
successfully attained by laying down wide slats of wood, with spaces of an
inch or more between them. It is not advisable to have spaces wider than
1-1/2 inches, otherwise there is a certain amount of danger to the limbs of
individuals who have to work or supervise in the building. In any case, it
is very convenient to have pathways of planks running the whole length of
the floor, so that the supervision is made more convenient. If this is
done, there can be no objection to the custom of suspending the rubber of a
lower chamber from the slats of the floor of the upper room. At present, in
some drying-houses, this means of suspension is used, but no planks are
laid down, and it becomes necessary to walk over the drying rubber. This is
a detail, but it is one which does not make for the improvement of rubber,
and the expenditure of a small sum would be sufficient to rectify the
matter.
From every point of view, it would be desirable to have the floor of the
packing-shed (or the packing-room in a combined house) raised from the
ground, to a height of, say, 3 feet; or the height of a bullock-cart or
motor-lorry. Not only is ventilation improved, but there would be a great
saving in labour. Packed cases could be wheeled directly on a level with
the cart or lorry.
A great many estates favour drying-houses of one storey. These are
eminently suitable, provided that the site is suitable, and that the
relative dimensions of the house are favourable to efficient ventilation.
It is a common mistake to find buildings of which the breadth is out of
proportion to the height. Obviously, if the height is not considerably in
excess of the breadth, ventilation will be defective. For a single-storey
drying-house, the maximum height should bear the ratio to the breadth of
3:2, and in a house of this type specially long pieces of crepe can be
utilised. Naturally, in a house of two storeys, this factor is not likely
to be neglected, and if the lower room is used for packing purposes the
rate of drying should be rapid. Again, when a single-storey building is
contemplated, it is well to make strict examination of local conditions. If
the site is low-lying and surrounded by trees it will be clear that tall
buildings are required, and that a house of more than one floor is to be
preferred. Considerations of this nature would have prevented the erection
of some dry-sheds which do not give a satisfactory rate of drying.
VENTILATION.--No matter how many floors there may be in a drying-house, the
greatest attention should be given to the question of ventilation. It is an
elementary point in the study of ventilation problems that the best system
of natural ventilation is obtained by admitting cool air near or through
the floor and providing an exit for the warmer air at the highest point in
the building. It is not often that such a rule is infringed in the
ventilation of rubber drying-houses, but several of the older buildings
erred in this respect. In a good modern house there is a space (about 2
feet in height) all round the base of the walls merely closed with expanded
metal; this admits cool air. An exit for warm air is provided in the ridge
of the roof by either ventilation chimneys or by a jack-roof. The latter is
preferable, as it provides for a more free and uniform escape.
In some drying-houses, besides the ridge openings, the space along the
eaves is left open. This would seem to be undesirable, as it provides for
the entrance of outer air, which might combat the ascending warm air and so
interfere with the natural upward currents. Provided that a jack-roof or
other suitable openings have been installed, there is, therefore, no
necessity for the existence of open spaces at the eaves, and they probably
do more harm than good.
In the tropics, on days of sunshine, there must always be an upward current
of air in well-designed houses. Temperatures of 105° F. are easily recorded
in the ridge space of a building, while the temperature in the lower part
of the house may be at least 15° F. lower. On the floor of an upper room a
temperature of 90° F. is commonly noted, and in buildings with three
storeys the usual day temperature of the top room is about or over 100° F.
Even, therefore, when there is no trace of a breeze, there must be a
displacement of air in an upward direction, though it may not be detected
without tests being applied.
It is often asked whether a temperature of 100° F., such as is obtained in
the upper room, is calculated to injure the quality of the rubber. There
need be no fear on this ground; the experience of many estates goes to show
not only that no harm results, but also that the drying of the rubber is
expedited. There would seem to be no reason why crepe rubber should not be
dried at a temperature of 100° F. It must be understood, however, that
higher temperatures for crepe rubber are not recommended, as it has been
proved that the rubber is affected. The fact becomes obvious with continued
treatment at temperatures much above 100° F., for the rubber stretches and
breaks across the support.
WINDOWS.--Concerning the subject of window space in a drying-house, there
has been much discussion at various times. Years ago it was common to find
windows widely open with the sunshine streaming in. Naturally, tackiness
developed in some of the rubber, and care was then taken to keep the
windows closed. Thus the rooms were darkened and air excluded. There
followed a period in which windows were fitted with ruby-coloured glass to
keep out the actinic rays of the sun, which were responsible for tackiness,
and excess of light, which was supposed to be responsible for the rapid
oxidation of rubber. Unless special precautions were observed in the
processes of coagulation and preparation, it was not proved that the
exclusion of light prevented or lessened the natural oxidation of crepe
rubber. Since the introduction of sodium bisulphite for the prevention of
oxidation, there has been no cause to worry as to the possible effect of
light, as no perceptible darkening of the rubber takes place. It follows,
therefore, that no trouble need be taken to exclude light, although the
necessity for excluding direct sunshine still exists. Windows may be left
open as long as the sun does not reach them. This can usually be arranged
in a drying-house by manipulating the windows at intervals during the day,
so that those in the shady side of a building are always open, while those
on the sunny side are always closed. If it is thought that this
manipulation cannot be entrusted with success to the store coolies, the
case may be met by having all windows constructed on the louvre pattern, so
that, although the windows are closed all day, air and light are not
excluded. Should it be desired to retain the existing type of windows,
which open outwards, and to keep them open all day, a simple arrangement of
ruby-coloured cloth on an outstanding wooden frame may be placed within the
walls of the building, or the shutters of the windows may be hinged at the
top to open outwards. Unless there is a pronounced breeze, or it is
required to examine the rubber closely, there is no necessity to have
windows open, except in the case of a house in which the bottom floor is
used as a packing-room. The windows of this chamber may remain open during
the day, to advantage in sorting and packing, and also to the proper
ventilation of the building. Thus the direct rays of the sun are rendered
harmless, while air and light are allowed to enter.
HOT-AIR DRYING-HOUSES.--Mention has already been made of the existence of a
system of drying in which hot air is forced into a drying-house by means of
a powerful fan. Provided that the temperature of the hot air could be so
regulated as not to exceed 100° F., there would be merit in the system.
Such matter of regulation could be solved by having a duct in the main air
passage, through which cool air could be admitted in such proportion as to
modify the temperature of the hot air. As the process is worked at present,
the temperature attained is often well above 100° F., and there is a danger
of thin crepe placed in this house over-night being found upon the floor in
the morning. Unless the crepe is prepared thick and cut into fairly short
lengths, it will not bear its own weight at higher temperatures; and if it
is made thick, drying is impracticably prolonged. It is probable that, with
a temperature of 100° F., and a steady current of air, average thin crepe
would dry in such a drying-house within six or seven days. This would be
an improvement upon the usual rate of drying in most factories, although
several ordinary drying-houses are known in which thin crepe will dry
naturally in that period.
SMOKE-HOUSES.--No discussion of theoretical considerations regarding the
process of smoke-curing will be attempted here. We are concerned only with
the necessity for supplying a demand for smoke-cured sheet rubber. Broadly,
the process is akin to the smoke-curing of herrings, and the objects are
much the same--viz., (1) drying, (2) preservation--except that while
herrings are only dried partially, rubber should be dried perfectly.
On a small scale a primitive smoke-house could be built easily and cheaply,
and such a building might be fully as efficacious as the most elaborate and
expensive installation. In the early days of estates it was not uncommon to
see temporary smoke-houses constructed of wood, and roofed with "attaps"
(palm leaves). Some of the best rubber in the market has come from wooden
buildings, but naturally the risk of destruction by fire is considerable.
For imperative reasons it may be sometimes found necessary to smoke rubber
when the only available building is a single-storey one. As a temporary
measure, the building may be converted into a smoke-house by placing the
fires in pits sunk deeply into the ground, and effectively screened above
by iron baffle plates. But it is not advisable that smoking be continued in
such a single-storey building, as the best effects are not obtained, and
the risk of fire is far too great.
USUAL TYPES.--At first sight it would appear that the best type of
smoke-house would be one consisting of a tall building, covering a
comparatively small superficial area, and having a number of superimposed
chambers in which the rubber could be hung to dry. In practice there are
several solid objections which limit the height and the number of floors.
Chief among these is the question of temperature. If smoke-curing is to be
effective, a certain temperature must be attained and maintained. To obtain
such results in a house of excessive height would be difficult, if not
impossible, under normal conditions. It would be found that the chamber
immediately above the furnace-room would be overheated if the temperature
in the upper rooms was within the desired range, etc.
Until recent years smoke-houses could be classed as belonging to one of two
types:
(1) Those having external furnaces.
(2) Those having internal furnaces.
The number of the former existing at the present time must be very small,
as it has been shown that the arrangement of the furnace outside the house
is unsatisfactory in comparison with the other type of house. In discussing
the question of smoke-houses, therefore, it will be understood that the
standard type accepted is that having an internal furnace. In its original
form it was known as a "Kent" drier, and consisted of a tall two-storey
wooden building. The walls of the lower chamber had the form of an inverted
and truncated pyramid. By this arrangement it was possible to obtain from a
comparatively small fire sufficient smoke and heat to cure the product
placed in the room above. This is the principle upon which many
smoke-houses in Malaya are designed. On a very large scale it is not
claimed that the sloping sides of the lower chamber lead to economy in the
number of fires, but merely divert the smoke in an upward direction. It is
acknowledged that vertical lower walls are quite effective, and it is an
easier matter to fit in doors.
It may be noted that the usual type of smoke-house now in general use
consists of a building of two storeys, in the lower of which are situated
the furnaces, while rubber is hung on racks in the upper room. Sometimes
there may be a third storey, also used as a drying (curing) chamber. As a
rule the drying-room is one long unit, as also is the furnace chamber; but
in some cases they are subdivided by vertical partitions into smaller
chambers, for ease of working and better control. This applies with some
force in the case of very long houses standing in an open space. It is
sometimes found in such cases that at certain seasons the prevailing winds
have the effect of making drying and curing uneven in parts of the
building.
With these exceptions, the ordinary type of smoke-house functions very
efficiently, and is capable of drying average sheet (from standardised
latex) in a period ranging from seven to eleven days. Should the building
not be capable of such performance, in spite of the strict observance of
all rules laid down for the processes of preparation, then there is some
defect in ventilation or in the distribution of heat.
GENERAL VENTILATION.--The ordinary rules of ventilation in drying-houses
apply equally to a smoke-house. There should be a slow current of air and
smoke from the lowest point to the highest point in the building.
In spite of all that has been written on this subject, it is by no means
uncommon to encounter the idea that a smoke-house should be perfectly
closed in order to get good results. As to what must become of the (say) 25
per cent. of moisture which the rubber contains there is no knowledge. In
dozens of cases, when complaints regarding slowness of drying have been
investigated, it has been necessary to point out the need for providing a
rational system of ventilation.
Naturally only a slow current of air and smoke is required. The creation of
an appreciable draught would have the effect of increasing the fuel
consumption of the furnaces, raising dust from the ash, and of causing a
temperature higher than that which is known to be desirable. It will be
clear, therefore, that if there are to be any openings at the base of the
walls they should be small in area, and should have some device by means of
which the current of air can be efficiently regulated. In the usual case
the construction of the building is not calculated to render it air-tight,
and the necessity for providing special air inlets does not arise.
WINDOWS.--Windows are not strictly necessary, and are only intended to be
of service during the time in which coolies are at work within the
building. The operations of examining rubber, turning sheets, removing dry
rubber, cleaning racks and floors, and putting wet rubber into position,
usually occupy some hours daily. During this interval the windows should be
widely opened if the weather is favourable, and should remain so until the
fires have been lighted. It should not be forgotten that during the heat
of the day quite an appreciable degree of drying is possible. Advantage can
be taken of this; but there is no necessity to extend the interval unduly,
and it is of greater advantage to proceed with smoke-curing when the work
in the drying-chambers has ceased.
RACKS OF SUPPORTS.--Still referring to the usual type of smoke-house, it
may be remarked that in the upper room bays of racks run at right angles to
a central passage down the length of the building. Narrower passages run
between the bays of the racks to facilitate ease in handling and
inspection. The wooden supports may be placed about 3 inches apart
horizontally, and 15 or 18 inches apart vertically. A full bay of racks
should contain nine or more lines of support in each of the planes which
are 15 or 18 inches apart vertically. The number of these planes is
governed only by the height of the room, measured from the floor to eaves.
The supports should be of smooth timber, and need not exceed 1-1/2 inches
square in section.
It is usual and advisable to smooth off the rectangular edges of the
supports or bars, to avoid the incidence of splinters of wood adhering to
the rubber. The bars should not be fixtures, but may either be accommodated
in slots, or may rest _between_ two nails, so that it is possible to give
them a rotary motion by turning the projecting ends. This practice is
followed in smoke-houses, the idea being to move the drying sheets slightly
each day, with a view to the prevention of a pronounced mark across the
sheets.
Care should be taken to see that the vacant racks are thoroughly cleaned
before fresh rubber is placed upon them, otherwise a distinct dirty mark is
caused across the middle of the sheet. This mark usually cannot be removed,
even by scrubbing with water. Where this mark occurs regularly in all
sheets, attention should be turned to the openings beneath the bays of
racks, if open fire furnaces are employed. It will generally be found that
gauze of too wide mesh has been fitted. This should be removed or covered
with a finer gauze.
A more effective way of dealing with the trouble, provided other
precautions have been taken, is to have plenty of spare wooden bars. It
should be a rule stringently enforced that, as soon as racks are emptied,
the bars should be removed to the factory to be cleansed thoroughly. A
spare set should enter the smoke-house with each batch of fresh rubber. The
actual number of spare sets required could be limited to a two days'
supply, and the extra cost would be recouped easily.
FLOOR OF DRYING-CHAMBER.--The floor of the chamber is usually of planks,
except that the space under each bay of racks should be filled with
expanded metal. With the use of wood fires there is always a large amount
of light ash formed, which may find its way into the upper chamber. To
counteract this, screens of fine mesh gauze are laid over the expanded
metal. This gauze may be fitted into a movable wooden frame, so that when
it becomes necessary to clean it the whole may be removed.
The difficulty is that with furnaces of the "open-fire" type the rise of
dust is so great that the gauze screens soon become clogged, especially as
the slight tarry matter in the smoke condenses on the gauze, causing the
dust to adhere. With the better types of furnaces, the employment of gauze
screens is not necessary, as there should be very little rise of dust. It
is sufficient to use only expanded metal, to prevent any displaced pieces
of rubber falling into the furnace chamber.
FURNACES GENERALLY.--The crudest and dirtiest method of fuel consumption in
the preparation of smoked-sheet rubber is that of making a fire on the
ground. This is still a common practice, and should be condemned as being
both wasteful and harmful. Under prevailing conditions coolies will, in
spite of instructions, heap up a pile of logs in order to save themselves
the trouble of stoking the fire in small quantity and at regular intervals.
A small supply of water is kept at hand with which to quench the fire
somewhat if it threatens to cause trouble. Naturally a large quantity of
fine ash is thus thrown up, and the rubber above receives the deposit. If
the coolie does not happen to be sufficiently awake, of course a house
burns occasionally.
From this primitive type of furnace, others have been evolved. These
usually take the form of more or less shallow trucks, the majority of
which are similar in principle to the fire on the ground, except that the
container can be withdrawn from the house for the purpose of removing the
ash. Sometimes they are even more objectionable than the ground fire,
inasmuch as, being raised above the ground level, an under-draught through
fire-bars is caused, and consumption of fuel is so much the more rapid.
PITS.--It is clear that large fires are not desirable, and that combustion
should be slow, provided that the necessary temperature can be maintained.
The lines along which the development of furnaces needed to extend are
therefore plain. The simplest device adopted was the digging of pits in the
ground. Sometimes these pits received the addition of an iron truncated
cone which was movable. Naturally the combustion was slow, but sufficient
heat was obtained if the pits were large enough or in sufficient number. An
objection was that the ash had to be cleared _in situ_, and in the process
the earthen pits gradually increased in size. In all cases it was necessary
to suspend an iron baffle-plate above the furnaces to distribute smoke and
arrest any sparks.
"POT" FURNACES.--The next development was the employment of "pot-furnaces."
These consist of iron drums, sometimes merely resting on the ground, and
sometimes mounted on trucks for easy withdrawal. These drums radiate
sufficient heat if present in sufficient numbers, and the fuel consumption
is low. They are usually manipulated by starting a fire in the bottom and
packing in logs cut to the necessary length. Some have no lids, while
others are fitted with perforated caps.
It was considered necessary in some instances to punch a few small holes
near the base of the drum in order to ensure a very slight upward draught.
In a few cases this perforation has been exaggerated to the form of a
hinged door. Unless this can be closed with ease, and is closed according
to instructions, part of the object of this type of furnace is defeated;
fuel consumption is rapid, and the temperature is too high. In the original
form "pot-furnaces" have been found to be effective on many estates, and
are still employed with satisfaction.
IRON STOVES.--Working on exactly the same principle, on some estates one
finds small iron stoves in use. Sometimes broad pipes are attached for the
better distribution of the smoke; if this is the case it should be noted
that the pipes should have a slight downward slope, and that the "bend" at
the end should be turned downwards. In this way condensed moisture and
creosotic matter falls to the ground, and does not lodge in the pipe. The
life of the conduit is thus prolonged. Usually such stoves are in use where
the "head-room" of a smoking chamber is insufficient for other types, or
where the nature of the site does not permit of sunken furnaces being
installed. They are of value likewise on occasions where the fuel supply is
limited to a rich timber such as mangrove-logs ("bakau"), when it is
necessary to ensure a low combustion with low cost of fuel.
HORIZONTAL DRUM-FURNACES.--To overcome difficulties inherent to drums or
"pot-furnaces," the next development was that in which the drum was made to
assume a horizontal position, and adapted ingeniously to a simple system of
working from the outside of the building. Reference to the drawings given
will explain how this is effected. In the first illustration (No. 2) it
will be noted that the drum is supported upon brick pillars, with one end
projecting through the wall of the building. At the other end a short
chimney is mounted, having within it a "damper" which is adjustable from
the outside. Over this chimney is suspended a simple baffle-plate, made
from a Chinese iron cooking-pan. The outer end of the drum is furnished
with a hinged and latched door, in which a small air-regulator is
accommodated.
In the second set of drawings (No. 1) the drum is increased in size and
fitted in a special manner for incorporation with a distinct type of
building. Such a scheme was first put into effect by Mr. R. C. Sherar, the
manager of Third Mile Estate, Seremban, F.M.S., and for ease of reference
the house and furnace will hereafter be mentioned when necessary as the
"Third Mile" type.
[Illustration: "THIRD MILE" TYPE; HORIZONTAL DRUM.
This type of furnace is suitable for adapting to existing buildings with
perpendicular lower walls.]
[Illustration: "THIRD MILE" TYPE OF FURNACE, USED IN CONJUNCTION WITH
"THIRD MILE" SMOKE-HOUSE.]
It will be seen that the furnace has at the farther end a door for the
removal of ash. As this, if badly fitting, may result in too great a
draught, it is well to insist upon good workmanship. Other adjustable
air-inlets are provided, and the drum is enclosed in a brick chamber.
RATE OF COMBUSTION.--However successful this furnace may have proved in the
hands of trained coolies, one must feel that with such a number of
air-inlets (whether accidental or designed) there would always be present
the possibility of obtaining too rapid a combustion. In the original forms
of drums or pot-furnaces of various kinds, a very slow rate of combustion
was attained. Naturally a relatively larger proportion of carbon remained
unconsumed, and there was a small proportion of ash. In these respects the
furnaces resembled charcoal-burners. In point of fact, some estates used
this principle for the dual purpose of smoke-curing the rubber, and at the
same time obtaining a supply of charcoal to provide fuel for their
suction-gas engines. This is a consideration in times when managers are
desirous of discovering any devices which tend towards reduction of costs.
It will be clear that, under ordinary circumstances, the condition of what
remains after the combustion of the fuel gives an indication of the rate at
which the wood has burned, and this test should apply to all furnaces. That
in which there is the most ash and the least charcoal is the one least to
be desired. In direct connection with this consideration, one must
recognise that a fire which is sunk below the level of the ground exposes
the least surface from which heat may radiate; and hence, in order to
obtain the maximum benefit of heat from a slow-combustion furnace, it
should be above ground-level, or should have a superstructure from which
the heat may be dissipated.
Simple drum furnaces, with slow combustion, have the further advantages
that a "charge" of fuel will need no attention for possibly eight to ten
hours, and practically no ash is found to be ejected. These advantages have
great practical importance. The first minimises any disabilities arising
from neglect on the part of coolies, and the second makes for increased
cleanliness in the drying-chamber. While these advantages would appeal to
most estates, there would appear to be a further advantage to small
estates which have only temporary timber smoke-houses. With a slow rate of
combustion in a furnace of this type, danger from fire is diminished
considerably.
Bearing in mind the slow rate of combustion, and hence the comparatively
low temperature obtained, it will be plain that drum furnaces should be
employed in larger number than ordinary open-hearth fires; and the drums
can be so placed as to ensure the best possible uniform distribution of
heat and smoke.
Large furnaces are sometimes seen, with flues of brickwork. In view of the
foregoing remarks, it will be obvious that these tend to large fires and a
rapid combustion, and hence must be classed as undesirable.
BRICK STOVES.--Developing from "drum" furnaces, another type comes into
existence. In principle it consists of an enclosed brick furnace, with
feeding door, and a low conical dome surmounted by an adjustable cap or
spark-arrester. The rate of combustion can be influenced by a suitable
movement of the cap, which is operated by a screw. This type of furnace has
been installed on several estates by the engineering department of Messrs.
Harrisons and Crosfield, and is understood to give satisfaction.
PATALING TYPE.--With the exception of the "Third Mile" type already
mentioned, all the furnaces described are open to a strong objection, in
that the coolies have to enter a room, usually filled with hot smoke, in
order to attend to the fires. The mere opening of the door of the building
is sufficient to fan most fires into a blaze and to raise sparks. Apart
from these points, it is natural for coolies to avoid entering too often,
with the result that they generally stoke with the maximum load of timber.
Even should they not sleep the danger is clearly great.
[Illustration: SIDE SECTIONAL ELEVATION (PATALING TYPE OF FURNACE).]
[Illustration: PATALING TYPE OF FURNACE.]
To obviate these drawbacks, furnaces which are fed from the outside of the
building were designed. There have been various forms, but as they were
first installed on Pataling Estate, in the present form, they may be known
under the description of the Pataling type of furnace. They are eminently
satisfactory, and have a low rate of fuel consumption. They are very
safe, and in fact, if worked with average intelligence in supervision, can
be regarded as being fool-proof. There is practically no ejection of fine
ash, and no fine-mesh screens need be employed. They can be adapted to any
building having either vertical or sloping walls of galvanised iron.
[Illustration: LARGE SMOKE-HOUSE OF ORDINARY CONSTRUCTION, WITH SHIELDED
VENTILATORS PERMANENTLY OPEN.
In foreground, movable folding racks on which sheets "drip" in the open
air. This smoke-house is equipped with brick furnaces fed from the outside
(Pataling Estate).]
In essential the furnace consists of a shallow pit below ground-level,
lined with brick, and having a square brick superstructure rising 4 feet
above the floor of the building. On top of the brick walls rests a sheet of
boiler-plate perforated with small holes. The hearth being below
ground-level, and with the extra 4 feet of height above the floor, it
follows that if ash is disturbed it is confined.
From the drawings it may be seen that the pit is prolonged to the side wall
of the building, with steps leading up to the ground-level. The top and
sides of the opening are made with galvanised sheeting, forming a kind of
short tunnel in which the coolie may stand upright. The outer face of the
brick furnace forms the inner end of the tunnel, and accommodates the door
of the furnace. The bottom of the pit is filled up with clay and stones
almost to the level of the bottom of the door. This ensures a very shallow
hearth, and guards against an unduly large fire. Obviously it is not
desirable or necessary to make the hearth of fire-bars, as was done in one
instance, with the provision of a door below for removing the ash. This
would lead only to a strong draught being created, with a high rate of fuel
consumption.
[Illustration: BRICK AND CEMENT SUPERSTRUCTURE OF FURNACE INSIDE THE
BUILDING, BUT FED FROM OUTSIDE.
On the top of the superstructure rests a sheet of perforated boiler-plate.
The actual fire-pit is below ground-level, and to the left may be seen
parts of the sides and top of the downward approach, from the outside, to
the door of the fire-pit.]
The openings can be screened by a narrow sloping lean-to, which serves to
keep out rain, and provides shelter for the stock of fuel and the coolie.
The iron furnace-door should be well made, with an easily worked latch; but
it is not necessary that it should be perfectly fitting. Any slight
aperture will serve to provide the necessary air-inlet, but in any case it
should not be more than slight.
CONSUMPTION OF FUEL.--Regarding this furnace, it may be said in conclusion
that it is more satisfactory in general working than any other furnaces yet
encountered. Obtaining information from over sixty estates, on the question
of fuel consumption compared with output of rubber, it was found that, as
far as ordinary smoke-houses were concerned, the Pataling type of furnace
showed the lowest unit consumption of fuel.
[Illustration: GENERAL VIEW OF SHELTERS COVERING APPROACHES TO FURNACES.]
This was at the rate of slightly less than 1 lb of fuel per 1 lb of
thoroughly cured sheet rubber. The figure on some estates mounted as high
as 4-1/2 lbs. of fuel per lb. of rubber. Naturally this factor may have
been affected by failure to utilise the drying space to its fullest
capacity, but in the main the high rate of consumption could be attributed
solely to the deficiencies of the furnaces.
FLOOR OF FURNACE-ROOM.--As a rule no attempt is made to improve the natural
earthen floor. Whether open-hearth fires, truck furnaces, or drums are
employed, it is usual to find a floor with an inch or two of dust upon it.
Where all endeavours are directed in other directions towards cleanliness,
it appears strange that this should be overlooked. In contrast, houses
employing the Pataling type of furnace (or others) have concrete and cement
floors, which can be kept quite clean. Cleanliness should be as zealously
attempted in the smoke-house as in other departments.
[Illustration: NEAR VIEW OF SHELTER.
Steps lead downwards where the wall of the smoke-house has been removed.]
ROOF.--In any type of smoke-house, the roof should fit tightly at the
eaves, and the only vent should be in or near the roof-ridge.
In an ordinary smoke-house, the opening should take the form either of a
low jack-roof or of squat chimneys protected against rain. If a jack-roof
is chosen, it may be so low as to need no scheme of adjustment, or
otherwise adjustable swing shutters must be provided. The chimneys may be
made with such low fitting between the cap and the body that no interior
swinging flaps are required.
During the operation of smoke-curing the smoke vents must remain open to a
degree which is arrived at by experience. Failure to provide a
comparatively free egress for smoke and moisture will bring trouble in its
train. After a house has been in use for some time, it will be noted that
the timber becomes covered with a shiny tarry coating deposited by the
smoke. If the rubber remained in the house for an equal period, it would
take on the same appearance. During the interval between the entry and the
exit of the rubber some amount of deposit does take place, and it is this
mixture of creosotic substances which plays a part in fitting the rubber to
withstand growths of mildew which would otherwise form.
If proper smoke-vents are not provided, the moisture evaporating from the
sheets is unable to escape quickly enough, with the result that a great
deal condenses at night-time upon the inner surface of the comparatively
cool roof, and falls back upon the rubber in unsightly black "drips," which
leave a distinct mark on the sheet. Even if vents are open, this may happen
during seasons of rain. The temperature of the moist smoke in the
roof-ridge may be as high as 130° to 140° F., while the outer atmosphere
may have been cooled by rain to 70° F. Such a difference on the two
surfaces of the roof must lead to condensation within the house, with
consequent "dripping." It used to be the custom to drape sacking material
above the bays of racks in order to prevent the drops of liquid falling
upon the rubber; but often for want of renewal the last state was worse
than the first. Modern houses have often an inner lining, a few inches
below the roof. This is made of soft wood which receives any product of
condensation and absorbs it.
OTHER TYPES OF SMOKE-HOUSE.--So far we have confined the arguments to
smoke-houses of the usual type. There are others which vary in either
design and method of working, or in the material of the structure. Mention
may be made of the most prominent of these.
BRICK HOUSES.--Some houses are constructed of brick, and may have one or
two storeys above the furnace chamber. The floors are sometimes made of
ferro-concrete, and the furnaces may also be of this material. These brick
houses give satisfaction, but there would seem to be some difficulty in
obtaining and maintaining the desired temperature, although it is not quite
plain why this should be so. The principle of these buildings is the same
as that of the ordinary iron house, and the suggestions made in previous
paragraphs apply with equal force.
"THIRD MILE" TYPE.--Reference has been made to the "Third Mile" type of
furnace. This is an integral part of a smoke-house, which for clearness of
distinction may be known as the "Third Mile" type of smoke-house, the
original of which was erected on the Third Mile Estate, Seremban, F.M.S.
[Illustration: "THIRD MILE" TYPE OF SMOKE-HOUSE.]
In essence the design consists of a building, having two storeys for
rubber-drying, and a shallow inverted pyramidal base, ending on the ground
in "Third Mile" furnace, already described and illustrated.
It will be seen that the principles of ventilation employed are those
indicated for an ordinary house--viz., air-inlet near the ground (with
little draught), and smoke-vent at the roof-ridge. The windows shown in the
drawing are only for purposes of inspection of the rubber during the day,
and form no part of the scheme of ventilation during the hours of smoking.
It is claimed that the efficiency of the house is high. Certainly the work
of attending to the furnaces is simplified, and there should be small
ground for excuse if negligence is displayed.
JACKSON HOUSE.--This was brought into notice under the description of the
"Jackson Cabinet," and it was claimed that average sheets could be dried in
a few days. It consisted of a small house of one storey, having several
tiers of racks. Smoke and heat were generated in a small stove placed
outside the wall. A smoke vent was provided in the roof. These cabinets had
a certain vogue as part of a small unit installation, with a fair degree of
success. It is not clear, however, that such speed in drying is required.
(This point will receive further attention in a subsequent chapter.)
"DEVON" TYPE.--In its full original design this type owes its origin to Mr.
H. E. Nixon, General Manager of the Devon Estates, Malacca, where it forms
part of unit divisional installations worked under a scheme of
decentralisation.
The original units consisted of a building erected with an iron framework
covered with sheets of asbestos-slate, and a roof of galvanised iron.
The novelty in design lies in the utilisation of external platforms upon
which the racks of bars supporting the sheets of rubber may be drawn out of
the smoking chambers, and on which the racks are loaded and unloaded. By
this device it is possible to remove the contents of any compartment bodily
without interfering with the continuity of curing in the other
compartments. That is to say, smoking in such a house can proceed day and
night if necessary, and yet the rubber in any part of the house can be
examined, can be removed, or can be replaced without cessation of smoking.
It will be seen from the illustrations that the house is more or less of
the same general design as the "Third Mile" type, with the addition of
external platforms. It has two storeys for the reception of rubber; and a
basal furnace-room with sloping sides converging downwards into a pit
containing a large drum-furnace. This is mounted on a low truck, and
travels on a short length of railway.
