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Title: The Preservation of Antiquities
A Handbook for Curators
Author: Friedrich Rathgen
Translator: George A. Auden
Harold A. Auden
Release Date: September 14, 2014 [EBook #46851]
Language: English
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Transcriber's Notes:
Italics are surrounded with _ _ and Greek words have been transliterated
and surrounded with # #. Isolated Greek letters have been replaced by the
name of the letter (e.g. alpha). The oe ligature has been replaced by the
letters oe. Subscripted characters have been preceded by _ and surrounded
with {} Fractions have been represented as 1-1/2 (for one and one-half)
and where a range of fractions has been specified, the numbers have been
surrounded by {} to prevent confusion (i.e. {3-1/2}-{4-1/2} represents the
range three and one-half to four and one-half.) Dittos and dashes used to
represent text have been replaced with the indicated text.
Some presumed printer's errors have been corrected. In particular, the
use of c.c. has been normalized when periods were missing, the degree
symbol (°) has been added when it appears to have been missing,
and words and numbers in the Index were changed to match the spelling and
placement in the body of the text. Some additional presumed errors which
have been corrected are listed below with the original text (top) and the
replacement text (bottom):
Table of Contents Literature xiv
Literature xiii
p. 59 vesse
vessel
p. 62 Gay Lussac
Gay-Lussac
p. 65 16 Feb 1904
16 Feb 1894
p. 102 Konigsberg
Königsberg
p. 154 Royal Musums
Royal Museums
Footnote 1 "Lexikon d. gesamsen Technik,"
"Lexikon d. gesamten Technik,"
THE PRESERVATION
OF ANTIQUITIES
Time, which antiquates antiquities, and hath an art to make dust
of all things, hath yet spared these minor monuments.
(SIR THOMAS BROWNE, _Hydriotaphia_, cap. v.)
THE PRESERVATION
OF ANTIQUITIES
A HANDBOOK FOR CURATORS
TRANSLATED, BY PERMISSION OF THE AUTHORITIES OF THE ROYAL
MUSEUMS, FROM THE GERMAN OF
DR FRIEDRICH RATHGEN
Director of the Laboratory of the Royal Museums, Berlin
BY
GEORGE A. AUDEN, M.A., M.D. (Cantab.)
AND
HAROLD A. AUDEN, M.Sc. (Vict.), D.Sc. (Tübingen)
CAMBRIDGE:
at the University Press
1905
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
C. F. CLAY, MANAGER.
London: AVE MARIA LANE, E.C.
Glasgow: 50, WELLINGTON STREET.
[Illustration]
Leipzig: F. A. BROCKHAUS.
New York: THE MACMILLAN COMPANY.
Bombay and Calcutta: MACMILLAN AND CO., LTD.
[_All Rights reserved._]
AUTHOR'S PREFACE.
The increasing recognition of the importance of the preservation of
antiquities justifies the publication of a handbook dealing with this
subject. As far as I can ascertain, with the exception of a short
article[1] for which I am myself responsible, only one work has appeared
which covers the whole field--the "Merkbuch[2]" prepared by Dr Voss at
the request of the Government. But as this book only gives a selection
of the known methods of preservation, the need of a more comprehensive
publication will scarcely be denied.
In spite of my ten years' experience in the special Laboratory of the
Royal Museums and the frequent opportunities of learning the methods
in use elsewhere, which the journeys and correspondence arising out of
my duties have given me during this period, I do not feel competent to
produce a review of these various methods which will be at once exhaustive
and sufficiently critical. There are several reasons for this. In the
first place the individual methods have been but rarely published, and
even then through the most varied literary media; often they are only
casually mentioned in articles dealing with anthropological or historical
subjects. On the other hand, the value of an object to be dealt with may
prohibit an attempt at treatment, the success of which is not assured.
My own experience has been gained by trials with objects chiefly from
the Egyptian section, but also to some extent from the Antiquarian and
Numismatic departments of the Royal Museums.
This deficiency can only be remedied by a work such as that now offered
to the public, and it is to be hoped that this handbook will stimulate
the Curators of State, Municipal and Societies' Collections, as well as
private collectors and others interested in the subject, to make public
their further experiences in this field of archaeology. I take this
opportunity, therefore, of expressing the hope that I may receive other
communications bearing upon the subject and may thus perhaps at some
future date be able to produce a more complete work.
In using the book it will be noticed that for the proper understanding of
the first portion, which deals with the causes of destruction, a certain
amount of chemical knowledge is assumed. In the second portion, however,
the methods of preservation are treated from a more elementary standpoint,
and the simple apparatus and manipulations required are so described that
the treatment may be readily carried out by those who are unfamiliar with
chemical methods.
In conclusion, I take this opportunity of expressing my thanks to all
those who have given their help, and especially to Dr Otto Olshausen for
his continued interest in the work of the Museum Laboratory and in the
production of this handbook. Especially am I indebted to his extensive
knowledge of anthropological literature for many references which would
otherwise have escaped my notice.
TRANSLATORS' PREFACE.
Dr Rathgen has, in his preface, stated the aim of this handbook, and it
is with a desire to further this aim that we have prepared an English
translation.
Claiming but limited experience in this field of research we have only
added such explanatory notes as seem in some way to bear upon the subject
or likely to be useful in a handbook of this kind (viz. the method of
taking squeezes, Appendix A, and a few footnotes which are signed and
enclosed in square brackets). We take this opportunity of thanking Dr
Rathgen for his interest in our undertaking, for his kindness in supplying
much additional matter which did not appear in the German edition, and
also for the loan of the blocks for Figs. 22 and 23. Figs. 7, 9-12, 30-33,
and 37, are from photographs of objects treated by ourselves.
Our thanks are especially due to Dr W. A. Caspari, of the National
Physical Laboratory, for his invaluable help in the revision of the
translation, and for his advice and suggestions in reference to the more
technical aspect of the work.
YORK,
_December 1904_.
CONTENTS.
PAGE
Literature xiii
Part I.
The changes undergone by antiquities in earth and
in air 1
Limestone and clay 2
Iron 7
Bronze and copper 15
Silver 49
Lead 53
Tin 53
Gold 53
Glass 54
Organic substances 54
Part II.
The preservation of antiquities 56
i. Preservation of objects composed of inorganic substances
_a._ Limestone 56
_b._ Marble and alabaster 74
_c._ Earthenware 74
_d._ Slightly baked or unbaked clay 81
_e._ Fayence 86
_f._ Stucco and Nile-mud 87
_g._ Sandstone and granite 87
Appendix: Cement for earthenware. Restorations 87
_h._ Iron 89
1. Methods of preserving objects of iron
without removal of the rust 89
2. Preservation by steeping and subsequent
impregnation 92
3. Preservation by removal of the rust 102
4. Preservation of medieval iron objects 119
_i._ Bronze and copper 120
A. Methods of impregnation 122
B. Preservation by reduction 125
Reduction of oxidized copper coins 140
Cleaning copper coins with melted lead 143
C. Preservation by exclusion of air 144
Appendix: Method of bringing out worn lettering 146
upon coins
_j._ Silver 148
_k._ Lead and tin 149
_l._ Gold 150
_m._ Glass and enamel 151
ii. Preservation of organic substances.
_n._ Bones, horns, ivory 151
_o._ Leather 152
_p._ Textile fabrics, hair 153
_q._ Feathers 154
_r._ Papyrus 154
_s._ Wood 156
1. Dry preservation 156
2. Preservation in liquids 159
Protection against wood-worms, etc. 160
Preservation and cleaning of coloured
objects of wood 161
_t._ Amber 162
Care of antiquities after preservative treatment 162
Concluding remarks 164
Appendix A. Method of taking squeezes of
inscriptions 166
Appendix B. Zapon 168
Index 171
ILLUSTRATIONS.
FIG. PAGE
1. Limestone block with well-preserved surface 3
2. Limestone block with pitted surface 3
3. Limestone block showing destruction of surface 4
4. Potsherd showing saline efflorescence 5
5. Pottery showing sodium nitrate efflorescence 6
6. Portion of horse-trappings showing blue and green patina 35
7. Head of Osiris showing advanced condition of warty patina 38
8. Etruscan mirror showing warty patina 40
9. } { 42
} Bronze figure showing destructive patina {
10. } { 43
11. } { 44
} The same after treatment (Finkener's method) {
12. } { 45
13. Gay-Lussac's burette 62
14. Air-pump fixed to water-tap 68
15. Apparatus for impregnation by extraction of air 69
16. } Assyrian clay tablet showing incrustation 79
}
17. } The same after treatment 79
18. }
19. } Assyrian clay tablet before and after treatment 80
20. }
21. }
22. } { 82
} Babylonian clay cone before and after treatment {
23. } { 83
24. Water-bath for iron objects 94
25. Iron sword treated by Blell's method 108
26. Iron spear-head treated by Blell's method 109
27. Iron fibula treated by Blell's method 109
28. Application of Krefting's method 111
29. Iron spear-head treated by Krefting's method 112
30.} Iron pin before and after treatment by Krefting's method 113
31.}
32.} Iron object before and after treatment by Krefting's method 114
33.}
34. Piece of iron sword-blade with inscription revealed by Krefting's
method 116
35. Iron sheath after treatment by combination of Blell's and
Krefting's method 117
36. Hammer-heads for removal of bronze incrustations 120
37. Osiris showing cracking and destructive patina 123
38. Boeotian bridle showing cracked patina 124
39. Bronze bull showing warty patina 132
40. The same after reduction by Finkener's method 133
41. Bronze axe-blade before treatment by Finkener's method 134
42. The same after treatment by Finkener's method 135
43. Reverse side of same after treatment 136
44. Dagger-sheath before treatment by Finkener's method 137
45. Dagger-sheath after treatment, showing design 137
46. Roman coins before treatment by Krefting's method 142
47. Roman coins after treatment by Krefting's method 143
48. Method of mounting objects in air-tight damp-proof cases 145
LITERATURE.
Aarböger for nordisk Oldkyndighed og Historie, udgivne af det
kongelige nordiske Oldskrift-Selskab. Copenhagen.
Aarsberetning fra Foreningen till norske Fortidsmindesmaerkers
Bevaring. Christiania.
Annalen der Chemie und Pharmacie. Edited by Wöhler, Liebig and Kopp.
Since 1873: Liebig's Annalen der Chemie.
Antiquarisk Tidsskrift, udgivet af det kongelige nordiske
Oldskrift-Selskab. Copenhagen 1843-63.
Archaeological Journal. London.
Atti della Reale Accademia dei Lincei. Rome.
Berg- und hüttenmännische Zeitung. Leipzig.
Bibra, E. v. Die Bronzen und Kupferlegirungen der alten und ältesten
Völker. Erlangen 1869.
Bibra, E. v. Ueber alte Eisen- und Silberfunde. Nürnberg and Leipzig
1873.
Bischoff, C. Das Kupfer und seine Legirungen. Berlin 1865.
Blätter für Münzkunde. Hannoversche numismatische Zeitschrift.
Edited by H. Grote. Leipzig.
Chemiker-Zeitung (Dr G. Krause). Cöthen.
Chemisches Centralblatt (Arendt) Hamburg and Leipzig.
Christiania Videnskabs-Selskabs Forhandlinger. Christiania.
Comptes rendus hebdomadaires des séances de l'Académie des sciences,
publ. p. les secrétaires perpétuels. Paris.
Dingler's Polytechnisches Journal. Stuttgart.
Finska Fornminnesföreningens Tidskrift. Helsingfors.
Finskt Museum. Finska Fornminnesföreningens Månadsblad. Helsingfors.
Friedel, E. Eintheilungsplan des Märkischen Provinzialmuseums der
Stadt Berlin. 6th issue. Berlin 1882.
Graham-Otto's Ausführliches Lehrbuch der Chemie. 5th Edition.
Anorgan. Chemie von H. Michaelis. Brunswick 1878-89.
Hauenstein, H. Die Kessler'schen Fluate. 2nd Edition. Berlin 1895.
Journal für praktische Chemie. Edited by Erdmann. Leipzig,
Journal of the Chemical Society. London.
Keim, A. Technische Mittheilungen für Malerei. Munich.
Kongl. Vitterhets Historie och Antiqvitets Akademiens Månadsblad.
Stockholm.
Kröhnke, Chemische Untersuchungen an vorgeschichtlichen Bronzen
Schleswig-Holsteins. Dissertation. Kiel 1897.
Layard. Discoveries in the ruins of Nineveh and Babylon. London
1853.
Lepsius, C. R. Denkmäler aus Aegypten und Aethiopien. Berlin
1849-59.
Lueger, O. Lexikon der gesamten Technik. Stuttgart 1894.
Merkbuch, Alterthümer aufzugraben und aufzubewahren. Herausgeg. auf
Veranlassung des Herrn Ministers der geistlichen, Unterrichts- u.
Medizinal-Angelegenheiten. 2nd Edition. Berlin 1894.
Mittheilungen der naturforschenden Gesellschaft in Bern. Bern.
Mittheilungen aus der Sammlung der Papyrus Erzherzog Bainer. Vienna
1887-1889.
Morgan, J. de, Fouilles à Dahchour Mars-Juin 1894. Vienna 1895.
Muspratt's theoretische, praktische u. analytische Chemie. 4th
Edition. Brunswick 1883.
Neues Jahrbuch für Mineralogie, Geognosie, Geologie und
Petrefakten-Kunde, edited by K. C. von Leonhard and H. G. Bronn.
Stuttgart.
Polytechnisches Centralblatt. Leipzig 1835-75.
Polytechnisches Centralblatt. (Geitel.) Organ der polytechn.
Gesellschaft zu Berlin. Berlin 1888.
Prometheus, edited by Dr O. N. Witt. Berlin.
Publications de la société pour la recherche et la conservation des
monuments historiques dans le grandduché de Luxembourg. Luxemburg.
J. J. Rein, Japan. Nach Reisen und Studien im Auftrage der Königl.
Preuss. Regierung. 2 Vols. Leipzig 1881-1886.
Revue archéologique, publiée sous la direction de MM. A. Bertrand
et G. Perrot. Paris.
Schliemann, H., Ilios. Leipzig 1881.
Simon, E., Ueber Rostbildung und Eisenanstriche. Berlin 1896.
Sitzungsberichte der Alterthumsgesellschaft Prussia in Königsberg.
Verhandlungen der Berliner Anthropologischen Gesellschaft. Berlin.
Verhandlungen des Vereins zur Beförderung des Gewerbefleisses in
Preussen. Berlin.
Zeitschrift für Numismatik. Edited by A. v. Sallet. Berlin.
Zeitschrift für anorganische Chemie.
Zeitschrift für Ethnologie. Berlin.
PART I.
THE CHANGES UNDERGONE BY ANTIQUITIES IN EARTH AND IN AIR.
The greater number of those objects of antiquity which are composed of
inorganic materials, such as limestone, earthenware, and metals, owe the
commencement of any alteration in their character to the situation in
which they are discovered, since they are buried in ground which has been
at some period damp or wet, and has contained, moreover, salts soluble
in water. Amongst these salts the most usual is sodium chloride (common
salt), but this is mostly accompanied by potassium chloride, potassium
sulphate, magnesium chloride, and calcium sulphate; in short, by those
soluble salts which are found in sea-water. In the fine pores of Egyptian
antiquities, especially, such salts occur, and their presence is readily
explained by the fact that the land of Egypt was originally a sea-bottom.
The presence of salt in the soil of Egypt has been known for a
considerable period. Thus Karabacek[3], quoting from Volney's "Travels in
Syria and Egypt" (Jena, 1788, I. p. 13):
"Wherever one digs one finds brackish water containing soda,
sea-salt, and a small quantity of saltpetre. Indeed, when a garden
has been flooded for irrigation purposes, crystals of salt make
their appearance on the surface after the water has evaporated or
has been soaked up by the soil."
In the dry climate of Egypt, objects saturated with salt keep better after
their removal from the ground than in our climate, where the variations
in the temperature and in the hygroscopic condition of the air produce a
partial deliquescence in wet weather, and in dry weather a re-formation of
crystals. The continued alternation of these processes gradually loosens
the surface of limestone or earthenware, or induces certain chemical
changes in objects of metal and in both cases leads to their destruction.
Limestone and Clay.
The series of changes are particularly well illustrated by the Egyptian
grave of Meten[4], the stones from which are now in the Royal Museum in
Berlin. The three illustrations here given show: (1) an undecayed block of
limestone, (2) a block with pitted surface, and (3) a block the surface of
which was formerly covered with hieroglyphics, but which is now totally
destroyed by flaking. The blocks of the latter kind were found in the
lowest layer, or lowest but one, while those blocks which were above
were the best preserved. As the amount of salt present scarcely varied,
these specimens offer a striking illustration of the greater influence of
moisture in the deeper soil than at the higher levels.
[Illustration: Fig. 1.
Limestone block, surface well preserved.]
[Illustration: Fig. 2.
Limestone block with pitted surface.]
[Illustration: Fig. 3.
Limestone block showing destruction of surface.]
Baked clay, particularly that of Egyptian ostraca (i.e. fragments of
pottery showing inscriptions), exhibits similar changes, as is shown in
the accompanying illustrations. The surface of some fragments is found
to be almost completely covered with a layer of salt, which, apart from
impurities of clay and dust and remains of the black lettering, consists
of almost pure sodium chloride; only a trace of magnesium sulphate being
found on analysis.
In contrast with this very loose superficial incrustation, the inner
portions of the ostracon contained considerable quantities of sulphates.
Figure 4 represents a fragment with granular efflorescences of sodium
chloride, and also fine needles of magnesium sulphate[5]. As a general
rule the amount of salt is small compared with the bulk of clay or
limestone: thus it was found by titration that three separate fragments
contained 0·13, 0·20, and 0·48% calculated as sodium chloride, and in one
series the average of 16 fragments was 0·13%. But the percentage of sodium
chloride has often been found higher, more especially in larger objects of
baked clay, being in one instance as high as 2·3%. The disintegration of
the surface is due to the mechanical action of moisture which results in
the scaling off of portions of the surface. This does not however exclude
a chemical action of the salts upon the clay, especially when this has
been only slightly baked. Thus by merely washing such fragments in cold
distilled water, not only sodium and magnesium compounds but also those
of aluminium and calcium may be removed. The soft powdery patches, which
some limestones show instead of scales, are also evidences of chemical
action; thus in one case a cuneiform tablet[6] of dolomitic stone showed
decomposition at those spots where the salt was firmly deposited as an
incrustation, and here the stone, elsewhere smooth and hard, was found,
on washing away the salt, to be soft and porous.
[Illustration: Fig. 4.
Potsherd showing saline efflorescence of sodium chloride and magnesium
sulphate.]
Although, as has been already remarked, sodium chloride generally
constitutes the bulk of the salts present, and only in rare cases, as I
have for instance shown in an Egyptian Fayence and in several Greek clay
vases, is the amount of sulphates greater, yet there are in collections
clay objects (Fig. 5) covered with needles of sodium nitrate[7] (Chili
saltpetre) where the nitric acid has been contributed by the decomposition
of organic substances; and here the presence of nitrates proves inimical
to antiquities just in the same way as a coating of limewash may be seen
to be destroyed by the so-called wall-saltpetre[8].
[Illustration: Fig. 5.
Pottery showing efflorescence of sodium nitrate.]
Iron.
If in some cases it may be uncertain whether the destruction of
antiquities of limestone or earthenware has been due to mechanical or to
chemical influences, this uncertainty is excluded in the case of metallic
objects, of which those of bronze and iron chiefly come under the notice
of the antiquary.