[Illustration: GENERAL VIEW OF DOUBLE "DEVON" TYPE OF SMOKE-HOUSE.
The platforms are common to both units. Building of brick with iron roof
(Batu Caves Estate).]
Each of the curing-rooms is divided into four compartments (making eight
compartments in all). These are closed by swing doors, each of which is the
full width of a compartment, and has a slight overlapping edge. Through
these doors light railways run into the house and out upon the platforms.
On the rails "bays" of racks run, and when fully loaded they are easily
moved. The racks were designed with a frame of stout hard wood, but light
angle-iron could be utilised.
[Illustration: GENERAL VIEW OF DOUBLE "DEVON" SMOKE-HOUSE AND FACTORY
BUILDINGS.
Timber in foreground cut to length for stoking. Note water-tower and engine
cooling-tanks adjacent to factory.]
The chimney style of smoke-vent has an internal butterfly flap, which is
controlled by means of a wire from the outside. In the ordinary course of
smoke-curing, it is advised that this flap should be permanently open so as
to reduce the possibility of internal condensation of moisture and
creosotic matter. The exact degree to which it should be open must be found
by experience.
[Illustration: VIEW OF PLATFORM OF "DEVON" SMOKE-HOUSE; DOORS OF
COMPARTMENTS OPEN, AND ONE RACK PARTIALLY WITHDRAWN.
Note below each rack opening through which smoke rises, covered with wire
netting.]
Although reference has been made several times to compartments, it should
be understood that the chambers are not subdivided internally by means of
partitions. There exists only the external effect of compartments in the
form of the eight swinging doors which allow for the withdrawal of, or
insertion of, any one unit of racks at any time without interference with
the bulk of the rubber.
[Illustration: DOUBLE "DEVON" SMOKE-HOUSE OF BRICK, WITH ROOF OF CHINESE
TILES, SHOWING LOADING PLATFORMS WITH RACKS WITHDRAWN FROM SMOKING
CHAMBERS.
Federated Engineering Co., Ltd., Kuala Lumpur.]
[Illustration: SIDE VIEW OF PRECEDING PHOTOGRAPH, SHOWING EXTERNAL
ARRANGEMENT FOR STOKING FURNACES.
Federated Engineering Co., Ltd., Kuala Lumpur.]
DETAILED DESCRIPTION.--As enquiries are often received it is permissible to
reproduce the following detailed description of the original house. This
appeared in the Fourth Local Report (Malaya) 1916, issued to subscribers by
the Rubber Growers' Association.
"The house has a steel frame-work, 22 feet long, 16 feet wide, and 22
feet high. Of the length, 14 feet is occupied by the platforms, and 8
feet by the chambers. These measurements can be varied. The whole of
the width (16 feet) is occupied by compartments of which one series is
placed above the other.
[Illustration: FRONT VIEW OF DOUBLE "DEVON" TYPE OF SMOKE-HOUSE.
Glenmarie Estate: Batu Tiga Co.]
"_Platforms._--The loading verandahs or platforms are of ordinary
'seriah' timber.
"_Compartments and Furnace Chamber._--These are enclosed with Bell's
'Poilite' sheets, each of which measures 8 feet by 4 feet by 3/16
inch. The sheets are affixed to the steel stanchions, doors, etc., by
galvanised bolts (1 inch by 1/4 inch) which pass through iron flats
measuring 8 feet by 2 inches by 3/8 inch (about). These iron flats
hold the sheets at the edges. The dimensions of the compartments are 8
feet by 8 feet by 4 feet.
"_Racks._--These are eight in number, and measure just under 8 feet by
8 feet by 4 feet. The capacity of each is roughly about 450 lbs., of
dry sheet rubber. The racks are mounted on 6-inch iron wheels, running
on rails of stock size, 'T' iron (1-3/4 inches by 1-3/4 inches by 1/4
inch).
"The sheets are hung on split bamboos. To prevent these projecting
over the edge of the rack and catching in the doors when the rack is
moved in or out, a thin strip of wood, about 1/2 inch high, is nailed
along the sides of the rack.
[Illustration: SIDE VIEW OF DOUBLE "DEVON" TYPE OF SMOKE-HOUSE.
Building constructed of galvanised iron. Shows door to furnace
chamber, and ventilator.]
"_Furnace._--This is of the type that aims at slow combustion. It
consists of a cast-iron cylinder, 3 feet in diameter and 4 feet high,
carried on a truck made of a sheet of boiler-plate, and mounted on
small wheels, so that the whole can be moved easily out of, and into,
the furnace chamber for easy cleaning and stoking.
"The furnace chamber is a pit lined with concrete, just wide enough
to take the trolley, and about 12 feet long. The top of the furnace,
which is almost flush with the ground-level, consists of a sheet of
zinc or galvanised iron with numerous holes about 3 inches in
diameter. Over these holes are strips of mosquito gauze, as flame and
dust arresters (see note below). There are no holes in the sides or
bottom of the cylinder.
"Over the furnace is hung a baffle-plate, measuring 4 feet by 4 feet.
Above this, on the first floor-level, the bottom of the compartments
is covered with wire netting, to prevent any rubber dropping
accidentally into the furnace chamber. The furnace chamber is fitted
with an iron-frame door, swinging on perpendicular hinges.
"_Method of Stoking._--The timber used is a mixture of jungle wood and
rubber-tree wood, cut to lengths of about 1-1/2 feet. In the ordinary
way the furnace is charged at 6 p.m., and at six-hour intervals a
little more fuel is added, but a new charge is not necessary. During
the daytime, when the heat of the sun is sufficient to raise the
temperature appreciably, a smaller fire is maintained.
"_Temperature._--There is no difficulty in maintaining a temperature
of 120° F. By continuous smoking, average sheets prepared from
standardised latex can be fully cured in five days. This represents
110 hours of smoke-curing, which is at least equal to ten days'
intermittent smoking in an ordinary house.
"_Capacity._--There are eight racks, each accommodating 450 lbs. of
standardised sheet rubber. The loading capacity of the house,
therefore, is 3,600 lbs. As each charge is cured in five days, the
monthly output may be 21,000 lbs.
"The cubic capacity is 2,048 cubic feet. As there are no gangways,
etc., this is fully utilised. This gives a rate of monthly output
capacity to over 10-1/4 lbs. dry rubber per cubic foot of drying
space; an excellent figure much in advance of values obtained in the
great majority of ordinary smoke-houses."[18]
[18] Since the above was written, it has been found possible to eliminate
the gauze. A mild steel top has been made, perforated with 2-inch holes.
Practically no dust is ejected from the furnace, and there are no flames.
Owing to shortage of supplies during the War, similar buildings have been
erected with frames of well-seasoned hard wood, which was protected by
strips of asbestos-slate or galvanised iron. The latter material was also
substituted in the covering walls. Later, houses were erected of brick,
with other minor modifications. As a natural development, the latest
buildings consist of two of the original houses face to face, under a
common roof, and served by common platforms.
As originally designed, the house was intended to meet the needs of a small
estate, or a division of an estate, having a maximum output of about 20,000
lbs. of sheet rubber per month. The possibility of an extension of this
idea has been shown to be great.
The furnace has been described as situated in a pit. Situated on a bank or
on sloping ground, it was easy to arrange for withdrawal of the furnace. In
some cases this has not been possible, and various modifications have been
effected. The most satisfactory yet encountered is that in which a shallow
brick pit is surmounted by an iron cone, about 4 feet in height. This is
fitted with a cap having small perforations. The fire burns in the pit, and
the heat is radiated by the cone. It would have been more effective to have
allowed greater height in the furnace chamber, and to have employed the
travelling drum-furnace as in the original design.
In order to avoid interference in draught by a space between the bottom of
the doors of the compartments and the platform (due to the presence of
rails), the floor of the platform is laid level with the top of the rails;
or to the bottom of the doors is attached a swinging flap, notched for
accommodating the rails when in position.
* * * * *
There are in use houses of other designs, which all more or less vary only
in some modifications from the types described. Hence they do not call for
special comment. Recently a rather distinct departure has been noted in a
structure designated the "Barker" smoke-house.
BARKER PATENT.--In essential this consists of a long narrow structure
erected with an appreciable slope from one end to the other. At the lower
end is a small furnace enclosed in a brick compartment. The smoke from this
furnace travels up the slope to the other end, at which the rubber enters.
The sheets are hung on bars which are attached to a unit framework. This
frame slides, by its own weight, upon timber side supports. A sufficient
number of these units occupies the full effective length of the structure.
The removal of "stops" at the lower end enables the foremost frame to be
removed, and the succeeding frames slide into a new position. Thus the
freshly prepared sheets, entering at the higher end, gradually and
automatically move towards the furnace as the frames of dry rubber are
removed from the lower end.
[Illustration: THE NEW "BARKER" TYPE OF SMOKE-HOUSE: A SMALL UNIT.
The racks slide automatically from top to bottom on withdrawal of the lower
frames through door at front. The furnace is contained in the brick
compartment at the lower (front) end.]
Thus far only small units have been seen. It is claimed that, properly
prepared, sheet rubber can be smoke-cured in about five or six days, and it
is stated that installations have been in successful working for
sufficiently long periods to prove their efficacy. The device is better
known in Java and Sumatra than in Malaya. The capacity of a unit building
is stated to be 7,000 lbs. per month, calculating on a six days' cycle of
working. In a more recent design provision is made at the lower end for a
water tank, into which all rubber can be discharged in case of fire.
CHAPTER XVI
_OTHER BUILDINGS (continued) AND SITUATION OF BUILDINGS_
SORTING-ROOM AND PACKING-ROOM.--It is in these departments that most
factory installations are lacking. More often than one cares to
acknowledge, sorting and packing are done under conditions which place a
premium upon poor work. As a consequence, consignments of rubber are often
marred by the inclusion of defective specimens. The result is that
shipments may be rejected when tendered against contracts, or that
allowances in price have to be made. In many instances it would not be fair
to lay the blame upon the manager or an assistant, as it is obviously
impossible for an individual to inspect every piece of rubber. Neither
would it be strictly fair in some cases to ascribe the fault to pure
carelessness on the part of the coolies.
Often the only provision made for this important work is the lower room of
a drying-shed, which may also contain hanging rubber. Under these
circumstances, space is cramped, and the light often poor. Small defects
may pass unnoticed, and the general surroundings do not conduce to keen
work.
Where, for economic reasons, the sorting and packing operations are
conducted in the drying-shed, there should be ample space free from hanging
rubber, and it should not be possible for wet rubber placed in the upper
room to drip upon the dry rubber below or upon packed cases. There should
be plenty of light, and for this reason windows should be ample. Usually
the window-frames are fitted with wooden shutters, which are preferably
hung on horizontal hinges from the top of the frame. By this device it is
not necessary to close all windows during a shower of rain, and rubber may
be stacked near a window with reasonable chance that direct sunlight will
not be allowed to fall upon it.
In dealing with smoked sheet, it is advised that the rubber to be examined
should be placed upon tables facing the windows, so that each piece may be
scrutinised in a strong light.
Crepe rubber also is best examined in a strong light, but preferably with
one's back towards the source of light or at an angle to it. For this work
coolies usually are most efficient when sitting on the floor.
It will be clear from the foregoing remarks that the best conditions would
be secured in a separate building especially constructed. A single room
would be all that is required; at one end sorting could be undertaken,
while packing could be done at the other end. No hanging rubber should be
allowed in the room.
The floor should be of hard timber, and raised from the ground, to the
height approximately of a bullock-cart or motor-lorry, as the case may be.
The boxes of rubber could thus be transported by small hand-trucks on a
level with the transport vehicle, reducing labour to the minimum.
The ventilation of the building should be good, especially if cases of
rubber are to be stored therein; and the entire structure should be
weather-proof.
STORE-ROOMS FOR RUBBER AND STORAGE.--The question of storage of rubber in
factory buildings has always possessed importance, but has demanded
increased consideration recently.
From experience in this country, it is clear that cement floors for
store-rooms or packing-sheds are the least suitable. They are often visibly
damp, especially in the early morning. To allow rubber, packed or unpacked,
to remain upon a cement floor in the tropics, is to court trouble from
moulds, external or internal. If the employment of a cement floor is
unavoidable, the rubber and boxes should be raised on wooden supports,
giving a clearance of at least 3 or 4 inches, and there should be clear
ventilation space between tiers of boxes.
Experience indicates that the best type of floor is that already advised
for sorting and packing rooms--_i.e._, a good hard timber floor raised at
least 3 feet above ground-level. Apart from the advantage in labour
specified in the previous paragraphs, this provision of ample ventilation
space below the floor is a great consideration in the preservation of the
timber. Raised store-rooms become essential in low-lying districts which
are at all subject to flooding, yet the writer has seen many boxes of
rubber damaged by flood-water entering a packing-room situated on the
level.
The question has often been raised recently as to the length of the period
during which rubber may be safely stored in this country. The answer can be
only supplied by experience, of which up to the present we have none
possible of being classed as reliable. Whatever storage may have been done
in the past has been influenced greatly by the unsuitability of the storage
accommodation, and the fact that often the rubber was not prepared with a
view to prolonged storage.
While the market demand was strong, rubber was being shipped and passed
into circulation, at a rate which did not demand investigation of the
subject of local storage. In the year 1918 conditions were such as to bring
the matter into prominence, and we were able to tender advice on the lines
given in this chapter. The necessity passed, but has again arisen.
Our experience goes to prove that if rubber is properly prepared and
thoroughly dried before packing, it will remain in good condition for a
period of a year or more in this country. How much beyond a year it may be
kept remains to be determined. The assumption of "proper preparation"
leaves great room for reservations.
In the case of crepe rubbers, there is no great difficulty, provided that
the recognised methods and formulæ are employed, and that the rubber is
packed only when perfectly dry. Under those conditions, the higher grades
of crepe remain apparently unaffected on storing. Any appreciable
deterioration may be attributed to defective preparation or external
causes, such as accidental damage by water.
The prolonged storage of lower grade rubbers is attended by more risk,
especially in the case of the lowest grade (earth-scrap) from estates which
neglect the practice of regular and frequent collection of the raw product.
The same reservation applies to crepes made from tree-scrap which is not
collected daily. In these types of crepe rubber "tackiness" may be
initially present only in small degree, but the final damage may be
immensely greater by close contact of the folded rubber during prolonged
storage.
When we come to discuss the possibility of storage of smoked sheets, the
difficulties become immensely greater. We have yet no reliable experience
as to the keeping properties of this grade when properly prepared, fully
cured, correctly packed, and stored under the best of local conditions. It
is understood, of course, that in the qualification by the term "local"
conditions, we assume it to be more difficult to store rubber generally in
Malaya than in a temperate climate. The average temperature and humidity of
the atmosphere are here much more favourable to the development of mould
growths than would be the case, say, in Great Britain.
In discussing this question, as far as it refers to the preservation by
storing of smoked sheet rubber, it is not fair to draw conclusions as to
the likely behaviour of packed rubber from data based upon observation of
loose specimens. We have samples of smoked sheets prepared in 1910, and
these, superficially, appear to have remained unchanged. No mould is
present and, as far as intermittent observation enables us to judge, moulds
have never been incident. Whether such rubber would have been preserved in
this condition had it formed part of a packed case, is a point upon which
we have no experience; neither can we give any opinion. It seems true,
however, that loose specimens "keep" better than bulk samples of the same
preparation.
It cannot be argued that the present good condition of these old specimens
may be due to correct preparation. In those days methods and formulæ were
rather haphazard, especially in view of the fact that the daily variability
of dry rubber content of latices was not then recognised.
One would rather submit the factor of adequate smoke-curing as the chief
influence in the superficial preservation of smoked sheets. Ten or eleven
years ago it was considered advisable to allow the rubber to remain in the
smoke-house for a period extending well beyond that necessary for ordinary
drying. As a result, very dark rubber was produced, which was thoroughly
impregnated with the products of wood combustion. There would seem to be
little doubt that this procedure was responsible for the prolonged freedom
from mould growths.
Market standards have varied to some degree since, with a tendency to
prefer a paler product than that in vogue, say, six or seven years ago.
Moreover, standardised methods of preparation have been introduced, with
the result that sheets of a desirably high standard can be produced in from
ten to fourteen days, when smoke-curing is conducted only during night
hours. Some estates are equipped with smoke-houses which, by continuity of
working day and night, provide smoke-dried rubber in from five to six days;
but the actual hours of smoke-curing are approximately equal to those of
the ordinary type of house.
This tendency towards the production of sheets paler in colour than the old
standard is probably largely responsible in the present for the commonly
observed incidence of surface moulds on stored smoked sheets, and also for
some complaints of "under-curing," where the term specifically refers to a
failure to dry and cure the rubber thoroughly. Boxes of smoked sheets,
which had been stored for varying periods up to five months, were recently
inspected, and, in the majority of instances, surface moulds were found to
be plentiful. In all cases it was observed that the trouble was intensified
where boxes of rubber were stored in contact with cement floors.
This "under-curing" is not a question solely of the duration of
smoke-drying, although probably the modern practice of curtailing the
period has exerted a great influence. To make this clear, it may be stated
that, given two batches of uniformly prepared wet sheets, it would be
possible to smoke-cure them for equal periods in different houses, so as to
produce one batch very much paler in colour than the other, although the
total hours of actual smoke-curing would be identical. In order to produce
such effects, all that is necessary is to employ different timbers for fuel
or different types of furnaces. In the one case there would be produced
heat and very little smoke, while in the other the necessary heat would be
obtained plus plenty of smoke. The best results naturally are obtained by
the employment of the happy medium, and if smoked sheets have to be stored,
the ordinary period of smoke-curing should be prolonged to an interval
consistent with the capacity of the smoke-house.
All precautions taken in preparation and curing can be nullified, as
already indicated, by unsuitable storage conditions.
TOOL-SHEDS AND STORE-ROOMS.--In some factories it is the rule to see lime,
cement, spare rolls, sieves, and a general heterogeneous assortment
occupying part of the rubber-drying rooms. The inconvenience is often
great; and it certainly seems that these stores and tools are of sufficient
value to be accommodated in suitable buildings.
SITUATION OF FACTORY BUILDINGS.--There can be no doubt that a great deal of
the "spot" disease trouble, and the general slowness of drying, can be
attributed in many factories to the unsuitability of the site chosen.
Probably the idea which actuated those responsible for the choice of site
generally was proximity to a water supply. This would account for the fact
that a number of factories are situated in valleys or near swamps. More
often than not, also, the actual clear space is very limited, and rubber
trees grow close up to the walls of the buildings. Under such
circumstances, it is difficult to see how these buildings can be anything
but dark and damp, and it is not difficult to understand the slow rate of
drying. In a few cases the sites chosen proved to be so unsuitable that the
estates were confronted with a very serious problem, the solution to which
was, either the erection of another complete set of buildings in a more
suitable spot or the installation of artificial driers.
It must be laid down as an axiom that the first essential in a suitable
site is that water may be brought to it easily, but, as already indicated,
this does not mean that the buildings need be placed in actual proximity to
the water-supply. The mistakes made by pioneers in this work are not likely
to be repeated, and it is common now to note well-designed and
comprehensive schemes in which the water is pumped to a reservoir placed at
a suitable elevation, whence the supply is gravitated to bungalows, coolie
lines, and the factory. The importance of securing a plentiful supply of
good water for factory purposes cannot be exaggerated, and it is a point
which is only thoroughly appreciated on estates where smoke-sheet rubber
has to be prepared.
The second essential, but of equal importance, is that there shall be an
ample open space on which the sun may shine all day. There must be no trees
too near the buildings, and there should be no adjacent swamps. Preferably,
the site should be on a raised position, so that it will be impossible for
surrounding trees to cut off sunshine, even when they are fully grown. From
such an arrangement it will follow that the factory will be light and airy,
and the drying-houses will receive the maximum of benefit to drying from
direct sunshine on the roof and walls. There can be little doubt that these
considerations play a most important part in determining the rate of drying
of the rubber, and where comparisons are made between the rates of drying
in various drying-houses all these factors enter into the question and
contribute to the total result. Presuming that the thin crepes made in two
factories are equal in thickness, it is not uncommon to find that in a
drying-house, situated in a wide open space, the period of drying may be as
low as six or seven days; while in another drying-house, situated near a
swamp and surrounded by trees, the period may be as high as eighteen days
to twenty-one days. The figures quoted are not fictitious, but are facts
actually noted in the course of the writers' experience.
A great deal also depends upon the exact position of buildings. Thus, to
obtain the maximum of light in a factory, it will be obviously beneficial
to erect it with the long sides running east and west, so that the windows
face the north and south, and the large end doors face the east and west
respectively. At first sight it would appear that the best position for the
machines would be on the north side of the building where no sun can enter;
but a moment's consideration shows that the south side would give the best
results. By the time the sun has come round to the south, it is usually
high in the heavens, and the direct sunshine does not fall very far into
the room. Even should it play upon the machines for an hour or two during
the day, no harm could result to the rubber which was being worked, as no
piece would remain there a sufficiently long time to be injured in the
slightest degree. Placed in this position, the maximum benefit of light
would be obtained, whereas if the length of the building ran east and west,
the machines would have only either the morning or afternoon light.
[Illustration: SUGGESTED ARRANGEMENT OF BUILDING.]
While it is advisable to erect a factory running east and west, the
drying-houses should run north and south. In this position the maximum wall
area will be exposed to the sun during the day, and it will be possible to
manipulate the windows of the drying-rooms so that those along one side are
open, and it will never be necessary to close all the windows at any time
of the day. Thus the windows facing east will be closed, and those facing
west will be open until after midday; then _vice versa_. With such an
arrangement a more uniform temperature may be obtained than by any other
arrangement of the buildings. If the building ran east and west, the
windows on the north side could remain open all day, while those facing
south would have to remain closed practically all day. The south side of
the house would be heated by the sun, while the north side would remain
cool, and the rates of drying would be correspondingly unequal. The total
wall area heated by the sun at any time of the day would be less in this
position than if the house ran north and south.
Similarly, to obtain the best drying effect during the daytime in a
smoke-house the building should run north and south. By this means the
temperature will be maintained to the maximum possible by sun heat, and the
rate of drying will correspond.
_References to Sketch Plan._
Drying-house No. 2 should be of two storeys, and unless a separate sorting
and packing room is to be built, No. 1 should also have two floors (see
previous notes on packing-rooms).
In the factory--
_V_ shows the position of the verandah, which may be quite open and
only divided from the inner room by
_S_, a wall composed of very strong expanded metal, which allows light
and air to enter the factory.
_T,T_ are the glazed tile tanks for the reception of latex, scrap
rubbers, and bark-shavings.
_M_ shows the position of the machines on the south side of the
factory, with the direction of extensions, and
_E,E_ the compartments in which the engines are bedded. In these
positions it is possible to obtain direct drive to the machines.
_D,D_ are large double swing or sliding doors (the latter for
preference always). These, while suiting transport of rubber, provide
also for a free draught of air.
If possible the scrap-washing machine should be placed outside the wall of
the factory, and tanks for the reception of scrap rubbers may then be
situated in convenient proximity.
Economy of labour is obtained by grouping all factory buildings as closely
as possible, but it should be borne in mind that smoke-houses should be
regarded as a possible source of danger from fire. This point has a
practical bearing upon rates of insurance, and it is essential that the
smoke-house should be situated at a minimum of 50 feet from any other
building or group of buildings. In this connection, also, it may be noted,
as being of further practical interest, that, in the insurance of
smoke-houses, preferential rates are given to those having a good type of
slow-combustion furnace.
CHOOSING A FACTORY SITE.--Sufficient has been written to make it clear that
the choice of a site for factory buildings is a matter demanding weighty
consideration. Much, of course, depends upon the planted area, and the rate
at which it comes into bearing. Under certain circumstances which will be
obvious, it is permissible to instal first a group of buildings of a
temporary nature only, the future site and permanent buildings to be chosen
later when the main portion of the estate comes into bearing. Often,
however, one finds that, from lack of forethought, the estate has been
committed to considerable expense in the establishment of equipment, which
later is proved to be unfavourably situated with regard to the majority of
the area in ultimate bearing. In such case, transport of latex is fraught
with difficulties and may be expensive.
In the instance of an estate which will gradually come into bearing, it is
not easy to decide whether a temporary installation shall first be
provided, or whether, in anticipation of future demands, a complete
equipment shall be erected. So much depends upon the financial aspect of
the question, and upon the rate at which areas will come into bearing. As
far as is possible, the best policy would be that of a compromise under
which the site would suit later requirements, and the factory would be so
planned as to be capable of future easy extensions both of buildings and
machinery.
It is not possible to lay down any definite data as regards requirements
based on acreage, or to make comparisons between any two estates of similar
acreage. The important factors determining such requirements are:
(_a_) Area.
(_b_) Shape of the estate.
(_c_) Topography of the estate.
(_d_) Available supplies of water.
Naturally the ideal site for factory buildings would lie in a central
position, given other favourable conditions.
CENTRALISATION OR DECENTRALISATION.--It is the experience of a number of
estates that, all other conditions being favourable, there is a limit
beyond which the centralisation of factory work leads to an unwieldy
position. We are not here concerned with the few extremely large estates
running into tens of thousands of acres. In those cases the total area
would be divided into economic sections. The argument there would resolve
itself into a discussion on the size of an economic section. This, in turn,
would be dependent upon the type of main product, involving the question of
transport of latex or coagulum, and the possible provision of batteries of
heavy machinery.
The differentiation between the transport of latex and coagulum,
respectively, is a most important one, and has a powerful influence in
determination of the maximum of centralisation possible. Whereas properly
prepared coagulum may be safely transported by bullock-cart, light-railway,
or motor vehicle for many miles, latex, on the other hand, demands very
careful treatment. Anti-coagulants may be employed to preserve fluidity,
but only within certain limits. Even under these conditions, other factors
(chiefly climatic) exert an influence which renders the transport of latex
for any distance a matter of anxiety.
It will be plain, therefore, that the limits of centralisation of factory
work are much narrower for the preparation of sheet rubber than is the case
when crepe rubber is to be made. In actual experience the preparation of a
high standard and a high percentage of smoked sheet is attended with
considerable difficulty in those cases where the factory processes have
been ultra-centralised. Apart from the difficulties inherent to the
transport of latex in a state of good preservation, there is the added
difficulty of dealing quickly with large volumes of latex brought from
various quarters. None of these should be allowed to remain standing if the
best results are to be secured; but obviously there must at times be some
congestion. Even on a small scale it is often found that the latest batches
of latex are unfit for the preparation of good sheet rubber, and the
trouble may be easily exaggerated when working on a large scale.
The centralisation of work on crepe preparation, therefore, is limited only
to a comparatively slight degree by distance of transport, and in the main
only by the size of the necessary equipment of machinery and drying
accommodation.
The successful preparation of sheet rubber is, on the contrary, governed
chiefly by the factor of transport. With this consideration in view,
several large estates, preparing sheet rubber as the chief grade, have
found it necessary to decentralise the factory work, with very satisfactory
results. Outlying sections are given uniform and complete equipments of
necessary buildings on a small scale, and hand-driven light machines.
Uniform coagulating tanks are installed, and the methods and quantities of
chemicals employed are carefully standardised. Experience has shown that
often the best sheet rubber coming to the market has been prepared on small
estates; and the same applies to the product of these decentralised
stations on large estates. There is no _a priori_ reason why the product
from one station should differ in the smallest particular from that of
another, apart from minor fluctuations which are due to variable weather
conditions affecting the latex. If the contrary is found to be the case, it
indicates failure on the part of the person responsible to follow the
regular rules and methods.
In the natural scheme of development of a large estate, it would be
necessary, of course, to have a comparatively small centrally situated
factory, equipped with power and heavy machinery for working scrap rubbers
in the preparation of crepe grades below No. 1 in quality. As the yield per
acre increases, or the area in bearing expands, it would be advisable later
to increase the size of the central factory and buildings so as to permit
of the preparation of a proportion of the crop in the form of No. 1 crepe
rubber, in order to be able to comply with prevailing market demands under
which preferential rates fluctuate between pale crepe and smoked sheet.
PART IV
THE FINISHED RUBBER
CHAPTER XVII
_DEFECTS IN CREPE RUBBERS_
GENERAL STYLE OF FINISH.--Broadly, there is no single and definite style of
finish, but on the whole it may be stated that the greater proportion of
crepe rubbers are prepared in a thin form and with a close-knit texture or
finish.
Very little thick or blanket crepe is now made on estates in Malaya, so
that beyond the mention of that type little need be written. A fair amount
of blanket crepe is sold in the Singapore market, but it should generally
be regarded as re-made rubber--_i.e._, it may have been prepared from thin
crepes, or from native pale sheets, in local rubber-washing factories. In
appearance these crepes have a rough finish, and vary in colour according
to the crude material employed.
The general preference of the market at present is for a thin,
smooth-finished crepe, with a close-knitted surface--_i.e._, free from what
is described as "laciness." What effect this looseness of finish can
possibly have upon the quality of the rubber is not understood, but the
standard type set up by the market must be comparatively free from small
holes.
Under existing conditions governing the sale and purchase of rubber,
various "standards" are set up. These really have no bearing upon the
intrinsic qualities of the rubber, and are concerned almost entirely with
superficial attributes. They are necessary in the absence of any proper
scheme of evaluation for the establishment of certain standards of
comparison, which imply that the rubber is apparently clean, free from
certain recognised defects, and has been carefully prepared--as far as can
be determined by a superficial examination. Thus the question of "finish"
has attained disproportionate importance, but must be respected when
preparing rubber for sale.
Under ordinary conditions, thin crepe rubber, as it leaves the finishing
machines, has what may be termed "deckled" edges. On many estates, in order
to comply with market conditions, the edges of the wet crepe are trimmed,
and the trimmings re-made into lengths of crepe. This is done under the
impression that the market price is influenced by the evenness of the edges
of crepe rubbers.
Again, it sometimes happens that, owing to "wear" of the rolls, the
finished dry crepe may show a faint but distinct pattern of mark--a diamond
or a horizontal bar. Since these are not accepted under the "standard"
comparisons, rubber exhibiting these characteristics does not obtain the
top market price. In other words, these innocent and innocuous marks are
regarded as defects and penalised accordingly.
Enough has been written to show how very important becomes the question of
finish. It will be acknowledged that the superficial qualities demanded in
the "standard" market type can be reproduced by any estate having adequate
machinery and ample facilities for drying and handling the rubber.
Methods of preparation and formulæ for the employment of chemicals are so
well laid down that, up to the stage of machining, no difficulty need be
encountered. But the standard type of finish in the dry crepes cannot be
obtained unless the estate factory is fully equipped with the three types
of rolls necessary--_i.e._, macerators, intermediate crepers, and smooth
finishing-rolls. This subject has received full discussion in Chapter
XIII., and is here only mentioned with the view of emphasising the point
that no estate can be blamed for a lack of "finish" in crepe rubbers if the
equipment of machinery is inadequate or in poor condition.
If, on the other hand, the factory has ample machinery for requirements,
and a good finish cannot be obtained on the thin crepe, then it is time the
rolls were attended to and changed, or that the ratios of the driving
pinions were altered.
DIRTY EDGES.--It seems to be almost impossible to keep old machines clean,
and it is equally difficult to keep the edges of crepe free from oil and
dirt. Usually these dark edges are to be found on crepe which is rather
wide, and it will be noticed that where wide crepe is made, unless special
precautions are taken, the edges of the rubber often pass under the edges
of the hopper and so pick up dirt and oil. On most machines it is a great
mistake to attempt the preparation of wide crepe; nothing but narrow crepe
must be made. To obtain this it is necessary to decrease the width of the
hopper placed above the rolls. This can easily be effected by blocks of
heavy hard wood, cut to shape and fastened in position.
Sometimes the dark edges of crepe are due to another cause. Rolls may be
gradually worn in the middle, so that to obtain a good finish it becomes
increasingly necessary to tighten up the screws which regulate the distance
between the rolls. It thus happens that just at, and beyond, the edges of
the rubber the rolls grind upon each other, and fine particles of iron and
graphite are transferred to the rubber. In such a case it is evident that
either the rolls must be "turned" or that a new pair of rolls must be
substituted.
IRON-STAINS.--One of the causes of iron-stain on rubber has been mentioned
in the preceding paragraph. This particular kind of iron-stain must not be
confounded with rust-stain, and gives a dark dirty colour. It results from
the grinding together of the rolls, and is usually noticed in the finishing
of fine pale crepe. For this operation it is necessary to screw up the
rolls tightly, and it will be plain that, whenever the rolls are vacant of
rubber, there is a tendency for them to grind upon each other, thus setting
free fine particles of iron and graphite. In order to avoid this, one must
be careful to see that between the working of each length of fine crepe the
rolls should be occupied with another piece of rubber, which may be kept
for the purpose. In some factories this trouble apparently does not exist,
while in others the amount of wear on the rolls is surprisingly great, and
the damage done to the rubber is excessive. The only way in which this
difference can be accounted for is that there must be a great difference
in the quality of the roll material. Some rolls seem to be excessively
soft, and from these contamination by iron-stain is great. For this reason
rolls are sometimes rejected, and there would appear to be an objection to
any but chilled steel rolls for the final stage of finishing crepe rubbers.
RUST-STAINS.--Rust-stains, on the other hand, throw the responsibility
entirely upon the labour and supervision of the factory. Rust is formed
upon the rolls when they are at rest, and any one passing pale rubber
between the rolls before they have been thoroughly cleaned is guilty of
culpable negligence. Even when apparently clean, a piece of lower grade
rubber should be passed through the rolls several times so as to remove any
slight trace of rust remaining.
Rust-stains have also been caused in a few cases by the large knives which
are used to cut up lumps of coagulum, or by allowing freshly coagulated
rubber to come into contact with iron vessels in the factory.
A similar appearance has been traced in a few instances to contamination of
the coagulum in transit by the dust of the reddish rock (laterite) employed
in localities for road-making.
OIL-MARKS.--The origin of oil-marks in crepe has already been described in
Chapter XIII. The whole question resolves itself into one of cleanliness,
moderation in lubrication, and supervision. The machines should be
inspected every day, and once a week rolls may be swabbed down with a 10
per cent. solution of caustic soda applied by means of a piece of cloth
fastened round the end of a stick. Immediately after this operation water
should be turned on and the rolls set in motion, so that all traces of
caustic soda are thoroughly removed. If possible, lubrication by oil should
be substituted by grease lubrication through screw caps.
Particular attention should be paid to the back of the machines. None but
the individual in charge of engines should be allowed to lubricate the
machines, and he should be held responsible for any excess of lubricant.
As a rule oil-marks are restricted to the edges or the proximity of the
edges of crepe, but sometimes the streak is to be found in the middle of
the length. In such a case it is almost certain that the oil or grease has
been picked up by the rubber in the tray. It sometimes happens, if the
"liners" of the bearings are eccentrically worn, that a few drops of dirty
oil or a particle of grease are squirted out to some distance. These
usually find a resting-place in the tray, and the contamination may then
appear in any part of the rubber.
It will be clear, therefore, that all trays beneath machines should be
examined as the probable source of danger from contamination by oil and
dirt. If the trays are as wide as or wider than the effective portion of
the rolls, they should be discarded. In their place (except sometimes in
the case of the macerating machine) all that is necessary is a movable
piece of board, in width not less than from 4 to 6 inches shorter than the
width of the rolls. Any oil or grease ejected from the bearings will thus
be allowed to fall clear of the board; and defects due to oil streaks,
etc., will be very much diminished, if not entirely obviated.
This point in connection with the damage possible by the existence of wide
trays is commended to the notice of manufacturers of machines for
plantations, as it is common to find that trays are made which contravene
the rule prescribed by experience. In fact, trays on some machines have
been so designed as to act as "traps" for all dirty matter exuding from the
bearings. Not only so; they are sometimes made of such a shape and height
that oil or grease lodging upon the edges act as a "wipe" to the rolls,
thus increasing the possibility of contamination. Until this defect was
investigated, it was common to note continued contamination of pale crepes
in spite of all precautions taken in cleaning the rolls at frequent
intervals. The trouble due to this cause is intensified when the same
machines are employed for the preparation of scrap-rubber crepes and No. 1
crepe. Small pieces of scrap find their way towards the bearings and lodge
on the edges of the trays. Unless a thorough inspection is made before
proceeding with the working of the No. 1 (pale) grade, contamination may be
continuous.
DIRT.--Streaks due to the presence of dirt (as apart from oil or grease
contamination, or that due to pieces of oxidised scrap) are unusual, and
when they do appear their origin seems to be somewhat of a mystery. It
could scarcely be advanced that the dirt was picked up on the machines, as
it is difficult to imagine where such dirt could come from. In one or two
instances there has been fairly clear evidence that the dirt was contained
in the coagulum, and the only explanation fitting the case is that it fell
into the latex after straining and during the course of coagulation. On
cutting open lumps of coagulum brought in from the field division, it has
sometimes been noticed that dirt is included, and the foregoing explanation
is the only reasonable one. How it was possible for dirt to get into the
latex must be left for explanation to those better acquainted with the
conditions under which the latex was coagulated.
HOLES.--On some estates it would seem impossible, with the existing
machines, to make really good crepe. The complaint is that, if thin crepe
is attempted, it is invariably found to be "full of holes"; and as,
apparently, the presence or absence of small holes in crepe rubber is a
factor which influences buyers, this defect must be avoided at all costs.
Why this matter of small holes in thin crepe should weigh so heavily with
buyers is a matter which the writers are not in a position to explain. As a
matter of fact, the presence of small holes is most generally an indication
that the rubber has received the minimum amount of working on the rolls
consistent with good washing. Further working would only be undertaken with
the idea of so consolidating the rubber as to get rid of holes in order to
meet the market scheme of valuation.
This is usually achieved by making a very thin crepe and rolling together
two lengths when wet. The resulting crepe may be slightly thicker than
ordinary, and the method employed may be usually detected by the appearance
of the edges unless these are trimmed.
GREENISH AND TACKY STREAKS.--Occasionally one meets cases in which pale
crepe exhibits streaks varying in colour from a decided green to an almost
black in which the greenish tinge is scarcely perceptible. Experience
indicates that these streaks are much more dangerous than they appear
superficially, inasmuch as they contain traces of brass from the "liners"
of the bearings. The presence of the copper in brass is responsible for a
gradual disintegration of the rubber, commonly recognised as "tackiness."
In fact, copper may be said to be a "poison" to rubber, and every effort
should be made to avoid possible sources of contamination. The effect may
be proved easily and perceptibly by fastening together several pieces of
crepe rubber by means of a brass "paper-fastener." In course of time a salt
of copper, green in colour, will be formed, and it will be found that the
portions of rubber in contact with the fastener have "perished" and become
tacky.
This contamination of crepe rubber may take place in two ways:
(1) By the ejection of actual particles of brass from the bearings of
machines, due to eccentric grinding of the "standards" of the rolls
upon the brass "bushes." These particles are carried by exuded oil or
grease into trays, and thence to the rubber.
(2) By the action of an acid lubricant upon the brass, with the
formation of a metallic soap which has a decided green colour, unless
obscured by the dark colour of the oil or grease. It is transferred to
the crepe rubber in the manner indicated above.
The inevitable effect, apart from the superficial defect, is incipient
tackiness. The extent to which this may develop will depend upon the amount
of the copper compound present, but it should be remembered that an
exceedingly small trace is capable of causing a disproportionately large
amount of damage. This effect is further magnified if the "tacky" piece of
rubber is packed in close contact with previously unaffected rubber.
When the defect is discovered, the affected portions should be cut out, and
the cuttings should be burned. To mix them with the lowest scrap grades, as
may be done thoughtlessly, is only inviting further trouble.
Besides the source of danger already indicated, it may be found, but far
less frequently, that contamination may arise from the presence in the
rubber of small pieces of the brass mesh which is generally used for
straining latex.
The view appears to be held in some quarters that these tacky streaks and
patches in crepe rubber may arise from contamination with oil or grease
alone. This does not agree with our experience. An experiment was made to
test the point using fresh oil and grease drawn from drums in stock,
specimens of the same lubricants to which traces of a copper salt were
added, and samples of lubricants taken from the bearings of several
machines. The treated pieces of rubber were placed in contact with
untreated pieces of crepe which served as "blanks."
Notes were made at intervals extending over a period of two years. The
conclusions arrived at were:
(1) Although there was surface discoloration, no tackiness had been
caused by fresh (unadulterated) lubricant; neither were the "blanks"
affected.
(2) In the majority of specimens upon which had been smeared a small
streak of lubricant taken from the bearings of machines, tackiness had
supervened, and had developed likewise in the contact "blanks."
(3) In all cases where a trace of copper salt had been used to
adulterate the fresh lubricant, tackiness was to be noted in the
course of a short period (a week upwards) after the rubber was dry.
Development was slow, but progressive, over the full period of
experiment, and the "blanks" in contact were affected. The degree of
affection was determined by the proportion of copper salt employed. In
the worst cases the affected strip of rubber had deteriorated and
disintegrated to such a degree as to cause a distinct longitudinal
gap, the edges of which appeared to consist of a moist gummy substance
of a deep syrup colour. The adjacent blanks in some cases exhibited a
similar appearance in lesser degree, or were merely affected by a
characteristic brownish stain.
These observations regarding the possibility of damage to crepe rubbers
from the existence of brass "liners" or "bushes" in the bearings of the
machines lead to the natural query as to whether the use of brass is
necessary. Experience shows that it is not necessary. Machines in use for
years have been running with plain bearings of iron or other metallic
substances. Satisfaction is obtained without the use of brass.
COTTON AND OTHER FIBRE.--One of the most frequent complaints made against
low grade crepes is the presence of fibre--generally classed in a wholesale
fashion as "cotton-waste."
It is true that some years ago most of the complaints were genuine in
referring the cause to cotton-waste. The defect arose chiefly owing to the
careless use of this material in the factory. Lumps of waste when discarded
were often thrown to the ground, and became mixed with the heaps of scrap
rubber and bark-shavings awaiting attention. The fault was one of sheer
negligence, and nothing can be advanced in extenuation. Even when the
soiled waste was thrown into the external drains, it often returned to the
factory mixed up with the scraps of rubber recovered by means of the
drain-screens.
As far as the complaint concerns itself with cotton-waste only, the remedy
is plain, and lies in the power of the management by reason of the ability
to restrict the use of "waste" only to the engine-drivers and mechanics.
In the vast majority of cases, however, the defect arises from
circumstances beyond the direct control of the factory, and under
conditions which make it difficult to check the evil. Although against
instructions, and for the purpose of fulfilling other orders, some coolies
persist in using pieces of cloth for cleaning cups. In course of time,
unless the practice is detected, this cloth becomes coated with rubber.
Careless coolies throw it away, when it may be collected by the individuals
who gather earth-scrap; or it may be brought into the factory in the
tappers' scrap-bag.
Cases have been known in which the fibrous matter observed in the dry crepe
rubber was of such a nature as to indicate that the source might be
attributed to leaf-stalks which had passed through the scrap-washer. It is
an easy matter to condemn the sorting as being careless, but it is another
matter to instil into the mind of factory coolies such a respect for easy
and sane precautions that the practice of them will be continued when the
eye of the supervisor is not fixed upon the workers.
It will be clear that contamination by fibrous matter should be limited
practically to the lowest grades of rubber.
The appearance of cotton-waste in high-grade crepes must be most unusual,
and the writers have not yet seen a case in a drying-house. That it does
occur, however, seems to be evident from brokers' reports. It is extremely
difficult to imagine how the waste enters the rubber. One possible
explanation is that a coolie may have been cleaning the rolls
surreptitiously with waste, which may have passed later into the rolls
together with rubber. Another explanation was offered in one factory by the
observed fact that coolies engaged in cutting up coagulum, ready for
passing into the machines, kept a wad of waste for the purpose of keeping
the knife-blade clean. This may have found its way into the rolls. It must
be recorded that in the course of many years of experience no case has been
seen in any drying-house of contamination of the higher grades of crepe by
fibrous matter.
BARK AND GRIT.--With ordinary machines and the usual process of working, it
would seem impossible to wash and macerate some of the scrap rubbers
sufficiently to free them entirely from bark. This applies specially to the
grade of rubber prepared from bark-shavings. Specimens have been handled in
which it was practically impossible to detect bark, but in such instances
the amount of working necessary would be such as to interfere seriously
with the regular working of the factory. Even with the employment of
special scrap-washing machines, complaints of the presence of bark in dry
crepe have been received, but it is certain that this mode of operation
reduces the quantity of bark to a minimum. While fully realising that the
amount of working it is possible to give in proportion to the existing
machinery and the output per day is limited, it must be recognised that the
working of lower grades of rubber is usually insufficient, and that where
possible it is the duty of estates to pay more attention to these lower
grades. A considerable improvement in this direction has been noticed of
recent years. It is not uncommon to encounter managers who fail to
appreciate that complaints regarding the presence of bark in the lower
grades are founded on legitimate grounds, and that they are not frivolous
objections put forward for the purpose of depressing the price of the
article. The sooner such an idea is jettisoned the better. There would
appear to be a good future demand for the lower grades, and it is only
natural that consumers will be willing to pay the best price only for an
article which is clean.
The same arguments apply to the complaints regarding the presence of sand
and grit. The quantity of the latter found in low-grade crepes from some
estates is surprisingly high. Its presence can often be shown by the simple
device of spreading a piece of crepe over the upturned and hollowed palm of
one hand, while striking the rubber with the other hand.
The incidence of bark in higher grades of crepe may be due to inadvertence
or to gross negligence. In the former class one might put those occasions
on which pieces of bark are embedded in lumps of naturally coagulated
rubber. A piece of bark-shaving may fall unnoticed into latex and be
partially responsible for the coagulation which takes place. This piece of
coagulated lump may be massed with others, and hence, unless each small
piece is cut up, the bark is not perceived. Or again, by some unknown
means, a piece of shaving may drop into a jar of latex, and so become
embedded in the coagulum. Sometimes this becomes evident on cutting up the
rubber, but it is quite as likely to pass unseen. On the whole, the
presence of bark in first-grade rubber is most unusual, and should be seen
before the rubber is packed.
In the class due to negligence may be included cases in which careless
coolies place the cup upon the ground before tapping. Pieces of shavings
fall into the cup, and coolies are too lazy to pick them out. More often
than not coagulation in the cup is caused. As it is impossible for the
European staff to supervise each individual tree tapped, some cases must
continue to pass unheeded. Sometimes bark-shavings are brought in with the
latex, and if a broken sieve is being used, these, with other impurities,
pass into the jar, and are embedded in the coagulated rubber. This must be
classified as negligence, for no manager would willingly allow the use of a
broken sieve. Again, naturally coagulated lump rubber on arrival at the
factory sometimes contains evident pieces of bark, leaves, and stems of
leaves. For lack of supervision the average coolie would not think of
picking out these obvious impurities, and would pass the whole mass into
the machines.
OXIDATION STREAKS.--Since the introduction of sodium bisulphite defects due
to streaks, caused by portions of the coagulum becoming oxidised, have
practically ceased to exist. In the usual course, and without the use of an
antiseptic agent, the freshly coagulated rubber has a surface darkened by
oxidation. Unless this dark surface were carefully cut off, there would
result a crepe containing dark streaks caused by the mixture of the
oxidised surface portion with the bulk of the paler coagulum. The presence
of oxidation streaks in No. 1 crepes, now being made, would imply either
that no anti-oxidant substance was in use, or that the quantity necessary
to prevent this surface oxidation is exceedingly small. Although the price
obtained would appear to be influenced by the presence of oxidation
streaks, no evidence can be obtained that the actual quality of the rubber
suffers to the same degree as does the appearance--_i.e._, there is no
evidence to show that a pale rubber, in which surface oxidation has been
inhibited, is intrinsically superior to one in which slight natural
oxidation has been incident.
"YELLOW LATEX" STREAKS.--This appearance of "yellow-latex" streaks in not
common, and may be accounted for by incomplete mixture of two different
latices. It is a fact of common observation that, when a new portion of
bark is being tapped for the first time, there is a distinct yellow tinge
in the latex excluded. As tapping progresses, this colour vanishes; usually
it may persist for a period varying from two weeks to more than a month.
Should this latex be poured into ordinary latex without thorough mixing, it
is sometimes found that, when the crepe rubber is dry, there are distinct
yellow streaks. It should be remembered that, as the rubber content of the
latex from first tappings is high, this latex is lighter than latex which
is more dilute, so that the mixed latices must be well stirred with a broad
paddle to obtain intimate mixture. It would be much better to keep yellow
latex apart, and coagulate it separately, if at all possible. In such case
the resultant crepe may be of a distinct canary yellow in tint.
In scrap-crepes of the higher grade this distinct yellow colour is often
visible in streaks which indicate the presence of tree-scrap, etc.,
obtained from recently opened tapping areas.
BISULPHITE STREAKS.--These, again, arise from defective mixing. In the dry
rubber it is seen that there are streaks of colourless rubber in a general
mass, which may be of varying shades of yellow; or, a length of exceedingly
pale rubber is apparently streaked in patches with a darker shade of
colour. A solution of sodium bisulphite is heavier than latex, and there
would be a tendency, therefore, for the chemical to sink in the large
mixing jar. Unless stirring is thorough it is possible that portions of the
latex would not be in contact with sodium bisulphite while others receive
more than a fair share. Especially would this effect be seen where
coagulation takes place quickly, and experience bears out the truth of the
suggestion. Another factor which has some bearing on the point is the
strength of solution in which sodium bisulphite is used. In the ordinary
course of working, the acid coagulant is added immediately after sodium
bisulphite has been stirred in. Should a strong solution of the bisulphite
be used, and if coagulation takes place quickly, it is easy to see that the
possibilities of obtaining a uniform and intimate mixture are small.
Probably in no factory is the sodium bisulphite now added to latex in
powder form, but it has been found that if care is not taken to see that
all the bisulphite has dissolved before the solution is added to latex
streaks may result in the dry rubber. The undissolved particles sink to the
bottom of the coagulating jar or tank, and there slowly dissolve, forming
local strong solutions. The effect upon the rubber in the vicinity of these
strong solutions is much more marked than in the bulk of the coagulum, and
hence lighter streaks or patches appear in the dry rubber. In spite of
apparently complete mixture by good stirring, it will be seen that it is
possible, therefore, to have failed in this direction if any undissolved
powder remains in the solution of sodium bisulphite.
"SPOT" DISEASE.--Few managers of estates preparing pale crepe rubbers are
unacquainted with this defect. It is manifested by the appearance of small
coloured spots varying in density (_i.e._, number to a unit area) and
differing in hue. The most common colours are black and orange, but "spots"
of brick-red, yellow, violet and ruby and green tints have been noted, the
last named very seldom. Sometimes in place of definite "spots," or
colonies, the colour is spread over practically the whole surface of the
rubber as a "flush."
These coloured spots, or "flushes," indicate infection by minute fungi,
which are present in the latex prior to coagulation. The infection of the
latex takes place in the field by means of spores, which are only visible
with a microscope.
It is not feasible to discuss any method of preventing this infection of
latex by air-borne spores, as the eventual preventive measures are so
simple. But it may be believed that under ordinary weather conditions most
latices are infected before reaching the factory. It is likewise true that
even fine pale crepes shipped in perfect condition may contain
possibilities of trouble in the form of "dormant" spores, the development
of which may commence and continue if favourable conditions arise.
The subject of "'Spot' Diseases" has been treated fully in previous
publications,[19] and it is not proposed here to enter into any lengthy
discussion.
[19] "Preparation of Plantation Rubber," Sidney Morgan, 1913. "Spotting of
Plantation Rubber," Keith Bancroft, 1913; Bulletin No. 16, F.M.S.
Department of Agriculture. "Spotting of Prepared Plantation Rubber," A.
Sharpies, 1914; Bulletin No. 19, F.M.S. Department of Agriculture.
If any reader is desirous of producing the defect experimentally, all that
is necessary is to prepare a piece of crepe rubber of rather more than
ordinary thickness, roll it up while wet, and place aside for some days.
This experiment reproduces the conditions favourable for the development of
the spores, and spots of various colours may result. It will be clear that
the chief factor influencing the result is the continued presence of plenty
of moisture.
This condition may be created inadvertently in the course of factory
practice, if piles of crepe rubber are allowed to remain for any
appreciable period before hanging to dry. For this reason batches of wet
crepe should always be placed on edge, to allow free drainage of surface
moisture, if the rubber cannot be taken at once to the drying-sheds.
[Illustration: THREE SPECIMENS OF FINE PALE CREPE SUFFERING FROM "SPOT"
DISEASE.]
The condition also is provided if the thickness of the crepe is
excessive. In some factories, having no smooth-roll finishing machines, the
crepes may have a distinct raised pattern upon them. It is usual to note
that if "spot" disease appears in such crepes, it is incident to much
greater degree in the thicker portions of the rubber--_i.e._, upon the
raised pattern.
The direct connection between the rate of drying and the appearance of
coloured spots or flushes is thus established, and it only remains to adopt
precautionary measures which will lead to an avoidance of delay (1) between
machining and hanging, (2) in drying.
It is indicated, therefore, that, if spot disease is to be avoided, the
prime consideration is the preparation of a thin crepe which will dry
quickly under average conditions. It may sometimes happen that even very
thin crepes will sometimes be found affected on some estates. In such
instances, it will be found that the design or situation of the
drying-house is at fault, and that specially favourable conditions for the
development of the fungi have been created by excessively wet weather.
Should the trouble persist in spite of the preparation of the thinnest
crepe, it would be advisable either to abandon this form of No. 1 product
or to consider the installation of artificial aids to drying.
We have not yet encountered any case in which it was found necessary to
treat the latex with an antiseptic or disinfectant substance for the
prevention of "spot" disease. There appears to be an idea held in some
quarters that sodium bisulphite may be so employed as a fungicide. This
does not agree with our experience, which is confirmed by Sharpies
(Bulletin No. 19, F.M.S. Department of Agriculture).
In 1913 experiments with chinosol were undertaken at the Pataling
Laboratory of the Rubber Growers' Association, and an account of the method
of treatment was given in a printed report issued to subscribers. Dr. P.
Arens,[20] of the Malang Experimental Station (Java), has also recommended
the use of chinosol. The substance is expensive, but is effective in very
small quantity. On the whole, given average conditions in factory practice,
such aids should not be necessary, and where keen supervision is not
available may lead to other difficulties.
[20] "Guide to the Preparation of Rubber," Arens, 1918; Communications from
the Experimental Station (Malang, Java).
It has already been remarked that it is possible for "spot" disease to
develop in dry rubber which previously gave no evidence of the presence of
fungi. The condition necessary to such an occurrence is supplied by the
presence of moisture. Thus, to state instances which are by no means
uncommon, if a box of rubber is allowed to remain exposed to rain, or is
damaged by flood-water, or by sea-water during transit, or (sometimes) if
the rubber is packed in a damp case, the crepe on arrival at its
destination may be found to be affected to a degree dependent upon the
extent of wetting and the duration of the wetting period.
No means are known by which these coloured spots, due to the growth of
chromogenic organisms, can be removed from the rubber. Naturally, although
they may be present in the darker lower grades of crepe, they are not so
easily visible as in pale crepe. It follows, therefore, that every possible
precautionary measure must be taken when pale crepe has to be prepared.
We are often asked whether it is possible for an infected piece of rubber
to affect sound rubber hanging in the same building; and whether, in case
of "spot" disease appearing, it is necessary to disinfect the drying-house.
In a general sense, the answer to both queries is in the negative. It has
not been proved possible to transmit the disease from one piece of crepe to
another, except by the closest possible contact and in the presence of an
abundance of moisture.
A dry crepe, even when in close contact with an infected dry specimen, has
not been found to be affected.
Unless, therefore, pieces of rubber are pressed together, under favourable
conditions as to moisture, there has been observed no transfer of disease.
Similarly it has not been found that the presence of spotted rubber in one
part of the drying-house has been responsible for an outbreak of disease
in another part of the same building. Furthermore, after the removal of
diseased rubber from the drying-shed, freshly prepared rubber may be hung
on the same supports without becoming affected, and without any
intermediate treatment of the wooden bars, providing the crepe is thin and
weather conditions are good. In our experience, no case has been observed
in which the disease has been communicated to freshly prepared rubber by
reason of the previous presence of affected rubber. In our opinion,
therefore, any scheme for disinfecting the interior of a drying-house, as a
preventive measure against the spread of "spot" disease, is unnecessary.
All other things being equal, it is plain that much will depend, as to the
incidence of coloured spots, upon the design and situation of the
drying-house. Sufficient has been written in previous chapters to indicate
the importance of these points as affecting the rate of drying, upon which
hinges the possibility of the appearance of "spot" disease.
In conclusion, the chief points in any discussion of this subject may be
summarised thus:
1. No coagulum should be left without working for longer than the
ordinary period. Otherwise, the prevailing conditions are very
favourable for the development of the disease.
2. Thin crepe only should be made. The quicker the rate of drying the
less possibility is there of the coloured spots appearing.
3. Crepe should never be allowed to remain folded overnight, and
batches of folded wet crepe should be placed on edge to drain off
surface moisture. The rubber should be hung to dry as soon as
possible.
4. Several species of fungi causing coloured spots have been
recognised, and it has been proved conclusively that it is possible to
infect latex and also fresh coagulum.
5. As far as our present knowledge goes, it appears that infection
takes place chiefly, if not entirely, by means of the latex in the
field-vessels. It may take place during transport also, or even during
coagulation.
6. While it is certain that infection can be caused by contact, it has
not yet been shown that infection of the finished wet rubber takes
place in the drying-houses by means of air-borne spores--at least,
under ordinary drying conditions.
7. There is reason to believe that no further infection takes place
once the rubber is well into the drying stage, and that dry rubber is
not infected even by contact. From this one might infer that, as long
as rubber remains dry, infection cannot take place during the voyage
to the port of consignment.
8. Coloured spots do not appear until the rubber is about half dry,
because that period is necessary for the development of the fungus to
that stage in its life-history when it excretes colouring matter. The
fungus in its earlier and colourless stage may have been present from
the time the latex entered the cup.
9. The natural habitat of the fungi would appear to be decaying
vegetable matter in the field.
10. Finally, if it is found impossible to be rid of fungoid-spot
disease after having exercised all care and observed all known
precautions, nothing remains but to supersede the ordinary drying
process by some system of quick drying, such as the vacuum-drying
process or a hot-air draught system, in which the rubber dries so
quickly that any possibility of appearance of "spots" is entirely
removed.
SURFACE MOULDS OR MILDEWS ON CREPE RUBBER.--Defects of this nature are most
uncommon in the higher grades of crepe rubber, but cases of affection in
the lower grades are not rare.
It will be evident from all previous discussions that the incidence of
these moulds must be due to an extremely slow rate of drying. The necessary
conditions would be supplied by one or more of the following causes:
(_a_) Making the crepe too thick.
(_b_) Hanging the crepe in a badly ventilated or badly situated
building.
(_c_) Occasionally by abnormally wet weather.
(_d_) Allowing piles of crepe to remain too long before hanging.
(_e_) Using excessive quantities of deteriorated sodium bisulphite. In
short, any factor contributing towards a retarded rate of drying may
be responsible for the appearance of surface mildews. The last
mentioned cause is of not infrequent occurrence. Knowing the chemical
to be of poor quality, relatively more is used to produce the desired
anti-oxidant effect. Unless the rubber is particularly well washed on
the rolls, there remains within it a residue of sodium _bisulphate_,
an oxidation product of the bisulphite. This is hygroscopic to some
degree--_i.e._, it takes up moisture from the atmosphere. Hence
drying is delayed, and even should mildews not develop the chemical
may sometimes be seen on the surface of the rubber as a whitish
"bloom."
The enumeration of the possible causes of mildews on crepe rubber is
sufficient to indicate the necessary precautions to be taken, and the
discussion will not be extended further.
TACKINESS IN RUBBER.--"Tackiness" is a term used to denote a deterioration
of rubber which renders it sticky, and, beyond this, implies that some
physical and chemical change in the nature of the substance has taken
place. In fact, it is no longer "rubber," but an oxidation product
containing much resinous matter. It does not behave as rubber, and hence
its value is much depreciated.
With modern ideas of erection of factories to guard against the
introduction of direct sunlight, it was hoped that this defect had
practically ceased to exist. In one grade of rubber it would be expected
that tackiness would continue to appear. Earth-rubber, often exposed to
direct sunlight for a week, would naturally become tacky, and this
tackiness cannot be avoided unless the earth-scrap is to be collected more
frequently. But in many cases even the higher grades of rubber show signs
of tackiness. Experiments have been carried out at various times and in
various places to determine the cause of tackiness. For some time the
theory of bacterial origin was in favour, but none of the experimental
results was convincing. Bacteria may be present in tacky rubber; but, on
the other hand, many cases of bacteria in rubber have been observed in
which there was no tackiness. Experiments were made by one of us some years
ago with a view to testing the bacterial theory by inoculating latex with
small pieces of tacky rubber. In opposition to the results which were
stated to have been obtained, there was no spread of tackiness. Other
investigators have obtained similar results. One writer proposed to explain
tackiness as caused by excess of moisture. This perfectly simple
explanation unfortunately displays only a profound ignorance of the
subject, and does not take into account the fact that tackiness is incident
in rubber after dryness has been reached. It need not be pointed out to
planters in Malaya that wet sheets of rubber are often exposed to direct
sunlight by workers of native holdings, with no resulting harm as long as
plenty of moisture is present in the rubber.
TACKINESS THE RESULT OF A SLOW PROCESS OF CHANGE.--As stated above,
tackiness does not appear until the rubber is dry, and even then it is to
be noted that it is possible for tackiness to appear in rubber arriving in
London, which showed no indications of tackiness when packed for shipment.
TACKINESS CAUSED BY TRACES OF COPPER SALTS.--Spence, as the result of
investigations, has pointed out that none of the various theories put
forward to account for tackiness--viz., the action of bacteria, premature
putrefaction, oxidation, excess of moisture, the action of enzymes,
etc.--have any basis in scientific proof, and believes that the cause of
tackiness cannot be directly attributed to bacteria. It has been stated
that the only known way of causing rubber to become tacky is to expose it
to sunlight or heat. While agreeing that in the ordinary way this statement
is correct as far as one rules out the employment of chemical substances,
it must be pointed out that tackiness of the worst degree may be caused by
the presence of traces of copper or copper salts. This point has already
been touched upon in a preceding paragraph dealing with the defect of
"green streaks" in pale crepe rubber.