From the first piece of metallic iron which he possessed man must have
soon become acquainted with its untoward property of rusting, but even at
the present day opinions differ as to the origin of rust, and the cause
of its rapid spreading. It has long been known with certainty that iron
containing but little carbon (wrought iron) rusts with greater ease than
iron which is rich in carbon (cast iron or steel), and that the rust is
a compound of iron with hydrogen and oxygen (hydroxide). That rust is of
variable composition may be inferred from the variations of shade from
yellow to dark brown which are met with.
Widely different views are held on the question of the production of
rust. Some[9] maintain that iron rusts only in the presence of water
containing free oxygen and carbonic acid (CO_{2}) in solution, a ferrous
bicarbonate being first formed; the bicarbonate is then converted into
ferrous carbonate, which finally yields the hydrate with evolution of
carbonic acid. This carbonic acid continues to attack further areas of
metallic iron. Others[10] maintain that, while the formation of rust _may_
proceed as described, carbonic acid is not necessary, and that free oxygen
_alone_ causes rusting when atmospheric moisture is condensed upon the
surface of iron. That iron remains free from rust when in a solution of
caustic potash or soda is said to be due to the absence of free oxygen
and not to the removal of carbonic acid. Spennrath holds, in opposition
to the opinion of Axel Krefting[11], that rust once formed cannot act
as an oxidising agent, except by virtue of its power of condensing
water and retaining it in its pores. Similarly E. Simon finds the chief
cause of the corroding action of rust in the property of absorption,
that is surface-condensation of gases. This condition is comparable to
that of liquefaction, and produces rapid chemical action. Under certain
circumstances ferrous hydrate is formed instead of ferric hydrate,
particularly when iron is subjected to vibrations, as Tolomei[12] has
observed in iron rails etc. Stapff[13] believes that mixtures of ferric
hydrate with ferroso-ferric oxide, which possess a similar composition to
forge scale, are formed under the influence of thermal waters. According
to Irvine[14] rusting proceeds rapidly when two kinds of iron, such as
cast and wrought, are in contact, since their electro-chemical relations
may result in a voltaic couple. The electric current brings about the
decomposition of the water, and the evolved hydrogen, being in the nascent
state, combines with the nitrogen dissolved in the water to form ammonia,
as had been previously observed by Akermann[15]. Similarly, electric
currents are said to be caused by the contact of ferroso-ferric oxide with
metallic iron, thus causing a further oxidation of the iron[16].
The presence of certain neutral salts, especially sodium chloride (common
salt), has a very marked influence on the destruction of iron[17].
When iron filings are exposed to air and moisture, oxidation takes
place; the action is, however, according to Krefting, far more intense
in the presence of an alkaline chloride. A mixture of iron filings and
sodium chloride exposed to moisture is converted in a few days into
a black powder which has the following composition:--11·4% FeO, 80·0%
Fe_{2}O_{3}, 8·6% H_{2}O, thus resembling the "iron-black" of Lemery;
on extraction with water the filtrate is found to be alkaline and to
possess a tallow-like smell[18]. Without entering further into Krefting's
researches, we will quote the hypothesis with which he concludes:
"The iron probably combines with small quantities of chlorine from
the sodium chloride, causing alternate reduction and oxidation,
and this, owing to the ease with which iron salts pass from one
stage of oxidation to another, very soon gives a visible result
in the formation of rust:
Fe + 2NaCl = FeCl_{2} + 2Na
2Na + 2H_{2}O = 2NaOH + H_{2}."
If these results be compared with observations made upon the condition
of iron objects which have been excavated, it is evident that these are
in many cases exposed to the action of the air to a lesser extent while
buried, and that their decomposition will advance more rapidly when they
have been withdrawn from their protective covering of earth. The condition
of the objects differs according to the kind of iron, the length of
time during which they have been buried, and the character of the soil
in which they are found. In one place objects are found covered with a
slight layer of rust only, in another with a thicker layer, in another
there remains but a small core of metal, or even none at all, or the
layer of rust may be intermingled with particles of earth or clay. The
rust may be uniform in colour and hardness in one case, and in another
soft areas, generally light in colour, may alternate with darker, harder
patches, while frequently the harder layer is found below the lighter and
softer, etc.--conditions which depend on the occurrence of the various
iron compounds. The behaviour of all, however, when placed in collections,
even in the driest of rooms, is the same; all sooner or later undergo
change, and portions of rust become detached, until in the course of time
every trace of the original metallic core is oxidised. A closer inspection
generally shows in these cases small brownish, glistening bubbles[19]
which prove, when touched, to be drops consisting of chlorine compounds
of iron surrounded and permeated with oxides. Krefting[20] gives as the
average of a series of analyses of the rust on northern antiquities the
following composition:
Ferric oxide 7·05
Ferrous oxide 12·7
Carbonic acid 3·9
Calcium oxide 0·58
Magnesium oxide 0·09
Ferrous chloride 0·260}
Calcium chloride 0·280} 0·61% Soluble
Magnesium chloride 0·023} salts.
Potassium chloride 0·018}
Sodium chloride 0·027}
Water chemically combined 8·0
Moisture 1·50
Organic matter 0·97
Thus the chief part in this rapid decomposition is played by the chlorine
compounds, as indeed was previously determined[21] by the experimental
proofs already given. If ferrous chloride is present the further
decompositions can be explained by such equations as those given by
Olshausen[22].
6FeCl_{2} + 3O = Fe_{2}O_{3} + 2Fe_{2}Cl_{6};
2Fe_{2}Cl_{6} + 2Fe = 6FeCl_{2}.
The equations do not claim to give a complete statement of the reactions,
for other reactions take place at the same time; thus ferric hydrates and
carbonates and perhaps also intermediate compounds of oxygen and chlorine
occur; they show however that in the oxidation of ferrous chloride,
oxides and ferric chloride are produced, so that new and hitherto intact
particles of the metal continually react with the ferric chloride.
In many cases the action of the chlorine is not only seen in objects
placed in a collection, but also in freshly excavated objects. Not
infrequently iron objects are found which are covered with large hard
blisters, and are thus more or less deformed. The interior of these
blisters consists of a mixture of ferrous chloride with oxides, but the
shell has become so hard by complete oxidation that it can only be removed
with hammer and chisel.
Iron objects found in peat differ from these chlorine-containing specimens
which are found in soil, and although sometimes much corroded, many are
well preserved. Blell[23] is of the opinion that if peat is free from
tannic acid, the finds will be well preserved, while the theory advanced
in the Merkbuch[24] is that tannic acid acts as a preservative. The
latter view is probably the more correct, for although ordinary tannic
acid seldom occurs in peat, yet peat contains a series of compounds which
are tanning agents, such as ulmic, humic, and crenic acids. These form
iron compounds which, being insoluble in water, protect the metallic iron
beneath from further action. If, however, the peat contains sulphates, and
especially if it contains free sulphuric acid, only much corroded iron
is likely to be found. Moreover the physical condition of the peat may
vary; thus it may be dry or damp or even submerged under water, and this
variation will exercise some influence upon the condition of the iron.
Iron objects which are covered with the black, so-called "noble" rust
(Edel-rost) usually prove very stable. This, like forge-scale, is a
ferroso-ferric compound in which there is a preponderance of ferrous
oxide where it is in contact with the metallic iron, and of ferric oxide
in the outer layer. "Noble" rust is probably in nearly all instances the
result of the action of fire, which may have been used in funeral rites,
or may have been accidental; very rarely can it have been produced by the
reactions mentioned above, as has been suggested by Stapff.
Iron which has been in contact with the bone ash of burnt corpses has
certain characteristics. When entirely surrounded with bone ash objects
are well preserved[25], and only covered with a thin layer of oxide. How
far the ash has acted as a preservative, I will not hazard an opinion,
having seen but few specimens, and these had been already varnished to
preserve them.
Under certain conditions the phosphoric acid of the bones forms a thin
bluish layer of iron phosphate, corresponding in composition to vivianite
(Fe_{3}P_{2}O_{8}.8H_{2}O), as was pointed out by Jacobi in a series of
objects in the Saalburg Museum at Homburg. These objects also are quite
durable.
In earth so full of sodium chloride as is that of Egypt, objects of iron
will be readily corroded, and the explanation given above will account for
the paucity of iron remains of Egyptian origin. It is difficult, however,
to find a satisfactory explanation for the fact that objects found in
sea-water are specially well preserved. It may be that, in spite of the
presence of free oxygen in solution in the water their complete insulation
from the atmospheric air has resulted in the preservation of the objects,
as is the case with those which have lain in a stream of fresh water.
Bronze and Copper.
Copper and its alloys are subject to the same far-reaching changes
as iron, but the action is less rapid. Bronzes of widely different
composition have to be dealt with to ensure their preservation, and to
a less extent, copper also[26]. According to von Fellenberg[27] bronze
objects may be classified according to the material in which they have
been found, i.e. peat mud, water, or earth.
"(1) Bronzes from peat mud are covered with a black earthy mass,
which can be easily removed by water and brushes, the alloy then
assumes its metallic lustre and the characteristic colour of
bronze. The complete preservation of the pure metallic surface
of the bronzes, in the same condition as they were when they
were submerged, is easily accounted for by the enclosure of the
metal in mud of organic origin under several feet of water which
effectually excludes the oxygen of the air.
(2) The bronzes found in water, as for example in the beds of
lakes and rivers, are less perfectly preserved. They have usually
a thin coating of a calcareous deposit, which however often allows
the lustre and colour of the metal to appear in places. When such
bronzes have dark or green coloured patches or spots, the layer
is very thin and may be removed by treatment with acids, which
allows the metallic colour to become visible. Bronzes preserved
in water still retain the same definite edges and points which
they possessed when they entered the water. If bronzes which
are markedly incrusted with verdigris are found in water in all
probability they had lain in the ground a considerable time before
being covered with water, and oxidation had penetrated deeply into
the metal before immersion.
(3) Bronzes found in the earth or in graves appear covered with
a fine green crust of verdigris which may be either light or dark
in colour and which often has a vitreous lustre. This is generally
known as Patina.
This crust varies in thickness from that of writing-paper to
several millimetres. If the green crust be filed away, or better,
removed by dilute nitric or sulphuric acid, the bronze is found to
possess a reddish colour; below the crust of cupric carbonate is
found a layer of cuprous oxide, which may be removed by ammonia,
thus revealing the metal with its characteristic colour and
lustre. This condition is characteristic of the slow oxidation of
bronze in moist earth. The layer of cuprous oxide between the pure
metal and the external crust of copper carbonate has been shown by
the examination made by Dr Wibel to be a product of the reduction
of copper carbonate by the metallic copper of the bronze. Bronzes
belonging to this category have often lost their former metallic
properties, and if of small diameter have often been completely
converted into cuprous oxide, surrounded by a lustrous green or
blue crust of carbonates. If a metallic core remains, it is found
to be crystalline, brittle, and non-coherent, that is, it flies
to pieces under the blow of a hammer. Fine ornamentation and
sharpness, whether of edge or of point, have often disappeared.
This does not occur with bronzes preserved in water."
In another volume of the series[28] von Fellenberg states that basic
copper chloride occurs as a constituent of patina.
A few lengthier quotations may be conveniently given here, in part
verbatim, in part abstracted from literature which is not readily
accessible.
Reuss[29] states that it has been hitherto generally assumed that copper
is first converted into cuprous oxide which is then converted into a
green hydrated oxy-carbonate which is separated from the metal by a thin
layer of cuprous oxide. The specimens examined by him, however, showed no
such dividing layer, the metal being either directly in contact with the
malachite[30], or else separated from it by a black or bluish layer of
cupric oxide. He further draws attention to the occurrence of irregular
knobs two to three lines in height which consist, in part, of azurite[31].
Neither oxides of tin nor chlorine were found. The alteration of the
bronze he explains by the prolonged oxidising action of water containing
carbonic acid.
In an exhaustive memoir Wibel[32] describes the various kinds of patina
as malachite, copper-oxychloride, and azurite, with admixtures of tin
oxide, silver, iron oxide, lead chloride and copper chloride. He discusses
also the occurrence of the cuprous oxide layer which is said to have been
described by Sage as early as 1779. After detailing the observations of
Davy, Hünefeld, and Picht, that the metallic copper exists partly in alloy
and partly free as crystals in the layer of cuprous oxide, he continues
as follows[33]:
"The process of decomposition in bronzes has been regarded as
a slow oxidation, in which cuprous oxide marks the first and
incomplete stage, while the carbonates represent the later
completed phase. The formation of both these substances was
assumed to be due to moist oxidation, on bronzes as well as
in those superpositions of copper, cuprite, and malachite,
so frequently found in minerals. Indeed, no other process of
formation of the carbonates is conceivable; moreover cupric oxide,
if really present, would be naturally regarded as a product of
oxidation. The other substances, such as tin oxide, which are
occasionally found, would be produced in part by similar simple
processes, in part by the simultaneous action of particular salts,
the chlorine compounds, for instance, by the presence of water
containing sodium chloride. Similarly the production of cuprous
oxide was usually attributed to an incomplete oxidation of the
copper, although it might very well be the result of an inverse
process, viz. the reduction of pre-existing cupric oxide."
From the following considerations Wibel thinks that he is
justified in his assumption that the layer of cuprous oxide is
the result of reduction. Firstly, by no means all bronzes which
have been dug up, even though from the same excavation, show the
layer of cuprous oxide. Secondly, the cuprous oxide layer is in
the crystallized state. Thirdly, 'all the facts of chemistry
show that the formation of cuprous oxide can only take place
by reduction, given the ordinary conditions of temperature and
pressure.' Finally, in addition to oxygen and carbonic acid, many
salts, those of ammonia for example, occur in the spots where
bronzes are found and favour the formation of copper salts. Wibel
also quotes in support of his views the experiment of Bucholz[34],
that a strip of copper, the upper half of which is immersed in
a layer of distilled water, and the lower half in a concentrated
neutral solution of copper nitrate carefully poured beneath it,
becomes coated with copper and cuprous oxide.
He continues:
"Bronze objects are attacked by waters which contain oxygen,
carbonic acid and a greater or less percentage of salts. Such
soluble salts as are formed are removed by solution, while the
bronzes become covered, according to circumstances, with an
insoluble layer either of carbonate or of oxide, whereby the
form of the objects is preserved. The water then penetrates by
capillary action through the porous coating into the interior, and
attacks further portions of the metal, forming a layer of soluble
cupric salt; a portion of which is able to pass by diffusion
through the external layer. For the same reasons the liquid,
bounded as it is on one side by the metal and on the other by the
almost insoluble crust, shows varying degrees of concentration:
thus all the conditions necessary for the Bucholz process are
fulfilled. If the water is rich in salts, a concentrated copper
solution is formed and even metallic copper may be deposited
from it (i.e. the 'copper crystals' of bronzes); but if, as is
usually the case, the water contains only small quantities of
salt, cuprous oxide crystals only are formed. The fact that the
process takes place chiefly in the pores made by the water itself
is readily understood, because of the comparative quiescence of
the liquid; and that it causes a marked progressive change in
the object arises from the continual exchange of a portion of the
copper solution already formed with fresh solvent from outside.
Where the absence of carbonic acid or other circumstances hinder
the formation of an almost insoluble crust, the reactions detailed
above may, under favourable conditions, take place directly upon
the surface of the bronze; if, on the other hand, there is a too
rapid change of liquid (as for example in very wet localities),
the process may altogether fail to set in, since the necessary
conditions of rest, etc. are wanting. Since the absence of the
necessary conditions may arise from a number of purely accidental
causes, it will be easily understood, that bronzes from one
and the same grave may show the same percentage of carbonates,
but very dissimilar percentages of cuprous oxide. In short all
actually observed conditions in which bronzes are found are
accounted for by the explanations given above."
The following extract is taken from the section dealing with patina in
Bibra's "Bronzes and Copper Alloys[35]":
"The conditions under which Patina is formed, or rather the
conditions under which copper alloys are gradually decomposed,
are variable in the extreme. The four main factors which may
be instrumental in determining the chemical changes may be thus
stated:
(_a_) The composition (qualitative and quantitative) of the
particular alloys.
(_b_) The mode of smelting and the original manipulation of
the components, such as a good or poor mixing, fine or coarse
grain, etc.
(_c_) The locality in which the alloy has lain.
(_d_) The length of time during which the alloy has been
exposed to the particular conditions.... Marked differences
may appear in the extent and nature of the chemical changes
shown by the same alloy; thus one fragment while underground
may have been enclosed in an urn containing bone ash and dry
sand, while another fragment may have been in contact with
decaying animal matter."
From what has been said above, the variations in the composition of patina
may be readily explained. The composition has been found to be:
(alpha) Basic carbonate of copper.
(beta) Basic carbonate and sulphide of copper.
(gamma) Malachite (normal carbonate of copper), with occasional
admixture of cuprous oxide and azurite (acid carbonate of copper)
[Stolba].
(delta) Crystalline cuprous oxide, according to Wibel[36] a
reduction product of the carbonate of copper, by the action of the
copper of the bronze.
Lastly, copper chloride has been occasionally found in patina
[Haidinger][37]. This is only to be expected from the varying character
of the localities in which the statues or bronzes are found. The author
has himself noticed on board ship, how objects of copper and brass,
which are exposed to the salt spray, develop a durable coating of copper
oxychloride[38] (atacamite).
In conclusion, reference may be made to a statement of Chevreul[39],
who, after examination of both hollow and solid specimens of Egyptian
statuettes, states that the bronze is of an excellent quality and that it
occurs in four different conditions. He describes these four conditions,
three of which are undoubtedly patina or altered copper, as follows:
(alpha) A green deposit with patches of blue.
(beta) A blood-red mass.
(gamma) A reddish coloured bronze.
(delta) Ordinary bronze unaltered in appearance.
The first in this category represents the ultimate stage of decomposition
of bronze and forms the outer incrustation of the statuettes. It is
a compound of copper chloride and copper oxide and water in the same
proportions as in Peruvian copper oxychloride (atacamite); the blue parts
contain water, carbonic acid and cupric oxide. It is in fact the blue
hydrated copper carbonate.
(beta) The blood-red substance consists chiefly of cuprous oxide
with an admixture of tin oxide. It contains chlorine, apparently as
cuprous chloride, sometimes in considerable quantity.
(gamma) The reddish colour seems to be due to the tin undergoing
more alteration in the course of time than the copper.
(delta) The well-preserved bronzes are remarkable for the excellent
quality of the alloy.
Chevreul continues:
"Copper and tin have thus undergone gradual changes from without
inwards into chlorides, oxides and carbonates; the tin has been
converted into oxide, the outermost layer of copper into oxide
and chloride, while the layer in contact with the unaltered bronze
beneath can only be oxidised into the suboxide."
In a fissure in a statuette he found crystals of blue basic carbonate of
copper, chloride of lead and hydrated oxychloride of copper.
Bibra himself examined the patina of several bronzes and found it to
consist mainly of sulphate and carbonate of copper.
To complete the quotation from Chevreul's work we may observe that he
finds the cause of the formation of the patina to be the action of air,
of water containing salt, and of carbonic acid. It is interesting that
Chevreul succeeded in restoring a small bronze containing chlorine by
reduction in a stream of hydrogen.
In the year 1865 M. A. Terreil[40] published the analysis of a bronze
patina containing chlorine. The result is as follows:
Bronze. Patina.
Copper 85·98 57·27
Tin 12·64 8·40
Lead 1·09 1·02
Zinc 0·50 0·46
Iron trace 1·61
Lime (CaO) 0·13
Chlorine 5·35
Carbonic acid (CO_{2}) 4·25
Alumina 9·86
Water 4·40
Oxygen 7·25
------ ------
100·21 100·00
So too at a meeting of the Association for the Promotion of Industries in
Prussia, Elster[41] referred to the existence of chlorine in patina, and
regarded this as a proof that the patina upon antique bronzes was actually
intentional on the part of the manufacturers.