In the course of laboratory experiments tackiness has often been induced by
the use of traces of copper salts. The rate at which tackiness is induced
appears to be dependent upon the amount of copper salt used, but once it
begins, the rubber molecule is very rapidly broken down, and resins are
formed. As the formation of resins is accompanied by the inclusion of
oxygen in the chemical constitution, it would be expected that dry rubber
becoming tacky should increase in weight. This is found to be the case, and
to give an idea of how this weight increases with the progress of
tackiness, the results below may be studied.
It will be seen that the maximum quantity of copper sulphate used amounted
to 0·025 per cent, (approx.) upon the weight of latex taken. Now it is
highly probable that only a fraction of this quantity was retained in the
rubber on coagulation, the remainder being in solution in the serum.
Furthermore, as the rubber was well washed and worked down to thin crepe,
_the total quantity of copper salt remaining in the dry crepe must have
been exceedingly small_. Yet the effect is most marked and should impress
upon all managers the necessity for guarding against any possible
contamination caused by brass or copper.
----------+-----------------+--------------------------------------------
_Sample._ | | _Weight of Rubber._
+------+ +------+--------+--------+--------+----------
| | | After |Further |Further | Percent-
|_Amount of Copper Salt._| When |Interval|Interval|Interval|age in In-
| | Dry. |of Four |of Seven|of Three|crease in
| | | Weeks. | Weeks. | Weeks. | Weight.
---+------------------------+------+--------+--------+--------+----------
| | Grms.| Grms. | Grms. | Grms. |
1 |0·02 grms. copper | | | | |
|sulphate per 100 c.c. | 430 | 441 | 482 | 488 | 13·5
|latex | | | | |
2 | Ditto | 428 | 439 | 481 | 486 | 13·55
3 |0·01 grms. copper } | | | | |
|sulphate, per 100 c.c.} | | | | |
|latex } | 962 | 987 | 1035 | 1036 | 7·7
|0·01 grms. copper } | | | | |
|acetate, per 100 c.c. } | | | | |
|latex } | | | | |
4 |0·025 grms. copper | | | | |
|sulphate, per 100 c.c. | 502 | 513 | 558 | 560 | 11·5
|latex | | | | |
---+------------------------+------+--------+--------+--------+----------
In view of the effect thus produced by the addition of traces to latex of a
copper salt, and the observed effect on rubber of contact with copper
salts, one may imagine the result produced some years ago when on an estate
smoked sheets were washed with a solution of copper sulphate as a remedy
for surface moulds!
With the exception of this chemical action we know of no other means by
which tackiness is produced, beyond those of direct sunshine and heat.
Cases governed by these two causes are common on estates. They are confined
chiefly to the lowest grades of scrap rubber, when the component raw
materials have been exposed to the sun for a period before being brought to
the factory.
It is now comparatively rare to find cases of tackiness in the higher
grades of crepe, and when they occur, one may look for evidence of gross
carelessness in the admission of direct sunshine. Usually this means the
failure of some individual to regulate window shutters according to the
position of the sun in the sky. More rarely does it happen that tackiness
may have been induced by placing thin crepe rubber too near the iron roof
of the drying-shed.
Regarding the question as to whether tackiness may be communicated by
direct contact, opinion appears to be divided. It has been stated that
sound rubber left in contact with tacky specimens was found to be
unaffected after two years. On the other hand, it is claimed that tackiness
has been induced in a sound rubber by infecting it with small pieces which
were tacky. In a preliminary article on the effect of copper and copper
salts upon pieces of dried and sound crepe[21] it was noted, after one
year, that tackiness had been communicated from the treated portion to the
"blank" in contact. There is sufficient evidence to warrant the injunction
that tacky rubber should be excluded from contact with sound rubber. If
shipped it should be packed separately.
[21] Report I., 1916 (Sidney Morgan), Rubber Growers' Association (Malaya).
Compounds have been put upon the market which assumedly claim to be cures
for tackiness. These are merely palliatives, consisting of starch, talc, or
chalk powders, which counteract stickiness.
NO CURE FOR TACKINESS.--At the present stage of our knowledge, there
appears to be no cure for tackiness. Neither do we see the necessity for a
cure when the phenomenon may be avoided by taking simple precautions, which
may be briefly summarised thus:
(1) Any permanent openings through which it is possible for direct
sunlight to enter, whether large or small, should either be totally
closed or provided with some substance which cuts off the direct
effect of the sunlight--_e.g._, ruby glass or ruby glazed cloth.
(2) Rubber should under no circumstances be placed near any source of
heat.
(3) No rubber should be hung in a drying-room in such a position
adjacent to a window or door that it is possible for sunshine to reach
it, even should coolies neglect to obey rules.
(4) Instruments or vessels of copper or brass should not be used where
acids are employed.
LACK OF UNIFORMITY IN COLOUR.--The complaint is far less real than it was
a few years ago. The introduction by the Rubber Growers' Association of the
"Metrolac" led to uniform dilution of latices varying in rubber content.
Previously the only known method of obtaining uniformity in colour and
appearance was that by which latices from all fields were mixed together in
bulk. Even so the uniformity applied only to the one bulking operation, and
any other day's results might show considerable variation from the first
standard.
This does not take into account any observed differences in shade of colour
attributable to natural oxidation which might vary in intensity from day to
day. The introduction of sodium bisulphite as an anti-oxidant exerted a
great influence upon the colour of pale crepes generally; but considerable
variation would still have been notable but for the adoption of the scheme
for uniform dilution, in addition to the use of small quantities of
anti-oxidant.
On most estates it is now possible, with slight exceptions due to abnormal
conditions, so to treat the latex that the pale crepes prepared on any one
day differ in no perceptible degree from the product of any other day.
Where this is not the case it must be suspected that there has been some
carelessness in manipulating the latex or the chemicals. Attention has been
drawn to the fact that there may be exceptional cases, when the determining
factors lie beyond the control of factory processes--_e.g._, heavy rains
causing over-dilution of latex, the yielding of "yellow" latex from newly
opened areas, etc. But on the whole there is now no reason why the general
average product from any estate should not be uniform in colour and
appearance. Furthermore, it should be possible for large groups of estates,
by the adoption of uniform methods, to produce similar rubber from all the
plantations. Moreover, apart from some differences caused by factors which
still need determination, the total product in a general sense should not
only be uniform in appearance but uniform in physical and chemical
properties.
BLOCK RUBBER.--This mode of preparation is employed only in comparatively
few instances. The block is prepared from crepe rubber, which has been
dried either in a hot-air drier or in a vacuum chamber.
There is another type of block which is made by placing layers of dry crepe
under considerable pressure. This is not the true type of block, and the
layers are quite distinct--_i.e._, they do not amalgamate. Usually this
pressed rubber consists of lower grades of crepe, and it should not be
popular, inasmuch as it leaves too wide an opportunity for the inclusion of
dirt, bark particles, and other impurities, which cannot be seen generally
on account of the protective colour of the rubber.
In the true type of block, the layers are in a plastic condition, due to
heat, when they leave the drying-chamber; and being immediately submitted
to great pressure the result is a homogeneous mass in which the layers
disappear by amalgamation. Only the best grade of crepe is employed, and
given the absence of defects in the layers there should be no complaint
regarding the final block.
Prepared in slabs which are three or four inches in thickness, the product
is easily handled, and should be sufficiently translucent to make it
possible to distinguish the shape of the hand when held between the block
and the light. This is not possible when blocks are made of greater
thickness.
The only complaints which it should be possible to lodge against block
rubbers are:
(_a_) The inclusion of dirt and other matter.
(_b_) The use of layers of crepe which have some defect.
(_c_) The inclusion of air-bubbles.
The remedy for (_a_) and (_b_) lies in the hands of the factory
superintendent. The last ground of complaint is dependent upon the style of
preparation of the original layers of crepe.
When layers of crepe are placed one upon the other, and submitted to great
pressure, it is natural to suppose that air would be contained in spaces,
and would be unable to escape. To guard against this, it would seem
necessary to prepare the crepe thin and with a fairly good surface finish.
It must be obvious to all acquainted with the processes involved in the
preparation of block rubber, that no possibility exists for the presence
of air-bells actually enclosed _in_ thin crepe. When the vacuum-dried crepe
is folded preparatory to the blocking process it is apparent that between
the layers there must always be a considerable volume of air, a small
proportion of which is bound to be retained owing to the nature of the
surface of crepe rubber.
That this has always been true of the preparation of block rubber cannot be
denied. It is possible, of course, for one type of block to show the
presence of air-bells more than another type, the proportion of air
enclosed in blocking depending upon the nature of the crepe of which the
block is composed. A block built up of layers of smooth, fine crepe would
be expected to contain less air-bells than a block composed of layers of a
rough crepe.
Block rubber has been seen which was free from air-bells, but this was the
thin variety of block prepared for show purposes with far greater care,
probably, than would be expended in commercial preparations.
CHAPTER XVIII
_DEFECTS IN SHEET RUBBER_
Before proceeding to deal with defects in the rubber as it is put upon the
market a brief account will be given of faults which may be noted in the
preparatory stages.
MILKY RESIDUE OR SERUM.--If the serum is not clear after the ordinary
period allowed for coagulation, it indicates one of the following possible
causes:
(_a_) Failure to obtain complete mixture by thorough stirring.
(_b_) Insufficiency of acid solution. This may be real or indirectly
due to the presence of an excess of anti-coagulant such as formalin or
sodium sulphite.
(_c_) In cases where other coagulants than acetic or formic acids have
been employed the failure may be due to an excess of, or an
unsuitable, coagulant--_e.g._, hydrochloric acid.
COLOURED SURFACE BLOTCHES AND UNPLEASANT ODOUR.--Sometimes the surface of
the coagulum exhibits yellowish or bluish streaks and patches. It will be
found generally that the yellowish colour is possessed by a slimy
substance, of offensive odour, which may be scraped from the surface.
Either insufficient acid has been used, or the mixing of latex and
coagulant has been at fault.
DARK DISCOLORATION OF THE RUBBER.--This may be stated to be a natural
process when fresh rubber is exposed to the atmosphere. It is usually
described as "oxidation," and it will be noted to be absent, or to occur to
less degree, on those portions of the rubber which are protected from the
atmosphere by being below the surface of the remaining liquid. This surface
change may be prevented (see Chapters VIII. and IX.) by the use of small
quantities of sodium sulphite (for preference) or bisulphite.
SOFT COAGULUM, SPONGY UNDER-SURFACE, TEARING OF COAGULUM.--If the whole
mass of coagulum is too soft, while coagulation appears to be complete,
over-dilution of the latex has occurred. This may apply also to the case in
which the under-surface only is spongy and soft. If coagulating-tanks are
employed, the upper edge may be comparatively hard, while the lower is soft
and weak. Often the spongy portion may adhere to the partitions. This
prevents the natural rise of the coagulum, due to retraction, as the mass
"sets." The pull between the free upper portion and the adhering lower edge
causes splitting and tearing of the coagulum, with marked porosity (spongy
appearance). The two factors to receive attention are the standard of
dilution and the condition of the surfaces of the partitions. If these have
minute cracks into which latex can penetrate, and in which coagulation
takes place, the boards should be discarded. Given the conditions indicated
above, the tearing and splitting of rubber in coagulating tanks is
sometimes augmented by the practice of flooding the tanks when coagulation
is judged to be complete. The surface water finds its way downwards between
strips of coagulum and the partitions, thus increasing the upward tension
between the free and adhering portions. The main idea governing the
practice of flooding the tanks is to prevent "oxidation" (darkening) of the
upper edges. If a small quantity of sodium sulphite is employed as an
anti-oxidant and to retard coagulation, it is not necessary to flood tanks.
"PITTING" OF SURFACES.--In pan coagulation this "pitted" appearance is
usually limited to the under-surface, while coagulum prepared in tanks may
exhibit the defect on both faces. The existence of these numerous "pits,"
or small depressions, points to the presence of bubbles of gas which have
been unable to escape freely. As the formation and retention of gas-bubbles
is not a normal occurrence in coagulation, we are led to infer that some
special conditions must have arisen. These may be supplied by one or more
of the following contributory causes:
(_a_) The latex had begun to "sour" before arrival at the factory or
while waiting to be treated. This premature coagulation is usually
checked or diminished by the employment of anti-coagulants (see
Chapters VIII. and IX.). It is generally accompanied by the
appearance of enclosed gas-bubbles in the dry rubber.
(_b_) There may have been a slight insufficiency of coagulant, or the
admixture was not thorough, thus allowing a slow putrefactive change
to take place in the incompletely coagulated areas.
(_c_) The wooden partitions may not have been effectively cleansed.
The existence of a thin slime, of bacterial origin, is sometimes
noted. This is accountable for putrefactive effects in the surfaces of
the coagulum, or in the serum, giving rise to the formation of gases.
If these cannot escape freely, by reason of adhesion between the
coagulum and the partitions, "pitting" occurs.
THICKENED ENDS OR EDGES, AFTER ROLLING.--As a rule these defects may be
ascribed to the employment of too rich a latex, or faulty manipulation.
Even if the standard of dilution should be correct it sometimes happens
that, in the preliminary rolling of a long strip of rubber, coolies begin
in the middle, rolling with a forward pressure and tension towards the ends
of the strip. This is generally not so much the fault of the coolie as
being due to the lack of proper facilities for preliminary rolling. The
table should be about 3 feet in height, so that ease of working is obtained
merely by natural pressure due to the position in which the worker stands.
The use of a heavy wooden roller would contribute towards this result,
inasmuch as it obviates the use of force, and the pressure is almost
entirely in a vertical direction.
MIS-SHAPEN SHEETS.--It is sometimes noted that sheets may be wider and
thicker at the ends than in the middle. Manipulation alone, as indicated
above, is not solely responsible. The primary cause is to be traced to
over-dilution of latex, giving a very soft coagulum which responds too
readily to tension and pressure. Faulty treatment in rolling exaggerates
the tendency for the strip of sheet to become narrow and thin in the
middle, wider and thicker at the ends.
THICKENED PATCHES, TORN SHEETS, "DOG-EARS," CREASES.--These elementary
defects are all due to careless working. While occasional errors cannot be
avoided, there is no real excuse for the continuance of trouble to any
degree, under average supervision.
Thickened patches are often caused in conjunction with torn sheets, and
the trouble may be ascribed to faulty practice in allowing too heavy a pile
of wet strips to accumulate before machining. Or a comparatively small pile
may have been transported some distance. It is difficult to separate the
strip, and occasionally the separation is only effected at the expense of
two sheets, one of which is torn and the other has a portion of the first
strip adhering to it.
"Dog-ears" due to the folding over of corners of the sheets, and creases
due to the rumpling of the coagulum, are generally the result of haste and
lack of average care. Machine coolies, more often than not, will not be at
any pains to straighten out folds before passing the coagulum through the
rolls.
GREASINESS BEFORE SMOKING.--Under ordinary methods of working this should
never be encountered. It may be taken to show that the machined rubber has
been allowed to remain, either hanging or in piles, far too long before
entering the smoke-house. The appearance is most marked if the rubber has
remained in a cool and moist atmosphere--_e.g._, if it has been hanging
over-night in a closed and badly-ventilated factory. In a marked degree
this is to be observed in the preparation of air-dried sheets, unless they
are exposed, when freshly prepared, to the action of the sun for a period.
This period, in the case of rubber prepared on native small-holdings,
generally extends over several days--until the sheets are more than half
dry.
In the preparation of smoked sheet, the greasy appearance and the cause
outlined contribute to a defect which is eventually described as
"stretching rusty."
SURFACE BLEMISHES.--The coagulum, during coagulation and subsequently, can
be contaminated in various ways. In most cases a little intelligence or
increased care would prevent the occurrence of these defects.
When the coagulum remains over-night, in the absence of a cover, it is not
uncommon to note the presence of dirt (from the roof above, or blown in
from the outside), the droppings of mice and rats, flies and small insects.
In theory these should be seen and removed by the factory hands. In
practice, except while under immediate supervision, the extraneous matter
is often rolled into the soft coagulum.
A fairly common cause of this surface contamination is the exhaust from the
power-unit; generally the worst offender is a steam-engine. Grit and smuts
continually find their way into the factory, alighting on the tables, in
the latex, in the water, and on the freshly prepared rubber. They are
rolled into the soft rubber and lead to marked depreciation in the selling
value. The radical remedy seems obvious, but is often beset with many
difficulties not unconnected with financial considerations.
Other superficial blemishes, such as those due to the presence of rust
marks, oil or grease patches, etc., are self-explanatory, if a little
thought is brought to bear upon them; and it is not proposed here to
discuss such defects more fully.
* * * * *
Having now dealt with certain defects which are visible in wet rubber, we
come to the discussion of others which are only perceptible either during
or after the drying period. As far as is known no plantations of any size
now prepare sheets other than in the form of smoke-dried rubber, with the
exception of a few which make a special form of thick and partially
air-dried product known as "slab" rubber.
It is not proposed, therefore, to treat in any detail with air-dried sheet
rubber. Certain obvious defects are common to both air-dried and
smoke-cured sheets, and these will be first discussed.
UNEVENNESS OF APPEARANCE.--This lack of uniformity may refer either to size
or colour, or to both. Apart from any other contributory causes, this
variation is due, in pan sheet, to a neglect to standardise the dilution of
all latices, or to lack of uniformity in the quantity of standardised latex
placed in each receptacle.
Where tanks are employed all sheets from the same tank should be of the
same size before rolling, and any subsequent disparity in thickness and
length must be attributed to some alteration in the width of the gap
between the rolls of the machines.
Unless all latices are standardised by means of an instrument, it is of
course probable that the content of one tank may be found to differ from
that of another.
In a general sense, whether air-dried or smoke-cured sheets are considered,
a thin strip will dry more quickly than a thick one, and should be paler in
colour when viewed by transmitted light--_i.e._, when the rubber is held
between the eye and the source of light.
It is necessary, therefore, to guard against the possibility of variations
in thickness caused by faulty manipulation. The distance between the
squeezing rolls (smooth) and between the marking rolls (patterned) should
be adjusted and should remain set until the conclusion of work. In a
factory having nothing beyond average requirements in equipment of machines
it should not be necessary to have to interrupt the work of the smooth
rolls or "markers" by having to make adjustments. This is, however,
inevitable if there is only one smooth-roll machine, as it is always
desirable to reduce the thickness of the coagulum by at least two stages
through even-speed smooth rolls. In some factories there are three light
power-driven smooth-roll machines, the gaps between pairs of rolls being
set so as to obtain a gradual thinning effect upon the fresh coagulum,
which is then passed once between patterned rolls. With such equipment it
is found possible, in some cases, to omit the preliminary hand-rolling, and
the strips of coagulum from the tank are passed direct through rolls set
with a wide gap. This work demands much care, as it is necessary to avoid
any distortion of the coagulum which may be caused by its own weight and
length.
VARIATION DUE TO OXIDATION.--The subject of oxidation has been mentioned in
the opening paragraphs of this chapter. It will have been learned that
oxidation is a natural process, and that it may be prevented by the
employment of anti-oxidants such as the sulphite or bisulphite of soda. In
earlier days it was sometimes prevented by steeping the thin rubber in very
hot water.
In the absence of an anti-oxidant the degree of oxidation may vary daily
and in different batches of latex on any one day, so that there is always
the possibility of a lack of uniformity due to oxidation effects. This
would be more evident in air-dried sheets than in smoke-cured rubber, as in
the latter case the darkening of the surface would be masked by the colour
induced by the smoke-drying process.
To obviate this variation anti-oxidants are used on most estates, but the
accidental or misinformed abuse of these chemicals may lead to further lack
of uniformity. Hence it is necessary to follow carefully the formulæ
prescribed by experience.
COLOUR OF SMOKED SHEETS.--It may be of interest to note that the effect
known as oxidation is attributed to the presence of micro-organisms called
enzymes (ferments) in the latex. It can also be produced artificially in
various ways--_e.g._, by the use of the crude product of wood-distillation
(pyroligneous acid) as a coagulant, or by the addition to the latex of
small quantities of a phenol such as carbolic acid. It is thus possible to
prepare in sheet form a rubber which has the appearance of having been
smoke-cured, although it may never have been in a smoke-house.
It will be clear, therefore, that apart from other causes, the colour of
the cured sheets may be influenced by oxidation of the fresh coagulation,
and by the constituents of the smoke. It follows that the smoke from
timbers which are richer than others in certain chemical bodies set free by
combustion will produce a rubber darker in colour.
There is thus no real connection between colour and period of cure,
although in a general sense the longer the interval the darker the colour.
Similarly it is now plain that when anti-oxidants are employed in excess
the paleness of the rubber is in no degree truly indicative of the period
during which the rubber has been smoke-cured.
The influence of the effect of the hypsical condition of the wet rubber
upon the final colour must be thoroughly grasped. One may take two sheets
of apparently the same thickness, and smoke-cure them in juxtaposition
within the same house, only to find that one dries much more rapidly than
the other. As a consequence, the first, when fully cured, will be of a
medium golden brown colour; while the other, owing to protracted smoking,
will be dark. Evidently there must be some distinct difference between the
two in physical condition prior to the smoking. Here the factor involved is
the rubber-content of the latex. Given two pieces of coagulum of identical
thickness, but prepared from latices of different dry rubber content, it
will be obvious that to reduce them to similar thickness, more pressure
will be necessary in one case--_i.e._, that piece of coagulum will be much
more dense (more consolidated)--while the other will be comparatively soft
and porous. Into the latter warm smoke can penetrate much more easily, and
the internal moisture can escape more rapidly. The full cure may be made,
say, within twelve days, while the tougher sheet may demand up to twenty
days.
To attain uniformity in colour, therefore, the following points must be
studied and controlled as far as is possible:
(_a_) Uniform dilution of all latices.
(_b_) Uniform dimensions of coagulating receptacles.
(_c_) Uniform volumes of standardised latex.
(_d_) Uniform quality and quantities of chemicals.
(_e_) Uniform methods of manipulating the coagulum.
(_f_) Uniform conditions of fuel and accommodation during
smoke-curing.
SURFACE GLOSS.--In the choice of fuel there is room for control if one has
good timber available. This point opens up a discussion on the vexed
question of "over-smoking," as the term is sometimes applied to a
pronounced dry glossy appearance of the surface of sheets.
Three main factors are involved:
(1) The nature of the fuel.
(2) The ratio between furnace capacity and the capacity of the
smoke-house.
(3) The rate of combustion.
Obviously any fuel which yields an excessive quantity of tarry matter
or creosotic substance would conduce to the formation of a heavy glaze
on the rubber. Such fuel, therefore, should at most only be employed
as the smaller portion in a mixture with "dead" timber.
It is impossible to lay down any general rules for the guidance of estates,
as the timber available varies so widely in nature. Experience must be the
only guide, and it should not be difficult to obviate the defect. Even so,
there must be minor differences between the results obtained on estates, so
that it is not possible to make strict estimations of the smoke-curing
period by an examination of the surface appearance of rubber, even under
the best of conditions. Some estates find that the rubber has a distinct
gloss in ten days, while others may smoke-cure for twice that period
without difficulty. Evidently, therefore, the question of available fuel is
of prime importance. It may be remarked that very satisfactory results are
always obtained from the use of fairly dry timber obtained from thinned
rubber trees, mixed with the "dead" timber of old logs and stumps found on
the estates.
Obviously if a smoke-house has a superabundance of furnaces, producing more
heat and smoke than is required, glazing will result. The point is tested
by the average temperature maintained and the average rate of drying. The
result of a high temperature would be the possibility of volatile tarry
matter being driven in excess to the upper chamber. That this effect is
eventually produced even at optimal temperatures is evident from an
examination of the wood-work within the upper room.
It is clear, also, that the rate of combustion exerts an influence. In a
general sense a rapid consumption of fuel would augment the quantity of
tarry matter passing into the upper chamber over any given period, and the
possibility of glazing would be increased.
On the other hand, it is possible that a surface glaze might be formed if
the temperature were uniformly too low, especially if the rubber were
rather thick. The rate of drying would be so slow, that if a timber rich in
tarry matter were employed, the rubber might be exceedingly glossy.
In order to guard against the appearance of a heavy glaze, then, the
following points must be observed:
1. The fuel must be carefully selected by experience.
2. The sheets must not be thick. No sheets should be thicker than 1/8
inch measured in average section across the ribs.
3. The temperature must not be too high. An average working
temperature of 120° to 125° F. should be ample.
4. If the sheets are fairly thick, a low average temperature should be
avoided. No lower average than 110° F. should be allowed.
DULL, BLACK SURFACE.--This is the opposite of the previous case, and
generally is accompanied by a fairly heavy darkening of the surface due to
"oxidation" effects. The fuel used is too "dead," and needs the addition of
some substance containing a fair amount of creosotic matter. The appearance
of the rubber does not justify the assumption that it has been over-smoked.
As a matter of fact, this type of rubber often becomes affected by mildew
fairly rapidly, thus showing that the smoking has been inefficient.
It may happen that an estate is in the habit of using a fuel which gives
even to a thin sheet a heavy glaze in a comparatively brief period. The
general custom is to soak such sheets in cold water, and then to scrub the
surfaces, sometimes using soap, in order to cleanse the rubber and free it
from the glaze. This operation may be done too well, in which case the
rubber will have a dull appearance, and may be rather more liable to
develop surface mildew after a time.
MOIST GLAZE, GREASINESS OF SURFACE.--This describes the condition of sheet
rubber when taken from the smoke-house. Sometimes the greasiness does not
develop until the rubber has been out of the smoke-house for a day or two.
As far as experience shows at present it may be due to two causes:
(_a_) The use of an excess of sodium bisulphite or sodium sulphite.
The use of sodium bisulphite is not recommended generally for
sheet-making. It may cause the rubber to be too pale in colour, and
the abuse of it may delay the drying unduly. In the latter case, a
trace of the salt may remain within the rubber or upon the surface. If
so, as the substance remaining is fairly hygroscopic, it will take up
moisture from the atmosphere and may cause the surface of the sheets
to have a moist and shiny appearance. The moist surface deposit comes
away upon the hand when the sheets are touched, and is difficult to
remove entirely. On some estates a very small quantity of the
bisulphite is employed, as it is found to be of service in the
prevention of bubbles, but in unskilled hands the method is open to
abuse, and is, therefore, not recommended for general use.
A large number of estates now use sodium sulphite in very small
quantities as an anti-coagulant and a preservative for latex in the
field. The abuse of this very useful substance carries its own
penalty. The substance is hygroscopic; and if an excess is present the
drying period will be protracted, and the sheets will have a very
moist surface film.
It may be found sometimes that only some of the sheets are affected.
This indicates that, whereas uniform quantities of a solution of
sodium sulphite have been served out in all fields, the proportion may
have been excessive in the case of fields giving a latex of
comparatively low rubber content. What suits the latex from old trees
may be excessive probably for the latex of younger trees. This is not
an infallible rule, as in the case of older fields in which immature
bark is being tapped, it is to be noted that the dry rubber content of
latex may be less than that of latex obtained from younger trees.
This type of moist glaze is not easy to remove. Ordinary surface
washing had but a temporary effect, and the trouble recurs. The only
way of dealing with the difficulty is to soak the sheets for days in
running water (or in successive changes of water) and to re-smoke
until dry.
(_b_) The second type of moist glaze is not so difficult to deal with,
and may be removed as a rule by washing the sheets when the rubber is
otherwise apparently dry.
It appears to be mainly a matter of unsuitable fuel for smoking and of
failure to provide adequate ventilation. A large number of estates
have been "thinning-out" or are doing so systematically. The logs thus
obtained are often used as fuel in the very green stage. The smoke
thus generated must be moist, and if the building is entirely closed,
this moisture must be deposited eventually upon the rubber and racks.
Some estates have surmounted the difficulty by opening up the
roof-ridge slightly so as to allow the moisture to escape with some of
the smoke; but if the logs from rubber-trees are to be used, they
should be stacked in the sun for some time. Even then, preferably,
they should not be used alone. A judicious admixture of dead and
rotting jungle-timber appears to give very satisfactory results.
VIRGIN SPOTS AND PATCHES.--If the description really indicates the defect
it simply means that portions of the sheets are not dry. When cut they
exhibit the typical whitish, opaque appearance described as "virgin." It is
doubtful whether any rubber put upon the market as No. 1 product nowadays
can have this complaint levelled at it. In the extreme case it points to
gross negligence on the part of the packer.
Sometimes what are taken to be small spots of "virgin" are really patches
of tiny air or gas bubbles. The point can be easily determined by cutting
through the patch and examining the cut edges.
SURFACE MOULDS OR MILDEW.--During the last two years, complaints regarding
the incidence of "mouldy rubber" (_i.e._, relating chiefly to the presence
of mildews on the surface) have become increasingly common.
To judge by the comments of producers, who as a rule never again see their
rubber after it leaves the estate, one would infer that the defect is
imaginary, and that the complaints are made solely with a view to
repudiation of contracts or the general cheapening of an article of
commerce. They can often point out, with a certain amount of truth, that
there has been no change in the methods of preparation or curing, and that
previously there were no complaints.
It is forgotten, however, that in former years the smaller output of rubber
was taken into consumption more rapidly than of recent years. That is to
say, the interval between smoke-curing and the employment of the rubber in
the manufacture of goods did not demand such a long period of storage.
Hence the effects of smoke-curing are now more likely to vanish.
Going still further back in the history of plantation rubber, we can point
to the time when smoked sheets were allowed, or had, to remain in the
curing-sheds for very extended periods. Loose specimens of rubber prepared
during that decade still exhibit no signs of mildew growth.
In later years a demand arose for sheets paler in colour than the old type,
and in order to meet that demand, a change had to be made in methods. This
led to a system of working whereby it was possible to smoke-dry sheets
thoroughly in from twelve to fourteen days. This interval was further
reduced on many estates, until some were producing rubber which appeared to
satisfy all requirements after only five or six days' curing. This does not
refer to the case of estates having smoke-houses of "continuous-working"
type, but to those on which smoking was confined practically to the hours
of night. Under former conditions of rate of production and consumption,
this short period of smoke-curing would possibly have been ample; but even
this is very doubtful, as often the rubber would not stand the relatively
short journey from the estate to Singapore without mildew-growth being
incipient. We have often received specimens of rubber sent from estates for
criticism, and have noted that within a comparatively brief period mildew
was to be seen.
The whole matter resolves itself into a question of thorough efficiency of
smoking. This is not dependent on duration of smoking alone, but involves
other factors, such as the kind of fuel employed, the rate of combustion of
fuel, the average temperature sustained, the ventilation of the
smoke-house, and the situation of the building. Other occasional
contributory factors are contemporary adverse climatic conditions and the
possible abuse of an anti-coagulant such as sodium sulphite.
It has been shown that after a time, given suitable conditions involving
the presence of moisture, moulds may appear on sheets which were apparently
fully smoke-cured, and that under the same conditions other and older
samples were unaffected. It is argued that the latter sheets had evidently
been smoked more efficiently than the others. Hence it is fair to assume
that, except under very special conditions, which do not apply to the
ordinary procedure in the shipping, storage, and sale of rubber, moulds
will not develop upon sheets which have been properly smoked. The term
"properly smoked" signifies efficient smoking for all practical purposes
under ordinary procedure, and implies or includes all the advantageous
factors which have been discussed or alluded to in preceding paragraphs.
Without discussing in wearisome detail conditions which may give rise to
the incidence of mildew on properly smoked rubber, it may be pointed out
that the following are favourable to the growth of moulds:
(_a_) Storing sheets in a damp place before packing.
(_b_) Packing sheets in wooden cases which are not thoroughly dry.
(_c_) Piling up cases of rubber in a badly ventilated store-room.
(_d_) Placing the cases on a cement floor.
(_e_) Wetting of cases by sea-water or by rain during transport, etc.
BLACK STREAKS, SPOTS OR PATCHES.--The origin of these is not difficult to
trace. They are caused by drippings from the roof, and contain condensation
products from smoke plus moisture. The ventilation of the roof-ridge should
receive attention, and if the trouble persists it will be necessary to
place some absorbent screen below the sloping roof. Sackcloth is sometimes
used, but leads to a worse state of affairs unless changed frequently. In
most modern smoke-houses having an iron roof there is an inner lining of
soft timber.
There scarcely seems a necessity to discuss the case in which an iron roof
has become perforated by the action of smoke. The remedy is too obvious to
describe.
WHITISH OR GREY STREAKS.--This is a very uncommon defect, and is generally
to be traced to a building in which fairly new galvanised sheets have been
employed. The zinc surface becomes oxidised, and the whitish powder which
is formed "flakes," or is carried away by drops of moisture condensing on
the surface of the iron sheets.
RUST.--Sometimes if a sheet is stretched forcibly and allowed to retract
quickly, the hitherto clear surface will be seen to be marred by a "rusty"
deposit. The rubber is then described as "stretching rusty," and its value
is depreciated.
This defect has caused more trouble during recent years than any other. It
is not proposed here to argue the question as to whether the presence of
this film, which appears when some rubbers are stretched, is detrimental to
the physical qualities of the product on vulcanisation. With the mere
statement of opinion that it could do no apparent harm, we may pass to the
aspect of the case as it affects the buyer and consumer. If one were to
judge by the attention drawn to the appearance of smoke sheet-rubber after
it has been stretched and allowed to retract, one would imagine the defect
to be of comparatively sudden and recent incidence. This is not so. The
peculiarity must have existed for years, and perhaps became more marked as
so many estates abandoned the former common practice of allowing varying
quantities of water to be placed in the collecting cups. As the substances
which cause the defect to be visible are partially soluble in water, it
would follow that when working with the very dilute latices which were
characteristic of the earlier years of the plantation industry, the
remaining liquid in the pan after coagulation would contain an appreciable
quantity of soluble substances which would otherwise have been retained in
the coagulum.
Conversely, the richer the latex, the greater the percentage of protein
matter retained in the coagulum. In the case of very rich latex, it must be
within the knowledge of every manager that the quantity of remaining liquid
in the pans would be almost nil. We may assume that the greater part of
these soluble proteins would be enclosed in the structure of the rubber,
but as the fresh coagulum must retain a quantity of liquid amounting to
from 60 to 70 per cent. by weight (we are now referring to rich latices),
it follows that some of the soluble protein matter must be removed when the
coagulum is placed under pressure. Even after the pressure is released more
of the contained liquid will exude from the surface of the rubber; and from
experience it is easy to imagine that this exudation, becoming
progressively feebler, will continue until the rubber begins to dry. Then,
with the evaporation of the surface moisture, the protein matter, either in
original form or as a degradation product, remains on the surface of the
rubber as a thin, solid film or crust. As drying continues, the interior
moisture escaping through the pores of the rubber evaporates, leaving
behind the substances hitherto held in solution. Should, however, the sheet
be thick and/or the temperature of drying low, the rubber may dry first on
the outside, forming a thin skin of dry rubber, which delays further drying
indefinitely.
It will be seen, therefore, that sheets which have been prepared from rich
latex or from too deep a layer of comparatively dilute latex will have a
surface film of dry protein matter. Moreover, these sheets will be slow in
drying, and in all probability will have a surface gloss and a dark colour.
Hence it is not difficult to understand that some brokers regarded the
presence of the so-called "rust" as an indication of over-smoking.
To show that this is not so, and further that the presence of rust has
nothing whatever to do with smoke-curing, it may be stated that _the
presence of this protein film may be seen on unsmoked sheets_ which have
been prepared from rich latex, from too deep a layer of more dilute latex,
or from some thick sheets which have been rolled only very lightly. In
fact, the presence of the protein film was noted on unsmoked sheet in 1910,
when it was seen to resemble a thin yellowish glaze which could be scraped
off with a pen-knife. Later, sufficient of this substance was removed from
some very thick air-dried sheets, or thin slabs, to fill a small test-tube.
When the sheets were bent or twisted, the apparent surface of the rubber
(_i.e._, the protein glaze) cracked in all directions. In the case of
sheets prepared from less rich latex, the surface film naturally is
extremely thin, and no cracking is observed.
If the fresh sheets are placed in a smoke-house, the drying film will take
up colour from the constituents of the smoke, and it will be invisible.
Somewhat analogous to the instance of a transparent glass giving a visible
and opaque powder when crushed, so the transparent film on stretching
breaks up into a visible powder which is lighter in colour than the rubber
on which it is superimposed.
It will be noted that since the introduction of standard methods of
preparation, involving uniform dilution of latex, say, to a content of
1-1/4 or 1-1/2 lbs. dry rubber per gallon, complaints as to "rust" have
decreased considerably.
It is to be further noted as a peculiar fact that while two estates may be
apparently working on identical lines, both as regards manipulation of
latex and subsequent treatment of the coagulum, the rubber of the one may
always be free from rust, while that of the other is often, if not always,
condemned for the alleged defect. Obviously, in such a case, there must be
an initial difference between the two latices as regards the percentage of
proteins present; or there must be some small unrecognised difference at
some stage of working.
It will now be clear that "rust" is caused by a film of matter which is
formed on the surface of the pressed coagulum, being there deposited by the
exudations from within the rubber and through the pores. It is, therefore,
necessary to avoid any conditions which will favour the formation of this
deposit--_e.g._, allowing sheets to remain too long in a moist atmosphere
before placing in the smoke-house.
At present there would seem to be only two methods which are successful in
the prevention of a "rusty" appearance in the dry rubber. Singularly
enough, the two methods appear to be directly opposed in principle. They
are:
A. THE HOT-WATER TREATMENT.--This method has been in constant use on
estates which have old trees giving rich latices. These latices are
always diluted to a uniform standard daily. Some estates which
formerly suffered from the defect now experience no difficulty, and in
other instances, where no complaint has yet been received, the
treatment has been followed consistently.
(1) After the sheets have been through the marking rolls, it is the
general custom to allow them to drip for about three hours. This
interval is really excessive for the mere draining away of the surface
water, but as a rule it is just sufficient to allow a portion of the
liquid retained in the rubber to exude. It has been shown that this
liquid may contain some protein matter in solution. Sometimes in the
case of thick sheets which have been subjected to pressure so much of
this matter is exuded as to form a thin surface slime which is
distinctly evident to the touch. If the sheets are allowed to hang
overnight, the presence of the exuded matter may be detected also by
its odour.
(2) Obviously, any method which will remove this surface film should
be of great benefit. It is found that the best results are obtained by
allowing sheets to drip for about two hours, and then placing them in
hot water for five or ten minutes. The water should be hot as the hand
can conveniently bear, and it need hardly be pointed out that the same
water should not be used for the whole day's output. For preference
there should be three or four vessels, each capable of holding a fair
proportion of the total number of sheets, and frequent changes of hot
water.
(3) After remaining in the hot water for the period mentioned, the
sheets are removed singly, each one being surface washed or swilled as
it is taken out.
(4) _It is important to see that the sheets are now well washed or
scrubbed under running cold water, or in frequent changes of water._
The reason for this procedure is plain. If the sheets are merely hung
again to drip after removing from the hot water, some moisture is
bound to remain on the surface of the sheet. As this surface moisture
contains some protein matter in solution, it is evident that, as the
water evaporates, the solid protein is again deposited on the surface
of the rubber. This would explain why some estates were unsuccessful
with the hot-water treatment. It is not essential that the running
water should be cold; it may be conveniently lukewarm if drawn from
the cooling tanks of the engines. But it is essential for the best
results that there should be running water, so that the substance in
solution is carried away. If the sheets are merely washed in a large
vessel, which has been filled with clean water, it must be obvious
that, by the time some scores of sheets have been washed, the protein
matter in solution on the surface of the sheets has been transferred
to the washing water, so that the later sheets of the batch are liable
to show the defect again on drying.
B. The second method is much more simple, and entails no extra labour
such as is demanded by the first method. A successful issue, however,
is rather more uncertain, and the method appears to give the best
results with sheet-rubber prepared on young estates or from more
dilute latex.
In this method, the sheets after rolling are allowed to drip for a
very short interval, so that the surface water is mainly removed. The
sheets are then placed in the smoke-house, and smoking is commenced at
once. In some cases where the defect had appeared continuously for a
long period, it was found to vanish entirely as soon as the method was
adopted; but when tried on some of the older estates, the results were
very doubtful, and a return was made to the hot-water treatment.
The explanation of the action which takes place is rather obscure, but
two theories may be advanced.
(_a_) It may be assumed that the interval given for dripping is too
brief to allow for the exudation of the internal moisture containing
dissolved protein matter.
In such case, the rubber is still in a highly porous condition, and it
might be advanced that the heat of the smoke may help to maintain that
condition. Thus the contained liquid might evaporate so quickly as to
leave behind the dissolved substances in the minute cellular structure
of the rubber. In other words, instead of the internal moisture
exuding slowly to the surface in liquid form, it may leave the
rubber, even in the first stages, in an evaporated condition, just as
it does in the subsequent stages of drying. Thus no dissolved protein
matter would be brought to the surface of the sheet and be deposited
there.
(_b_) The other theory also demands the first assumption propounded in
the preceding theory, but subsequently perhaps is less feasible as it
assumes a chemical action of which we have no definite knowledge.
The idea is that as the rubber is in a porous condition, and is placed
quickly in an atmosphere of smoke, the heat may maintain that
condition to such a degree, that some constituents of the smoke may
enter the rubber and cause the precipitation _in situ_ of the protein
matter held in solution by the contained water or other liquid. The
contained liquid would be water which has in solution possibly a very
slight trace of the coagulant employed, of sugars, of protein matter,
and of inorganic salts. Of these the substances which would evaporate
would be probably the water and the coagulant in most cases. If a salt
had been used as a coagulant, the dissolved trace would be deposited
within the rubber in this case, whereas if a rich latex had been
employed or a thicker sheet made from more dilute latex, some of the
salt would be brought to the surface and there deposited together with
the protein matter. This has actually been experienced in practice,
and it has been possible to remove minute crystals from the edges of
the rubber so prepared.
It will be evident that in order for either theory to contain an element of
probability, the rubber must be soft (porous) when placed in the
smoke-house, and must also be fairly thin. It is observed in all cases
where the method has been successfully employed that both these conditions
are generally fulfilled--at all events the rubber is fairly thin. When
thicker sheets are made, either from rich latex or from a deeper layer of
comparatively dilute latex, the method is not uniformly successful.
OTHER VIEWS ON "RUST" CAUSATION.--Later experimental work on "rust"
formation by Hellendoorn[22] leads to the observation that "rustiness" is
caused, not actually by the deposition of original serum-substances, but by
the decomposition thereof, under the action of aerobic micro-organisms.
[22] "The Cause of Rustiness in Sheet-Rubber," H. J. Hellendoorn, Archief
voor de Rubbercultuur, October, 1919 (Communication from the Central Rubber
Station, Buitenzorg, Java).
Without going into a full discussion of the subject, the following points
noted in the experimental work may be quoted:
1. Rustiness could apparently be produced at any time merely by
keeping freshly rolled sheets for periods varying from twenty-four to
forty-eight hours in a moist atmosphere.
2. Sheets placed immediately in a temperature of, say, 110° to 130° F.
never showed "rust"; but if air-dried at ordinary room temperature,
"rust" might appear.
3. "Rust" can be prevented by soaking freshly prepared sheets in
dilute solutions of disinfectants--_e.g._, formalin, sodium
bisulphite, or chinosol.
If subsequently the sheets are hung for any length of time in a moist
atmosphere, the protective effect of the disinfectant gradually
vanishes and "rustiness" may be produced.
The same disinfecting effect may be obtained by the use of steam or
hot water. It was found that there was less liability to the formation
of "rust" when sheets were immersed in water at a temperature of 95°
to 120° F., whilst steeping at 140° F. gave complete freedom.
4. It was shown that the micro-organisms which cause decomposition of
the serum-products flourish only in the presence of air--_i.e._, they
are aerobic in character. It is not uncommon to find, therefore, that
"rust" may be incident only on those parts of a sheet which have been
exposed for some time to air and moisture before being placed in a
warm smoke-room.
5. The optimal temperature for development of the particular organisms
appeared to be about 100° F., in a moist atmosphere.
6. Soaking the sheets in water (except the short immersion in hot
water, which we recommend), even for a period extending over a week,
does not hinder the formation of "rust."
7. Rustiness may be prevented by placing the sheets in a sufficiently
warmed smoke-house as long as there is adequate ventilation and a
moist atmosphere does not persist.
The simplest means of prevention is to soak the sheets first for a
short period in water, and then to hang them to drip for a few hours
in a well-ventilated place, outside the factory and under cover.[23]
[23] We advise and practise hanging sheets in the open, without shade or
cover.
It will be gathered that, although there may be a slight difference between
our previous views and those advanced by Hellendoorn as to the exact cause
of formation of the "rusty" film, the general conclusions are identical
with those given by us in preceding paragraphs and previously advised in
the Malayan reports of the Rubber Growers' Association.
BUBBLES.--The presence of bubbles in sheet-rubber has for years been the
bane of some managers' existence, and the bone of contention between
sellers and buyers. Taking the argument down to bed-rock, producers urge
that the presence of bubbles has no influence upon the ultimate quality of
the rubber on vulcanisation. They assert that the alleged defect is merely
a peg upon which to hang an unreal grievance, serving the purpose of the
buyer under the existing conditions of sale. All this may be true, but as
long as the present system continues, it must be recognised that "kicking
against the pricks" is a futile recreation.
The sympathy of the writers is certainly on the side of the producers,
inasmuch as they realise how extremely difficult, and even impossible at
times, it is for the most careful individual to prepare sheet-rubber free
from this blemish.
Much has been written, and many have been the discussions, on this vexed
subject; and it is recognised that sometimes, in spite of all precautions,
there may suddenly be an incidence of bubbles in rubber which is ordinarily
free from them. It must be allowed that climatic conditions and
physiological variations affecting the metabolic functions of the trees
exert an influence which is difficult at times to combat, and often beyond
human control.
The contributory causes are many and varied. It should be premised that,
although the defect is described as "air-bubbles," it is seldom that the
appellation is strictly correct. Rarely do the bubbles contain air. In the
vast majority of cases they contain gases in minute quantity. These gases
may be considered to arise, broadly, from some decomposition of substances
(other than rubber), contained either in the coagulum or in the serum. In a
general way, if this decomposition is evidenced by an unpleasant odour, it
is described under the term of "putrefaction." We are not concerned here
with the question as to how far such decomposition may be ascribed to a
purely chemical action, or to the indirect result of the presence of
certain bacteria or other micro-organisms. Suffice it to state that, at
least as far as field operations influence the result, the decomposition is
generally to be attributed to the work of micro-organisms. Conditions
favourable to the incidence and development of these are to be found when
absolute cleanliness in all details is not aimed at.
With this preamble we may proceed to classify possible causes of the
formation of bubbles into two groups:
(_a_) Those originating in field operations.
(_b_) Others which may arise after the arrival of the latex at the
factory.
IN THE FIELD.--Decomposition may be caused by:
(1) Spouts, buckets, and cups being dirty. Regular cleaning is
necessary. If the buckets are allowed to be taken to the lines by
tappers, trouble may ensue. Within the writers' experience it has been
shown that an otherwise baffling case of premature flocculation of
latex was traced to the presence of acid substances in the buckets,
which had been used by coolies for preparing their food.
(2) Delay in commencing work. This means similar delay in collecting
the latex which is exposed to greater heat than under ordinary
circumstances.
(3) Exposure to the sun's rays. The heating of the latex may provide
improved conditions favourable to the development and action of
micro-organisms.
(4) Allowing latex to stand too long before collection. This usually
is the result of giving tappers too great a task.
(5) The addition of water to the latex, either purposely or
accidentally, in the form of rain. The water may be slightly acid in
character, or it may carry micro-organisms from the bark into the
latex.
(6) Tapping trees at too great a height. The latex generally has an
abnormal distance to travel before reaching the cup.
(7) Sometimes the latex from old trees, or from trees after wintering
(just prior to full renewal of leaf), contains more than the usual
proportion of substances (_e.g._, sugars), which are capable of
effecting flocculation or coagulation.
(8) Too great a distance for transport. The trouble arising from this
cause is likely to be much increased if the journey has to be made
over bad roads. In such case the physical action augments the effect
likely to be produced by long standing.
The foregoing do not include all possible causes, but serve to indicate the
directions from which trouble may be mainly anticipated. It will be plain
that any latex which exhibits symptoms of premature coagulation (or minute
flocculation) should be regarded as a potential source of bubbles in
sheet-rubber.
It will be equally obvious that the employment in the field of any harmless
substance of an anti-coagulant nature is to be encouraged. This point is
discussed in detail in Chapter V.
IN THE FACTORY.--As a general rule it may be understood that the mischief
has been done before the latex is handled at the factory. Sometimes it is
perceptible from the peculiar appearance of the latex, and in such case the
batch should not be used for the preparation of sheet-rubber. Often it is
found that only the last to arrive at the store is visibly affected. This
should not be mixed with other apparently normal latex, as it is capable of
acting as a "leaven" to the bulk.
Contributory factors in the store are:
(1) Lack of cleanliness of utensils, particularly of coagulating
dishes or tanks.
The trouble becomes acute sometimes where wooden tanks are employed.
Unless the tank and the partitions are thoroughly and regularly
cleansed, the wood may become coated with a bacterial slime, which is
capable of causing what may be termed "fermentation" of the latex
layers in contact.
The tank should be thoroughly cleaned occasionally with a weak (5 per
cent.) solution of sodium bisulphite. The partitions should be
scrubbed and placed in the sun twice or three times a week.
(2) Allowing latex to stand too long before treatment. This point
needs no further expansion.
(3) The use of a latex of too high a rubber content. Such latices are
difficult to handle in order to secure uniform mixture with the
coagulant.
(4) The use of too concentrated a solution of coagulant. In
conjunction with (3) there may be a rapid and irregular coagulation,
giving rise not only to decomposition in parts (and subsequent
formation of gas), but also to the formation of true "air-bubbles" by
inclusion of air during stirring.
(5) The use of insufficient coagulant. Coagulation is slow and
incomplete.
(6) Defective straining and skimming. Small flocculated particles of
rubber may pass, or be rubbed through, the strainer. If allowed to
remain, they act as local points of danger.
(7) The proximity of the coagulating latex to some source of heat, or
exposure to sunlight.
(8) Any delay of drying in the preliminary stages, either before or
after the rubber enters the smoke-house.
BLISTERS.--This description aptly fits the case in which sheet-rubber in
the smoke-house exhibits large bubbles of gas which distend the surface of
the rubber. When subjected to pressure, small "balloons" are formed, which
burst with a perceptible report. It was formerly the belief that this
defect was occasioned solely by an abnormally high temperature. That such
is not the case can be shown by the experience of estates which have had
only the rubber of a particular day or short period affected under normal
factory conditions.
At the same time it is not disputed that the heat of the smoke-house exerts
an influence (causing expansion and distension), but it is advanced that
the gases had begun to form before the rubber entered the house.
The view held is that decomposition had supervened or was taking
place--probably from one or more of the causes enumerated in the preceding
paragraphs. The heat of the smoke-house only serves to exaggerate the
effect. It is acknowledged that the degree of decomposition must be
initially greater than in the ordinary incidence of "bubbles."
Beyond this point we are not in a position to put forward any definite
supposition as to the apparently haphazard occurrence of the phenomenon.
It is to be noted, fortunately, that the defect is comparatively rare, and
seldom appears on estates which employ an anti-coagulant in the field.
While we have examined persistent cases, one of which led to a temporary
discontinuance of the preparation of smoked sheet rubber, we have not yet
been able to arrive at any satisfactory idea of the exact conditions
governing the incidence of "blisters." Our investigations only lead us to
two observations:
(_a_) That blisters have appeared on the rubber of some estates after
wintering, and during the period of new leaf-development.
(_b_) That the defect has been noted on other estates during a period
when there were frequent but not heavy rains, and when there was a
comparatively low average temperature.
In either case, as the factors are beyond human control, it would be
expected that without any change being made in estate procedure, the
trouble would vanish as mysteriously as it appeared. This is our
experience; but as showing the possible intensive effect of a high
temperature in the smoke-house, it may be remarked that very infrequently,
in a batch of sheets exhibiting ordinary bubbles, a few hanging directly
above the furnaces show signs of a slight blistering effect.
"SPOT" DISEASE IN SHEET RUBBER.--That "spot" disease may appear in
air-dried sheets was evident at the beginning of the outbreak in the spring
of 1911. The first cases noticed took the form of pink and bluish "blushes"
spreading over the whole of the sheets. Later, fungoid spots began to
appear. These mainly took the form of red or black blotches, and were very
unsightly. As "spot" disease cannot develop in smoked rubber, the obvious
and simple course to adopt was to smoke-cure the sheets. When it is stated
that "spots" do not develop in smoke-cured rubber, it is understood that
the smoke-curing must be efficient and must commence as soon as the rubber
has been rolled, and the surface water has drained away. If the sheets are
allowed to air-dry for a few days, the disease may develop, and then
smoke-curing will not get rid of the coloured patches. The operation of
smoke-curing will not get rid of the coloured patches. The operation of
smoke-curing may tone down the colour, but the spots would still remain
evident.
SUPPORT MARKS.--It frequently occurs that one sees across the middle of
smoked sheets a wide mark. This is where the wooden support in the
smoking-chamber has been. As a rule, even in the most careful cases a faint
mark may always be seen, but in many instances this mark is exaggerated to
such an extent as to point to lack of care on the part of the store
supervision. If bays of racks remain empty over-night, they may possibly
become covered with a light sprinkling of fine wood-ash and tarry deposit.
Wet rubber placed upon these racks will pick up and retain the impurities,
and more often than not they cannot be washed out. It is incumbent upon the
manager to see that empty racks are thoroughly cleansed before placing wet
rubber upon them. The better plan is to arrange that the bars can be
removed easily from sockets. There should be in stock sufficient "spares"
for, say, two days' rubber. When the dry rubber is removed, the bars should
likewise be taken away, to be cleansed and kept in the factory until again
required. This will ensure that fresh rubber always rests upon a clean
support.
On some estates, in order to guard against a pronounced "bar-mark," sheets
are moved and turned daily.
In other smoke-houses the upper surface of the bar is chiselled in concave
form, so as to admit of the passage of smoke below the surface resting on
the bar.
STICKINESS.--This is not to be confounded with "tackiness," from which the
rubber does not recover. Stickiness is only temporary, and may be remedied.
As a general rule, it is due to packing sheets, which have not a good
raised "ribbing," and which may have been coated with light tarry deposits
(see Glaze). This surface film may be removed by washing the sheets, or
scrubbing them, with cold water. Usually a further two days' air-drying
will make the rubber fit for packing; and if the smoke-curing has been
efficient, there should be no need to anticipate trouble from mildew. Some
estates adopt this practice daily with success, as a form of insurance
against complaints of surface deposits.
RIBBING, SURFACE PATTERN.--While we know that the passing of sheets of
rubber between rolls, causing a particular raised pattern to appear, has no
effect upon the actual quality of the rubber, there is a great deal of
practical advantage gained.
The practice ensures an increase of superficial area which is an aid in
drying, improves the appearance of the rubber for selling purposes, and is
of distinct advantage in enabling the rubber when packed to travel in
better condition. Sheets do not become so closely packed; sampling and
general handling are easier on delivery.
As long as the plane surfaces are sufficiently and regularly distorted,
there would seem to be no limits to the type of pattern or "mark" which may
be placed upon the rubber. But in actual practice the variety is small. The
most popular type of "ribbing" is that best described as a small diamond
effect, produced by a pair of rolls cut with closely placed narrow grooves
running spirally. The spirals travel in the same direction on both rolls,
producing close-cut ribbing running in opposite directions on the surfaces
of the sheet. On sheets of standard thickness, the result approaches a
diamond effect.
A few other patterns are employed, notably that producing longitudinal
stripes of varying thickness. On the whole, the type of pattern would seem
to be immaterial, if the points already indicated are achieved.
It is seldom one encounters a case nowadays in which the "marking" is
unsuitable, but a few estates may be using an old type of patterned roll on
which the full diamond grooving is cut. As this appears on both sides of
the sheet of rubber, and as the ribbing does not coincide, a blurred effect
is seen when the sheet is viewed against the light.
THICK ENDS, "SHEET CLIPPINGS."--It rarely happens, even with good equipment
and average supervision, that the preparation of smoked sheet is
unaccompanied by slight defects. For instance, in spite of rules and
regulations regarding manipulation of the coagulum, it is not uncommon to
find that some sheets, after rolling, have slightly thickened ends. In the
ordinary course of events these might delay drying considerably. It is the
practice on some estates to cut off these thickened ends while the rubber
is still wet. The pieces are then machined into crepe form, but as no
sodium bisulphite may have been used, the resulting rubber cannot be
classed as No. 1 Standard Crepe.
The other alternative is to trim the ends when the bulk of the rubber is
thoroughly smoke-dried. The moisture containing portions are then returned
to the smoke-house until dry, and are subsequently packed without further
treatment as "smoked-sheet clippings." It will be plain that, except in the
particularity of form, these clippings differ in no degree from the
original sheets; and, owing to extra smoke-curing, may arrive in even
better condition. One must be prepared, however, to find that a slightly
lower price is offered. Whether the price obtained would be comparable with
that commanded by the crepe made from wet sheet slipping would depend upon
general ruling market conditions, and the degree of care exercised in
guarding against the inclusion of any inferior pieces of rubber. In
ordinary factory practice, there could be no room for abuse under the
latter clause.
OTHER INFREQUENT DEFECTS.--This chapter will be closed with a reference to
other small defects which, although infrequent, cannot be classed as minor
complaints. In point of fact, when they occur, they assume an importance,
in the eyes of the consumer, which is not, perhaps, sufficiently
appreciated by producers.
DIRT.--Trouble from this source should be absent, but carelessness on the
part of packing coolies may be responsible for occasional complaints. How
the dirt is incident may remain a mystery, but it has been noted that
sheets have at times been thrown upon a cement floor. A certain amount of
loose dust may thus adhere to the rubber.
ASH.--The source of this surface deposit scarcely needs indication. Where
open-hearth furnaces are employed, and the wire-mesh floor screens are not
perfectly sound, fine ash may find its way into the upper chamber. If this
trouble is persistent in spite of precautions, the sheets should be
surface-washed and air-dried before packing.
BARK.--Complaints of the presence of particles of bark in sheet rubber used
to be fairly frequent, but are now less common. The trouble may be traced
to the use of defective straining sieves when the latex is being handled.
SPLINTERS.--The use of packing-cases of unplaned soft timber is responsible
for complaints of this nature on delivery. Without here discussing the
larger question of the ideal packing case, it is sufficient to emphasise
that the interior surfaces of wooden chests should be planed. The cases are
often so damaged in transit, that splinters of wood may be found throughout
the contents. The device adopted on some estates may go far to prevent
this. The cases are first lined with loose sheets, and finally other sheets
are arranged to overlap at the top of the case. The bulk is thus enclosed
in a wrapping of sheets, and any splinters or other deleterious substances
are confined to the surface of the mass.
PART V
GENERAL
CHAPTER XIX
_CHOICE OF COAGULANT_
Almost without exception, the agent employed in the coagulation of
plantation (_Hevea_) rubber is acetic acid, or in some cases formic acid.
Under ordinary trade conditions supplies are always obtainable at
reasonable prices, but during the recent War the question of possible
substitutes was brought greatly to the fore. Fortunately the subject of
coagulation and coagulants had been previously studied to such effect in
laboratory practice, that there would have been small difficulty in
prescribing agents other than acetic acid in cases of expediency. As far as
our knowledge extends, all the possible substances which have the power of
coagulating latex have been tested. They include mineral acids, organic
acids, compounds known chemically under the general term of "salts,"
alcohols, sugars, etc.
The heading of this chapter must be seen to "beg the question," inasmuch as
it leads to the assumption that a coagulant (in the popular sense) is
necessary to secure coagulation. In point of fact, methods are sometimes
employed which depend upon no artificial coagulant to produce the desired
effect. To these methods reference will be made later.
In this section it is proposed to describe briefly the more important
agents which are used, or might be used, in effecting coagulation. In the
class of those which are not in common use some could be used as
expedients, while others are only of scientific interest.
ACETIC ACID.--There is no need to enter into a discussion of the merits of
this agent. In practice it remains the cheapest and safest coagulant known
at present.
FORMIC ACID.--This agent is equally as safe to use as acetic acid, and as
easy to handle. Bulk for bulk its coagulative power is higher than that of
acetic acid. Its pre-war shipping price, when taken in conjunction with its
coagulative power, was slightly below that of acetic acid, but local prices
put the balance in favour of the latter. If prevailing costs put it on
terms of parity with acetic acid, there would appear to be no reason why
formic acid should not have a widely-extended use on plantations.
CITRIC ACID, TARTARIC ACID.--The acids of the extracted juices of most
tropical fruits consist, to a large degree, of citric or tartaric acids.
These can be used in place of acetic acid as satisfactory coagulants in
case of emergency; but the questions of availability of supplies and of
costs preclude their more general adoption.
OXALIC ACID.--This is a satisfactory coagulant as far as observed effect is
concerned. It produces a rubber paler than ordinary coagulants (without the
use of sodium bisulphite), as it has the nature of an anti-oxidant.
It would not be a safe agent in the hands of coolies, as it is classed as a
poison.
SULPHURIC ACID.--During the War, in a period of shortage of acetic acid and
of high prices, this agent was used with success on some estates.
It scarcely need be remarked that it is a dangerous substance to handle,
and that its employment must be accompanied by close European supervision.
At prevailing prices during the War it was very much cheaper than acetic
acid, and even at the present reduced cost of the latter the advantage
still lies with sulphuric acid.
It must be emphasised, however, that the abuse of this agent to any but the
slightest degree is harmful to the resultant rubber. Hence its use would be
sanctioned _only in the absence of the commoner, and much safer,
coagulants_.