E. Priwoznik[42] has described a rare kind of patina which formed a
coating 5 to 7 mm. in thickness composed of three layers consisting
of a reniform or botryoidal incrustation of an indigo blue colour. The
uppermost layer which was also the thickest consisted of 33·22% of sulphur
and 66·77% of copper, and was therefore cupric sulphide, CuS (which is
known in the mineral world as Indigo Copper or Covelline). The second
layer, which existed only in patches, was 0·5 mm. in thickness and of
a blackish colour; it consisted of cuprous sulphide, Cu_{2}S with 15%
of tin. The third layer which, like the second, was incomplete, formed
a fine black powder, and consisted of 59·8 Cu_{2}S, 23·2 Sn and 3·4% of
water. The patina had been produced by the action of soluble sulphides
or of sulphuretted hydrogen upon the copper, while the sulphur compounds
themselves had resulted from the decay of organic matter in the soil in
which the bronze was found.
Mitzopulos[43] described the green patina of the copper alloys found in
Mycene as malachite and atacamite upon a reddish layer of cuprous oxide.
Another analysis of patina was made by J. Schuler[44]. The bronze in
question had a grey outer layer, which passed gradually into a light
green friable layer 2 mm. in thickness. A detached portion of this layer
of patina, dried in a desiccator over concentrated sulphuric acid with a
loss in weight of 9·44%, gave the following analysis:
Tin oxide 49·13%
Copper oxide 22·46%
Lead oxide 3·53%
Iron oxide and aluminium oxide 1·75%
Silica and insoluble matter 6·16%
Carbonic acid determined directly 6·35%
Carbonic acid determined by ignition 9·15%
Water determined by ignition 14·43%
Schuler calculates from these figures that the patina contains:
60·92% H_{2}SnO_{3}
34·55% CuCO_{3}, CuH_{2}O_{2}
4·51% (PbCO_{3})_{2} PbH_{2}O_{2}.
The analysis of the bronze itself was as follows:
Copper 89·78%
Tin 6·83%
Lead 1·85%
Cobalt and Nickel 0·90%
Iron 0·28%
Schuler makes the following observations:
"Whilst the percentage of copper in the alloy is high (89·78%) and
the percentage of tin is low (6·83%), the percentage of copper
in the patina (metallic copper 19·84%) is smaller, that of tin
(metallic tin 42·67%) considerably greater. The percentage of lead
in the patina has also slightly increased. One of the causes of
this alteration in the proportion of the metals may lie in the
fact that basic carbonate of copper is soluble in water containing
free carbonic acid, whilst tin hydrate is insoluble. Another cause
may be found in the action of water which contains in solution
ammonia and ammonium carbonate produced by the decomposition of
organic matter. Confirmative evidence of this supposition is the
presence of small quantities of ammonia in the patina[45]."
Schliemann[46] asserts that bronze objects are destroyed by copper
chloride, and another reference to the presence of chlorine is made by
Krause.[47]
Arche and Hassack[48] give the following details as the result of their
analyses of three specimens of bronze:
I. II. III.
Copper 66·97 73·40 71·98
Lead 17·27 14·77 18·37
Tin 11·98 5·09 7·20
Antimony 1·28 3·33
Arsenic Trace 0·82
Iron 1·00 0·31 0·89
Sulphur 1·50 2·28 1·56
They obtain the following formulae and composition for the patina of the
three bronzes[49]:
I. | II. | III.
CuCO_{3}, 2CuO_{2}H_{2} 85·83 | CuCO_{3}, 3CuO_{2}H_{2} 95·11 | 56·08
2PbCO_{3}, PbO_{2}H_{2} 13·01 | 4·49 | 24·62
SnO_{3}H_{2} 1·16 | 0·40 | 19·30
Reference may be here made to an article by Mond and Cuboni[50] published
in the Report of the Academy of Florence, from which the following extract
is taken:
"By the terms 'rogna' or 'caries' of bronze, archaeologists
designate a peculiar change, to which ancient bronzes, as statues,
coins, vases, etc. are sometimes liable when preserved in museums.
This consists in a species of efflorescence of light green
colour at one or more points upon the surface, which spreads by
degrees, like oil over a sheet of paper, destroying the surface
and converting the interior of the bronze into an amorphous
whitish-green powder. The rapidity with which this destruction
proceeds varies much according to circumstances which are not yet
sufficiently known. Sometimes the destructive spot grows so slowly
that it is hardly perceptible even after some months; sometimes
it grows very rapidly, numerous spots form, spread, and unite,
until in a few months an ancient coin may be entirely destroyed.
In this way antiquities which are valuable for their history, or
for their workmanship, are sometimes more or less injured by this
development of patina, which archaeologists regard as a plague in
their collections."
Mond and Cuboni believe that the growths above described are caused by
Bacteria. Although they have not succeeded in producing the appearances
of spreading patina by transference of cultures of bacteria to intact
bronzes they think that their observations sufficiently support this
supposition, which they believe is further strengthened by the fact
that bronzes exposed for a quarter of an hour to a temperature of 300°F.
(150°C.), whereby any bacteria would be killed, showed no further change
after a period of six months. The following is an extract from an article
by Berthelot[51]:
"Copper objects, which have been buried in the earth for several
centuries, are found to be covered with a green patina and with an
earthy layer of varying thickness which has the same colour. The
metal itself is to a greater or less depth converted into cuprous
oxide. After removal the patina returns; in other words, the metal
shows further growths, and when in contact with the atmosphere
of our climate is in all cases by degrees converted into dust.
These facts are well known to every collector and archaeologist,
who designate the specimens thus affected 'métaux malades'....
Analysis shows that the superficial green layer consists in great
measure of atacamite (cuprous oxychloride) agreeing with the
formula 3CuO, CuCl_{2}, 4H_{2}O. There are also found traces of
sodium salts. The changes which have been observed are produced by
salts from the soil, especially sodium chloride, held in solution
by water. In fact a few drops of salt water placed upon a copper
plate are sufficient for the formation of oxychloride.... This
reaction is the result of the simultaneous action of the oxygen
and of the carbonic acid of the air upon the copper and upon the
sodium chloride in the presence of moisture, as is represented by
the following equations:
4Cu + 4O = 4CuO
4CuO + 2NaCl + CO_{2} + 4H_{2}O =
3CuO, CuCl_{2}, 4H_{2}O + Na_{2}CO_{3}.
Thus the continuous transposition which, under the influence of a
salt-containing water, often acting in large volume, converts the
metal into oxychloride, is readily explicable: while the process
whereby the small quantity of sodium chloride originally present
in an excavated bronze may cause its destruction after it has been
placed in a museum is the following:
When the reactions given above have resulted in the formation of a
certain amount of copper oxychloride, it is to be supposed that a
small quantity of sodium chloride comes into simultaneous contact
with the oxychloride and with the metallic copper. A slow reaction
takes place and a double compound of cuprous chloride and sodium
chloride is formed. The remaining portion of copper is converted
into cuprous oxide:
3CuO, CuCl_{2}, 4H_{2}O + 4Cu + 2NaCl =
Cu_{2}Cl_{2}, 2NaCl + 3Cu_{2}O + 4H_{2}O.
The solution of the double salt is also in turn oxidized by the
air which penetrates the whole mass. The result of the reaction
is therefore sodium chloride, atacamite, and copper chloride:
3Cu_{2}Cl_{2} + 3O + 4H_{2}O =
3CuO, CuCl_{2}, 4H_{2}O + 2CuCl_{2}.
The copper chloride which remains, if in contact with air
and copper or even cuprous oxide, is similarly converted into
oxychloride:
CuCl_{2} + 3Cu + 3O + 4H_{2}O = 3CuO, CuCl_{2}, 4H_{2}O.
The cycle is thus complete, and its constant recurrence under the
influence of oxygen and moisture is the cause of the destruction
of those objects containing copper which are imbedded in earth,
and even of those which are preserved in our museums."
Finally a memoir by Villenoisy[52] should be noticed, the first portion
of which is devoted to a proof that the patina of ancient bronzes is due
to natural causes and is not the result of the art and methods of the
metal-workers of the ancient world. The second portion deals with the
various kinds of patina and their formation, as the following excerpts
will show:
The following substances may be mentioned as capable of attacking
alloys:--Ordinary oxygen, which has but a slight action on copper
in the dry state but a more vigorous action in the presence of
moisture, or as ozone; sulphur also, ammonia, carbonic acid,
and organic substances. Water has no direct influence, but acts
as a solvent. The metals or metalloids of the alloys can unite
independently with oxygen, sulphur, or carbonic acid, etc. to form
oxides, sulphides, or carbonates; or again they can react among
themselves and produce copper stannate or lead stannate. Ammonia
will form ternary compounds or play a catalytic part. Whatever
processes may result in the formation of patina, the changes which
occur are too slow to allow their imitation and examination in the
laboratory. The four metals which are found in ancient bronzes,
viz. copper, tin, zinc, and lead, are particularly liable to
certain changes. Copper forms chiefly cupric and cuprous oxides.
The first of these is soluble in ammonia; the latter combines with
ammonia to form a substance which is colourless, but which becomes
blue on exposure to air. Tin forms stannic acid which probably
produces stannates with copper and lead. Zinc becomes zinc
oxide, lead is converted into oxides. Sulphur, as sulphuretted
hydrogen, causes the formation of metallic sulphides. Ammonia
has a threefold action, viz. it causes and furthers hydration,
it is an energetic solvent, and it forms double salts. This
last-mentioned action is particularly important in the formation
of patina. Carbonic acid in the presence of moisture attacks
copper, lead and iron, and, as a carbonate, exists in every
metallic oxide which is exposed to the air. Several combinations
of copper with carbonic acid are known, while lead is readily
converted into lead carbonate by oxidation. The part played by
the carbon compounds resulting from the decomposition of animal
and vegetable substances has hitherto received little attention,
but this decomposition of organic material is probably the chief
cause of the beautiful blue patina. The action of oxygen will
depend upon the composition of the metal, upon the locality, and
upon numerous other circumstances, while the colour of the patina
will vary accordingly.
Villenoisy proposes to classify patina into three groups:
(1) Blue patina, with grey to blue-green and apple-green
tints.
(2) Dark green patina.
(3) Black patina.
1. The blue patina produced by the action of ammonia upon the
products of previous oxidation does not destroy the outer form of
the bronzes, but is nevertheless unfavourable to the preservation
of the metal, since the substratum of the patina is a porous
mass, consisting of lead stannate and lead carbonate mixed with
ammoniacal copper carbonate. The specimen has frequently an intact
appearance, as if covered with a thin layer of oxide only, whilst
in reality all traces of metal have already disappeared, and
slight pressure often suffices to break the bronze into pieces.
The nearer the colour of the patina approaches to grey, the less
solid is the bronze likely to be, a result which is no doubt
caused by the presence of lead carbonate. This type of patina has
often a yellowish colour, especially on prominent parts, where,
being porous, it has retained in its superficial layers substances
which were in suspension in the subsoil water. The occurrence of
a pale fine-grained patina of a uniform colour is in almost all
cases due to the scaling off of patina belonging to this type.
2. Whilst blue patina is generally formed on bronzes which have
been buried in earth, the dark green patina is formed both in the
earth and also in the open air. The presence of lead seems to be
an obstacle to its formation. This dark green patina consists of
variable proportions of basic copper hydrate and copper carbonate.
The green layer frequently rests upon one of a red colour, a
circumstance which proves that the dark green patina is almost
always the result of two successive reactions: cuprous oxide
is first formed and subsequently takes up water and carbonic
acid. Tin is present as copper stannate. The cuprous oxide,
which is generally regarded as unaffected by air, is perhaps
drawn into further reaction through the agency of ammonia. In
those situations where there is a flow of rain water a certain
translucency of the green patina is often produced, and this is
also possibly caused by ammonia. Unlike the blue patina, the dark
green variety assists the preservation of bronze.
3. Black patina is probably due to a variety of circumstances.
The substances which enter into its composition are cupric oxide,
lead oxide, lead peroxide, copper sulphide and lead sulphide. If
bronze does not contain lead it is blackened only by the action
of sulphur. The rarity of black patina is no doubt due to the
rapid oxidation of the copper on the originally rough, unpolished
surface, which leads to the formation of a green patina.
These extracts show how little value can be attached to a classification
of bronzes from the character of the patina present: the views upon
the subject are so divergent, while the actual composition of the
incrustations which form the patina and their external appearance are so
widely different. In fact only two groups of bronzes may be distinguished,
i.e. those which show patina and those from which patina is absent.
The first group comprises almost all the bronzes which are found in peat,
which show, with rare exceptions, a metallic, often somewhat darkened,
surface. Their state of preservation depends upon the nature of the peat
in which they are found, but the metal surface has, in the majority
of cases, become somewhat rough and etched, although all the details
are clearly distinguishable. More rarely one side retains the original
polished surface while the other side is much corroded. If a much corroded
bronze is found, the peat in which it has lain has probably contained
free sulphuric acid (see also p. 13). All bronzes found in water must
be included also in this group. The second group will then comprise all
bronzes with an oxidized patina.
The classification given by Villenoisy seems entirely unsuitable, for it
does not by any means exhaust all the kinds of patina which may occur.
Thus no mention is made by him of the frequent occurrence of a patina
which contains chlorine. If we separate the dark brown and the blackish
patina, in so far as these two colours are pure, from those of a green
colour, the first two varieties cannot be regarded as groups, because
the tones of colour differ too much, and because, as Villenoisy himself
observes, widely different patinas often occur on one and the same bronze.
The durability of a patina upon a bronze cannot be judged either by the
outer appearance or by the chemical composition alone. The fact that there
has been no alteration in the outward appearance for many years offers
no guarantee against further changes taking place. Thus a Minotaur[53]
in the Berlin Museum, which for many years had shown no sign of change,
was eventually found to be completely covered with numerous bright green
spots over its entire surface. My own opinion is that the only patina
which is really stable is that which consists of combinations of oxygen,
hydrogen and carbonic acid with the metal, somewhat similar to those
analysed by Schuler (see page 24), and by Arche and Hassack (see page
27). The presence of sulphides, and even of sulphates, does not seem to
be injurious.
If a patina is to deserve the name of a good, sound, or, as it is termed,
a "noble" patina (Edel-patina), the original contours of the bronze with
all its markings must be distinctly visible. For this the patina must
not be too thick, must be of moderate hardness, and above all must have
an enamel-like surface. Apart from chemical influences, such a patina
can only have been formed in those cases in which the alloy has been
homogeneous, fine-grained, dense and not porous, and when its surface has
been so smooth that oxidation has taken place very slowly. Under these
conditions the colour of the patina may vary greatly, for it may be bright
green, blue, or of darker shades from yellowish to brown, or even black.
These latter tints often denote patina layers of very slight thickness. My
own observations confirm Villenoisy's view that the brown and the black
patina are for the most part due to the presence of lead in the bronze.
Rein[54] holds the same opinion in regard to Japanese bronzes.
Certain forms of patina are not necessarily prejudicial to the
preservation of bronzes, i.e. the green and blue varieties which have the
composition of malachite (CuCO_{3}, Cu(OH)_{2}) and azurite (2CuCO_{3},
Cu(OH)_{2}), both of which are very often found on the same bronze.
This variety of patina shows a crystalline structure. The simultaneous
formation of both varieties, which is due to the greater exposure of one
part of the bronze than another to the action of moisture, is well shown
by a specimen in the Berlin Museum[55] (Fig. 6). This consists of the
frontal portion of a Boeotian bridle, over parts of which leather straps
had probably been tightly fixed. Those parts which had been thus somewhat
protected from moisture were covered with blue azurite, which contains a
smaller quantity of water. But the crystalline structure of these kinds
of patina has often the disadvantage that the surface of the bronze is
no longer clear, and consequently engraved markings and even stamped
impressions are not visible. On page 142 may be seen illustrations of
Roman coins, some parts of which are totally illegible. More frequently
met with than these varieties or than the so-called "noble" patina, is
that in which the bronze presents a more or less rough and pitted surface,
light or dark green, or even grey in colour if there is a large proportion
of lead present. More rarely the tint is blue or brown. The behaviour of
such kinds of patina varies greatly, but durability is for the most part
assured if, under the layer of green oxide, a reddish layer of cuprous
oxide is found. This rule is perhaps not invariable, for I have often
found cuprous oxide present under the so-called spreading patina, but
absent beneath one which is undoubtedly durable.
[Illustration: Fig. 6.
Portion of bronze horse-trappings showing blue and green patina.]
Two instances may be here quoted as confirming Wibel's view in reference
to the reduction of cupric oxides to cuprous oxides and even to metallic
copper (see page 17)[56]. In removing a sandy crust saturated with copper
salts from a large Egyptian bronze[57], small crystalline masses of copper
were seen here and there, separated from the metal beneath by a layer
of cuprous oxide to which the admixture of tin gave a whitish tint. The
copper was mostly deposited in slight depressions upon the surface of the
metal and could be easily removed. Similarly, upon an Etruscan mirror
exhibited in the Berlin Museum[58], reduced copper can still be seen
forming red spots upon the lighter coloured surface of the bronze, which
has already been freed from cupric oxide. The copper also can be removed
with comparative ease, and is observed to be separated from the bronze
by a thin whitish layer of tin oxide. A quantitative analysis of a small
piece showed 100% of copper.
As has been remarked above, the layers of oxide frequently enclose grains
of sand and even fragments of clay, earth, and ferruginous particles, so
that the original contours of the bronzes are often indistinct or entirely
obliterated (see Figures 41-43). These incrustations may occasionally
be removed by a careful use of the hammer, but they are often so firmly
united with the bronze, which is itself so oxidized, that removal by
mechanical means is no longer possible.
[Illustration: Fig. 7.
Head of Osiris, showing advanced condition of warty patina[59].]
These incrustations are however not so injurious as the tuberous and warty
patina. Figure 8 shows an Etruscan mirror covered with a patina which
generally results in the progressive destruction of the bronze[60].
[Illustration: Fig. 8.
Etruscan mirror showing warty patina.]
The following series of quantitative determinations of chlorine obtained
from the examination of bronzes in the Berlin Museums, shows conclusively
the destructive influence of chlorine in the production of patina:
Percentage of chlorine
Dark green "noble" patina (wine pitcher, Ant. Misc. Inv. 7161) 0
Green patina on a layer of cuprous oxide (Etruscan vase, Ant.
Fr. 1571) 0
Dark blue "noble" patina (Etruscan wine pitcher, Ant. Fr. 608) 0
Bright blue "noble" patina (Etruscan mirror, Ant. Misc. Inv. 7275) 0
Bright blue "noble" patina (lid of vessel, Ant. Misc. Inv. 6322,
292 _a_) 0
Hard greenish-yellow exfoliating patina upon a bright green,
softer patina (Roman saucer, Ant. Fr. 1601 _a_) 0
Bright green fairly firm patina, the colour rubbing off somewhat
in parts (handle of vessel, Ant. Fr. 1440) 0
A firm smooth green layer upon a brighter soft patina (mirror,
Ant. Fr. 136) 0
Blue crystalline patina (harness from Boeotia, Ant. Misc. Inv. 8579) 0
Rough dark green patina (situla, Ant. Misc. Inv. 8509) 0
Greenish "noble" patina (sword, Ant. Fr. 1144) trace
Rough green softer patina, with admixture of earth (funnel, Ant.
Misc. Inv. 8582) trace
Dark green, compact warty patina (mirror, Ant. Fr. 32) trace
Green warty patina, with translucent cuprous oxide (mirror, Ant.