In view of the possible incidence of such an emergency, the following
notes are given. It is impressed that strict adherence to the rules must be
given.
HANDLING SULPHURIC ACID.--(_a_) Always use glass or glazed earthenware
vessels.
(_b_) Pour slowly and avoid splashing. Drops finding their way to clothing
or other fibrous material will destroy it locally; and if thrown upon any
part of the body may cause painful burns.
(_c_) When diluting this agent always remember to pour the acid into the
water (_i.e._, the lesser into the greater), and never _vice versa_. Pour
the acid carefully and slowly down the side of the vessel, and stir well.
(_d_) Should strong acid be spilled, do not throw water upon it. A supply
of sand or dry earth should be kept close at hand. Throw this upon the
acid.
STORING SULPHURIC ACID.--(_a_) Jars or cases should be handled as seldom,
and as carefully, as possible. If the acid is contained in a case, the top
should be plainly indicated.
(_b_) Stocks should be stored in a detached building which should not be
damp. Jars or cases should not stand on a wooden floor if possible.
(_c_) See (_d_) above.
BUYING SULPHURIC ACID.--(_a_) Commercial acid of specific gravity 1·84 is
the best of its kind. It contains impurities which are non-injurious to
rubber preparation.
(_b_) It is always advisable, if possible, to buy the acid in small jars
containing not more than 100 lbs. each. Smaller jars, with a content not
exceeding 50 lbs., would be preferable.
(_c_) If the acid is bought in jars, it should be stipulated that the
stoppers be covered with a plaster head, and that the containing crate or
case should have prominent labels or marks indicating the top of the case.
FORMULA FOR USE OF SULPHURIC ACID.--It will be understood that as this
formula has been calculated for working with latex, having a consistency of
1-1/2 lbs. dry rubber per gallon, it applies in a strict degree only to
such latex. In other cases, where the dilution of the latex is not known,
the formula will serve as a basis for experiment until the correct quantity
has been discovered.
(Sulphuric acid of specific gravity 1·84.)
NOTE.--The directions must be followed carefully, and glass measuring
vessels should be used if procurable.
(_a_) Measure out 1 pint of strong acid, and pour it carefully and
slowly _down the inner surface_ of a jar containing 20 gallons of
water. Do not pour it directly into the water.
The heavy acid will sink to the bottom of the jar, and a good mixture
must be obtained by stirring well.
(_b_) Of this solution (which is approximately 1 per cent. by weight),
use 1 gallon to 20 gallons of latex.
Readers are doubtless now well aware of the corrosive action of strong
sulphuric acid, and we scarcely need point out that even the dilute acid
should not be kept in contact with the usual iron vessels found in
factories. The mixing of solutions should be done in one of the glazed
earthenware jars commonly in use.
The formula given above works out at approximately 1 part strong acid to
2,000 parts of latex (of dry rubber content 1-1/2 lbs. per gallon). The
formula for using acetic acid with the same latex works out at about 1:
1,200. It will be apparent, therefore, that relatively sulphuric acid is a
more powerful coagulant than acetic acid. In terms of dry rubber obtained
from latex of the consistency indicated above--
1 lb. sulphuric acid will produce 300 lbs. dry rubber. 1 lb. acetic
acid will produce 180 lbs. dry rubber.
With both acids selling at the same rate, sulphuric acid would be more
economical in use; when its cost is less than that of acetic acid, which is
the normal condition, the economic advantage in favour of sulphuric acid is
augmented still further.
It may be found that the standard formula for sulphuric acid will not
always give a perfectly clear remaining serum, even though an attempt is
made daily to work to a uniform consistency for all latices. It is
inevitable that the manipulation of the latices should be slightly in error
on occasions, or that a small mistake might occur in preparing the solution
of acid. Hence a clear remaining serum after coagulation may be secured
less often than a slightly turbid serum. This is as it should be. The
minimum quantity of acid may be adjusted so closely as to give such
results. If a clear serum is obtained always, that should be an indication
of continual excess of coagulant. Naturally, if a milky serum is always
obtained, the reverse is the case.
As a last word on the subject, it may again be emphasised that the use of
sulphuric acid is not advised, except in an emergency; and that the
greatest possible care must be exercised in the observance of the strict
formula for use.
HYDROCHLORIC ACID, NITRIC ACID.--These mineral acids would prove more
expensive than sulphuric acid. In addition they are much more uncertain in
action. For example, the use of a certain excess of hydrochloric acid would
not hasten coagulation, but would prevent it. Above all their effect, in
excess, is deleterious to the rubber.
HYDROFLUORIC ACID.--This has a strong corrosive action on porcelain or
glass. Hence it has to be contained in bottles of gutta-percha or lead. It
is mentioned here merely because some years ago it found a use as a
coagulant, chiefly in Ceylon. It was sold in the form of a 10 per cent.
solution under the name of "Purub," and was the subject of a patent.
It is effective as a coagulant, and has also an anti-oxidant action, which
was its chief recommendation when cheap and harmless anti-oxidants were not
commonly known. It is comparatively expensive, and, as indicated above,
difficult to handle and store. In short, it has nothing to commend it, in
comparison with acetic or formic acids.
ALUM.--This substance has been used for years by native rubber producers as
a coagulant. It fulfils the desired purpose, and its popularity was
maintained because of the ease with which it could be stored and handled.
Unfortunately, this facility often led to the use of an excess, and native
sheets were often criticised as being brittle. Investigations have shown
that alum, even in minimum proportions, has an appreciably harmful effect
upon the quality of the rubber prepared by its use as a coagulating agent.
Its employment by native rubber producers has now been largely superseded
by acetic acid in some form.
PYROLIGNEOUS ACID.--This is otherwise known under the names of "crude
acetic acid" and "crude wood vinegar." Owing to the shortage of acetic acid
during the War, attention was directed towards the possibility of making an
effective coagulant locally by what is termed the "dry distillation of
wood"--_i.e._, the wood is not burned but heated in a retort. The enquiries
could be placed in two classes:
1. Those which aimed at making the pure, strong acid of commerce.
2. Those which sought information concerning a crude coagulant
(pyroligneous acid) on estates.
Regarding the first class, we can do no better than reproduce our remarks
published in the April local report of the Rubber Growers' Association for
1916--with the reservation that, on account of a threatened shortage of
timber, a local scheme might not now be feasible:
"Probably the most common enquiry encountered since the rise in the
price of acetic acid is concerned with the possibility of making
acetic acid in this country. It may be stated that the proposition is
a feasible one, even on a fairly large scale. We have the essentials
necessary for such a scheme in:
"1. A good supply of suitable timbers, the most valuable of
which, possibly, is mangrove timber, locally known as 'bakau.'
Other suitable timbers are known, but as far as preliminary
experiments show mangrove timber gives the best yield. At present
this timber is in great demand as a fuel for steam plants, but
with the extension of the local coal industry the timber may
become cheaper.
"2. There would appear to be less valuable timber which would be
suitable for heating the retorts. Or, local coal might be used.
"3. Supplies of lime at reasonable rates are available, as the
limestone formation in the peninsula is quite considerable in
extent.
"4. Supplies of sulphuric acid are available from Japan,
Australia, Burma, etc., even at the present time, although
naturally rates are higher than normal. Under ordinary
conditions, supplies from England and parts of Europe would be
much cheaper than at current rates.
"For the benefit of many readers perhaps a brief and nontechnical
description of the preparation of acetic acid would not be amiss,
and would explain the necessity for the essentials indicated
above. In brief, the process is as follows:
"(_a_) A suitable timber is heated in a closed retort. This is
termed 'dry distillation,' and results eventually in the
carbonisation of the wood--_i.e._, charcoal is obtained in the
retort.
"(_b_) Tar, vapours and gases are distilled over during the
carbonisation of the wood. These liquors and gases pass through
condensers. The gases pass away, while the condensed liquors
separate out into (1) wood tar, (2) a watery liquor called
pyroligneous acid or crude wood vinegar.
"(_c_) The pyroligneous acid is separated from the tar, and again
distilled to obtain the acetic acid present.
"(_d_) This crude acid is steam-heated with milk of lime, which
fixes the acid, forming calcium acetate (or acetate of lime).
"(_e_) Eventually the calcium acetate is taken out in the form of
a thick paste, which is spread to dry. When dry this 'grey
acetate' is the main source of all glacial acetic acid now made.
"(_f_) The acetic acid is released from the 'acetate of lime' by
the action of sulphuric acid. It is then distilled several times,
and under various conditions, in order to increase its strength.
In the past copper tubes were used for this purpose, but owing to
the fact that traces of copper were found to be injurious to
rubber, some works instal tubes of glazed earthenware for the
distillation.
"Such is the process in outline, and it will be seen that no proposal
to manufacture _glacial acetic acid_ on an estate could be considered
feasible, although it would not present any great difficulty on a
large scale and under skilled direction. Furthermore, the cost of the
plant would be far too great for any estate."
Although it is clear that pure acetic acid is beyond the scope of an
estate, crude pyroligneous acid has been produced on a varying scale in
this country and in Ceylon. In the latter country some success was obtained
by the distillation of coconut shells with comparatively inexpensive plant.
In this country, wood-distillation was practised on a few estates, but
improved facilities for obtaining pure acetic led to a termination of the
experiments, although sufficient crude acid could then be made at a
reasonable cost.
The pyroligneous acid obtained, is generally clear, after nitration, and of
a dark brown colour. It has a peculiar odour reminiscent of smoked
sheet-rubber, or of creosotic substances in general.
Its acid content depends chiefly upon:
(_a_) The kind of timber heated in the retort.
(_b_) The efficiency of the apparatus.
(_c_) Condition of the timber as to moisture.
(_d_) The temperature employed, and rate of working.
(_e_) The point at which distillation ceases (_i.e._, the duration of
interval between commencement of heating and cessation of collection).
Samples received from estates for testing purposes were found to contain
equivalents varying from 2 per cent. to 10 per cent. of acetic acid.
They were all suitable coagulants when used in quantity calculated from the
discovered acidity, but produced rubber darker than ordinary when
air-dried. This effect was not of much importance in the preparation of
smoked sheets, but to produce a pale crepe it was necessary to employ
sodium bisulphite as an anti-oxidant.
This darkening in colour is to be ascribed to the presence of traces of
phenols,[24] which are stated to exert an effect upon the rubber during and
after vulcanisation.[25] This subject will be discussed in another section.
[24] Whitby, _Journal Soc. Chem. Industry_, vol. xxxv., No. 9, 1916.
[25] See also "Preparation and Vulcanisation of Plantation Rubber" (Eaton,
Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture,
April, 1918.
With this provision the crude pyroligneous acid which can be produced on
estates, could be employed as a coagulant until such time as the price of
glacial acetic acid was so low as to make the production of the crude acid
non-profitable. This point would be determined from a knowledge of the cost
of production per gallon, and the percentage of acetic acid per unit. For
example, if the cost of production (including cost of timber for
distillation, cost of fuel for heating the retort, cost of labour, etc.)
was 60 cents per gallon of crude acid containing 9 per cent. of acetic
acid, that would be equivalent approximately to buying glacial acetic acid
at $30 per demijohn of 44 lbs.
SMOKED WATER.--A weak solution of pyroligneous acid may also be obtained
by passing smoke through water. With this object in view, a machine was
designed by the Federated Engineering Company of Kuala Lumpur. In this the
principle of retorting was not employed. Smoke was produced by ordinary
combustion in a compartment of the apparatus, and was drawn through water
by the action of a high-speed fan worked by hand. A solution, equivalent in
effect to a 2 per cent. solution of acetic acid, could be obtained at a
comparatively cheaper cost than crude pyroligneous acid produced by dry
distillation as it was then being practised. This was chiefly because of
the wasteful methods of fuel combustion, in the latter process, in the
heating of the retort.
CHINESE VINEGAR.--This agent was found to be a satisfactory coagulant, and,
_a priori_, there is no reason why it should not be suitable, as it is
essentially a dilute solution of acetic acid.
The qualities sold were generally colourless, and were probably the result
of acetic fermentation of rice.
Samples tested showed a varying content of acetic acid, ranging roughly
from 3 per cent. to 8 per cent.; but on this basis of valuation it was
found generally that the price bore no relation to the degree of
efficiency.
It was advanced not only that the vinegar was an efficient substitute for
glacial acetic acid, but that it was also cheaper. This latter claim was
proved to have no foundation in fact, even at the high price of acetic acid
prevailing during the period of stress. It is not likely, therefore, that
vinegar can displace acetic acid, except as an expedient.
SULPHUROUS ACID.--The anti-oxidant effect of sodium bisulphite and sodium
sulphite is due to the liberation of the gas, sulphur dioxide. This gas
dissolves easily in water, forming an acid solution called sulphurous acid.
This acid solution is an effective coagulant in fairly small quantity. Not
only so, but it produces, in addition, the anti-oxidant effect noted in the
employment of sodium bisulphite. It is thus possible to produce rubber
varying in shade of paleness by means of a single solution.
In the event of sulphurous acid being used, it would be necessary to
import cylinders of sulphur dioxide from which the solution could be
prepared in factories each day. There would be no insurmountable difficulty
in this, as it is only necessary to pass the gas through a series of closed
vessels containing water. Enough solution could be prepared at one time for
three or four days, but preferably the solutions should be as fresh as
possible. Altogether there would seem to be possibilities in the use of
sulphurous acid for preparing pale crepe rubbers, providing the cost is
within comparable limits with the commoner coagulants at present in use,
and that no adverse effect on the rubber can be shown to result. If the
cost did not exceed the combined cost of acetic acid and sodium bisulphite,
the employment of sulphurous acid solution might be worthy of
consideration. There is one drawback to the use of sulphurous acid
solution, and that lies in the proximity of the limits of the quantities
necessary for coagulation and that which is in excess, and prevents
coagulation. Thus, with ordinary field latex having about 20 per cent. dry
rubber content, the minimum necessary for coagulation per 100 c.c. of latex
is about 8 c.c. of a 1 per cent. solution. The maximum quantity possible
for use is about 15 c.c. of a 1 per cent, solution, so that great care
would have to be exercised in avoiding an excess of coagulant, otherwise
coagulation would be effectually prevented.
It is believed that the preparation of rubber by this method is the subject
of a patent secured by Messrs. Boake, Roberts, and Co., London.
SUGARS.--Coagulation may be effected by the addition of small quantities of
sugars.[26] These are assumed to be effective by fermentative conversion
into lactic and acetic acids. The presence of lactic acid is supposed to
have a twofold effect:
[26] "Preparation and Vulcanisation of Plantation Para Rubber" (Eaton,
Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture;
Gorter and Swart, Bulletin No. 6, West Java Expt. Station.
(_a_) As a direct coagulant.
(_b_) In its action upon certain organisms which, in the ordinary
course of events, would delay or prevent coagulation. Although work on
an experimental scale has been done, as far as we know no practical
application has been made of the employment of sugars as coagulating
agents.
VARIOUS SALTS.--Of experimental interest only it may be recorded that
coagulation has been effected by means of various chemical "salts"--_e.g._,
calcium chloride, barium chloride, magnesium chloride, sodium chloride,
aluminium sulphate, magnesium sulphate, sodium sulphate, etc. None of these
has been found to have any practical application, except, perhaps, calcium
chloride, which is used in small quantity as an accelerating agent in a
special process of anaerobic coagulation, which will receive mention in the
following chapter.
At one period during the War and the dearth of acetic acid, it was found
that there were available in England large supplies of the acid sulphate of
sodium (sodium hydrogen sulphate), which proved to be an effective
coagulant. Experimental work gave satisfactory results, but no practical
application resulted when supplies of acetic acid were again obtainable.
VARIOUS PROPRIETARY COMPOUNDS.--We have seen many proprietary coagulants
advertised and pass into the limbo of forgotten things. They can generally
be divided into two classes. The first embraces those founded upon a
woefully incomplete knowledge of requirements. The second covers those
which meet requirements, but for which exaggerated claims are made and
excessive prices charged.
As as instance of a substance which fell under both classifications might
be mentioned the case of "Coagulatex." Pretentious claims were made, and it
was emphasised that the liquid contained no _vegetable acids_. Acetic and
formic acids might be quoted as examples of vegetable acids, and as these
have been shown to be the most satisfactory coagulants now employed one
fails to imagine where lay the value of the guarantee given by the
advertisers of "Coagulatex."
On analysis the liquid was found to consist mainly of sulphuric acid,
against the indiscriminate use of which warnings have been given. Thus it
was a dangerous substance for common use.
Furthermore, comparing the value with its sulphuric acid content, it was
found that the price required for "Coagulatex" was roughly four times the
contemporary cost of commercial sulphuric acid in the Federated Malay
States.
Those in charge of estates should realise, therefore, that no proprietary
coagulants should be adopted until a proper report of tests, and a
comparative valuation, has been obtained from one of the research
laboratories.
CARBONIC ACID GAS, CARBON DIOXIDE.--Now of only scientific interest, it may
be noted that some years ago great claims were made for the use of carbon
dioxide gas as a coagulant. In actual practice we were unable to effect
coagulation by passing the dry gas into latex. It was suggested that the
original investigators were misled by failure to secure a dry and clean
gas. It would appear that probably the gas was prepared by the action of
hydrochloric acid upon marble or limestone. Unless intervening "washers"
and "driers" were used, the liberated gas, when passed into latex, would
carry with it traces of hydrochloric acid, which would effect coagulation.
ALCOHOL.--In the cheap form of methylated spirit, alcohol has been employed
by us as a speedy coagulant for many years. Latex run slowly into alcohol
coagulates instantaneously. The method has been in common laboratory use.
The employment of alcohol has also been made the part-subject of a patent
process of coagulation, to which reference will be made in the succeeding
chapter.
VEGETABLE EXTRACTS.--At various times experimental work has been directed
towards the use of liquids of purely vegetable origin, such as the juices
of tropical fruits, and of a waste product of tropical industry--the
so-called "milk" (or water) of ripe coconuts.
In the former class there is usually a natural acidity, but in coconut
water the acidity is chiefly the result of fermentation of the carbohydrate
(sugar) constituents.
These substances were all found to effect a more or less satisfactory
coagulation, but it is unlikely that they would be suitable for practical
application on a large scale.
As being more directly related to the subject of coagulation in general
than to coagulants in particular, a discussion of several special processes
will be relegated to the ensuing chapter.
CHAPTER XX
_SPECIAL METHODS OF PREPARATION_
Every year appears to bring forth some new ideas in the mode of rubber
preparation. Some of them are based in principle upon the oldest known
method--_i.e._, the native Brazilian process of making "Hard Para." Others
strike a new note, and in a few cases the claims put forward are
substantially confirmed by results. In other instances the claims are too
pretentious, and discredit may be brought upon schemes which, although
lacking in comparative success, are yet commendable for the ingenuity
manifested.
To the present not one of these new methods has been able to compete to any
marked degree in general practice with the established methods of ordinary
preparation. A few continue to find local application, but most have either
been abandoned or are gradually falling into desuetude.
We do not propose to discuss in fine detail all the various claims made on
behalf of these special processes, or to enter into controversies. The aim
is to present to the reader an outline embodying the main principles and
advantages claimed.
DA COSTA PROCESS.--Briefly, this was a method by which coagulation was
effected with smoke. The smoke was generated by the combustion of wood in a
special compartment, and was forced into latex by means of a jet of steam.
It was really only applied to the preparation of coagulum intended for
crepe form. The exact degree of coagulation effected was uncertain, and the
final colour of the rubber precluded it from being classed as a modern No.
1 product.
"BYRNE CURING" PROCESS.--This is a process for treating coagulum obtained
by ordinary methods.
It was the subject of a patent obtained by Messrs. E. J. and F. A. Byrne,
and at one time had a considerable vogue on estates. The chief claim
advanced was that the rubber produced was in all respects equal to Fine
Hard Para, and could be shipped while still moist without detriment to the
physical qualities.
The principle of the process was the treatment of coagulum, in either sheet
or thick crepe form, with vapours produced by the volatilisation of two
special fluids. This treatment was undertaken in comparatively small wooden
sheds, in which the coagulum was placed. The "smoke" was conducted into the
curing sheds from furnaces outside the building. The sheds were covered
externally with "felt" material to prevent leakage of the vapours, and a
very dense smoke was obtained.
The furnaces were specially designed, and consisted essentially of a
"hot-plate" heated by a powerful kerosene blast-flame. On top of the
machine were two reservoirs controlled by taps. In these were placed the
special fluids which were released in definite proportion. The composition
of the fluids was not divulged, but it is assumed that the principal
ingredients were (_a_) wood tar products, (_b_) crude pyroligneous or
acetic acid. The mixture of these, dropping on the hot plate at the correct
temperature, spontaneously volatilised, to form dense whitish fumes, having
an intense and not disagreeable odour of wood combustion. A duct led from
the back of the machine into the curing-shed, where the vapours were
distributed through perforations in the pipe.
The coagulum usually remained under treatment in the shed for three to four
hours, and then was removed for ordinary air-drying. When taken from the
curing-shed it had a pinkish colour, which later developed into a dark
brown by a natural process of oxidation. The exterior of the rubber, on
shipment, resembled the appearance of smoked sheets; while the interior, on
cutting, was seen to be still white. As packed for shipping, the rubber
contained from 10 to 15 per cent. of original moisture, for the usual sheet
form, and even more when "slab" rubber was prepared.
Originally either crepe or sheet rubber was made, but later the preparation
of the crepe form was displaced largely by "slab" rubber. These "slabs"
were really very thick sheets, which had been subject to only slight
pressure.
Still later the preparation of the "slab" form was displaced by "loaf"
rubber. This form was built up by winding together ordinary thin sheets
which had been subject to the "cure." Only slight tension was needed,
during the operation of winding, to cause close adhesion of the component
wet layers, and the final result was a "loaf" or roll dark in colour, and
apparently dry when examined superficially. On being cut, even after an
interval of months, the middle portion was still so moist as to be quite
white.
In course of time it was discovered that all the claims made for the
process could not be substantiated, and for various reasons (which need not
be detailed) most of the estates which had adopted the scheme reverted to
ordinary methods of preparation. At the time of writing few, if any,
continue to work the process. It appears to be agreed, as the result of
investigations, that in no degree does the process yield advantage over
ordinary methods.
FREEZING PROCESS.--A patent was secured a few years ago to cover a process
whereby coagulation was effected by refrigeration.
Latex remained for several hours in the refrigerating chambers of an
ordinary ice-making plant. The resulting solid mass, on being thawed,
yielded a coagulum appearing in no way to differ from that obtained by
ordinary methods of coagulation.
Provided the process exerted no influence for good or evil upon the quality
of the resulting dry rubber, the value of it would appear to depend upon
the relative cost of working, plus considerations of capital expenditure
and depreciation on the plant. At the present time it would be difficult to
imagine that the cost of preparation alone would compare favourably with
that sustained by ordinary coagulative methods.
Furthermore, beyond the expensive refrigerating plant, the usual machinery
of a factory would still be required if the ordinary market demands are to
be met.
Finally, it has not been found[27] that any advantage in the final physical
qualities of the rubber is obtained by the employment of this process.
[27] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham,
and Day), Bulletin No. 27, F.M.S. Department of Agriculture.
WICKHAM PROCESS.--This process, invented by Sir Henry Wickham, aimed at the
production of a rubber resembling Fine Hard Para. The principle employed
was that underlying the preparation of the best rubber in Brazil--viz.,
coagulation of superimposed thin layers of latex by the action of smoke and
heat.
In essential the machine employed consisted of a rotating drum into which
latex and smoke entered. The result was the formation of thin "skins" of
rubber which, coagulating _in situ_, formed a mass corresponding to "Fine
Hard."
That the rubber was fully satisfactory as to quality is acknowledged, but
economically and in practical utility the process was unsuccessful, the
rate of output being so low.
DERRY PROCESS.--The invention of Mr. R. Derry, late of the Singapore
Botanic Gardens, this in principle resembled the Wickham and other
processes. It aimed at a mechanical imitation of the native method of
producing Fine Hard Para.
In place of the rotating drum, an endless belt was used. This travelled
over pulleys, more or less horizontally placed. The upper of these could be
raised to varying height above the level of the other, and likewise could
be so adjusted as to tighten the belt.
The under layer of the belt impinged, in its travel, upon the surface of a
layer of latex contained in a shallow tray. The belt was operated by
hand-power, and the height of the latex trays was adjustable.
The trays of latex were situated at the lower end of the machine which lay
outside the smoking-chamber. It will be understood that the vastly major
part of the total length of belt was always within the chamber.
Smoke was generated by combustion of wood in an external structure, was
brought into the chamber by a wide duct, and was then distributed below
the belt by means of perforated pipes.
The thin film of latex picked up by the belt was coagulated partly by the
action of smoke constituents by evaporation due to heat. Assuming (1) that
the belt was of adequate length, (2) that the rate of travel was not
excessive, (3) that the latex was not too dilute, (4) that the temperature
of the smoke was sufficiently high, (5) that the smoke was sufficiently
dense and not too damp--then the process should be a continuous one.
It will be clear that success could only be obtained by a careful
adjustment of all these factors. The latex must, necessarily, be of a
fairly rich consistency (at least 2-1/2 lbs. dry rubber per gallon), but
unfortunately there is considerable difficulty in maintaining such latex in
a state of fluidity for the period demanded by this process, without loss
of latex. Naturally, the addition of an anti-coagulant would retard the
rate of output of the machine to a marked degree.
The layer of rubber thus formed on the belt was stripped off, and hung for
further air-drying, as it still contained a fair percentage of moisture.
As a really practicable method for treating plantation latex, the process
failed by reason of its low rate of output over a given interval. This
alone was sufficient to condemn it, apart from the facts (1) that it was
not shown to be a cheaper method than coagulation by acetic acid, (2) that
the resulting rubber was not proved to be of superior intrinsic value to
rubber prepared by ordinary methods.
SPONTANEOUS COAGULATION.--All readers will be aware of the phenomenon of
the curdling or souring of milk. The behaviour of _Hevea_ latex, under
certain conditions, may be taken to be analogous. Difficulty is experienced
in maintaining fluidity--a difficulty which appears to vary in great degree
according to locality, nature of soil, age of trees, the relative demand
made upon the trees by the system of tapping employed, etc.
It is sometimes found, before the latex reaches the store, that it may
exhibit one of various stages of premature (spontaneous) coagulation:
(_a_) To all appearances it may be quite fluid, but a close
examination shows it to consist mainly of a serum containing very
minute particles of rubber in suspension (microscopic coagulation).
(_b_) In a later stage these particles coalesce to form larger
"flocks" (macroscopic coagulation).
(_c_) The whole, or practically the whole, of the latex may have
coagulated, forming one mass of rubber with a milky residual serum.
Passing from this aspect of the question, it may be noted as peculiar facts
that:
(1) A shallow layer of latex is less likely to coagulate spontaneously
(_i.e._, without the addition of a coagulant) than a deeper volume.
(2) The shallow layer, and also the surface of the deeper volume
(where exposed to air), on standing will be found to develop a
superficial film of finely coagulated particles, yellowish in colour,
and having an offensive odour due to decomposition of protein matter.
(3) While this partial coagulation is confined only to the surface of
a shallow layer of latex, it will be found that below the surface film
of the deeper volume a much more definite coagulation has taken place.
The coagulation will be practically complete, and the coagulum, apart
from a spongy appearance, is normal in character. This coagulum is
free from the offensive odour noted above.
(4) On testing the surface film of both the shallow layer and the
deeper volume, it will be found to be _alkaline_ in character; whilst
the lower liquid surrounding the main portion of the coagulum in the
deeper volume of latex is of an _acid_ nature.
These observed facts are sufficient to indicate that there are apparently
_two distinct types of spontaneous coagulation_, and that the latter takes
place particularly where the latex is more or less out of contact with the
atmosphere. We may, therefore, differentiate thus:
(_a_) _In contact with air (aerobic)_: incomplete spontaneous
coagulation, accompanied by yellowish slime, offensive in odour and
alkaline in character.
(_b_) _Out of contact with air (Anaerobic)_: Practically or wholly
complete. There is no offensive odour under normal conditions and the
serum is acid in character.
It is concluded[28] that there are present in latex, on collection in the
field, two types of organisms. Those which work in contact with air
(aerobic) show a tendency to _prevent_ coagulation and to form an alkaline
yellow slime on the surface of the latex. The others, which work in the
absence of air (anaerobic), may, under favourable conditions, cause
complete coagulation unaccompanied by any decomposition or offensive odour
within a normal period. If air is rigidly excluded, the coagulum obtained
is quite satisfactory for all purposes.
[28] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham,
and Day), Bulletin No. 27, F.M.S. Department of Agriculture, 1918; "De la
Coagulation naturelle du Latex d'Hevea Brasiliensis" (Denier and Vernet),
_Comptes Rendus l'Académie des Sciences_, No. 3, July, 1917.
This type of coagulation, without the employment of a chemical coagulant,
and under anaerobic conditions, was the subject of a patent granted in 1914
to Messrs. Maude, Crosse and others. The process has been in use on Cicely
Estate (Perak) for some years. With subsequent slight modifications the
apparatus consisted in essential of a tank with a loose cover. The flanges
of the cover were sufficiently long to dip into a water-seal surrounding
the tank. Thus the cover may rise and fall without an inrush of air.
Coagulation, in fact, can be effected thus in any kind of air-tight
receptacle; and experimentally the reader can obtain a satisfactory result
by filling completely with latex the bottle which has a loose stopper.
Under the patent held the coagulum may be prepared either for crepe-making,
or for sheets by a modification of the tank.
The crepe when dry does not have the bright appearance of the ordinary
"Fine Pale" standard prepared with the aid of the anti-oxidant sodium
bisulphite.
Unfortunately the addition of this substance to the latex in normal
proportions is not possible under anaerobic conditions, as it is found to
prevent coagulation, probably owing to its sterilising effect upon the
anaerobic organisms.
To prevent the oxidation of the rubber in actual practice, the freshly
prepared crepe is soaked in a solution of sodium bisulphite before hanging
to dry. The resulting colour of the rubber is quite good.
It was shown by Eaton and Grantham that anaerobic coagulation is slightly
uncertain in action. Owing probably to variations in the composition of the
latices, or to the extent of infection by organisms, coagulation may one
day be complete and on other days less satisfactory.
They found further that, by the addition of small quantities of sugars,
coagulation under both aerobic and anaerobic conditions was improved. The
conclusion formed was that the addition of sugars created a medium
favourable to the development of anaerobic organisms and unfavourable to
those which cause decomposition of the natural nitrogenous constituents of
latex.
This work was confirmed by Gorter and Swart,[29] who attributed the action
to the conversion of sugar to lactic, acetic, and succinic acids by
fermentation.
[29] Gorter and Swart, Bulletin No. 6, West Java Station.
Denier and Vernet, whose work has already been mentioned, studied the
presence of the organisms in latex, and succeeded in isolating one which,
under anaerobic conditions, effects coagulation within twenty-four hours.
Sometimes to produce complete coagulation it was found necessary to employ
small quantities of sugars--_e.g._, 1 gramme per litre of latex (1:1,000).
It is to be noted also that the addition of small quantities of soluble
calcium (lime) salts to latex has much the same effect as the employment of
sugars. Recent investigations[30] showed that the addition of 0·5 to 1
gramme of calcium chloride per litre of latex caused complete coagulation
in closed vessels within twenty-four hours, a result agreeing with the
findings of Barrowcliff.
[30] "Archief voor de Rubbercultuur," Nederlands Indies, 1920, 4, 273.
On page 308 of the same publication, experiments on the effect of sugars
are described, in connection with _aerobic_ coagulation. Observations from
a further set of experiments tended to indicate a direct connection between
the effects of tapping and spontaneous coagulation. It is suggested that
heavy tapping causes a diminution in the latex of those substances which
act in some way as accelerating agents in coagulation--_e.g._, sugars. The
smaller the proportion of these substances, the slower and less complete is
natural (spontaneous) coagulation.
ILCKEN-DOWN PROCESS.--This process is the subject of patents granted in
1915 to Messrs. Ilcken and Down. It has been in fair prominence, and has
been tried experimentally on several estates and in public demonstration.
It is a coagulating process, and, in the original specification, employed
as agents a mixture of alcohol (in the form of methylated spirit) and
benzene (petrol), or alcohol with petrol and coal-tar naphtha. The mixture
was injected in the form of a fine spray into the latex, contained in a
tank specially fitted with paddles.
Later modifications covered the addition of a small quantity of glycerine;
or, failing supplies of that substance, coconut oil.
Many advantages are claimed for the process, but most of them cannot be
substantiated. The two chief claims are:
1. The production of a uniform standard of rubber.
2. The obtainment from a unit volume of latex of a greater weight of
rubber than can be obtained from an equal volume of the same latex by
ordinary coagulation with acetic acid. It is to be inferred that the
agents employed have the power of adding to the coagulum some of the
substances which usually remain in solution in the clear serum.
Regarding the first of these claims, it has been shown[31] that the rubber
is not uniform in its behaviour on vulcanisation, and that its variability
is similar to that of rubber prepared by other processes.
[31] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham,
and Day), Bulletin No. 27, F.M.S. Department of Agriculture, 1918.
The second claim has been the subject of much controversy. Experiments made
on estates under the supervision of, or in the absence of, the patentees
have given conflicting results. When varying factors have been eliminated,
the general conclusion was that no increase in weight of rubber was
obtained.
Private laboratory investigations led to a similar verdict, and Eaton[32]
records a confirmatory finding. More recently the claims made for the
process were investigated in Java[33] under varying conditions. Three
series of experiments were made:
[32] _Ibid._
[33] "Archief voor de Rubbercultuur" (De Vries and Spoon), Central Rubber
Station, Java, May, 1921.
(1) During the rainy monsoon and at a height of 1,800 feet.
(2) During the dry monsoon on a low-country estate.
(3) In the experimental gardens at Buitenzorg during bright sunny
weather and the most favourable conditions.
The agents used were (_a_) a mixture of alcohol and fusel oil, (_b_)
alcohol and petrol (benzene).
In these experiments no advantage in weight of rubber was obtained by the
Ilcken-Down process, and it would thus appear that the principal claim
fails to be substantiated.
The general composition of the rubber was approximately the same as
ordinary crepe obtained from undiluted latex. The rubber on vulcanisation
was found to be normal in behaviour, and was similar to the controls.
The coagulum ordinarily is affected by oxidation, and does not produce a
fine pale crepe. To remedy this defect the freshly prepared crepe is soaked
in a solution of sodium bisulphite and sulphuric acid.
It may be noted that in the recent experiments coagulation was effected in
vacuum in a specially designed wooden tank. From a study of the previous
section on "Spontaneous Coagulation," the reader will perceive that results
equal to those obtained by the Ilcken-Down process can be obtained
_without_ the necessity of using such agents as alcohol, petrol, or fusel
oil.
SLAB RUBBER.--This type of preparation has been the subject of much
discussion of recent years. There is nothing really special in the mode of
preparation, and in its original form "slab" rubber is only a thick sheet
which may be obtained by coagulation with acetic acid or other agents.
The coagulum, when removed from the serum, is subjected to comparatively
slight pressure, and the "slab" thus made is either placed to air-dry at
once, or may be subject to treatment in other liquids before drying.
The rubber is not allowed to remain until wholly dry, but is shipped while
still containing an appreciable percentage of enclosed moisture.
It is claimed[34] that the production of "slab" rubber by standardised
methods eliminates to a great degree the variability which at present
characterises plantation rubber, and that a fast-curing medium is obtained.
These claims will be discussed in later chapters dealing with the
vulcanisation of rubber, and demand no notice in this section.
[34] "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham,
and Day), Bulletin No. 27, F.M.S. Department of Agriculture, 1918.
From the producers' point of view, it may be noted that the preparation of
slab rubber is a simple process, but not altogether as pleasant probably as
might be desired, when undertaken in crude form.
The appearance of the partially dry slabs is unattractive, but that does
not signify if the quality of the vulcanised product satisfies
requirements.
For the average producer, the difficulty lies in having to meet the demands
of the general market. Even, therefore, if one assumes that the intrinsic
qualities of slab rubber are all that the claims advance, it would be
necessary for the producer to be assured of definite and regular sales.
At present it would probably be fair to state that practically all the
"slab" rubber being prepared is produced by those who are also consumers.
They are thus in the enviable position of being able to satisfy their
requirements as to the mode of preparation. Until such time, therefore, as
there exists a regular demand for "slab" rubber in the general market, the
vast majority of estates must proceed on ordinary lines of preparation.
PART VI
VULCANISATION
(BY DR. H. P. STEVENS)
CHAPTER XXI
_INTRODUCTORY DEALING WITH TREATMENT AND VULCANISATION_
In the foregoing chapters the methods of treating latex, coagulating,
rolling and curing, or drying, have been described in great detail. These
details will give the reader some idea of the precautions taken, and
procedure necessary to produce rubber which will be acceptable to the
market. The expressions "inferior rubber," "defective crepe," "poor quality
sheets," etc., are frequently met with, but these expressions must not be
taken to indicate any defect in the rubber for manufacturing purposes, but
merely that the rubber is defective for selling purposes--that is to say,
being unsightly, it will not fetch the full market price.
Raw rubber, as produced on the plantations, is almost invariably subjected
to the process of vulcanisation in the production of manufactured rubber
articles as we know them. Previous to the advent of plantation rubber, the
raw material was purchased by the manufacturer in a moist and impure
condition; frequently the rubber was adulterated with sand, dirt, and even
small stones. Consequently it was the invariable practice of the rubber
manufacturer to wash the raw rubber and convert it into crepe, which was
then hung and air-dried before use. The effect on the rubber, if of high
grade, was more severe than the washing and crepeing process on the
plantation, because the rubber was not a soft coagulum but generally dried
on the surface and semi-hard. The power required was considerable, and the
resulting crepe was consequently softer and more susceptible to heat than
plantation first latex crepe. Much of the "wild" rubber was soft and tacky
and inferior to "earth-scrap."
Vulcanising in its simplest aspect consists in mixing the rubber with
sulphur and heating the product under regulated conditions. The effect of
heat on the inferior grades of "wild" rubber is very marked. A soft,
sticky, and resinous material is transformed into a relatively tough and
elastic product. The effect of vulcanising on the better grades is less
marked, but immediately apparent. On the other hand, the effect of
vulcanising is least apparent on first latex plantation grades, because in
these we have a raw rubber prepared in a manner best suited to retain its
natural characteristics.
The need of vulcanising in the process of manufacturing rubber goods became
an axiom in pre-plantation days, and it is only quite recently that
attempts have been made to utilise raw rubber directly, without
vulcanisation, particularly for shoe soles. For this purpose a thick dense
crepe has been found satisfactory. Smoked sheet rubber is not generally
suitable, apparently owing to its microphysical structure. It is possible
that the process of rolling in the making of dense crepe compacts the
rubber particles, yielding a harder and more resilient product. The rolling
must not be carried too far, or the "working" of the rubber will
approximate to a preliminary mastication, and the product will be weakened.
The utilisation of crepe rubber directly has not yet been sufficiently
tested to enable a definite conclusion to be reached as to its future
scope, but it is obvious that for use in a raw state some modification in
preparation may be advantageous. The present method--_e.g._, coagulation
with acetic acid--does not yield the hardest and toughest rubber.
Hardness and toughness are actual drawbacks in the utilisation of rubber
which is required for vulcanising. When the output of plantation rubber
began to increase and to displace the inferior wild sorts, manufacturers
complained of the increased power consumption of their machines. The power
was required mainly to "break down" or "mill" the rubber preliminary to the
mixing with sulphur and other ingredients. It is obvious that a material
such as raw rubber cannot be mixed with powders such as sulphur with a
pestle and mortar, or in any simple form of mixing machine. This difficulty
was overcome by the earlier experimenters by immersing the rubber in a bath
of molten sulphur. The latter was gradually absorbed and "dissolved" in the
rubber, and the heat of the bath caused the dissolved sulphur to combine
with the rubber to produce vulcanised rubber. The limitations of such a
process are apparent. Thus the vulcanised rubber retains the form in which
it was originally shaped. Moreover, other ingredients, such as mineral
matters, cannot be dissolved or absorbed by the rubber in this manner. The
method eventually adopted consisted in "breaking down," "milling," or
"masticating" the rubber by passing it continuously between differentially
geared steam-heated rollers. By this means a high-grade rubber is converted
into a soft, plastic mass, which will "take up" sulphur, mineral matter,
and other ingredients as desired. The mixing operation may be carried
through on the same roller machine as was used for breaking down the
rubber, or separate machines of other designs may be adopted. Details of
the process will be found in books dealing with rubber manufacturing.[35]
It will suffice here to explain that when rubber is kneaded between two hot
rollers moving at different speeds the rubber forms a continuous band
around the slower moving roller, and if the distance between the rollers be
adjusted the excess of rubber held back by the nip of the rollers will form
a "bank" or moving wedge-shaped mass on the top of the nip. This closes the
space between the rollers, so that sulphur and powder placed on the rubber
pass round towards the nip, and are there driven into the rubber. In this
manner it is easy to mix, say, 10 per cent. of sulphur into the rubber
without a single particle falling through. In technical mixes where large
quantities of powders require to be mixed there is always some caking, and
part of the powder falls between the rollers into a tray underneath. This
is swept up with a broom and put back on to the rollers, the process being
repeated until the whole of the ingredients have been incorporated.
[35] For instance, "India-Rubber and its Manufacture," by H. L. Terry.
From this description it follows that, preliminary to mixing, it is
necessary to thoroughly masticate or "plasticise" the raw rubber. Much of
the "wild" rubber was of so inferior a quality that it very readily broke
down, and but little mastication was necessary. It was soft and resinous,
and readily took up the powders which were to be mixed with it. The better
grades of wild rubber, such as Fine Para, were more difficult to break
down, but not so difficult as most plantation rubber, because they had
already received a preliminary "working" in the process of washing and
crepeing, and we have already explained that such treatment takes more
power than the crepeing of the soft moist coagulum on the plantations. The
amount of "working" or "plasticising" produced in the rubber is connected
with the power expended; the greater the expenditure of power, _caeteris
paribus_, the greater the working effect on the rubber. Although the
manufacturers possessed a relatively soft rubber in the form of washed Fine
Para, it was customary in most cases to employ this rubber in conjunction
with washed lower grades to produce a soft plastic material for further
treatment. Now, however, the manufacturer has little else but plantation to
deal with, and most of it more difficult to break down than washed Para
crepe. This is the reason why a hard, tough rubber is no longer a
desideratum with manufacturers, although originally taken as an indication
of good quality. For the majority of purposes they want something which
will break down easily. Hence if a rubber could be produced answering to
these requirements, without loss of vulcanising quality, it would be
preferred.
Having incorporated sulphur and other ingredients, the plastic mass is
sheeted and run between layers of calico to prevent the superimposed sheets
from adhering. From this "calendered sheet" the article, whatever it may
be, is built up. The calender rollers are heated so as to keep the rubber
compound plastic. There is a limit to the thickness of the sheet which can
be produced. It is a difficult operation to perform satisfactorily so as
to yield a smooth surface and a sheet free from enclosed air. When cool the
rubber hardens and is readily handled. The object to be manufactured is
then built up from the calendered sheet. Thus in the manufacture of a motor
tyre the tread is built up on the casing or carcase by laying the sheets on
the canvas and rolling these with a hand or power operated roller, so that
they adhere firmly, the first layer to the canvas of the casing and
subsequent layers to one another. This rough description will suffice to
illustrate how important it is that the rubber when mixed should be plastic
enough to give a smooth sheet, and to allow the sheet to be manipulated in
building up the article in process of manufacture. The testing of rubber in
regard to its plasticity and power to absorb finely divided mineral matter
will be discussed in a later chapter. We may, however, point out here, that
the mineral matter is not generally added as an adulterant, but because of
certain specific properties it confers on the product.
To proceed with our outline of vulcanisation, we have now arrived at the
stage at which the goods are built up and ready for vulcanising. For this
purpose they are generally enclosed in some manner, either in metal moulds
bolted together, or tightly wrapped in cloth, as, _e.g._, in the
manufacture of inner tubes, hose, etc. In the latter case, you can detect
the cloth mark on the finished product. Sometimes the rubber is
spewed--that is, driven out of a barrel by means of an endless screw
revolving in it. In this way rubber tubing, perambulator tyres, and such
articles, may be made. More recently even tyre treads and the shaped rubber
for band tyres (heavy solid tyres) have been extruded in this manner, for
the process is much cheaper than building up a tyre from calendered sheet,
and then cutting the mass to shape by hand. But for spewing the rubber mass
must be very soft and plastic; this condition is not obtainable unless the
raw rubber originally used can be made thoroughly plastic without damage.
Nor can it be effected with a rubber mass containing much finely divided
mineral matter, as this hardens the mixture.
For other purposes the rubber is swollen in a solvent, such as coal-tar
naphtha, and subsequently masticated; the soft dough is then shaped or
spread on cloth, and vulcanised after allowing the solvent to evaporate.
Here, again, the properties of the raw rubber are of immense importance.
Thus, the more plastic the dough, the less solvent required, and the less
there is to drive off before vulcanising. The plasticity of the dough will
depend on the plasticity of the raw rubber, and so forth. It is evident
that the physical properties of the raw rubber are of great importance.
They directly affect the manufacturing operations up to the vulcanising
stage, and indirectly affect the results obtained on vulcanising.
The actual vulcanising consists of heating the mass of mixed rubber for
a definite time and at a definite temperature, each "heat" being chosen
to suit the particular mixture. These data are arrived at
empirically--that is, by trying a number of "heats" and choosing that
which appears the most suitable. The suitability will depend on the
nature of the article, the service to which it is to be put, and the
time it is intended to last. All vulcanised rubber goods, whatever the
process, have a limited life or period during which they can be relied
on to give useful service. After a time, vulcanised rubber tends to
harden, cracks appear on the surface when the article is bent or
stretched, and eventually the rubber becomes rotten and "perished." This
tendency varies with the quality of the original raw rubber and the
conditions of vulcanising. Before plantation rubber was available, the
manufacturers were dependent on inferior wild grades for a great part of
their output, and, consequently, the goods made from these inferior
rubbers never showed very good mechanical properties and soon
deteriorated. The severest critics of plantation rubber have admitted
the advantages to the manufacturers of the replacement of the lower wild
grades by plantation rubber.[36] But even the best grades give a
vulcanised product which rapidly deteriorates if the vulcanisation is
carried too far. This results from too long heating, or too high a
temperature, and the product is termed "overvulcanised" or
"overcured."[37] The appearance of the product is deceptive, as the
physical properties are remarkably good if the overvulcanising is not
more than 50 to 100 per cent. in excess of the normal cure. Only in the
case of very much overvulcanised rubber do we obtain a product which is
brittle from the beginning.
[36] See Williams, "The Rubber Industry," 1914, p. 284. It must also be
remembered that the inferior wild grades were derived from latices often
containing a large proportion of "resinous" matter, and which could not
yield a really high grade of vulcanised rubber whatever the care and skill
employed in preparation.
[37] The terms "curing" and "vulcanising" are generally employed as if
synonymous. Twiss has suggested that the former be applied in regard to a
change in physical properties, and the latter to the chemical change
whereby sulphur is combined with the rubber. The term "curing" is also
applied to the process of preparation of raw rubber. This must be kept in
mind so as to avoid confusion.
The degree of vulcanising will vary with the type of article to be
produced, and where a long life is desired, the tendency will be to
"undervulcanise"; but if the best mechanical properties are desired, the
tendency will be towards "overvulcanising," or, more correctly, "fully"
vulcanising. These considerations are aptly illustrated by reference to
pneumatic tyres. The inner tube need not possess high tensile strength,
provided that it is easily distensible, for the reason that, during use, it
is protected by the casing of the tyre proper, which confines and supports
it against the air-pressure applied. Inner tubes are therefore cured to
give a long life without developing the maximal physical properties. On the
other hand, the casing and tread of the tyre are required to withstand
severe mechanical conditions--particularly the constant flexing of the
cover, and the abrasion of the road surface. Tyres are not stored for any
long period, and, when put into service, have a limited period of useful
life. Consequently it is needful to develop maximal mechanical properties,
and vulcanisation is therefore carried further than in the manufacture of
inner tubes.
The rate of cure is controlled by a number of factors in addition to the
period and temperature of vulcanisation, in particular by the proportion
and nature of the other ingredients, especially sulphur and accelerators,
and also by the rubber itself. The main complaint as regards plantation
rubber is that it varies excessively in this respect. This matter will not
be discussed here, but is only introduced in order to explain the
importance of a constant rate of vulcanising to the manufacturer.
Plantation rubber should, therefore, be prepared so as to be as uniform as
possible in this respect, and the earlier part of this book gives full
details of the precautions advised, and in many cases adopted on the
plantations. Unfortunately, it is impossible to secure uniformity of
methods among all producers, even when they are Europeans, to say nothing
of the native producers, who account for perhaps one-third of the output.
Hence the importance of branding the rubber whenever possible, so that the
manufacturer may identify the rubber he purchases. If found satisfactory,
he can then secure further supplies from the same estate.
CHAPTER XXII
_TESTING OF PLANTATION RUBBER_
This subject may be subdivided into (_a_) Tests on the raw rubber; (_b_)
tests on the vulcanised rubber.
The tests on the raw rubber may be carried out (1) on the sample of sheet
and crepe as received. For this purpose the rubber is cut into a strip,
which is clamped between grips and gradually stretched to breaking-point.
The ring testing machine can be adapted for this purpose by replacing the
rollers with clamps. As the thickness of the samples to be tested will
vary, it is advisable to cut the strips of such a width that the
cross-sectional area of all test pieces is the same--say, 40 sq. mm. The
method is applicable to both sheet and crepe rubber. (2) Tests may be made
as to the behaviour of the rubber during milling or mastication. Small
batches are milled under uniform conditions, preferably in an enclosed
masticator such as Baker and Perkins supply. The power taken (as measured
by the current taken to drive the motor actuating the machine) and the time
are recorded. A further test may be applied to the milled or masticated
rubber, to ascertain the amount and the time taken to incorporate a finely
divided mineral matter, such as carbon black, zinc oxide, or one of the
refined clays.[38] The results are not very exact, and the difference in
plasticity and dryness noted are usually less than found when working with
full-sized machines in the factory. (3) The rubber, either raw or
masticated, may be "dissolved" in a "solvent," such as benzene, and the
viscosity of the "solution" measured. Generally speaking, the less viscous
the solution, the more plastic the rubber.
[38] Bulletin Rubber Growers' Association, January, 1921, p. 43; August,
1921, p. 340.
The testing of vulcanised rubber has been treated in such detail in the
recent works of Whitby[39] and De Vries[40] that a few special points only
will be dealt with here. The preparation of samples for testing involves
first the sheeting of the mixture of rubber, sulphur, and other
ingredients, if any. The sheets may be 1 to 2 mm. thick. They are soft and
adherent, and must be kept between layers of calico to prevent adhesion. A
sheet of rubber is then built up by laying three or four sheets evenly upon
one another, and pressing together to form a sheet 5 mm. thick. The thick
sheet is then roughly cut to shape and vulcanised in a mould by heating in
steam under pressure. From the vulcanised sheet so obtained the rings for
testing are cut (45 mm. internal diameter. 5 mm. face, and 4 mm. thick).
Rings obtained in this manner will not vary in diameter or thickness
(reckoned as sections of a tube), as these are controlled by the size of
the punch, but will vary a little in the face, as this is controlled by the
thickness of the sheet, which depends on the completeness with which the
mould is closed. More recently smaller moulds have been adopted, one mould
for each ring, and an annular space for moisture to develop a pressure
during vulcanising and prevent porosity. The moulds are vulcanised in an
oil bath, or oven of some description, in which a constant temperature is
maintained. I have adopted for some years a third method. The principle is
that used in the factory for making annular-shaped rubber articles, such as
washers, rings, elastic bands, etc. An aluminium mandrel, 45 mm. external
diameter, is taken, and the thin rubber sheet is wrapped round this, so as
to build up a tube about 4 mm. thick, the surplus rubber is cut off, and
the edge bevelled with a wet knife. The manipulation will vary somewhat
with the type of compound to be treated; thus, in some cases, it is
sufficient to well roll the tube with a hand roller to secure adhesion. In
other cases it is better to wipe the sheet of compound with a rubber
solvent previous to rolling. In the latter case time must be given for the
solvent to evaporate before vulcanising. The tube is next tightly wrapped
in wet cloth, and is then ready for the vulcaniser. Or the tube may be
enclosed in moulds which form an outer circular shell and take the place of
the cloth, but for most purposes, and in particular for the rubber-sulphur
mixing usually employed, it is sufficient to use cloth to obtain even and
regular tubes. The tube, after vulcanising, is slipped on to a wooden
mandrel and cut into rings on a lathe. Of these rings the internal diameter
is constant, for this is formed on the mandrel, also the face, which can be
cut accurately in the lathe, but the external diameter, and consequently
the thickness, may vary a little.
[39] "Plantation Rubber and the Testing of Rubber."
[40] "Estate Rubber."
It appears, therefore, that all methods result in rings of approximately
the correct size, and it is usual to check, and, if necessary, make an
allowance for variation in dimensions. It is not possible to do this, even
approximately, with soft rubbers, as the rubber gives under the pressure of
the micrometer. No doubt a photographic method would give more accurate
results, but would take too long. I have found that a very close
approximation is obtainable by weighing the rings as the specific gravity
of the standard rubber mix is known. It is not necessary to weigh each
ring, but the whole five or ten taken for testing may be weighed together.
The next point that arises is the choice of a formula for the test mix.
Practically all the work to date has been carried out on mixtures of rubber
with 7 to 10 per cent. of sulphur. For some purposes--_e.g._, detecting
variation in rate of cure--this mixing is satisfactory, but for other
purposes it is not. Nor is the behaviour of a rubber-sulphur mixing a sure
guide to the behaviour of one containing other ingredients, such as
litharge. Thus, two samples vulcanised satisfactorily when mixed with
sulphur only, but one of them gave unsatisfactory results in the presence
of litharge. It has long been recognised that mineral ingredients may
modify the product when vulcanised, but the modification is not necessarily
uniform. Consequently, tests should also be made, when practicable, with
vulcanised rubber containing other ingredients in addition to sulphur.
As regards physical tests on the vulcanised products, these usually involve
determination of breaking load and elongation at rupture (usually recorded
as final length--that is, including the original length reckoned either as
unity or as 100 units). Simultaneously a load-stretch curve is recorded on
an autographic attachment. The type of curve varies with (1) state of cure,
or degree to which the rubber is vulcanised; (2) proportion of sulphur
and/or other ingredients; (3) specific nature of the rubber used. The last
factor is almost negligible compared with the two former--at any rate for
average quality rubber. As (2) is kept constant for any batch of tests, or
even for every test, it follows that the load-stretch curve is mainly
dependent on the state of cure, and the degree of vulcanising may be
measured by comparing either the elongation produced at a given load or the
load produced at a given elongation. Either set of figures is readily
determined by measuring up the load-stretch diagram.
The peculiar type of the curves has long been a subject of comment and
speculation. Special properties have been attributed to the "slope" or
inclination of the upper and approximately straight portion of the curve.
According to the writer's investigations, the "slope" is largely dependent
on the degree of vulcanisation, so that it is difficult to "place" as an
index of the specific nature of a rubber.[41] Moreover, it has recently
been shown that the peculiar type of curve given by vulcanised rubber is
the result of plotting the load against the sectional area of the
unstretched test piece, whereas this area decreases progressively as the
test piece stretches. If this decrease be allowed for, the curve obtained
is an equilateral hyperbola.[42] Preliminary experiments with rubber
compounded with large proportions of finely divided mineral matter, such as
carbon black, show that the load-stretch curves obtained autographically
are likewise reducible to equilateral hyperbolæ.
[41] Bulletin R.G.A., October, 1921, p. 397.
[42] _Hatschek Journal Soc. Chem. Ind._ 1921; _Trans._, p. 251.
CHAPTER XXIII
_THE PROPERTIES OF RUBBER_
This section, like the last, is divisible into two subsections. The first
deals with raw rubber, the second with vulcanised rubber.
We have already explained that, until recently, rubber was not used in the
unvulcanised condition, but that the excellent physical properties of
plantation rubber have made this possible. It is interesting to compare the
physical properties of raw rubber with that vulcanised with sulphur. A
compact sample of crepe as received from the East will give breaking strain
of over 30 kilos per sq. cm. and over 300 per cent. elongation. When mixed
with sulphur and vulcanised, a breaking strain of 150 kilos and elongation
of 1,000 per cent. are not unusual. It is possible that crepe rubber would
give higher figures if it could be prepared in the form of a compact ring,
as used for tests on vulcanised rubber. In any case, the figures for
vulcanised rubber are much in excess of those for raw crepe rubber. It must
also be remembered that a breaking strain of 150 kilos is not permanent
with vulcanised rubber, for reasons which will be explained later.[43] To
obtain a reasonably permanent vulcanised product, the vulcanisation would
not be carried further than to give a figure of 100 kilos. On the other
hand, raw rubber is remarkable on account of its great permanency, although
subject to some physical changes at ordinary atmospheric temperatures.
Tensile tests, although valuable, do not tell us all about the physical
properties of a sample of rubber. Abrasion tests, or tests designed to
measure resistance to wear and tear, would be more valuable, but,
unfortunately, these properties do not lend themselves to simple tests.
There are grounds for believing that raw rubber is superior in some
respects to fully vulcanised rubber, if prepared without the addition of
finely divided mineral substances which exert a toughening effect.
[43] _Journal Soc. Chem. Ind._, 1916, p. 872.
Sheet rubber gives results in some ways inferior to compact crepe rubber
when subjected to physical tests. Tensile strength seldom exceeds 15 kilos,
but the elongation is usually higher--up to 600 or 700 per cent. That is to
say, it stretches more, but breaks more easily. If, however, we take into
consideration the diminution in sectional area of the test piece during
stretching, it will be seen that crepe and sheet rubber have compensating
properties.
As this matter of sectional area reduction during stretching is important,
both for raw and vulcanised rubber, it may be briefly referred to here.
When rubber is stretched, the volume does not appreciably alter--at any
rate, as regards uncompounded rubber. Hence the reduction of sectional area
on stretching bears a simple relationship to the amount of stretching. If
we double the length of the test piece, we halve the sectional area; if we
treble the length, we reduce it to one-third, and so forth. Hence, if we
multiply the breaking strain by the final length (_i.e._, length at break,
taking the original length = 1), we obtain a figure, the "tensile product,"
which embodies both breaking strain and stretching capacity. In effect it
gives us the breaking strain calculated on the sectional area at the
_moment of rupture_ of the test piece. Adopting this formula, we obtain for
crepe--
_Tensile _Final Length--i.e., _Tensile
Strength._ Elongation + 1._ Product._
30 × 4 = 120
and for smoked sheet
15 × 8 = 120
The difference in properties between crepe and sheet may probably be
attributed to the heavier rolling of the crepe; which compacts the rubber.
But if the crepe is rolled too much, the tensile strength falls, and there
is no increased elongation to compensate. For the same reason, crepe which
has been rerolled in this country is inferior to crepe as received direct
from the plantation. At the most it is permissible to unite two or three
layers of thin crepe to a thicker one by a single passage through even
speed rollers, if the physical properties of the original rubber are to be
conserved.[44]
[44] Bulletin R.G.A., February, 1922, p. 64.
Attempts to prepare crepe for use in a raw state, by rerolling uneven or
irregular surfaced crepe in this country, only result in a rubber with
inferior physical properties. Nor can sheet be rerolled to give crepe of
good physical properties. The power required to break down the sheet and
the heat developed, even on cold rollers, are an indication of physical
properties destroyed. For subsequent vulcanisation this is not a matter of
importance, because the vulcanising process restores to the rubber the
properties which are lost in the process of rolling and milling or
mastication.
Raw rubber has been used to some extent for proofing purposes, as for the
manufacture of material for hoods of motor-cars. In this case no attempt is
made to preserve the physical properties. The rubber is masticated, mixed,
taken up with solvent and spread on the cloth exactly as if it were to be
vulcanised.
VULCANISED RUBBER.--We have already explained that the properties of
vulcanised rubber are dependent, to some extent, on the specific nature of
the raw rubber, or what De Vries terms the "inner qualities." That is to
say, differences appear on vulcanising which are not apparent from the
tests made on the raw rubber. Indeed, no investigation or analysis of the
raw rubber can enable one to foresee exactly how the rubber will behave on
vulcanisation. This illustrates the deficiency in our knowledge of
vulcanisation. When dealing with soft, resinous, or decomposed rubbers, it
is safe to anticipate a weak vulcanised product; but when we come to deal
with a number of samples of "standard" crepe or sheet--_i.e._, sheet or
crepe passing a certain standard of appearance--it is found that
differences in vulcanising properties cannot be foreseen. This matter is,
however, not so great a drawback as might be imagined, for reasonably well
prepared consignments of standard crepe or sheet differ but little from
one another, and the difference is mainly in the ease with which they break
down, or the rate or speed with which they vulcanise, and not with the
properties of the vulcanised product. Many of the plantation scrap grades
are equal to or nearly equal to "standard"; but some of these, as also the
rubber produced by native holders, show appreciable variation, and are the
source of most of the complaints which emanate from manufacturers. We shall
consider in turn the different grades and the effect of the usual surface
defects, such as mould, spots, etc.
CREPE RUBBER.--Oil marks and tackiness are the most serious defects from
the manufacturing standpoint. In the first part of this book we have shown
that damage caused by the so-called oil marks is not due to the oil, but to
traces of copper from the bearings of the machines. There are several
metallic compounds which cause deterioration of rubber both raw and
vulcanised, but copper is the most deadly, and rubber showing signs of
deterioration is rightly rejected by the manufacturers.
The only other defect of crepe rubber which has any bearing on its use in
manufacture is mould. Crepe rubber very seldom shows the ordinary surface
moulds not uncommon in sheet-rubber. There are, however, microscopic
growths which cause the development of coloured spots referred to in detail
in the earlier part of this book. The rubber hydrocarbon itself does not
appear to be affected by the moulds, but some of the serum constituents are
altered, with the result that the rubber vulcanises more slowly than it
otherwise would do. For this reason, crepe rubber with coloured spots may
give rise to trouble in the factory.