Misc. Inv. 3312) trace
Green and blue crystalline patina (Buto, Aeg. 13135) 1·7
Bright green cracked and warty patina (muzzle of the harness
from Boeotia, Ant. Misc. Inv. 8579) (see Fig. 38) 1·7
Green firm warty patina (Etruscan mirror, Ant. Fr. 53) 2·1
Completely oxidized Cyprian bronze fragment (Ant.) 2·2
Green cracked patina upon a thick layer of cuprous oxide (bronze
fragment from Troy) 4·0
Completely oxidized Cyprian bronze fragment (Ant.) 4·2
Bright green efflorescent patches upon dark tuberous patina
(bronze fragment, Ant.) 5·9
Bright green powdery patina in the hollows of a darker smoother
patina (Horus, Aeg. 11010) 6·7
Bright blue powdery moist patina (Aeg. 12663) 7·4
Green and blue patina mixed with grains of sand (Buto, Aeg. 13132) 8·3
Bright green cracked patina (bronze fragment from Troy) 9·3
Bright green powdery patches, dark green rough patina (cup, Ant.
Fr. 1654) 10·2
Thick greenish black tuberous patina (Besa, Aeg. 9716) 10·8
Green firm patina, with brighter patches (Buto, Aeg. 13787) 11·3
Bright green powdery patina (Isis with Horus, Aeg. 14078) (copper) 12·5
Green tuberous and cracked patina (Horus in the lotus flower,
Aeg. 2409) 13·1
Bright green powdery excrescences (Buto, Aeg. 13787) 13·9
Bright green soft patina, with a dark and somewhat firmer surface
(door hinge from Babylon, Aeg. V.A. 2185) 15·1
A due consideration of these figures must lead to the conclusion that as
a rule a malignant patina is one which contains chlorine. That traces of
chlorine are found in many cases of benign patina need cause no surprise,
for frequent handling alone may suffice to bring about such a condition.
Nor is this rule invalidated by the fact that a patina which is proved to
contain chlorine (e.g. that of the mirror[61] depicted on page 40), has
remained unchanged for years under certain conditions, for the formation
of patina depends upon various causes, and it often happens that a bronze
carries a patina which outwardly seems to have stood the test of years,
yet internally oxidation has continued and becomes outwardly visible only
when some mechanical injury to the patina allows variations of temperature
to exert a greater influence. A specimen is often regarded as bronze,
whereas in reality it does not even contain a metallic core, but consists
merely of cuprous oxide, copper oxychloride, tin oxide, etc.[62], and
is therefore incapable of further change. On the other hand it is not
surprising to find a patina, which, although containing no chlorine,
affords but a poor protection to the bronze, for in this case the cause
may lie in the non-homogeneous and porous nature of the alloy.
This list shows in addition that this high chlorine-content is a
distinguishing feature of the patina of Egyptian bronzes, as is only
to be expected from the character of the Egyptian soil (vide pp. 1,
2 _et seq._); in fact, although in most cases qualitatively only, I
have proved the existence of chlorine in each Egyptian bronze without
an exception. The destructive nature of chlorine is not often apparent
in bronzes recently excavated, which usually show an apparently sound,
dark green patina with a smooth surface, sometimes like malachite or
azurite; personally I have not met with any bronze object from Egypt which
could be said to have a patina deserving the name of "noble" patina.
Not till some time, or it may be not till years after the objects have
been placed in museums does the change become apparent, as has been so
strikingly described by Mond and Cuboni (see page 27). The varying amount
of moisture in our atmosphere undoubtedly influences the spread of the
patina, which, if the application of a preservative is delayed, gradually
eats into the bronze. The adjoining figures (Fig. 9 to 12) of the same
bronze before and after the process of preservation show distinctly such
ravages, whereby the surface has been in some places eroded to a depth
of 2 to 3 mm. In other cases, especially hollow bronzes, the thin walls
have been completely perforated. The explanation of these processes is
found in the experimental work of Krefting[63], and also in the treatise
by Berthelot, from which extracts have already been given. The theory
enunciated by Mond and Cuboni, that the "wild" or spreading patina is
due to the action of bacteria, cannot now be maintained, for not only
do chemical reactions give an adequate explanation of the process,
but these observers have failed to transplant the bacteria; nor were
the experiments of Dr Stavenhagen, undertaken at our request, more
successful. That certain bacteria are capable of attacking metal, as
for example the metal lettering on books, is an established fact, while
the universal distribution of bacteria will naturally lead to their
presence upon bronzes and their patina. The application of heat checks
chemical change by driving off the moisture, and therefore arrests the
spread of a patina for some time, until by penetrating the oxidized layer
the moisture and carbonic acid can again act upon the patina and the
underlying metal. As has been already stated in the passage from Dingler's
"Polytechnic Journal" quoted above, I have observed the renewed formation
of efflorescence upon a bronze statuette which had been thus sterilised.
This, it may be urged, was a case of re-infection: it is, however, strange
that Mond and Cuboni do not refer to chlorine as a component of the
patina. The presence of chlorine may have been overlooked; it cannot well
have been absent, for in every case of rodent patina I have found without
exception chlorine in the bright green efflorescences, whatever may have
been the original source of the bronze.
[Illustration: Fig. 9.
Bronze Pasht showing destructive patina.]
[Illustration: Fig. 10.
The same after treatment (Finkener's method).]
[Illustration: Fig. 11.
Bronze Pasht showing destructive patina.]
[Illustration: Fig. 12.
The same after treatment (Finkener's method[64]).]
Nor am I able to endorse the statement of Friedel[65] that a spreading
patina is characterised by a peculiar and disagreeable smell, although
some oxidized bronzes have a distinct smell which it is not easy to
describe.
The presence of chlorine is particularly dangerous to those bronzes which
consist of a casing of metal of variable thickness around a core of sandy
clay, the object of which has been to economize metal. These constitute an
important class amongst Egyptian bronzes. The chlorine often exists in the
core as sodium chloride, and can thus attack the metal from both sides.
Moreover, the structure of many Egyptian statuettes of a later period is
very porous and spongy, and thus presents a large surface to destructive
agencies. On sawing through the support of an Osiris[66] numerous small
bright spots were found, upon examination with a lens, to be small pores
filled with a salt solution. A few days later the action of the carbonic
acid had begun, and the bright spots of moisture were represented by small
green patches. The following figures show the absorption of moisture and
of carbonic acid by this specimen and by another Osiris from the Egyptian
collection.
I. _Base of Osiris._
Weight, air-dried 14·6554 gr.
After one day in the desiccator 14·6514 gr.
Air-dried, after one day 14·6540 gr.
Air-dried, after ten days 14·6576 gr.
Air-dried, after one month 14·6599 gr.
Air-dried, after two months 14·6623 gr.
Air-dried, after four months 14·7261 gr.
After a prolonged period in the desiccator 14·7033 gr.
Air-dried, after one day 14·7254 gr.
Air-dried, after one month 14·7321 gr.
Air-dried, after two months 14·7362 gr.
Air-dried, after four months 14·7381 gr.
II. _Osiris._
Weight, air-dried 77·7522 gr.
After some time in the desiccator 77·7397 gr.
Air-dried, after one day 77·7462 gr.
Air-dried, after ten days 77·7548 gr.
Air-dried, after one month 77·7582 gr.
Air-dried, after two months 77·7617 gr.
Air-dried, after four months 77·7704 gr.
After being heated to 200°C. in the drying stove
and lying one day in the desiccator 77·5967 gr.
Air-dried, after one day 77·6752 gr.
Air-dried, after one month 77·8044 gr.
Air-dried, after two months 77·8191 gr.
Air-dried, after four months 77·8320 gr.
Air-dried, after seven months 77·8444 gr.
After four days in the desiccator 77·8225 gr.
These figures show that in the first case the absorption of carbonic acid,
oxygen, and water proceeded at first slowly, but more rapidly after three
months, as was evidenced also by the appearance of marked efflorescence on
the oxidized surfaces. The Osiris, which was more highly oxidized, showed
a more rapid increase in weight from the first. The increased action after
the heating was also manifest externally, for at the end of a fortnight
the bright green efflorescences had made their appearance. In this case
therefore the heating recommended by Mond and Cuboni, so far from proving
beneficial, actually induced a more rapid decay.
The patina layer, as Schuler has also observed, often contains a greater
proportion of tin than does the alloy; a result which is manifestly due
to the solution and removal of the copper salts by the subsoil water.
The bright efflorescences of an Egyptian statue of Buto[67] contained
10·49% of tin, while the percentage in the metal itself was only 7·66.
In certain circumstances it may even result that an object which was
originally composed of bronze is represented only by tin oxide[68]. The
small proportion, and occasionally the complete absence, of copper is the
result of the action of ammonia which may arise from the decomposition of
dead bodies and of carbonic acid, both of which agents, with the help of
oxygen, attack the buried bronzes, and, dissolving the copper compounds
by the subsoil water, leave only the insoluble tin oxide.
Upon the whole the foregoing remarks upon bronzes are equally applicable
to objects of copper, which however appear to possess a greater power of
resistance to the destructive action of carbonic acid and moisture, even
where salt is present. This is probably due to the fact that the absence
of tin and lead precludes any interaction between the compounds of these
metals and those of copper. Copper objects with a sound so-called "noble"
patina apparently do not occur.
Silver.
Unless alloyed with a large amount of copper, in which case they show
green efflorescences similar to those of bronzes, silver objects are
almost always covered with a layer of soft silver chloride (horn-silver)
of varying thickness, AgCl, or of the harder silver subchloride, Ag_{2}Cl;
and when these compounds form a thick layer, they often show a warty or
more rarely a cracked surface. If the layer of chloride is thin, incised
designs upon the silver will be visible both before and after removal of
the chloride. The two chlorine compounds frequently appear together in
distinct sharply defined layers of different colours, that nearer the
silver being the layer of subchloride. This is especially well shown
on fragments of silver from the Hildesheim silver-find[69]. Upon one
fragment[70] the layer of silver chloride was about twice as thick as that
of the silver subchloride. Being unable to separate them I determined the
silver and the chlorine of both layers together with the following result:
Silver 74·52. Chlorine 21·90.
Now for 2AgCl, Ag_{2}Cl 74·52 silver would correspond to 18·11 chlorine
only, while for AgCl the proportions would be 74·52 silver to 24·15
chlorine. Since the specific weight of silver subchloride is greater than
that of silver chloride, these figures prove that the subchloride is also
present.
Between the metal and the silver chloride there is often a thin powdery
layer consisting of finely divided cupric oxide, or silver sulphide,
and occasionally of gold, if, as is frequently the case, the silver is
auriferous. The presence of gold may, however, also point to the existence
of gilding. The silver chloride often shows a reddish or brown colour
on the surface, due probably, in some cases, to the adherence of minute
quantities of the earth in which it was found, but partly also to the
action of light upon the silver chloride.
Thin black layers upon silver, as also the so-called silver tarnish,
result from the formation of silver sulphide, from contact with decaying
organic substances which have contained sulphur.
When placed in museums silver objects remain unaltered, and no further
chemical changes take place.
Any other changes which have been observed will be gathered from the
following extracts.
Church[71] analysed a specimen of silver upon which two layers were
distinguishable. The outer semi-metallic layer consisted of metallic
silver, with traces of chloride, sulphide, and iodide of silver, together
with copper carbonate and a small quantity of gold; the inner layer, which
was soft, grey and powdery, had the following composition:
Silver 94·69%
Gold 0·41%
Copper 3·48%
Lead 0·28%
Antimony with traces of arsenic and
bismuth 1·21%
As the composition of the sound metallic core was identical, it is evident
that physical and molecular changes only had taken place similar to those
observed by Warrington[72] as early as 1843.
Silver objects found in Mycene are said by Mitzopulos[73] to show three
layers, the outermost of which has a red colour and is not markedly
friable, consisting of silver oxide; the second is tough and consists of
silver chloride (horn-silver); while the third, that next to the metal, is
similar to the outermost layer. Mitzopulos thinks that the chlorine must
have been brought by rain water, since there are neither sea nor springs
of water in the neighbourhood.
Schertel[74] distinguished two layers in fragments of silver from the
Hildesheim silver find, the outermost of which proved to be silver
chloride:
Silver 75·43% found, 75·31% calculated for AgCl
Chlorine 24·51% found, 24·69% calculated for AgCl
Beneath this layer was a very thin, almost black, brittle layer of silver
subchloride:
Silver 87·0% found, 85·89% calculated
Chlorine 12·8% found, 14·11% calculated
Between the metal and the latter layer was a small quantity of dark
powder, which Schertel recognized as gold. He thinks that the layer of
silver subchloride seems to indicate that the water, which permeated the
surrounding clay, contained chlorides, and first converted the copper into
copper chloride; that the copper chloride together with the silver then
formed silver subchloride and cuprous chloride. Should the subchloride
again become chloride, it would be able to attack the silver afresh. The
slowness of the process, when the silver and copper in association with
it had been converted into chlorine compounds, allowed the gold to be
deposited as a fine powder upon the intact metal.
A silver coin rolled out into a thin plate, after remaining in a solution
of common salt for six months, was found to have lost 27·7% of its copper,
so that the plate became brittle, especially in those parts where it was
thinnest.
Bibra[75] gives a similar explanation of the conversion into silver
chloride. He believes that the reddish colour which is occasionally seen
on silver at a fresh fracture must be due to the presence of cuprous
oxide.
The following extract is taken from the section which deals with silver
in the work of Berthelot[76] previously quoted:
"Silver chloride is for the most part produced by the sodium
chloride dissolved in the subsoil water, which acts in conjunction
with the oxygen and the carbonic acid of the air:
2Ag + O + (n + 2) NaCl + CO_{2} = 2AgCl, nNaCl + Na_{2}CO_{3}.
But this reaction differs from that which takes place in the
case of copper in that it does not proceed continuously except
in the presence of a considerable quantity of salt water only,
as for instance in the sea. In museums the alteration goes no
further than corresponds to the minute quantity of sodium chloride
contained in the object. On the other hand in an earth which
contains salts, the continued presence of water can bring about a
more or less marked change, and in some cases even a stable silver
subchloride may be formed."
Lead.
Objects of lead have always a white appearance due to the formation of
lead carbonate, as has been already mentioned above in connection with
bronze. The carbonate is also often mixed with oxide.
Tin.
Objects made of tin[77] are frequently found in pile-dwellings in a good
state of preservation. They are, however, occasionally covered with white
or brown layers of hydrated tin oxide, while in some cases oxidation has
advanced so far that no trace of metallic tin is left in the hard grey
masses of oxide which result.
Gold.
Gold is found to be unaltered, or there is at most a thin layer of silver
chloride, which is the result of the action of sodium chloride upon the
silver which the gold usually contains. Gold objects often have a red
coating, which has been found to consist of ferric oxide, and is due to
extraneous deposits which have been fixed by the silver chloride. I have
not been able to prove the presence of gold chloride[78], and it does not
appear possible that water containing sodium chloride can have the power
of acting upon gold. If the ferric oxide is removed mechanically, some
of the gold will naturally be removed with it, and this can be readily
ascertained on analysis.
The degree of brittleness in objects of gold depends upon the changes
which have taken place in other metals, especially silver, which are mixed
with it.
Glass.
Ancient glass, which is for the most part lime-soda silicate, exhibits a
dull, rough surface with the well-known iridescence. The alkali is removed
from the glass by the action of moisture, oxygen and carbonic acid, while
the silicic acid remains in the form of minute scales, which cause the
iridescence by interference. According to Bunsen the chemical action of
the gases of the atmosphere on glass is facilitated by the condensation
of water upon its surface; for the water thus condensed absorbs large
quantities of carbonic acid. In certain circumstances almost the whole
of the alkali is withdrawn from the glass. An analysis of glass of this
kind, together with a discussion of the chemical reactions involved, is
given in Muspratt's "Chemistry[79]."
Glass objects which are markedly iridescent undergo gradual decay even
under museum conditions; this is probably due to the continued action of
carbonic acid.
Organic Substances.
The changes which organic substances undergo are various; thus, while
leather becomes hard, papyrus becomes brittle. Like all other organic
material they may undergo those destructive processes which are due
to the growth of moulds or to the agency of various bacteria. They are
also liable to be attacked by maggots, moths, and other insects. It is
unnecessary here to describe in detail these numerous and varied changes;
a few special cases only need be mentioned.
Acid peat, in which iron objects perish, is found to have a good
preservative action upon wool and horn, whilst vegetable fibres are
destroyed. On the other hand, in pile-dwellings wool and horn substances
have disappeared. Olshausen[80] thinks that animal fibre is destroyed by
simple decay brought about by the oxygen in solution in ordinary water,
whilst in peat the immense quantity of vegetable matter takes up the
oxygen which can therefore no longer serve for the oxidation of wool and
similar material.
Under certain circumstances woollen textures are found to be remarkably
well preserved in oak coffins, as may be seen in the Museum at Copenhagen.
Bones, horn, and ivory show great variety in their behaviour, which
depends of course on the nature of their surroundings. Thus for instance
in acid peat sometimes the animal matter only is preserved[81], while
in graves, beyond a few remains of tooth enamel, there is often nothing
to show that they have enclosed bodies. Burned bones are generally found
to resist decay, for the destruction of the animal matter leaves them no
longer liable to further decomposition[82].
Amber objects are well preserved in water or in peat, but if they have
lain in earth, they are darkened and often friable.
If organic substances, such as wood, etc., have lain in the immediate
neighbourhood of oxidized bronze, and are thereby saturated with copper
compounds, they show a very good state of preservation, which continues
after they have been placed in a collection. Similarly the remains of
fabrics upon iron objects, which are permeated with rust, are sometimes
found in good condition.
Objects imbedded in salt (sodium chloride) are in certain circumstances
found in a good state of preservation and continue so, as is shown by the
skins, leather and wooden articles which are exhibited in the Salzburg
Museum.
As a general rule absence of moisture in the earth is essential for the
preservation of organic substances, and is the cause of the splendid
condition in which objects of organic material are found in Egypt.
PART II.
THE PRESERVATION OF ANTIQUITIES.
The object with which it is proposed to deal should first be photographed,
and from different sides if necessary; for the external appearance is
often changed during the process of preservation, and it is advisable
that a representation of the specimen in its original condition should
be kept in case any injury should befall the object, which however rarely
happens if proper caution be observed. For this reason in the Laboratory
of the Royal Museums at Berlin all bronzes are photographed before
treatment, as also are all limestone blocks. Thus the 125 blocks from the
Grave-chamber of Meten were each separately photographed. It is only in
certain cases that this rule is not observed, as for instance in the case
of the numerous Egyptian ostraca, i.e. fragments of earthenware showing
inscriptions which had been previously copied.
I. Preservation of Objects composed of Inorganic Substances.
(_a_) Limestone.
The method formerly employed for the preservation of decaying and
crumbling limestones was that of simple impregnation, and this is still
followed in some cases which will be subsequently described. But as the
active agents of destruction are not removed by this method the result
is not always satisfactory, and an attempt is now made where possible
to remove those salts which are soluble in water, especially the sodium
chloride, by the simple process of steeping in water.
If the presence of salt in a limestone is evidenced by a crumbling
surface, or by the taste when touched with the tip of the tongue, the
question will arise whether it will bear steeping, or whether the
destruction is so far advanced that, on being placed in water, the
limestone will fall to pieces.
If fractures or cracks can be actually seen in the stone, steeping is
contra-indicated, but if the condition is less manifest, a preliminary
test should be applied.