SHEET RUBBER.--The commonest defect is mould.[45] This is usually of a
light surface type, easily brushed off, and numbers of vulcanising tests
failed to trace any reduction in rate of vulcanising or other defect due to
this. In spite, however, of the harmlessness of light surface moulds, they
are looked upon with suspicion by the manufacturer. Occasionally samples of
smoked sheet are offered contaminated with a "heavy" type of mould. The
sheet feels damp and "heavy" or flabby, and contains an excess of moisture;
sometimes a moist exudation is noticeable on the surface, and "virgin"
patches are present. Such sheet vulcanises more slowly than F.A.Q. samples,
but does not necessarily show other defects after washing and drying.
[45] Bulletin R.G.A., February, 1921, p. 97; April, 1921, p. 190; June,
1921, p. 243; November, 1921, p. 472.
"Stretching rusty," as already explained, is due to a dry film on the
surface of the sheet, and according to a recent investigation, this film
consists, not of serum substances, but of a microscopic mould growth, which
presumably grows on the serum substances. A sample of sheet which stretches
rusty gives the rubber a "dry" appearance, and for a long time
manufacturers mistook the surface film for resin. On the assumption that
such rubber was "resinous" they rejected it, and to this day it is regarded
as a defect, although it has no influence on the vulcanising properties of
the rubber.
It is hardly necessary to point out that defective appearance, such as is
due to thickened edges, faint markings, bubbles, and so forth, have no
effect on the vulcanising properties of the rubber. They only point to some
irregularity or carelessness in preparation. The only justification for
distinguishing between rubber of good and bad appearance is that the former
bears the impress of careful preparation, and is therefore more likely to
be uniform in rate of vulcanising.
Similar considerations apply to the colour of smoked sheet, which may vary
from a pale yellow-brown, through various shades of red-brown to dark
brown. There are various factors affecting the colour, but the buyer can
see but one--viz., the "degree" of smoking--and the rubber, from his point
of view, may be undersmoked or oversmoked. No doubt the degree of smoking
affects the vulcanising properties, but to a less extent than was at one
time imagined. In a recent paper[46] it has been shown that the average
breaking strain and rate of cure of a number of samples of smoked sheets
were practically the same for light as for dark sheets.
[46] Bulletin R.G.A., December, 1921, p. 521.
VARIATION IN PHYSICAL PROPERTIES.--A very large number of tests on
vulcanised specimens of plantation rubber have been carried out. The
rubber was almost invariably mixed with 7 to 10 per cent. of sulphur, and
no other ingredient, and vulcanised to give the maximal breaking load.
Unfortunately, this determination is subject to a very appreciable
experimental error, so that a large number of determinations are necessary
to give a reliable figure. It is quite impracticable to make a large number
of determinations in routine testing, on account of the labour involved. It
is usual to make five, or possibly ten, determinations, although some
investigators have been content with two. It is generally conceded that any
exceptionally low figures should be ignored, as probably caused by some
flaw or irregularity in the test piece. On the other hand, a study of
actual determinations shows an occasional excessively high figure, and it
is questioned whether this also should be left out of account. Others
ignore all except the highest figure, and take this to represent the true
breaking strain. As a consequence, the figures published by different
workers show considerable variation. De Vries has analysed a large number
of the figures obtained in systematic examination of estate samples, and
has constructed curves to illustrate the results.[47] It is open to
question how far the variations shown are attributable to experimental
error. The figures show, however, that the variation in breaking strain is
relatively small, and not very different for crepe and sheet rubber. In our
opinion, undue importance should not be attached to very high or
exceptionally high figures for breaking strain, which are occasionally met
with. Provided the figure does not fall much below the average, the sample
may be regarded as satisfactory. It is very seldom that any sample of first
latex estate rubber does not show satisfactory figures.
[47] "Estate Rubber," p. 466.
THE RATE OF CURE OR RATE OF VULCANISATION is subject to more exact
measurement, whether this be based on the physical or the chemical
properties of the rubber. If the testing machine be provided, as is usual,
with an autographic attachment, the position of the curves traced on the
recording paper gives a measurement of the rate of cure. These load-stretch
curves, to which reference has already been made, take up a definite
position in accordance with the physical properties; it is only the length
of the curve, or the point where it terminates (which gives the breaking
strain and elongation at break), which is largely fortuitous.
As a measure of rate of cure we may take the actual measurements made on
the record.[48] It is convenient to measure the elongation produced by a
load of 130 kilos per sq. cm., as all fully vulcanised rings of soft rubber
should give higher breaking load figures. For less cured or weaker samples
a lower figure may be taken, such as 60 kilos. We have found that when
fully vulcanised to give the maximal breaking strain, the elongation at a
load of 130 kilos is in the neighbourhood of 850 per cent. (final length
950 per cent.). This applies to ordinary samples of estate rubber under the
conditions of testing indicated above. If, however, the proportion of
sulphur be considerably reduced, or mineral ingredients in a fine state of
division be added to the mixing, or accelerators, whether organic or
inorganic, be employed, the above relationship no longer holds. Nor does it
hold with regard to plantation rubber prepared in an exceptional manner,
as, for instance, matured coagulum or "slab."
[48] Bulletin R.G.A., June, 1921, p. 246.
There is a second method of determining the rate of cure--namely, by
analysing a vulcanisate produced under standard conditions, and determining
the amount of sulphur which has entered into chemical combination with the
rubber. For this purpose the weighed sample is cut thin or creped thin, and
exhaustively extracted with acetone to remove any "free" sulphur--that is,
sulphur not in combination with the rubber. The sulphur remaining is then
determined and calculated as a percentage of the raw rubber contained in
the sample taken. This gives the so-called coefficient of vulcanisation.
If we compare the coefficient with the time of cure at a constant
temperature for an ordinary sample of plantation rubber, they are found to
be approximately proportional, so long as the sulphur is in sufficient
excess. The amount of combined sulphur is, therefore, an index of the time
vulcanisation has been in progress (under standard conditions of
temperature, etc.), and, therefore, the coefficient is a measure of the
rate of cure.
The change in position of the load-stretch curve is not directly
proportional to the time of heating, and it therefore follows that it is
also not directly proportional to the coefficient. For ordinary samples of
crepe and sheet the relationship is, however, not very far removed from
proportionality. This applies particularly to sheet rubber. The
relationship is readily seen on plotting one against the other and tracing
the curves. For sheet we get an almost straight line; for crepe there is
some curvature.[49] For ordinary estate samples of sheet and crepe rubber
the maximal breaking strain is obtained when the coefficient reaches
approximately five units, so that this corresponds to the elongation of 850
per cent. at a load of 130 kilos.
[49] Bulletin R.G.A., June, 1921, p. 246, October, 1921, p. 398.
Either physical or chemical methods may, therefore, be used for determining
the rate of cure of ordinary sheet or crepe rubber, but great care must be
taken when interpreting the results obtained with rubber prepared in an
unusual manner. The rate of cure may be expressed in terms of the time
taken to vulcanise the rubber at a constant temperature (in our case 138°
C.), so as to give an elongation of 850 per cent. at a load of 130 kilos,
or to give a coefficient of five units. The higher the figure so obtained,
the slower curing the rubber. To express the results more directly as rate
of cure, we have adopted the plan of taking an average crepe rubber,
calling the rate of cure 100 units, and expressing the rate of cure of
other samples in these terms. Thus, a sample which gave a coefficient of
four only, in the time taken by the standard to give a coefficient of five,
would have a rate of cure four-fifths of the standard, that is, 80; or if a
sample takes only two hours to give an elongation of 850 per cent., whereas
the standard takes three hours, the rate of cure of the sample will be 3/2
of standard or 150.[50]
[50] _Journal Soc. Chem. Ind._, 1918, p. 280.
As stated, the coefficient is approximately directly proportional to the
time of cure; it is also independent of the proportion of sulphur, if in
fair excess, and in the presence of inert ingredients. It is also
independent of the amount of mastication given to the original raw rubber,
however great. On the other hand, the position of the load-stretch curve is
variously modified by these factors--in some respects, therefore, the
coefficient is a more reliable index. However, the coefficient is
influenced by accelerators, so that here also great care must be exercised
when interpreting results. For the purpose of detecting variations in rate
of cure, it is best to choose a mixing which is particularly sensitive. In
the first place, there must be an ample excess of sulphur; and in the
second place, no ingredient should be added which will complicate the
load-stretch curves, and no accelerators should be present which may
possibly tend to obscure the vulcanising properties of the rubber itself.
It has been found, therefore, that the best mixing to use consists of
rubber with an excess of sulphur--say, in the proportion 9:1 without other
ingredients. The rate of cure of a specimen of plantation rubber is
attributed to the presence of certain natural vulcanising catalysts,
because it is found that carefully purified raw rubber (that is, with the
resinous and nitrogenous constituents removed) vulcanises very slowly or
hardly at all, but that on replacing the extracted matter the rate of
vulcanising is restored. The natural catalysts contained in the extracted
matter are influenced to a varying degree by some of the common ingredients
of manufactured rubber articles. This applies particularly to litharge
(oxide of lead), to which reference has already been made. Thus, acetone
extraction of raw rubber to remove resinous matter has but little effect on
the vulcanising properties of a mixture of rubber and sulphur. But if
litharge be a constituent, it is found that acetone-extracted rubber will
hardly vulcanise at all. From this, it follows that a rubber giving a low
acetone extract may be found to vulcanise exceptionally slowly in a mixing
containing litharge, whereas it shows no such defect when compounded with
sulphur only.[51] Litharge is used to a very large extent, as it has a
balancing effect in a rubber compound--that is to say, it allows of
appreciable variation in vulcanising conditions, without corresponding
alteration in the state of cure.[52]
[51] _Journal Soc. Chem. Ind._, 1916, p. 874.
[52] _Ibid._, 1915, p. 524.
INFLUENCE OF VARIOUS FACTORS IN RAW RUBBER PREPARATION ON THE "RATE OF
CURE," OR "RATE OF VULCANISATION."--As the capacity of a rubber for
vulcanisation depends on the presence of small quantities of accessory
substances in the serum which act as catalysts, the rate of vulcanisation
(or curing) will depend on the nature and quantity of such substances
present in the rubber. A very small quantity of these substances has a
considerable influence on rate of vulcanising, and as the substances are
difficult to isolate and identify, our knowledge of their formation and
chemical nature is not as definite as is desirable. Substances have been
isolated having the characteristics of "simpler bases." Bodies of this
class are formed by putrefaction of organic matter, and can be separated in
much larger quantity from coagulated latex, which has been allowed to
putrefy before working up than from such which has been worked up without
giving time for an appreciable amount of putrefaction to take place.
Further, rubber from putrefied coagulum vulcanised much faster than that
ordinarily prepared, so that we are justified in connecting the
putrefaction bases with the rate of vulcanisation. Moreover, it has been
shown that any treatment of the latex or coagulum which inhibits the
development of putrefactive organisms also prevents the rubber vulcanising
as fast as would otherwise have been the case.[53] Also, the crude bases
isolated from fast vulcanising rubber have the power of increasing the rate
of vulcanisation when added to ordinary slow vulcanising rubber.[54]
[53] Eaton and Co-workers: See Bulletin No. 27, F.M.S. Department of
Agriculture.
[54] _Journal Soc. Chem. Ind._, 1917, p. 365.
On the other hand, there are one or two facts which are difficult although
not impossible to fit in with theory. Thus, although the putrefaction bases
are very easily soluble in water and acetone, they cannot be removed by
washing on the creping rollers, or by acetone extraction. This may be due
to the power of colloidal substances to retain other crystalloidal
substances, such as the bases, which, in consequence, cannot be washed out.
A parallel case is the retention of small quantities of water soluble
substances in the soil. Also, the theory does not explain why rubber
obtained by evaporation of latex at relatively high temperatures is fast
vulcanising, although the possibility of putrefaction is excluded.
As regards practical results, it follows that the rate of vulcanisation (or
cure) of a sample of rubber will depend on the time allowed to elapse
between the collection of the latex and treatment till the rubber is dry,
as also on atmospheric conditions. Thus, slow drying will result in an
increased rate of cure, for it gives an opportunity for putrefactive
organisms to play a part. The results will, however, be influenced by the
extent to which the rubber was washed previous to hanging, and so forth.
Smoking is an antiseptic process and will, therefore, tend to inhibit the
action of micro-organisms and produce a slower vulcanising rubber. On the
other hand, sheet contains more serum than crepe, so that there is more
food material for growth of micro-organisms. The net result is to give a
rubber (sheet) which usually vulcanises a little faster than crepe.
Among other factors controlling the rate of cure, special mention should be
made of the nature and amount of coagulants. Weak "organic" acids, such as
acetic, lactic, tartaric, etc., used in the minimal proportions (1 to 1,200
of standardised latex in the case of acetic acid), give the fastest
vulcanising rubber; "strong" mineral acids, such as sulphuric acid, even
when used in the minimal proportions (1 to 2,000), yield slower vulcanising
rubber. Acid salts, such as alum, are intermediate in effect. Increased
proportions of coagulant cause a reduction in rate of vulcanising with all
coagulants, and the effect is least noticeable in crepe rubber,
intermediate in sheet rubber, and most pronounced in "slab" rubber
(discussed below).[55]
[55] Bulletin R.G.A., July, 1919, p. 39; September, 1920, p. 343; November,
1920, p. 433; October, 1921, p. 393; March, 1922, p. 134.
OTHER TYPES OF PLANTATION RUBBER.--We have up to now confined our attention
to ordinary thin air-dried crepe and smoked sheet, as almost all plantation
rubber is now marketed in one or other of these two forms. There are,
however, other types, to which reference has been made. Of these, the most
important is the thick blanket crepe, made chiefly in Ceylon by rolling
together thin crepe, which has been artificially dried (Colombo drier or
vacuum drier). The heat of the driers causes a surface stickiness, which is
got rid of by rolling several thin layers together to give one thick one.
This rubber vulcanises at about the same rate as ordinary thin crepe, for
the relatively high temperature of drying does not appear to influence the
rate of cure. The rubber is generally softer than air-dried crepe, and is
easily "let down" in naphtha; it is, therefore, suitable for some solution
work. Generally speaking, the properties of blanket crepe do not differ
materially from ordinary thin crepe. Another type of rubber seldom met with
is matured slab or crepe, prepared from it. This type of rubber is being
made in small quantities on one or two estates, who supply direct to the
manufacturer. The method of preparation has already been described. It is
unsuitable for sale in the open market, as it contains a variable amount of
moisture, has the various surface defects such as slime, mould, and "rust,"
and there is the additional disadvantage that it is not easy to judge of
its cleanliness or freedom from coarse impurities by inspection. If the
slab rubber be creped and air-dried on the spot, the product is of
satisfactory appearance, except that it is of low colour and may be
streaked. As the crepe so produced vulcanises almost as fast as the
original slab, the crepe embodies all the advantages of a fast curing
rubber with few of the disadvantages of the slab itself. We have made
experiments from time to time, and found that by a judicious use of sodium
bisulphite it is possible to produce a fast vulcanising crepe rubber
sufficiently even and light in colour to satisfy the Standards Committee.
A fast curing raw rubber is not necessarily a desirable type for all
manufacturing purposes. In the vulcanising of large masses of rubber, a
slower rather than a faster vulcanising rubber may be desirable, so as to
give ample time for the heat to penetrate and spread evenly throughout the
mass. But for many purposes a fast curing rubber enables a larger output to
be obtained, so that artificial organic accelerators are coming more and
more into use. The addition of such accelerators might be obviated, if a
suitable fast curing rubber were available, but it is essential that such
rubber should be uniform. It is just in this respect that slab rubber or
crepe made therefrom is found to be deficient.[56] The rate of cure depends
on the functions of wild bacteria, which are naturally sensitive to changes
of conditions, such as temperature, etc. The coagulated rubber depends on
chance circumstances for infection, and, as a natural result, the activity
of the bacteria and the nature and amounts of active vulcanising agent
produced will vary and be difficult to control. Consequently, the rate of
cure of slab rubber shows considerably greater variation than ordinary
crepe or sheet.[57] This, in our opinion, is the main difficulty of
utilising "slab," or crepe prepared from it. Experience in other
industries, using micro-organisms, has shown that the only method of
control has been to replace the wild growths by cultures of some particular
strain, as, for instance, in yeasts for brewing. To control the rate of
cure of slab, it might be possible to use a special culture for the
purpose.
[56] Bulletin R.G.A., January, 1920, p. 6; January, 1921, p. 47.
[57] _Ibid._, January, 1920, p. 68.
Other less usual methods of preparation, referred to in the earlier part of
this book, do not call for particular mention, as the properties of the
rubber do not differ much from ordinary sheet or crepe. It is mainly a
matter of variation in rate of cure.
This short account of the vulcanising properties of plantation rubber would
not be complete without a reference to Fine Hard Para, the premier rubber
of the Amazon. This rubber has come to be regarded as the standard
high-grade product with which plantation rubber may be compared, and many
manufacturers are still of the opinion that it is unsurpassed by any
plantation product. Yet, when subjected to the ordinary vulcanising tests,
we find that samples of Fine Hard Para give figures very similar to average
plantation rubber; indeed, it is not difficult to find specimens of
plantation rubber which give appreciably higher figures on testing. It is
claimed, however, that Fine Para is more uniform than plantation rubber,
and can be relied on always to give the same results. Yet tests on a series
of Fine Hard Para specimens gave variations in rate of cure similar to
those found for plantation. Some figures were published, which tended to
show that the variation was smaller for Fine Para, but it turned out that
each of the samples taken for examination consisted actually of a number of
slices cut from different balls, so that greater uniformity was not
unexpected.[58] The superiority of Fine Para is, therefore, somewhat of a
mystery. It is probable that some manufacturers prefer to use it because
they feel safer with it, and know actually how it will behave from long
experience. In one respect Fine Para is possibly superior to most
plantation rubber--that is, for the preparation of raw rubber solution for
sticking the seams of waterproof garments, and for similar purposes. The
method of preparation may well influence the strength of the raw rubber
when used for this purpose. Plantation rubber has been prepared in the same
manner as Brazilian Para, in particular on an estate in Java. The product
resembles Brazilian Para in appearance. Vulcanising tests gave satisfactory
figures, but, as already stated, this would not serve to show that the
rubber was equal to Brazilian Para from the manufacturer's standpoint.
[58] Bulletin R.G.A., September, 1920, p. 347.
INDEX
Acetic acid, 74, 279
Acid, acetic, 74, 279
--, carbonic, gas, 289
--, formic, 279
--, hydrochloric, 282
--, hydrofluoric, 282
--, mixing, with latex, 79
--, nitric, 282
--, oxalic, 279
--, pyroligneous, 282
--, quantity of, 76
--, sulphuric, 279, 286
Acids for coagulation, effect of, on rate of cure, 323
--, quantities necessary for modern requirements, 78
Air-drying, aids to normal, 143
--, of crepe, rate of, 138
--, progress of, 140
Alcohol, coagulation with, 289
Alum, coagulation with, 282
Anti-coagulant for transport, 61
Anti-coagulants, 46
Artificial driers, 133, 148
Ash on sheet, 276
Assembling cases for shipment, 156
Bags for packing, 154
Bakau, 147
Bales for packing, 154
Bark in crepe, 232
-- -- shavings, 56, 123
-- -- sheet, 276
Bases in vulcanised rubber, 322
Basket plants, 10
Blanket crepe, properties of, 324
Blemishes of surface, 252
Blister in sheet, 272
Block rubber, 129, 246
Breaking down of rubber, 304
-- load of test piece, 311
Bubbles in sheet, 269
Buildings, 159
Bulking latex, 69
Byrne curing process, 270
Calendered sheet, 304
Carbon dioxide, 289
Carbonic acid gas, 289
Cases, choice of, for packing, 153
Catalysts, natural, in rubber, 312
--, vulcanising, 312
Centralisation of factories, 221
Chinese vinegar, 286
Chinosol, 238
"Chula" drier, 148
Clippings, sheet, 275
Coagulant, 111
--, choice of, 74, 278
--, quantity of, 113
Coagulation, 74, 111
-- centres, 62
--, premature, 46
--, spontaneous, 294
-- with alcohol, 289
-- -- sugars, 287
-- -- various salts, 288
Coagulum, soft, 249
--, spongy undersurface of, 249
--, tearing of, 249
--, transport of, 59, 63
--, working of, 103
Coefficient of vulcanisation, 319
Collecting latex, 38
-- pails, 48
Collection, advantages of early, 60
Combustion, rate of, in smoke house, 191
Compound crepes, 126
-- -- No. 1, 151
-- -- No. 2, 151
Contents of cases, weight of, 156
Copper salts, cause of tackiness, 243
Cotton fibre in crepe, 230
Creosotic substances, 146
Crepe, air-drying of, 132
--, bark in, 232
--, bearing of defects in, on manufacture, 316
--, bisulphite streaks in, 235
--, colour of fine, 114
--, dirt in, 227
--, dirty edges of, 225
--, drying houses for, 178
--, fibre in, 230
--, general style of finish, 223
--, grades of, 150
--, greenish, tacky streaks in, 228
--, iron stains on, 225
Crepe, No. 1 fine pale, 110
--, oil marks on, 226
--, oxidation streaks in, 234
--, rate of air-drying of, 138
--, rust stains on, 226
--, smoked, 130
--, surface moulds on, 241
--, weight increased in drying house, 141
--, yellow latex streaks in, 234
-- rubber, defects in, 223
-- --, lower grades of, 120
-- --, preparation of, 110
-- --, tensile strength of, 313
Cups, cleaning, 40
--, water in, 44
Cure, rate of, 318
Curing, 307
Da Costa process, 290
Decentralisation of factories, 221
Defects of sheet, infrequent, 276
Derry process, 293
Designs and "layout" of tanks, 176
Dirt in sheet, 276
Discoloration of rubber, dark, 249
Drains for tanks, 176
Drier, Colombo Commercial Company's, 136
Driers, artificial--for crepe rubber, 133
-- --, for sheet rubber, 148
--, "Chula," 148
--, vacuum, 134
Drum furnaces, horizontal, 189
Drying chamber, floor of, 187
-- --, arrangements of, 186
-- houses for crepe, 178
-- --, hot air, 182
-- --, ventilation of, 185
-- --, windows of, 185
-- of rubber, 132
--, period of, 145
--, rate of, effect on rate of cure, 323
Earth scrap, 124
-- --, collection of, 58
Edges, thickened, after rolling, 251
Elongation of test piece, 311
Ends, thickened, after rolling, 251
Engines, 170
--, position of, 174
Factories, 172
--, centralisation of, 221
--, decentralisation of, 221
--, number of floors, 174, 178
--, ventilation of, 180
--, windows of, 181
Factory buildings, situation of, 216
--, choosing site for, 220
--, ideal arrangement of, 162
-- operation, 65
Fibre cotton, in crepe, 230
Field maintenance, 13
Fine hard Para properties of, 325
First latex and other grades, percentage of, 59
Floor of drying chamber, 187
-- factories, 173
-- furnace room, 196
Formalin, 87
Formic acid, 74, 279
Formula for test mix, 311
Freezing (coagulation) process, 293
Fuel, consumption of, 196
Fuels for smoking, 146
Furnace room, floor of, 196
-- --, Petaling type of, 192
Furnaces, horizontal drum, 189
--, "pot," 188
Germination, 6
Grades, number of, 151
Grading, 150
Grafting, 8
Grass squares, 14
Greasiness before smoking, 252
-- of surface, 258
Grit in crepe, 232
Hand rolling sheets, 104
Hevea Brasiliensis, 1
Hot air drying houses, 182
Hydrochloric acid, 282
Hydrofluoric acid, 282
Ilcken-Down process, 298
Instruments, method of using, 100
--, recording, 144
--, standardising, 98
Lallang, eradication of, 15
Latex, bulking, 69
-- cups, choice of, 40
--, decomposition of, in the field, 270
--, first and other grades, percentage of, 59
--, first quality, 150
--, mixing acid with, 79
--, mixing sodium bisulphite solution with, 117
--, preliminary treatment of, 65
--, reception of, at the store, 65
--, standard, 96
--, standardisation of, 69, 110
--, straining, 67
--, transport of, 59
Light, importance of, in factories, 172
Litharge, 312
Load stretch curve, 312, 319, 320, 321
Low grade rubbers, fibrous matter in, 124
Lower grade rubber, care in manufacture, 129
Lubrication of machines, 166
Lump rubber, naturally coagulated, 120
Machinery, 159
Machines, access to, 169
--, adequacy of, 160
--, arrangement of, 168
--, lubrication of, 166
--, position of, 173
--, sheeting, 166
--, speed of, 162
Mangrove, 147
Marking sheets, 105
Metrolac, 98, 100
Michie-Golledge system, 138
Mildew on surface, 260
Milky residue on serum, 249
Mixing acid with latex, 79
Moist glaze of surface, 258
Mould on surface, 260
Moulds, surface, on crepe, 241
Natural catalysts, 312
Nitric acid, 282
Nurseries, 9
Overcured, 307
Overvulcanised, 306
Oxalic acid, 279
Oxidation, prevention of, 56
--, variation due to, 254
Packing, 150
--, bags for, 154
--, bales for, 154
--, cases, choice of, 153
--, folding for, 155
--, methods of, 156
--, rooms, 211
Pale crepe, No. 1 fine, 110
--, rubber, former methods of making, 118
--, sheet, 89
Patches, 259, 260
--, virgin, 259
Payment by result, 53
Perished rubber, 306
Physical properties of rubber, variation of, 317
Pits for smoke houses, 188
Pitting of surface, 250
Plantation rubber, testing of, 309
Planting, 1
Plasticising of rubber, 304
Plasticity of plantation rubber, 309
"Pot" furnaces, 188
Power units, 170
Premature coagulation, 46
Preparation, special methods of, 290
Pyroligneous acid, 282
Racks, 186
Rate of cure, 307
Raw rubber, physical properties of, 313
-- --, tests on, 309
-- --, uses of, 315
Recommendations, Rubber Growers' Association, 152
Recording instruments, 144
Ribbing of sheet, 274
Rolling, 251
Rolls, grooving of, 164
-- running hot, 165
-- -- "free," 165
Roof of smoke house, 197
Rubber, drying of, 132
Rubber Growers' Association, Recommendations, 152
Rubber, properties of, 313
--, smoking, 109
Rust, cause of, 267
-- on sheet, 262
--, treatment to prevent, 265
Scrap washers, 57, 124
Screw plug, unsatisfactory, 73
Seed at stake, 10
Seeds, 2
Selection, 5
Senang folder, 156
Serum, milky residue on, 249
Sheet, ash on, 276
--, bark in, 276
--, bearing of defects on, in manufacture, 316
--, blisters in, 272
--, bubbles in, 269
--, clippings, 130, 275
--, creases in, 251
--, dirt in, 276
--, "dog ears," 251
--, grades of, 151
--, infrequent defects of, 276
--, pale, 89
--, ribbing of, 274
-- rubber, artificial driers for, 148
-- --, defects in, 249
-- --, preparation of, 89
Sheet rubber, rolling and marking of, 102
-- --, tensile strength of, 313
--, rust on, 262
--, splinters in, 276
--, stickiness in, 274
--, style of, 101
--, support marks on, 273
--, surface pattern of, 274
--, thick ends of, 275
Sheeting machines, 166
Sheets, mis-shapen, 251
--, thickened patches in, 251
--, torn, 251
--, unevenness of appearance, 253
Short weights, 157
Skimming, 100
Skimmings, 122
Slab rubber, 299
-- --, properties of, 324
Slope, 312
Smoke curing of sheet rubber, 143
-- --, temperature of, 144
--, houses, 183
-- --, Barker patent, 209
-- --, Devon type, 200
-- --, iron stoves for, 189
-- --, Jackson type, 200
-- -- of brick, 198
-- -- rate of combustion in, 192
-- --, roof of, 197
-- --, "Third Mile" type, 199
-- --, types of, 198
Smoked crepe, 130
-- sheets, colour of, 255
-- water for coagulation, 286
Smoking, effect on rate of cure, 323
--, greasiness before, 252
-- rubber, 109
Smooth rolling of sheets, 104
Sodium bisulphite, 80, 114
-- --, abuse of, 85
-- --, care of, 116
-- --, deterioration of, 115
-- --, evaluation of, 115
-- --, making a solution of, 85
-- --, quantity of, 84
-- --, residual traces of, 86
-- -- solution, mixing, with latex, 117
-- sulphite, 86
-- --, deterioration of, 115
-- --, evaluation of, 115
Sorting, 150, 152
-- rooms, 211
Spewing, 305
Splinters in sheet, 276
Spontaneous coagulation, 294
Spot disease, 235
-- -- in sheet rubber, 273
Spots, 259, 262
--, virgin, 259
Standard latex, 96
-- sheet, 102
Standardising instruments, 98
Stickiness in sheet, 274
Stock solution, method of making, 75
Storage of rubber, 212, 216
Stoves, iron, for smoke houses, 189
Straining latex, 67
Streaks, 262
Stumps, 9
Sugars, coagulation with, 287
Sulphuric acid, 280, 286
-- --, buying, 280
-- --, formula for use of, 280
-- --, storing, 280
Sun-drying sheet rubber, 147
Support marks on sheet, 273
Surface blotches, coloured, 249
--, dull or black, 258
-- pattern of sheet, 274
Tackiness, cause of, 243
--, copper salts cause of, 243
-- in rubber, 242
Tanks, 90
--, care of, 94
--, designs and "layout," 176
--, drains for, 176
--, installation of, 93
--, situation of, 175
--, water-supply for, 177
Tapping, 38
--, former systems of, 29
-- knives, 38
-- systems, 38
Tartaric acid, 279
Tensile product, 314
Test mix, formula for, 311
-- pieces, making of, 310
Testing of plantation rubber, 307
Thick ends of sheet, 275
Thinning, 19
Timber for smoking, 147
Tool sheds, 216
Transport, 60
-- by coolie, 62
-- of coagulum, 59, 63
-- of latex, 59
Trays, 167
Treatment of rubber in the factory, 301
-- to prevent rust, 265
Tree scrap, 55, 122
-- --, care of, 56
-- --, oxidation of, 56
Trees per acre, 26
Trenches, silt catchment, 11
Uniformity, 90
-- in colour, lack of, 246
Vacuum driers, 134
Variation due to oxidation, 254
Vegetable extracts, 289
Ventilation of drying houses, 185
-- factories, 180
Verandas, 175
Virgin spots, 258
Viscosity of rubber solution, 309
Vulcanisation, 301
--, rate of, 318
Vulcanised rubber, 318
-- --, tests on, 309
Vulcanising, 307
-- catalysts, 312
--, "heat," 306
Washers, scrap, 57, 124
Washings, 122
Water-supply for tanks, 177
Weeding, clean, 13
Weights, "short," 157
Wickham process, 293
Windows of drying houses, 185
-- factories, 181
Working of rubber, 304
Yields, 23, 25
End of the Project Gutenberg EBook of The Preparation of Plantation Rubber, by
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