A large drop of water, e.g. about 25 cubic centimetre in volume, should
be placed on the surface of the stone, and any changes which take place
should be carefully noted. If the drop is not absorbed by the stone, it
may be due to a layer of dust or to previous saturation with solutions of
resin or varnish. Dust may be removed with a moderately hard brush or by
rubbing with the finger, but if a limestone has been previously saturated
with a varnish solution it will not absorb the water, and is therefore
hardly suitable for this treatment. If the drop is absorbed, an iron or
a steel point, such as the thick end of a medium-sized needle, should
be used to ascertain whether the limestone at the moistened spot shows
the same degree of hardness as elsewhere. If this is found to be the
case, especially if the pieces are of a large size, the test should be
repeated at other spots, including the back of the stone, for a hardened
layer on the front aspect may be the result of former treatment. If the
result of this examination is satisfactory and no soluble colouring is
observed on the limestone, the process of steeping may be applied. If
on the other hand the moistened area has become softer, or has become to
any extent swollen, or if any colours which may be present show signs of
disappearance or fading, treatment with water must be abandoned.
The difference of behaviour is easily explained, for limestones do
not always consist of lime only, or, more correctly, carbonate of lime
(CaCO_{3}), but often contain sand or clay, and the greater the amount of
clay the more readily the stone softens or swells. Even when a limestone
has borne this preliminary test satisfactorily it should be carefully
watched for an hour or two after immersion and should be at once removed
from the water should any further changes appear.
Steeping. The procedure to be observed is as follows. The rapidity with
which the salts may be removed varies directly with the quantity of water
used in steeping. The treatment of objects of small size presents no
difficulties; any vessel of glass, porcelain, or earthenware will serve
the purpose. Towards the end of the treatment distilled water should be
used, or in default of this, clean rain water should be used in preference
to that from a well. For larger objects (as, for example, the large
limestone blocks of the Meten Chamber mentioned above, some of which were
1 metre in length and 1/2 metre or more in breadth and thickness), it is
convenient to use wooden tubs fitted with a tap in front to draw off the
water, and so tilted by means of stones placed underneath that the tap may
be at the lowest point. The objects should not touch the bottom of the
vessel. Smaller pieces may be suspended or may be made to rest on glass
rings or supports of glass rods; while large objects should be laid on
blocks of wood so placed as to allow the tub to be cleaned when necessary
without removal of the blocks, the weight of which would otherwise entail
much labour. The blocks should be as near to the surface of the water as
possible, leaving a considerable depth of water beneath, for the heavier
salt-laden water sinks to the bottom, thus bringing into contact with the
limestone water with a smaller salt-content. The length of the steeping
must depend upon the size and porosity of the limestone.
Under certain circumstances phenomena make their appearance which must
not be neglected. Thus if the treatment extends over a considerable
length of time, the wooden tubs should be provided with lids to prevent
the access of light. This was found indispensable in the treatment of
the blocks from the Meten Chamber when Berlin tap-water[83] was used, for
when the tubs were open a large quantity of brown hydrated ferric oxide
appeared on the limestone, the roughness of which rendered its removal an
impossibility even with brushes. This oxide is produced by various forms
of algae and bacteria which developed in such numbers that the sides of
the tubs were frequently covered with a layer of slime, which under the
microscope appeared as a confused web of transparent threads[84]. This was
brushed off with soft brushes at least once a fortnight, for the slimy
covering impeded the access of the water to the limestone. That in this
case the ferric oxide was the result of the action of light was proved
by the fact that only those blocks which were placed near the windows
were discoloured, and that the discolouration was proportionate to the
amount of light which fell upon them[85]. Again, after the treatment in
the covered tubs some of the blocks became so black that they resembled
blocks of coal rather than limestone. After exposure to light for a day
or two, especially when the water had been drawn off, the discolouration
disappeared without leaving any traces. The colour was doubtless due to a
minute quantity of iron in the form of sulphide which, after oxidation in
the air and light, became invisible upon the light yellow limestone. Under
these circumstances the presence of sulphuretted hydrogen in the water,
possibly produced by bacterial action upon the sulphates, was attested by
the characteristic smell.
The enormous number of bacteria which develop in the water constitute a
great hindrance to the process of steeping, and as to boil such a quantity
of water as is required for these large objects is out of the question,
frequent changes of water and frequent cleaning of the stone, wooden
blocks, and tubs are the only remedies.
Examination of the Progress of the Steeping. The water should be changed
at first daily, then by degrees every two, three, or four days, later on
weekly only, until finally once a fortnight is sufficient. To ascertain
the progress and completion of the elimination the quantity of chlorine
in the wash-water may be determined by a simple method of titration[86].
The following short explanation may be of use, and the method is easily
learnt. If a solution of silver nitrate is poured into a solution of
common salt (sodium chloride), a white curdy precipitate is produced, a
process which the following equation will explain:
NaCl + AgNO_{3} = AgCl + NaNO_{3}.
The white precipitate is the silver chloride, whilst the sodium nitrate
which is produced at the same time remains in solution and is therefore
not visible. As always definite proportions of the two substances, silver
nitrate and sodium chloride, react upon one another, by the use of a
solution containing a known amount of silver nitrate we can determine the
amount of salt, and hence of chlorine present. By cautiously inclining a
burette (Fig. 13) divided into tenths of cubic centimetres[87], the silver
solution should be dropped into a beaker containing a definite volume of
the solution to be examined for chlorine; the level of the silver solution
in the burette should be read off before and after pouring out, and the
number of cubic centimetres of the silver solution required to precipitate
the chlorine will thus be known.
[Illustration: Fig. 13.
Gay-Lussac's Burette. 1/6 nat. size.]
The process when carried out in this manner has one defect, for it is
necessary to allow the precipitate to settle in order to see clearly
whether an additional drop of the silver solution will produce further
precipitation, or whether it will merely cloud the fluid; this defect can,
however, be remedied by means of a so-called "indicator." A few drops of
a concentrated solution of neutral yellow potassium chromate should be
added to the fluid to be examined, which is thereby coloured yellow; the
solution is then shaken or stirred with a glass rod, while the silver
nitrate is dropped in. Every drop of the silver solution will cause a
red precipitate, the colour of which however disappears on stirring so
long as there is any chlorine present; only when the silver solution has
precipitated all the chlorine does the red colour become permanent, and
thus the change of colour of the whole fluid from yellow to red shows
with exactness the complete precipitation of the chlorine. For practical
purposes all that is required is the so-called decinormal silver solution,
and from the number of cubic centimetres of this solution which are
required to precipitate all the chlorine the total amount of chlorine
present can be readily calculated.
In steeping smaller objects before examination the whole of the water
should be well stirred with a glass rod or poured two or three times from
one vessel into another: 100, 50 or 25 cubic centimetres are then poured
into a graduated glass or drawn up into a pipette. The water should be
drawn up by suction slightly above the level of the mark upon the stem of
the pipette, the upper end of which is immediately closed with the thumb.
By slightly raising the thumb the water is allowed to run off until its
upper surface is exactly level with the mark. The amount taken is then
placed into a beaker (for 100 cubic centimetres a beaker of 400 c.c.
capacity should be used), and, after the addition of a few drops of a
solution of potassium chromate, is examined by titration. In the treatment
of large objects, for which tubs are required, the necessary quantity of
water may be drawn by means of a long pipette from the bottom of the tub,
where the quantity of salt is always greatest, or through the tap at the
bottom of the tub (as was done with the blocks from the Meten Chamber,
in which case about 1 litre was drawn off into a glass out of which 100
c.c. were taken for titration). To obtain results which are comparable,
care must be taken that the object is always as nearly as possible in the
same quantity of water. After placing the larger blocks in the water, one
examination should be made during the first few days, when the titration
may require 20 c.c. or more of silver solution. There is no need to
examine for chlorine while the water is being frequently changed: indeed,
in order to economise the silver solution, this need not be begun until
the second month, when the water is changed every fortnight.
As has been stated above, it is only necessary to read off the number
of cubic centimetres of the solution used in the titration, for the
decrease in these figures is a sufficient indication of the progress of
the operation, while the diminution of the chlorine-content may be taken
as an indication of the simultaneous removal of the sulphates[88]. In
the treatment of small objects in distilled water, the process may be
regarded as complete if the red colour is obtained on the addition of from
one to two drops (i.e. about 1/10 to 1/5 cubic centimetre) of the silver
solution. If, when tap-water is used, and is being changed at intervals
of a fortnight or a month, the estimations give a constant result between
0·6 and 1·0, the treatment need not be carried further.
The accompanying table shows the figures obtained from three large blocks
from the Meten Chamber. They represent the number of cubic centimetres
of decinormal silver solution used for 100 c.c. of the water, which was
changed every fortnight. The first column on the left shows the dates upon
which the stones were placed in the tubs:
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
|3 Feb. | 7 Apr.| 4 May | 2 June|25 Aug.|22 Sep.| 4 Nov.| 1 Dec.|12 Jan.|
| 1890 | | | | | | | | 1891 |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
| | 3·0 | 2·5 | 2·3 | 1·4 | 1·0 | 0·9 | 0·8 | 0·8 |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
|25 Apr.| 9 May |18 July| 1 Aug.|25 Nov.|23 Dec.|16 Feb.|29 Aug.|12 Sep.|
| 1893 | | | | | | 1894 | | |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
| | 6·0 | 5·6 | 4·9 | 2·0 | 1·7 | 1·5 | 0·8 | 0·8 |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
|29 July|28 Oct.|25 Nov.|23 Dec.|20 Jan.|16 Feb.| 1 Aug.|29 Aug.|12 Sep.|
| 1893 | | | | 1894 | | | | |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
| | 3·0 | 2·0 | 1·5 | 1·3 | 1·1 | 0·7 | 0·8 | 0·7 |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
When repeated examinations gave a fairly constant result of 0·7-0·8 cubic
centimetre the process was regarded as complete, for Berlin tap-water
itself contains small quantities of chlorine compounds, 100 c.c. requiring
from 0·4 to 0·6 c.c. decinormal silver solution. Before using the water
from a well or from waterworks, it should be examined to ascertain the
number of cubic centimetres of silver solution required to produce the red
colouration. As the amount of chlorine compounds in the water may vary it
is advisable to repeat the examination[89].
The following table shows the rapidity with which salts can be completely
extracted from small pieces of limestone. The limestones were placed in
tap-water in three glass cylinders, each containing 2 litres; the amount
of silver solution required for the water was 0·45 c.c. per 100 c.c.
+-----------------------+--------+---+---+---+---+---+------------------+
|No. of days of soaking | | | | | | | |
| in water | 1 | 1 | 1 | 2 | 2 | 4 | |
+-----------------------+--------+---+---+---+---+---+------------------+
|1|Weight in grammes 107|3·4 c.c.|1·4|1·1|0·7|0·5|0·5|cubic centimetres |
| | | | | | | | | of decinormal |
| | | | | | | | | silver solution.|
| | | | | | | | | |
|2|Weight in grammes 66|3·0 c.c.|1·2|0·7|0·5| --|0·5|cubic centimetres |
| | | | | | | | | of decinormal |
| | | | | | | | | silver solution.|
| | | | | | | | | |
|3|Weight in grammes 47|3·6 c.c.|0·8|0·6|0·5| --|0·5|cubic centimetres |
| | | | | | | | | of decinormal |
| | | | | | | | | silver solution.|
+-+---------------------+--------+---+---+---+---+---+------------------+
These figures show the numbers of the c.c. of silver solution used for
every 100 c.c. of the water, which was changed after 1, 2, etc. days as
shown above. After 9 to 11 days therefore the stones could be declared
free from salt.
If the accuracy of the titration method be considered unnecessary, either
on account of the small number or size of the objects to be treated, or
for reasons of expense (the outlay required is however very small), a
solution of unknown strength may be used. A comparison between the degree
of turbidity produced on mixing the silver nitrate solution with the
tap-water, and that produced with the wash-water, will enable the progress
of the operation to be gauged.
Advantages and Disadvantages. Although steeping removes the cause of
decay, i.e. the salts contained in the limestone, and although permanence
may be considered as certain, there are certainly some disadvantages
connected with the process, especially when the pieces, on account of
their size, must remain in the water for some length of time. Some large
and very thick blocks from the Meten Chamber required to be soaked for
more than a year.
The small quantity of carbonic acid which is always found in water
dissolves small quantities of calcium carbonate, thus the sharp contours
of prominent parts may become somewhat rounded. Limestones which have
developed fissures may, on immersion, lose small portions which might
otherwise have remained attached, though probably for a while only.
In such cases it must be carefully noted from which block, and from
which part of it, the fragment has broken off, in order that it may be
replaced[90].
Limestones which are much cracked, or which are likely to fall to pieces,
should be wrapped round with gauze, or held together with twine, before
they are put in the water.
In addition to the permanent preservation of the object some other smaller
advantages of this method may be mentioned: for example, the layer of
dust which is often present is removed and thus traces of colours may be
brought out by the steeping which had been concealed by it. Thus certain
remains of colour mentioned by Lepsius[91] as being still visible in his
time upon some of the blocks from the Meten Chamber were no longer visible
when we took them in hand. Moreover traces of green colouring which were
visible after the treatment in the eyes of a few large figures in relief
were probably evidence that colours had formerly been present.
Drying. When the steeping is finished the limestone is taken out to be
dried. Small objects may be placed upon a glass ring, wooden tripod or
some such appliance, which admits air on all sides, and may thus be dried
by the air only. A piece of paper laid loosely over them will protect
them from dust. In winter a hot stove, or similar source of heat, affords
a satisfactory method of drying, but wet stones must not of course be
placed directly upon the hot iron stove plate lest spots of rust should
be produced upon the stone. Large blocks are preferably dried in drying
chambers in which in summer time a strong draught is obtained by opening
windows on opposite sides, and which in winter are strongly heated and
opened every now and then for a short time. The limestones should be laid
upon wooden blocks to allow air to pass beneath them, while they must
be guarded from dust both above and at the sides with sheets of paper.
Several months are often required to dry large blocks completely.
Impregnation. When limestones have been completely dried, especially if
they are soft, it is often advisable to impregnate them with one or other
of the impregnation agents. To economize material, large objects may be
painted over once or twice with a solution of the material chosen, but
smaller objects should be immersed in the solution until air-bubbles are
no longer formed. If there is a supply of tap-water with sufficiently
good pressure, rapid and complete penetration by the fluid can be ensured
by placing the object in a vessel containing the necessary fluid under a
bell glass, the air from which is then exhausted by a water air-pump[92].
Figure 14 illustrates the application of such an air-pump fixed to the
water-tap by means of an india-rubber tube which is firmly bound with
wire. An india-rubber stopper perforated to admit a glass tube is fixed
in the top of the bell glass, while the smooth ground edge and the
thick ground glass plate upon which it rests are smeared with grease or
vaseline. The side tube of the air-pump is connected with the interior
of the bell glass by an india-rubber tube which is sufficiently strong to
resist the pressure of the outer air, and thus when the tap is opened the
pressure of the flow of water carries with it the air from the bell glass
with which the pump is connected. If the water-tap is suddenly turned
off when the air is exhausted the pressure of the outer air will force
the water into the bell and cause it to mix with the solution of resin or
varnish. To prevent this, a stop-cock or valve should be inserted, or the
water-tap should not be turned off until the stopper of the bell-glass has
been cautiously raised. A second glass tube provided with a stop-cock may
be passed through the india-rubber cork and connected with a manometer to
measure the progressive action of the pump (Figure 15). When air-bubbles
cease to come from the object under treatment, the glass tap should be
closed and the manometer removed, after which the glass tap should be
again opened and the water-tap closed[93].
[Illustration: Fig. 14.
Air-pump fixed to water-tap.]
[Illustration: Fig. 15.
Apparatus for impregnation by extraction of air fitted to manometer.]
If the object is of some length but not too thick, the bell-glass may be
fixed on a strong glass cylinder of a similar diameter having a ground
edge (Fig. 15), into which the object and the impregnating solution are
then placed.
The following solutions, amongst others, may be recommended as suitable
for impregnation:
(1) Shellac dissolved in alcohol.
(2) Solution of gum-dammar[94].
15 grammes of dammar are dissolved in 130 grammes of benzine,
to which is added a solution of 20 grammes of clarified poppy
seed oil in turpentine. If the solution becomes too thick
it should be diluted with benzine and a small quantity of
turpentine.
(3) Rice water or tapioca water[95].
(4) Dilute size.
(5) Waterglass solution.
(6) Linseed oil dissolved in benzine.
(7) Linseed varnish dissolved in 3 parts of benzine or petroleum
ether.
(8) Solutions of stearine or paraffin wax in benzine.
(9) Collodion (free from acid). Zapon[96].
(10) Kessler's fluate.
It may be added that, as a general rule, solutions for this purpose must
be used as dilute as possible, for two immersions in a dilute solution are
preferable to a single soaking in a concentrated one, which often scarcely
penetrates into the pores.
As the preparation of solutions of shellac, gum-dammar, and of such
substances as resin, stearine, and paraffin, necessitates heating, and
as the solvents are very inflammable, it is advisable to make use of the
solution of linseed varnish in benzine. This solution may be obtained
at any time at any degree of concentration without the use of heat.
Although it has the advantage that it hardens more rapidly than a simple
solution of linseed oil it has also one disadvantage, for it gives a
somewhat darker colour to light-coloured limestones. No more of the
mixture of varnish and benzine should be prepared than is required for
the impregnation, for this solution, on standing, throws down a gelatinous
precipitate which is not re-dissolved even by heating. As this alteration
is accelerated by the action of light, the mixture should always be kept
in a dark place.
Collodion and zapon[96], on account of the expense, should only be used
for small objects. After impregnation the objects should be covered
with glass jars, cardboard boxes, etc., to prevent the precipitation
of moisture upon them, as the result of the rapid evaporation of such
volatile substances as benzine and ether upon exposure to the open air.
Rice water, tapioca water, or size (the latter of no greater strength
than 2%) are only applicable to specimens which are kept in dry rooms,
for in damp rooms they readily become sticky, and are liable to be
attacked by moulds. Waterglass solution, probably because it is generally
applied in too concentrated a form, instead of penetrating the object
has a tendency to form a pellicle, which readily strips off. Even dilute
solutions, however, are said to be unsuitable, from the liability to the
efflorescence of alkali salts.
In the case of marble objects and antique statues of porous limestone,
showing colours which are still bright on excavation, but which would soon
fade, Rhousopulos[97] recommends impregnation with a very dilute solution
(1 in 1000) of waterglass to preserve the colour. The solution should be
as neutral as possible: in any case not alkaline. This is several times
sprayed upon the object, which is allowed to completely dry between each
spraying.
A material which is suitable for large objects to which the solution can
only be applied upon the surface is Kessler's fluate[98], which is soluble
in water, and which hardens the limestone without completely closing the
pores. It offers the additional advantage that it is applicable to thick
limestone blocks, the dryness of which is not certain. The solutions
numbered 1-9 must only be used when the limestone is dry throughout its
mass. The fluate to be used in any particular instance must be decided
from the nature of the case. Those most generally applicable are magnesium
and zinc fluates and the so-called "double fluate."
The stones from the Meten Chamber were hardened in the following manner:
The limestone blocks were placed upright and the surface dusted by the
air-current from a Dechend's spray apparatus[99] which was then used
to spray them repeatedly with a solution of "double fluate" of sp. gr.
1·16. Owing, however, to the injurious effect of the fine spray of the
fluate upon the nose and lungs the stones were turned to a horizontal
position, and a solution of fluate of sp. gr. 1·38 was applied by means
of a large brush until the fluid was no longer absorbed. For the treatment
of limestones on which there are remains of colours the use of a solution
of shellac, gum-dammar, or collodion is recommended. Fluates should not
be applied until their suitability for the particular purpose has been
tested.
All specimens should be kept after impregnation in rooms which as far as
possible are free from dust, for the dust which falls upon the surface
will set in the varnish whilst it is hardening.
IMPREGNATION WITHOUT PREVIOUS STEEPING. If a preliminary examination
has shown that specimens of limestone will not bear steeping in water,
recourse can be had to impregnation only. The treatment of such specimens
must be thorough, for merely to paint the fluid upon the surface with a
brush almost invariably proves a failure. Instead of penetrating the stone
the impregnating medium forms a firm coating which is liable to be lifted,
and in parts broken, by the crystallisation of salts, and thus allows
the destructive processes to continue uninterrupted. Aqueous solutions,
e.g. size, cannot of course be applied, and as it is necessary to make
a preliminary trial of a fluate spray, it is generally found preferable
to make use of the varnish-benzine mixture. In spite of this, salts may
still make their appearance in the form of a crystalline powdery layer on
the surface, which can be wiped off with a wet sponge; any moisture must
however be removed with a soft dry linen cloth.
REMOVAL OF INCRUSTATIONS AND DUST. Incrustations of earth, lime, or
gypsum should be washed off with water or removed by mechanical means,
such as gentle rubbing with the finger. The solvent action of acids upon
limestone precludes their use for this purpose. Any dust which adheres
can be removed by rubbing with stale bread-crumb.
(_b_) Marble and Alabaster.
It is usually only necessary to clean marble with a soft brush and warm
water, with the addition perhaps of some good neutral soap. In rare
cases the presence of sulphates may perhaps cause some friability. The
crystalline structure of marble renders steeping futile, and accordingly
impregnation is resorted to. The use of Kessler's fluates may be
recommended. Adherent pitch or resin is best removed by a mixture of
alcohol and ether. Alabaster seems to remain permanently sound and may be
cleaned in the same way.
(_c_) Earthenware.
STEEPING. The same line of treatment should be followed as in the case of
limestones. A preliminary examination should always be made to test the
power of resistance in water, which is always satisfactory if the clay
has been sufficiently baked.
In the case of coloured terra-cotta care should be taken to ascertain
whether the colours are likely to suffer during steeping. There is no
danger of injury if the steeping is not too prolonged; in fact, the
removal of the dust during the procedure often brings out the colours
more clearly. If the Egyptian ostraca (clay fragments with black script)
require to be washed they should be carefully watched in order to preserve
the script, and therefore should be placed in the bath in such a way that
the lettering is visible.
These fragments are usually curved and bear the script upon the convex
side, care should therefore be taken that they are completely immersed,
and that no large air-bubbles prevent the access of the water to any
part of the under-surface. The writing is done with either lamp-black or
more rarely some form of iron ink, and is retained mechanically by the
porous character of the ostraca. In the latter case the characters may
be enhanced by the application of a dilute solution of tannic acid, which
sometimes proves useful also for limestone pieces.
If these fragments are sufficiently few in number to allow each to be put
into a separate glass vessel, the washing out of the salts is completed
so quickly that there need be little danger of obscuring the script. When
large numbers were to be washed and when the script was already indistinct
I have employed the following method: After examination as to their
fitness for immersion the fragments are placed on a wooden grating in a
tub, in which they remain for a couple of days, during which the water
is renewed once. They are then taken out and allowed to dry. All those
which still show the script distinctly are separated and their steeping
is completed, but the remainder, having been completely dried, perhaps
on the top of a warm stove, are brushed over once or twice with a dilute
(1:6) mixture of varnish and benzine in such a way that the surface is
only moistened, and when dry shows no gloss. The pieces thus superficially
varnished are kept in a dry place for about two months, until the varnish
is hardened; the process of washing out the salt is then begun again.
The thin coat of varnish fixes the script without interfering with the
steeping. The varnish solution must be dilute, for a thick coating will
partially peel off from the object in the course of the steeping, or will
remain in the pores in the form of opaque particles, and thus render the
script illegible.
The same difficulty which arose in the treatment of the Meten limestones
was frequently met with in the treatment of these ostraca. Those which
were of a dark brown colour especially, and to a less degree also the red
and the yellow, were covered with a slimy growth of algae. As the script
is easily destroyed no attempt should be made to remove these algae from
the side which bears the script even with the softest brush, although they
should from time to time during steeping be brushed from the underside.
The inconvenience caused by algae is, however, less marked in the
treatment of earthenware, the light and porous character of which renders
prolonged steeping needless, nor is there the same necessity to continue
the steeping for the purpose of chlorine estimation. The following results
were obtained in the treatment of 13 fragments, the average thickness of
which was 1 cm. [3/8th inch], with an average superficial area of 1/10 sq.
metre [4 inches]. The tub in which they were steeped contained 85 litres
[18-1/2 gallons] of water.
100 cubic centimetres of the tap-water used were found to require 0·5 c.c.
of the silver solution, and on each occasion this quantity of the water
was tested.
+------------+-----+-----+-----+-----+-----+-----+-----+-----+----------+
| Water | | | | | | | | | |
changed | | | | | | | | | |
| after | 1 | 1 | 1 | 1 | 2 | 2 | 2 | 4 | 5 days |
+------------+-----+-----+-----+-----+-----+-----+-----+-----+----------+
| 100 c.c. of| | | | | | | | | |
| the water | | | | | | | | | |
| used in | 3·0 | 1·3 | 1·0 | 0·8 | 0·9 | 0·7 | 0·7 | 0·6 | 0·6 c.c. |
| steeping | | | | | | | | |of silver |
| required | | | | | | | | | solution |
+------------+-----+-----+-----+-----+-----+-----+-----+-----+----------+
The water was changed at first daily, then every two days, and so on:
the steeping could therefore be regarded as complete at the end of a
fortnight.
A small figure of earthenware, which weighed only 28·9 grammes, was
steeped in 1-1/2 litres of distilled water, and gave the following result
for every 100 c.c. used:
Water changed after 2 days required 3·6 c.c. silver solution.
Water changed after 3 days required 0·4 c.c. silver solution.
Water changed after 4 days required 0·0 c.c. silver solution.
The steeping was, therefore, in reality complete after five days, and, as
the steeping water was thoroughly mixed before the withdrawal of the 100
c.c., the total quantity of sodium chloride contained in the figure can
be calculated as follows:
For the 15th part (viz. 100 c.c.) of the water 3·6 + 0·4, i.e. 4·0 c.c.,
of decinormal silver solution were used, which is equivalent to 15 × 4·0,
i.e. 60 c.c., of silver solution for the whole quantity. Now 1 c.c. of
this decinormal solution corresponds to 0·00584 gramme of sodium chloride;
the water therefore contained 60 × 0·00584 gr., or 0·35 gr. sodium
chloride. Thus the figure contained altogether 1-1/5% of sodium chloride.
In addition to the chlorine compounds, there was also a considerable
quantity of sulphates, the presence and disappearance of which were tested
by adding to a few cubic centimetres of the water a dilute solution of
barium nitrate or of barium chloride[100]. The soluble barium salts give
with sulphates a white precipitate or cloudiness of insoluble barium
sulphate. If therefore on the addition of a solution of barium nitrate
no cloudiness appears, even after some time, it may be concluded that
sulphates are no longer present in the water. When the ostraca have been
washed and dried, it is often possible to make the script more distinct
by varnishing them over with a varnish-benzine mixture (1:6).
It is advisable to subject friable objects of earthenware to the process
of impregnation (cp. the impregnation of unbaked clay, p. 81).
THE REMOVAL OF INCRUSTATIONS. Incrustations of earth or lime can be easily
removed if the earthenware has been well baked, but trial must first be
made with a drop of dilute hydrochloric acid, whether the earthenware
itself is not attacked by the acid. The specimen is then placed upon
a glass ring or suspended in water containing 2% of hydrochloric
acid[101]. This mixture, which must be renewed every 24 hours, will remove
incrustations which it would be difficult to remove by mechanical means,
while crystals of gypsum of considerable size, which are often found on
clay tablets of Assyrian origin, are easily dissolved in from two to four
days.
Figures 16 to 21 represent two Assyrian tablets which have been cleaned
by myself in this manner. It will be seen that the cuneiform characters,
which before treatment were almost invisible, are now distinctly legible.
[Illustration: Fig. 16.
Fig. 17.
Assyrian clay tablet with incrustations.
Before and after treatment.]
[Illustration: Fig. 18.
Fig. 19.
Fig. 20.
Fig. 21.
Assyrian clay tablet before and after treatment.]
After this treatment with acidulated water the acid must itself be
removed by careful washing in pure water. Here too a solution of silver
nitrate will serve as a test, for, so long as any chlorine, and therefore
any hydrochloric acid, is present in the water, a white precipitate or
cloudiness is produced.
The method of titration with yellow potassium chromate is not applicable
here, for the free acid prevents the appearance of the red precipitate.
The steeping must therefore be continued in distilled water until the
addition of silver nitrate no longer produces any cloudiness.
Baked earthenware which shows colouring, or which has incised lines filled
with substances containing lime, must not be steeped in acidulated water,
nor will ostraca bearing inscriptions in iron ink stand this treatment;
these are, however, fortunately rare: in fact amongst several thousand
fragments few have shown incrustations of lime or gypsum. Should any such
be found a cautious attempt should be made to remove the incrustations
by some mechanical means. Rhousopulos[102] carries out the cleaning of
Lecythoi[103] and clay vases, which are painted in water-colours and which
have a thin white incrustation, by dipping them into a 5% solution of pure
hydrochloric acid. As soon as the colours show the least sign of running,
or if an efflorescence makes its appearance, the vase is immediately
removed and allowed to dry. It is then dipped into distilled water and
allowed to dry a second time. Impregnation is not necessary.
"If the treatment is otherwise successful, but an earthy layer
remains upon the colour, the spots which are thus affected are
lightly touched with the finger whilst the object is still in the
liquid. Rubbing, or any sort of mechanical attack, is absolutely
out of the question."
This process evidently requires the greatest care and constant attention.
(_d_) Slightly Baked Or Unbaked Clay.
IMPREGNATION. If upon examination it is found that a drop of water softens
the clay, the same line of treatment must be followed as in the case of
limestones which exhibit a similar condition (see p. 73), i.e. they must
be subjected to the process of impregnation[104]. As the colour of the
clay objects is yellow-brown or red-brown, the varnish benzine mixture
will be the most suitable application for the purpose. A considerable
number of sun-dried Assyrian clay tablets treated in this manner have
given good results, and have undergone no change during the last five
years, in fact they may now even be laid in water without crumbling.
In the case of slightly baked or unbaked Babylonian clay tablets
the method formerly employed was merely to remove deposits of lime,
clay, gypsum, etc., by lifting or scraping them away with pointed or
wedge-shaped tools, for the soft clay would not stand treatment with
water, still less with 2% hydrochloric acid. The difficulty in avoiding
damage to the clay surface, when removing the deposit, makes this method
both tedious and risky. Warming to 200-300°C. in a drying oven, or in
an iron box embedded in sand, seldom aids the removal of incrustations;
moreover, this treatment has no hardening effect upon the clay, and thus
does not facilitate the removal of the injurious salts by soaking. A
further expedient therefore remains, that of heating the clay to higher
temperature, whereby it is fully baked and rendered capable of resisting
subsequent treatment with water or 2% hydrochloric acid. At the Royal
Museum this firing is done in muffle furnaces[105], the smaller of which
has a capacity of about one cubic foot, and is heated by six and twelve
Bunsen burners. The temperature is regulated in the same way as in
porcelain manufacture by the use of Seger's cones[105], which are placed
in the muffle, where they can be seen through the observation aperture.
To avoid cracking the heating must be gradual, the gas-supply being very
gradually increased. The firing must at first be adjusted to cone 022
[590°C.; Watkin, No. 1, 1094°F.]; the gas is then turned off and the
furnace allowed to cool as slowly as possible. To effect this the damper
is closed and all openings into the muffle are made up with fire clay.
The clay tablet is removed when quite cold (usually in 18-24 hours), and,
as a rule, much of the incrustation can then be removed by means of a
soft brush. Should the removal prove difficult, and a preliminary trial
have shown that it will bear the treatment, the removal of the deposits
will be assisted by soaking for two or three days in water. Should the
tablet prove capable of bearing treatment with 2% hydrochloric acid it
may remain in the acid for 12 to 18 hours. If necessary the acid may be
renewed once; it must then be thoroughly removed by steeping in ordinary
water and finally in distilled water, until the wash-water is free from
chlorides. After steeping, the tablets will be found somewhat softened
and occasionally coated with a slimy growth of algae, care must therefore
be used in changing or taking them from the water. The best way to handle
them is to place the fingers of the two hands under the tablet.
[Illustration: Fig. 22.
Babylonian clay cone before treatment.]
[Illustration: Fig. 23.
Babylonian clay cone after treatment--firing,
treatment with hydrochloric acid and steeping.]
After thoroughly drying the tablets first in the air, then in the drying
oven at a temperature of 212°F., supported on glass rings, it is well
to impregnate them. This can be best carried out by placing them, while
still warm, into melted paraffin wax, and raising the temperature to about
250°F. [120°C.]. The wax is allowed to cool to about 160°F. [70°C.],
when the tablet is removed upon a broad band of gauze, any excess of
wax is drained off, and the object is wiped with a soft cloth. The
benzine-varnish mixture or zapon may also be used for impregnation. If
heating in the muffle to Seger cone 022 is insufficient to allow of the
removal of the incrustations, or if the condition of the clay does not
warrant soaking in water or acid, the object must be again placed in the
muffle and fired to Seger cone 010 [950°C.; Watkin, No. 13, 1742°F.],
and, if softening occurs upon the application of water or acid after
exposure to this temperature, recourse must be had to a third heating to
Seger cone 05 [1050°C.; Watkin, No. 18, 1922°F.]. Higher temperatures
than this are not advisable, for the lime, sodium chloride, and other
salts found in some Babylonian tablets may partially fuse. During firing
therefore the appearance of the object must be carefully watched, and the
temperature lowered at once by reducing the gas-supply, if signs of fusion
are noticed.
ADDITIONAL METHODS OF IMPREGNATION. If clay objects have a smooth surface,
it is, according to the "Merkbuch[106]," advisable to impregnate them with
Belmontyl oil[107], for varnish in the course of drying gives a lacquered
appearance to the surface. According to the same authority the surface
of glazed vessels can be restored by impregnating them several times with
a mixture of poppy seed oil and benzine [20 grammes clarified poppy seed
oil in 270 gr. benzine, i.e. 1 in 13-1/2], and by subsequently brushing
them first with soft, then with harder brushes. There are, however, many
other substances used in different collections for impregnation, a few of
which are subjoined.
In the Museum at Vienna friable clay objects are laid for two or three
minutes in a dilute solution of warm size, and when dry are brushed over
with a solution of shellac; size alone, or a solution of shellac alone, is
frequently used for impregnation, or to give a coating. In the Museum at
Wiesbaden thin specimens are impregnated with a solution of white of egg,
brittle objects with dilute fish glue, while for hard objects a solution
of shellac or melted shellac is used.
(_e_) Fayence.
I have been able to wash out the sulphates from several Egyptian fayence
figures in spite of the glaze, the fissures in which allowed the water
to penetrate into the interior. The process of steeping, which was
necessarily somewhat prolonged, was tested from time to time by the barium
nitrate test (vide p. 77).
(_f_) Objects of Stucco and Nile-mud.
These are rare, and in almost all cases contain salts. As, however, they
will not bear steeping, they must be preserved by means of impregnation
only. The varnish-benzine mixture should be used for this purpose.
(_g_) Sandstone and Granite.
These scarcely need any special preservative process, but Kessler's
fluates are useful for the impregnation of weathered sandstones which are
exposed to the open air (see p. 72).
They can be cleaned by washing with warm water, while calcareous
incrustations may be removed by hydrochloric acid. A thick coating of
oil paint was successfully removed from an Egyptian statue of sandstone
by placing it in an alcoholic solution of soda. Oil colours and similar
substances may often be removed with ease and completeness from stone,
plaster, wood, etc., by placing the objects in air-tight vessels together
with a vessel containing alcohol. The alcohol vaporises, even at the
ordinary room-temperature, and causes a softening of the paint. The time
required for the treatment depends upon its age and hardness.
Appendix.
Cement for Earthenware. Restorations.
To fix together pieces of broken pottery good Cologne glue is useful,
but it has the disadvantage that it can only be used when warm. For this
reason it is better to use liquid fish-glue [Syndeticon], which may, if
necessary, be thinned with a little vinegar. Fire-clay dust in waterglass
is used in the Museum at Breslau. A thick ropy solution of shellac[108]
may also be mentioned, for the use of which the opposing surfaces must be
first moistened with alcohol.
Gum arabic and dextrin should not be used, for objects thus cemented
readily fall to pieces unless kept in perfectly dry rooms. This, however,
may also be said of earthenware which contains salts, if cemented with
glue or fish-glue. Previous steeping would obviate this difficulty.
Chalk, plaster of Paris, brick-dust, or fire-clay dust are often added to
the fish-glue, dextrin, etc. Without giving additional strength to the
cement, these substances may be of use in filling up small gaps between
the fragments to be cemented.
For filling up larger gaps the "Merkbuch[109]" recommends stone cement,
for the preparation of which it gives the following prescription:
"Mix 500 grammes of Cologne glue with three sheets of strong white
blotting-paper, or four sheets of white tissue paper, shredded as
small as possible, and boil until it becomes thick, stirring the
whole into a perfectly smooth pulp. Let it boil thoroughly, and
while stirring continually, and working with a stout wooden rod,
add 2-1/2 kilogrammes of very finely sifted dry purified whiting.
After working this mixture thoroughly, add 80 grammes of linseed
oil, which must be also thoroughly worked in. To preserve the glue
add 50 grammes of Venetian turpentine. This stone cement will take
any shade of colour if mixed with lamp-black or coloured earths."
(_h_) Iron.
The various methods for the preservation of iron objects which have been
or are still in use may be divided into two groups. To the one group
belong those methods in which the objects are preserved with their coating
of rust, or with the rust that has penetrated them; to the other group
belong those in which the removal of rust precedes preservation. The
former methods must be applied when the iron has been completely converted
into rust or when the rust has only left a small metallic core. These
methods may of course be used also for all iron antiquities.
The methods of the second group can be applied to those objects only which
still retain a strong metallic core, in which case the objects regain the
more or less grey or white surface of fresh unoxidized iron. These methods
are at present little known, and therefore but little used, for owners
and the general public are still accustomed to see in the covering of rust
the evidence of antiquity with which they are loth to part.
In addition to these methods, there are others which are of an
intermediate kind, either special or a combination of methods from both
these groups.
(1) Methods of preserving Objects of Iron without removal of the Rust.
IMPREGNATION. The earliest processes, which are to some extent still in
use in some collections, are simple impregnation methods, in which the
object is either painted once or more with the impregnating medium by
means of a brush, or is placed directly in the medium itself. In either
case the penetrating power of the solution used is directly proportional
to its fluidity.
The following media may be used for the purpose:
(1) Warm size.
(2) Warm isinglass solution.
(3) Solution of waterglass.
(4) Solution of shellac in alcohol.
(5) Rubber solution in carbon bisulphide. The mass after swelling
is dissolved in benzine[110].
(6) Copal varnish diluted with turpentine.
(7) Copal varnish mixed with linseed oil[111].
(8) Linseed oil.
(9) Linseed varnish.
(10) Linseed varnish mixed with an equal quantity of petroleum.
(11) Bees'-wax dissolved in turpentine.
(12) Bees'-wax dissolved in benzine.
(13) Petroleum.
(14) Vaseline.
(15) Melted paraffin.
(16) Oleate of lead: 100 grammes of olive oil, 100 gr. of lead
oxide, and 100 gr. of water are boiled until all the water has
evaporated and the mass has become grey. The mass is extracted
by shaking it with alcohol, and the residue is dissolved in
absolute ether, in the proportion of 100 gr. of ether to 5 gr.
of the substance. Before use it should be diluted with a little
ether[112].
(17) Speerschneider's mixture. This consists of 8 parts of rape
oil, 1 part of bees'-wax, 1 part of pine resin, and 2 parts of
benzene[113].
(18) Collodion, or the mixture used in the Museum at
Donaueschingen, which consists of 30 grammes of collodion, 2 gr.
of camphor, and 1 gr. of oxalic ether.
In addition to these materials, there are other mixtures of resin,
varnishes, and bees'-wax, with their appropriate solvents, but they do
not possess any special advantages as impregnating solutions.
After treatment with size or isinglass, iron objects may be given when
dry a coating of linseed oil, linseed varnish, solution of shellac, etc.
The materials numbered 7 to 10 in the above list should be applied warm to
enable the viscid fluids to penetrate the rust, for the more readily the
solution enters the object the better is the result obtained. Apart from
the fact that they are easily ignited at a high temperature, they must not
be heated beyond 230°F. [110°C.], otherwise objects which consist largely
of rust will fall to pieces[114].
In the process of impregnation a twofold result is aimed at, viz. to
prevent the rust from crumbling, and to exclude air from the specimen.
The application of heated linseed oil or linseed varnish is founded upon
the supposition that these substances enter into a chemical combination
with ferric oxide to form a stable compound; this is, however, disputed
by some modern authorities[115]. Neutral substances offer a safer method
for the exclusion of air, and of these melted paraffin is undoubtedly
the best. The paraffin must be quite pure and free from stearine, as
can be ascertained from the melting point; thus pure paraffin melts at
130°-150°F. [55°-65°C.], stearine at 160°F. [70°C.]. Paraffin with a
melting-point higher than 65°C. should be looked upon with suspicion.
In many collections the objects are heated before impregnation with media
which are insoluble in water, or they are exposed to the air for six
to twelve months after excavation. This latter proceeding is, however,
certainly inadvisable if the iron contains chlorine, and if this is the
case not one of these methods produces satisfactory results.
On the other hand, as almost all the iron antiquities which do not contain
chlorine compounds may be treated by the methods of the second group,
simple and direct impregnation is passing more and more out of use. Before
impregnation all soluble substances, especially chlorine compounds, must
be removed by steeping.
(2) Preservation by Steeping and Subsequent Impregnation.
KRAUSE'S METHOD. The water used for steeping should be preferably
lukewarm, and should be changed every twenty-four hours. It is even
better, at least for the first time, to lay the object in water and then
raise it to boiling-point, a measure which will allow the more ready
penetration of the water. As in the case of limestones, earthenware, etc.
(p. 59), care must be taken to place the objects as near to the surface of
the water as is possible. Small objects may be put in glass jars, large
ones in wooden troughs, tin vessels, or wooden boxes lined with zinc or
lead. Any little excrescences on the iron, which are frequently filled
with ferrous chloride, should be punctured to give the water unimpeded and
more speedy access. Crumbling objects should be held together by tightly
wrapping them in muslin. Curators must decide for themselves how far means
such as files, chisels, or small hammers may be used to remove the rust
or earthy material conglomerated by rust.
Although much recommended, the method of adding soda or lime water to
remove the chlorine as soluble sodium chloride or calcium chloride is, in
our opinion, inadvisable. Both these substances precipitate the iron from
the ferrous chloride or ferric chloride (which are soluble in water) as
insoluble hydroxide of iron, which more or less closes the interstices,
and thus impedes the access of water to the interior.
The process of steeping can here again be controlled by the use of the
silver solution (p. 62), for if there no longer appears any or only very
little cloudiness the steeping may be considered complete. The length of
time required for steeping depends upon the thickness of the rust and
the porosity or existence of cracks in it, and if the objects are of
considerable size, it may extend over several weeks.
After steeping the object should either be dried in the open air,
and later on a warm stove, or be placed for a few days in alcohol to
remove the water, after which the rapid evaporation of the alcohol will
quickly dry it. The steeping of iron objects in warm alcohol has been
recommended[116], but if their size is considerable the method is an
expensive one. This method has the advantage that the alcohol penetrates
the rust sooner than does water, and also prevents oxidation, which may
be actually produced by the water. It may perhaps be advisable to dilute
the alcohol, the usual strength of which is 95% to 96%, with about an
equal volume of water, for some salts are not readily soluble in pure
alcohol. When dry the object is warmed for a few hours in a mixture of
equal parts of good linseed varnish and petroleum. The petroleum serves to
dilute the varnish, which can thus more quickly permeate the entire mass
of iron and rust. On account of the inflammable nature of the mixture the
warming should be done over a water-bath. For small objects a cylinder
made of ordinary tin-plate, measuring from 6 to 10 inches [15 to 25
cm.] in diameter and 6 in. [15 cm.] in height, may be used. To increase
stability the lower half should be of a smaller diameter, and fitted
into an iron tripod. The same end is attained by soldering a ring round
the middle of the cylinder, which will rest on the ring of the tripod.
The cover consists of a number of copper rings gradually diminishing in
diameter, which fit closely into one another, thus enabling porcelain
vessels of various sizes to be used. For larger objects, such as swords,
two long rectangular troughs (Fig. 24) of stronger plate should be used.
The following sizes will probably be found useful: one about 40 inches
[100 cm.] long by 4 inches [10 cm.] broad, and 4 inches [10 cm.] deep,
and the other slightly larger. Handles should be fixed at the upper edges.
Three iron bars 1 inch [2-1/2 cm.] thick and 4 inches [10 cm.] in length
are laid across the bottom of the larger trough, on which the smaller is
placed. The space between the two vessels is filled with water to a depth
of 2 inches [6 cm.]. The trough is warmed on a stove, or better, where
gas can be had, by means of a number of Bunsen burners fitted with rose or
ring burners, over which the trough may be supported upon tripods. While
heating care must be taken that the water does not boil over, which can
be easily avoided by regulating the gas supply. As the water evaporates
further quantities should be added as required. After simmering for about
two hours, the objects should be removed and allowed to drain; they should
then be placed on a tripod, or on glass rings, on the warm stove in cold
weather, to accelerate the evaporation of the petroleum and the setting
of the varnish. In summer drying chambers may be used; these are sold by
dealers in physical and chemical apparatus, or can be made at little cost
by a tinsmith.
[Illustration: Fig. 24.
Water-bath. 1/15 nat. size.]
If the objects have been steeped in pure alcohol, or at least towards the
end of the treatment in three changes of alcohol, so that all the water
is replaced by alcohol, they may be dipped directly without drying in the
varnish mixture, for the alcohol evaporates in the varnish bath which is
at a temperature of 194°-203°F. [90°-95°C.]. As the varnish hardens, the
iron thus treated acquires a glazed surface; other means of impregnation
may therefore appear preferable, e.g. a solution of gum-dammar or melted
paraffin. For impregnation with the dammar solution the object must first
be dried, and the air-pump used in the way described on p. 68. On account
of the inflammable nature of the benzine heat must not be applied, nor
indeed is it necessary.
When impregnating with pure paraffin[117], specimens may be lightly
wiped with a cloth, but need not be dried. The paraffin may be heated
to 212°-248°F. [100°-120°C.] without danger, so long as it is kept from
direct contact with the flame. A thermometer should be used, and, as soon
as the paraffin has melted at a temperature of about 60°C., the object
should be placed in it by means of tongs. When the temperature has risen
above 212°F. [100°C.] the water is converted into steam, and causes a
brisk ebullition of the melted paraffin. The quantity of paraffin used
should, therefore, be such that its level remains at the least 2 inches
[5 cm.] below the upper edge of the vessel.
When the bubbles have ceased to rise, thus showing that all the water
is expelled, the paraffin should be allowed to cool to a temperature of
180°-190°F. [80°-90°C.]. The iron should be taken out with tongs, and
the liquid allowed to run off. It should then be wrapped, while still at
80°C., in soft blotting-paper or in a piece of old linen to absorb the
superfluous paraffin. If the surface of the object is very uneven, or if
there are deep cracks or holes in which the paraffin can collect, it will,
when cold, form a white mass, and should therefore, while still warm and
fluid, be soaked up with filter paper, or distributed evenly by means of
suitable brushes. The superfluous paraffin may also be absorbed by putting
the object in dry sawdust; any sawdust which remains attached can be
removed when cold with benzine, or it may be scraped from the spots where
it has collected with a knife or spatula. Any spots where the iron may
have become exposed may be covered with a thin coat of paraffin dissolved
in benzine.
EKHOFF'S METHOD[118]. The objects are laid for two or three months in
water which is changed every two or three days, a small quantity of
quicklime being added[119]. After this steeping, and after some of the
rust has been removed mechanically, the object is lightly dried and put
into heavy petroleum of sp. gr. 0·85 to 0·95, which is then heated up to
220°F. [105°C.]. A thermometer should be used to ensure this temperature.
This temperature being higher than the boiling-point of water, the water
contained in the object evaporates and causes the petroleum to bubble, as
in the method previously described. When all the water has been replaced
by the petroleum the bubbling ceases. After the fluid has somewhat cooled
down, the iron is taken out and is allowed to remain for about an hour in
sawdust, which absorbs the superfluous oil. Finally, while gently warming
the object over a warm, but not too hot, stove, it is coated over with a
mixture of 1 part of bees'-wax and 2 parts of turpentine, or better with
paraffin dissolved in benzine. Heavy petroleum, which we have found by
experience to be a suitable material, is preferable to varnish in so far
as the iron is impregnated by a neutral substance which is practically
liquid paraffin, but has the disadvantage of being highly inflammable and
of being difficult to obtain at so high a specific gravity.
STRABERGER'S METHOD. This method, for the description of which I am
indebted to Herr Straberger, has proved effective in the preservation of a
number of iron antiquities in the Museum at Linz on the Danube. Even iron
objects, which had been in bad condition and had undoubtedly contained
chlorine, have after treatment by this method shown no signs of change,
while the dull black surface has an agreeable appearance.
Straberger places the newly-excavated objects immediately into linseed oil
to prevent the access of air. After remaining in the oil for some time
they are taken out, wrapped in cloths saturated with linseed oil, and
removed packed in sawdust. Upon arrival they are unwrapped and put into
water, to which a small quantity of soda is added to remove the oil more
easily. The water is frequently changed, and the objects are meanwhile
cleaned mechanically with emery paper and hard brushes. Any blisters
are removed by the aid of a small hammer and chisel. After steeping they
are dried and smoked over a candle flame which is allowed to play over
the whole surface. The soot is then rubbed off with a cloth or soft
brush. Objects with a smooth surface may be rubbed with india-rubber.
The preservative action of this proceeding depends upon the fact that
during the smoking, in addition to the soot, oily products of combustion
are deposited from the candle flame, which prevent the access of air and
moisture to the iron.
"Objects which are much decayed or cracked should, when cleaned
and thoroughly dry, be again placed into linseed oil which has
been slightly warmed and should remain therein for a few days
before being smoked. Upon removal from this second oil bath they
should be lightly wiped and dried over a moderately warm stove
or in the sun. Patience is necessary, and nothing further should
be done until the oil has entirely dried in the fine cracks and
crevices and firmly binds the mass. The oil crust on the surface
is then loosened by soaking in a strong soda solution and wiped
off, after which the object is dried, smoked over the candle
flame, and the soot wiped or brushed off with a soft brush. The
smoking and wiping may be repeated if necessary."
Herr Straberger states that his treatment has been successful when
impregnation with isinglass and coating with shellac has failed.
The methods of Hartwich and Jacobi hold an intermediate place between
the above methods and those which will be subsequently explained. With
the former they have this in common that they do not call for the entire
removal of the rust and that they require the use of linseed oil; on
the other hand their application presupposes the existence of a strong
metallic core, otherwise when the rust is removed they will show merely
a skeleton of the original object. The existence of a sufficiently
substantial metallic core can be easily ascertained from the weight, for
an object, which consists solely or in great part of the oxide of a metal,
is much lighter than one of the same size which is largely metal. The ring
also affords a test, for an iron object, of which the greatest part is
metallic iron, gives a clearer note when struck than one which is chiefly
rust. A still more certain test is the use of a file or a drill (comp.
page 107).
HARTWICH'S METHOD[120]. This method is intended for objects of an
especially large size, the hard oxide coating of which does not allow
satisfactory steeping. Hartwich heats the object to redness, allows it to
cool slowly, and then scrapes off the outer layer which has been rendered
friable by this treatment. The subsequent procedure is that of Krause's
method, viz. warming in linseed varnish.
JACOBI'S METHOD. The method of preservation of iron antiquities used in
the Saalburg Museum at Homburg is described by Jacobi as follows: The
object is heated in the fire of a forge, which causes the chief part
of the rust to flake off, while any rust which still adheres is removed
when cold by water and brushing. The object is again held in the flame
with tongs and heated (smaller objects may be placed on an iron plate);
and during the heating is quickly taken out three or four times and each
time brushed over with linseed oil. Most of the linseed oil is thus burnt
and the deposition of carbon gives to the iron a black colour, while the
oil which has been partially burnt or hardened by the heat produces a
slight lustre. This process, as carried out at Homburg by a locksmith,
is that which blacksmiths ordinarily use to blacken iron objects and to
protect them from rust. The preservation has proved permanent, and only
in rare cases has it been found necessary to repeat the process. These
good results are probably due to the fact that the antiquities of iron
preserved in that Museum are for the most part found in good condition,
having very little rust and certainly containing only a very small amount
of chlorine. Iron articles which contain chlorine but which still have
a good metal core, after washing, drying, and a cautious preliminary
application of heat, are ready for treatment by Jacobi's method.
INLAID IRON OBJECTS require especially cautious treatment. Although I have
not had any personal experience in the treatment of objects of this kind,
good results have been obtained in several Museums, especially in that at
Mainz.
The following quotation from the "Merkbuch" (p. 75) describes the method
which is applied at Mainz, where it probably originated:
"Objects of this kind which are likely to have been originally
inlaid with silver, gold, copper or brass, as is frequently the
case with objects of the Merovingian period, are not placed in
alcohol after the steeping, but are warmed and dipped three or
four times into a hot dilute solution of isinglass. The heating
is necessary, otherwise the isinglass will set on the surface and
will not penetrate into the interior. When the object has been
dried and the isinglass has set, the layer of rust which covers
the inlaid ornaments is scraped off with a graving tool, and any
spongy hollow parts are filled up with a paste made of iron rust
and isinglass, before the inlaid work is cleaned. During the
scraping the object is held in the left hand on a little wooden
board covered with plush or thick chamois leather, to which it is
fixed as firmly as is necessary by means of a vice. In scraping
special care must be taken that the graving tool follows the lines
of the designs, for in scraping across the design it may slip
under the flat silver thread and raise it out of its place. When
the ornamentation has been completely laid bare, it is rubbed
with emery cloth and then polished with a brush and fine emery
powder. The piece is then dipped into a solution of gum-dammar,
and, when the surface is dry, emery is again used to remove the
varnish, which gives the silver a slightly yellow colour. The
object is then protected from the influence of air and moisture
by the transparent retouching varnish of Sohnée frères (Paris)."
A modification of Krefting's method (p. 108) has proved eminently
successful in the treatment of iron objects inlaid with silver.
Krause[121] recommends that the article be placed, with the inlaid surface
downwards, for 24 hours in a mixture of
10 grammes of 40% acetic acid,
10 grammes of ammonium chloride,
70 grammes of distilled water,
10 grammes of aluminium powder.
It is then removed from the bath, carefully brushed and washed, and,
if the inlaid work is not yet cleaned, is replaced in the bath. This
is repeated until the inlaid work is completely exposed. Spots of
ferroso-ferric oxide which are difficult to remove may be ground away
by an emery wheel, care being taken that the inlaid surface is held
against the lower side of the wheel (which must be rotated in the reverse
direction) so that it is always in sight.
All the methods of this group, which have been applied to many articles in
various Museums, exhibit one inherent defect, for any rust which remains
after treatment may cause the continued oxidation of the iron. The effects
of this action of rust are, I believe, extremely small, and it must at the
same time be admitted that iron antiquities, even if they have been well
steeped and afterwards impregnated, do not always remain in a permanent
and sound state of preservation. If in such a case the well-known small
watery bubbles should make their appearance, the steeping has undoubtedly
been insufficient. This evil can be remedied by gradually heating the
object to redness to destroy the impregnating material, and by a careful
repetition of the steeping and impregnation.
(3) Preservation of Iron Antiquities by Removal of the Rust.
STEFFENSEN'S METHOD (COPENHAGEN). The objects are carefully heated over
a flame and are then laid in dilute sulphuric acid. The sulphuric acid
dissolves a certain amount of the iron, and it is found by experience that
the chemical action is strongest at those spots where any rust remains,
and that this is detached by the hydrogen which is produced. When the
cleaning is sufficient, the iron is laid in a dilute soda solution to
neutralise the acid, and is afterwards well washed with water and dried in
an oven. When dry the iron is brushed over with a solution of bees'-wax
(or better of paraffin) in benzine, the evaporation of which leaves a
protective coating of bees'-wax or paraffin.
BLELL'S METHOD. The method proposed by Blell and applied by him to many
of the objects in his collection is distinct from that described above,
although in its earlier stages the principle is the same. The following
quotation is taken from the description of his method which the author
read before the Antiquarian Society[122] at Königsberg:
"If a specimen is found to have a sufficiently strong core of
iron it should be heated in the furnace to bright redness and
then dipped into water. The expansion of the iron caused by the
heat and the subsequent contraction caused by the sudden cooling
thoroughly loosens the layer of rust. Large iron objects with a
strong and firmly attached incrustation of rust will require a
repetition of the process. By this means not only is the rust
converted into a red powder which is easily rubbed off, but
the object itself is rendered more suitable for the subsequent
treatment. At the same time the heating process removes any
coating of oil, fat, etc., which may have remained from previous
attempts at preservation, and which would interfere with the
further stages of the process. Smaller or delicate specimens
should be treated in the flame of a spirit-lamp, but special care
must be taken that there is sufficient iron present. Sword blades
and other tools and weapons with sharp edges should be heated
only, for the sudden cooling may cause cracks in the cutting
edges."
To complete the removal of the incrustation of rust which has been
loosened by the heating process, or by the heating and sudden cooling,
the object should be placed
"in a well-stirred mixture composed of one part by weight of
sulphuric acid in nine parts of water. Bubbles of hydrogen will
immediately rise and the rust will begin to separate. In freshly
prepared acid objects which are not very rusty will be freed from
rust after four to six hours, those covered with a deeper layer
of rust in about twelve hours, but several days, or even weeks,
may be necessary. The duration of the process depends upon the
strength of the acid and the character of the rust, viz. whether
it is thick and solid, or thin and porous, and whether the iron
is of a soft, or of a hard character.
When first making use of this method it is advisable to use dilute
acid and to take out the objects several times in the course of
the day and examine them, while during the night they should be
taken out of the acid and placed in soft water[123].
For the acid bath and for rinsing it will be found convenient to
have two pairs of wooden troughs having the following internal
measurements:
(1) An internal length of 10 inches [25 cm.] by 7-1/2
inches [19 cm.] in breadth and 4-3/4 inches [12 cm.]
in depth, which will be useful for the larger number of
objects.
(2) For long narrow objects, e.g. sword-blades, and long
spear-heads, the internal measurements should be 40 inches
[100 cm.] long by 4 inches [10 cm.] broad and 3 inches [8
cm.] deep.
Small fragile objects are most satisfactorily treated in glass
vessels or glazed earthen pots or vases.
The acid must have free access to all parts of the object; if a
sword, for example, lies flat upon the bottom, the under-surface
apparently remains unacted upon by the acid. This should be
remedied by the use of a couple of small wooden supports.
Frequent rubbing with a cloth and forge scale[124] or coarse
sand greatly helps in removing the rust, but gentler treatment
is required for the smaller and more fragile objects. The rust
is often very firmly attached in some portions of the object,
and in this case those areas which have been already freed from
rust should be coated over with lard, which is free from salt, to
protect them from further action of the acid, while the pockets
of rust are alternately treated with acid and graving tools. No
particle of rust should be allowed to remain, for sooner or later
it will begin to spread, whatever precautions may be taken.
The action of the acid becomes less effective if it has been used
for several objects. A little fresh acid should then be added.
The more active the sulphuric acid, the brighter will be the grey
colour of the iron after the rust has been removed. If old acid
has been used the iron will be of a dirty grey colour, and should
then be placed into fresh acid for a short time until it assumes
a clear light grey colour.
The third part of the process begins with the removal of the iron
from the acid bath and has as its object the removal of every
trace of the acid, otherwise the rust will very quickly return
and cover the whole surface. The object is therefore immediately
and repeatedly rinsed in soft water and carefully dried; the
cheapest material for this purpose is cotton waste, but ordinary
linen-cloth must be used for objects with jagged edges, for the
threads will catch in the notches and hinder the drying. This
should be done without delay, or a change of the colour from light
grey to yellow will betoken a new formation of rust. Articles
showing a very complicated construction, which are however rare
from the Iron Age, should be packed in perfectly dry hot pinewood
sawdust, while those which are still more difficult to dry, for
example, coats of chain-mail, after thorough rinsing, should be
immediately put into a pan with melted lard, free from salt, and
boiled until the cessation of bubbling shows that all the water
has been driven off by evaporation.
They are then rubbed dry or are laid in hot sawdust, after
which they are brushed over with melted lard and placed in this
condition for at least half an hour in a moderately hot cupboard
until the fat has penetrated into the finest pores of the iron.
That this has really taken place may be proved by the use of a
file.
When by this means all trace of sulphuric acid has been removed
the fourth stage of the process is reached, viz. the removal of
the grease from the surface and the subsequent application of some
preparation to prevent the access of air and moisture. Most of
the grease is removed by placing the objects in a warm place on
blotting-paper. Any grease still remaining on the surface can be
entirely removed with a cloth or paint-brush by means of benzine.
If no restoration or repair is required nothing more is necessary
than to apply the protecting solution."
A white varnish has much to recommend it from its protective power, but
as it gives to iron an unsatisfactory gloss, it is preferable to use a
solution of bees'-wax in benzine.
Having made use of Blell's method in a number of cases I have a few
suggestions and modifications to offer. The heating should be carried out
carefully and gradually, lest the sudden conversion of the moisture in the
rust into steam should cause small explosions which would scatter pieces
of rust. There is no danger of this if the objects are heated in an oven;
they should not therefore be heated in an open flame. For smaller objects
I use a box six inches [15 centimetres] square, of strong tin-plate
loosely covered with an iron lid, or with a piece of asbestos sheet; but
if the objects are large, e.g. swords, spearheads, etc., I heat them on a
strong piece of tin-plate bent round to form a channel, and covered with
a long piece of asbestos sheet, the edges of which are bent over the edges
of the channel, to retain the heat as much as possible.
It is advisable, in my experience, to use the sulphuric acid well diluted,
e.g. in the proportion of 1 to 20, and to renew it several times if
necessary. In mixing concentrated sulphuric acid with water great caution
is required on account of the evolution of heat. The acid should be poured
in a thin stream into the water, but not vice versâ, and the mixture
should be constantly stirred with a glass rod. If a glass vessel is used
for the mixing, it must not be too thick lest the heat should cause it to
break, but the larger the proportion of water to the sulphuric acid, the
less considerable will be the rise of temperature.
For boring out rust spots which have eaten deeply into the iron a
dental drill can be used with success, and a great variety of drills
and milling cutters can be obtained. The rinsing, which Blell carries
out by moving the object to and fro close under the surface in a vessel
full of water, may be sufficient for thin iron objects, such as swords,
knives, spear-heads, and similar objects. Larger specimens should be
freed from the acid by putting them into a still more dilute solution,
and, when necessary, by steeping for a short time in water. It may also
be advisable to put the objects into dilute soda solution to neutralize
the sulphuric acid, but this does not do away with the necessity for
steeping in water. The brown coating of rust which may possibly follow
the steeping can be removed by the use of steel-wire brushes, which can
now be made of such fine wire that their softness almost equals that of
a moderately soft tooth-brush. Brass-wire brushes should not be used, on
account of the yellow colour which they give to the iron. I always put the
objects directly after steeping into clean fat heated to 250°F. [120°C.],
for brushing over with fat and warming in a stove often caused a slight
tarnish to cover the surface. I have also used paraffin wax instead of
fat.
For the method of restoring iron antiquities and of filling up large gaps,
the reader should refer to Blell's detailed account; it will here suffice
to quote his statement that a mixture of iron filings with tin filings
can be used for this purpose. These are melted and applied by the aid of
a blowpipe.
The accompanying illustrations represent iron antiquities which have been
treated by Blell's method: the sword (Fig. 25) proved, after reduction,
from its two ridges to be a scramasax; on the spear-head (Fig. 26)
treatment revealed a small copper ring at the most constricted part, while
the fibula, which previously had been a mass of rust, now shows the spiral
which had been totally disguised.
[Illustration: Fig. 25.
Iron sword treated by Blell's method.]
[Illustration: Fig. 26.
Iron spear-head treated by Blell's method.]
[Illustration: Fig. 27.
Iron fibula treated by Blell's method.]
KREFTING'S METHOD. The electro-chemical method of Krefting was
originally published in "Aarsberetning fra Foreningen till Norske
Fortidsmindesmaerkers Bevaring," 1892 (p. 51), but in "Finska
Fornminnesföreningens Tidskrift[125]" there is a translation into German
by H. Appelgren, and an additional series of observations and experiments
by him. His remarks are equally applicable to Blell's method, and the
following extracts and quotations from this paper give Krefting's method
of procedure and the circumstances under which it should be applied.
Small fragile objects such as fibulae, thin clasps and bracelets or those
which are much eaten away by rust, are not suitable for this mode of
treatment, thus:
"A knife which is much corroded, and which when taken out of the
earth shows a distinctive form (for example, that of the Early
Iron Age), may lose so much by the application of the electric
current that every distinct sign of its original character is
destroyed. The characteristic edges of a spear-head or of an
axe of the late Iron Age, or the equally characteristic point
of an iron sword, may, if the rust has eaten deeply into them,
be unrecognisable when removed from the electrolytic bath. A
sword, the hilt of which is inlaid with copper wire or is plated
with silver or gold, or the blade inlaid with inscriptions in
gold, silver, or copper, may be totally destroyed by incautious
treatment; for the ornamentation, if undermined by rust, may be
detached with the rust from the underlying iron."
On the other hand, objects of sound metallic iron covered with an
incrustation of rust about 1/25 inch [1 millimètre] in thickness may
be easily cleaned in this manner, but if on using a file the metal does
not appear at all, or only at a depth of 1/8 inch [3 millimètres], great
caution must be used. If there is reason to believe that there is gold
or silver inlaid work undermined by rust, Appelgren recommends that the
object should, as a preliminary, be laid in clean water, which should be
renewed every day. After some time, three weeks at the most, sufficient
rust will have been cleared away by carefully brushing with a steel
brush to lay bare the ornamentation, at least in part, and it can then
be ascertained whether there is any rust underneath which would, if
Krefting's method were used, cause the ornamentation to be detached.
The line of treatment is as follows: The metallic iron core is laid bare
by filing in several places. The specimen is then wrapped with strips
of zinc in such a way that the zinc is in actual contact with the bare
metal (Fig. 28). The whole is then placed into a 5% solution of caustic
soda[126]. Appelgren uses a solution of {3-1/2}-{4-1/2} lbs. [{1-1/2}-{2}
kilogrammes] of caustic soda in 2 gallons [10 litres] of water. The rust
is cleared away by voltaic action; the iron forms the negative pole, the
zinc the positive of a voltaic cell, in which the water is resolved into
its constituents, viz. oxygen and hydrogen. At the negative pole, i.e.
the iron, the hydrogen rises up in small bubbles and acts in part by
mechanically detaching the rust as in Blell's method, in part also by the
chemical conversion of the rust into metallic iron, or into a compound
which contains a smaller quantity of oxygen than does ordinary rust. The
oxygen combines with the zinc to form zinc oxide, which is dissolved in
the soda solution. The process is usually completed in 24 hours[127].
The black powder which is loosely attached to the iron is best rubbed off
with wet sand and fine wire brushes. Any hard pieces of black stable rust
(Edelrost), magnetic oxide of iron, which have not yielded to the electric
current should be removed by means of a small chisel. After rinsing the
object thoroughly in water, it should be placed in melted paraffin at
240°F. [115°C.], which will expel every trace of moisture. On removal the
melted paraffin should be allowed to drain off, and thus leave when cold
a protective covering upon the iron[128].
The following points should be observed in the application of the method.
Vessels of glass or glazed earthenware should be used for the reduction,
while long swords can be put into tall glass cylinders or into wooden
troughs, the interior of which must be coated over with paraffin. The soda
solution must be kept in a closed glass bottle[129]. It should be diluted
with water until the specific gravity, as shown by the hydrometer, is
1·06; the mixture will then contain about 5 per cent. of caustic soda.
During the reduction process the mixture frequently assumes a brownish
colour as the result of the presence of organic matter associated with
the rust. On account of the dissolved zinc which it contains it cannot
be used a second time, unless regenerated by boiling with quicklime. The
solution is, however, so cheap that this is scarcely worth the trouble.
The objects should be handled with metal tongs, and should not be touched
with the hand until they have at least been dipped or rinsed in water,
for the soda solution has an injurious effect upon the skin. A basin
containing vinegar, dilute hydrochloric or sulphuric acid should always
be at hand into which the fingers should be quickly dipped if they have
been in contact with the caustic soda. These materials will serve also
for cleaning the vessels used in the reduction process.
The zinc strips should be 1/4 to 1/3 inch [1/2 cm. to 1 cm.] in breadth,
and should be cut out of a piece of sheet zinc of moderate thickness, but
of sufficient pliability.
Any firmly fixed rust may be removed by mechanical means, such as the
graver, drill, etc., as has been previously mentioned. If in rinsing a
slight layer of oxide appears, although this is rare, it should be brushed
off with a steel-wire brush.
If one portion only of a specimen requires reduction (the other portion
having, for example, remains of wood attached, and therefore being
unsuitable for reduction), that portion only should be wrapped with the
zinc and immersed in the solution.
The results obtained by Krefting's preservation-process are quite as
surprising as those which are afforded by Blell's method. Figure 29, taken
from Appelgren's work, shows the lower portion of a spear-head before
and after treatment, by which it became apparent that the whole socket
was plated with silver, with two engraved and gilded animal figures. Fig.
34 represents a piece of a sword, on which an inscription was brought to
light by the reduction process.
[Illustration: Fig. 28.
Krefting's method. Iron spear-head wrapped with strips of zinc.]
[Illustration: Fig. 29.
Iron spear-head before and after treatment by Krefting's method.]
[Illustration: Fig. 30.
Iron pin from "Danes' Graves," Yorks. [Cp. Yorks. Phil. Soc. Report,
1897.]]
[Illustration: Fig. 31.
The same after treatment by Krefting's method, still showing chalky
accretions.]
[Illustration: Fig. 32.
Iron object from Lamel Hill[130], York. It appears to have been
originally rivetted to wood or leather.]
[Illustration: Fig. 33.
After treatment by Krefting's method.]
[Illustration: Fig. 34.
Piece of iron sword-blade showing inscription, after treatment by
Krefting's method.]
HARTWICH'S REDUCTION METHOD[131]. This method is only applicable to small
objects, because it necessitates the subjection of the objects to red-heat
in a glass tube in a current of hydrogen. By these means the hydrogen
combines with the oxygen of the oxides, which are thus reduced to metallic
iron. Owing to the explosive nature of a mixture of hydrogen and air,
this process should only be carried out by one who is conversant with
chemical methods, for results which are equally good can be obtained at
less expense by Krefting's method. For Hartwich's method a strong core of
metal is essential, for although objects which are entirely oxidized may
be thus reduced, the result will be the formation of a more or less loose
iron powder which is frequently in such a fine state of division that by
union with the oxygen of the air, in consequence of the great amount of
surface presented, it becomes red-hot with the formation of ferric oxide
as a combustion product.
It is advisable to apply a combination of Blell's or Krefting's method
with one of the first group (under certain conditions) to such iron
objects as are found, during the process of preservation, to be penetrated
by black stable rust to such a degree that the complete removal would
only leave a kind of iron skeleton. Fig. 35 represents such an iron
dagger-sheath[132], the dark spots upon it being rust. After heating
and cooling down and a short treatment with acid the removal of the
rust was proceeded with mechanically, but was not completed. The object
was then well steeped, and when dry was warmed in the varnish-petroleum
mixture[133].
[Illustration: Fig. 35.
Iron dagger-sheath after treatment by a combination of Blell's and
Krefting's methods.]
Iron objects, the size of which is inconsiderable, such as arrow heads,
small rings, etc., can be very quickly reduced, if they still have a
well-preserved core, by heating them for a short time in molten potassium
cyanide[134]. The cyanide may be melted in a porcelain crucible supported
by wire gauze on a tripod over a good-sized Bunsen burner, and the object
introduced by the aid of tongs. The reaction is accompanied by vigorous
effervescence and is soon complete. It is then taken out and dropped
into cold water. By repeatedly boiling in fresh quantities of water it
is thoroughly cleansed, then treated with paraffin wax, or the water may
be expelled by alcohol. It is then dried, and finally impregnated with
zapon. If the cyanide treatment is insufficient, any remaining rust may be
removed by drills or other suitable tools. Hitherto this method has only
been applied to a small number of objects, but there is no doubt that its
use may be largely extended. Owing to the poisonous nature of the cyanide
this method should be left to those who possess chemical knowledge.
The disadvantage of the process lies in the difficulty of fusing large
quantities of the potassium cyanide[135].
(4) Preservation of Medieval Iron Objects.
A complete treatise on this subject would be beyond the limits of a
handbook, the following observations, therefore, will be sufficient
for our purpose. The rust spots on objects of this kind are frequently
only superficial and can be removed either mechanically by rubbing with
pumice or emery, etc., or chemically by a concentrated solution of sodium
sulphide[136]. To prepare this, sodium sulphide is dissolved in water, or
flowers of sulphur are boiled in a solution of caustic soda. If the object
is too large for immersion, the solution may be applied with a brush, and
if the layer of rust is thick, the application must be repeated. After
treatment the object must be rinsed in water and dried.
Small articles can be freed from rust by immersion in strong fuming nitric
acid[137], for strong acid dissolves the rust only, while it induces
in the iron the so-called "passive[138]" condition in which it is not
acted upon even by dilute acids, and can be safely washed in water. When
thoroughly cleaned, the most suitable protective is some neutral substance
such as paraffin wax, vaseline, or paraffin dissolved in benzine, but any
of the numerous forms of oil or fat may be used.
(_i_) Bronze and Copper[139].
Well-preserved bronzes with a stable patina, such as the highly esteemed
glossy stable or "edel" patina, or that which, although not glossy, covers
the bronze with a rough and often crystalline coating, should not be
interfered with. Such bronzes as need treatment should be subjected either
to simple cleaning or to some appropriate method of preservation.
THE CLEANING OF BRONZES. Bronzes, the metallic substance of which is more
or less intact, while the surface is hidden under earthy or sandy material
cemented together by copper compounds, may be cleaned either by mechanical
or chemical means. When the materials forming the incrustation are more
firmly cemented together than they are to the material beneath (which
often still retains a polished surface), a small hammer may be used, but
more adherent portions require the use of small chisels, which can be made
to order in different shapes or sizes. I have used with advantage hammers
with striking surfaces like those shown in Fig. 36. The two on the right
are rounded so that they touch the object at one point or on a line only.
The process may be facilitated by the use of Springer's method. A warm
thick solution of glue should be spread upon the incrustation covering
the bronze. As the glue dries and becomes cool it scales off, carrying
with it some portion at least of the crust, thus leaving the metal clean.
That part of the glue which remains can then be readily detached by gentle
strokes with a hammer. The eyes should be protected when using the hammer,
whether on the incrustation or on the glue.
[Illustration: Fig. 36.
Hammer heads, natural size.]
OTHER METHODS. Since metallic oxides are scarcely, if at all, soluble in
water, washing with water, even when a brush is used, will remove only
earth or soil which is loosely attached. Compounds containing oxygen or
oxygen and chlorine are, however, more or less soluble in ammonia, and,
if they are thin and not too compact, after immersion for some time can be
removed with a brush. Thick compact layers are loosened with difficulty.
Immersion in 2-5% hydrochloric acid acts more effectively, while sulphuric
acid, nitric acid, and concentrated acetic acid have the same action. The
frequent use of these reagents is, however, strongly to be deprecated,
for it is impossible to remove the acid by simple washing with water
after the incrustation has been removed. The bronze should be washed and
placed in a very dilute soda solution or in dilute ammonia, after which it
should be again well washed with distilled water. As has been explained
in Part I., it is to chlorine compounds that the destruction of bronzes
is chiefly due, and these are actually produced by the hydrochloric acid
treatment. If the bronzes are not thoroughly washed, and this is no easy
matter, sooner or later efflorescences will make their appearance, and the
process of preservation must be repeated if the destructive action is to
be arrested.
Various attempts have been made to remove the incrustation by raising the
bronze to a red heat. This process is not recommended; for not only does
it give to the bronze an unpleasant appearance, but it detaches any inlaid
metal (gold or silver) or enamel which may be present.
In conclusion, it may be stated that, although the process is slow
and laborious, the best results are obtained by careful removal of
incrustations by mechanical means.
Preservation of Bronze and Copper Objects.
(A.) METHODS OF IMPREGNATION. The impregnation of bronzes, as of the
majority of antiquities, has for some time been carried out by the use
of solutions similar to those already enumerated for iron. These are
applied directly or after the specimen has been either steeped in water
or treated with dilute acids. This latter treatment, as has been already
stated, is to be avoided, and if used all acid must be washed out before
the object is dried. Steeping in water is of little use, because compounds
containing oxygen or chlorine are often insoluble in water, which will
at most only wash off loosely attached dirt or earthy material. The
impregnation process may therefore be applied directly, and this should
be done in all cases in which the surface is much corroded, warty (Figs.
7 and 8), or cracked (Figs. 37 and 38), or in which there is little or no
core of metal. Impregnation is also the only means of preservation when
the formation of oxides has raised inlaid metals or enamel in such a way
that the removal of the oxides would detach them. The "Merkbuch[140]"
recommends poppy seed oil and benzine mixture (p. 70) or the gum-dammar
solution. To obtain thorough impregnation this should be carried out by
extraction of the air, as has been already recommended in the case of
limestone (p. 68). The object must also be perfectly dry, which may be
insured either by exposure to moderate heat or by keeping it for some time
over anhydrous calcium chloride[141]. The object is placed under a glass
bell jar, the edges of which are smeared with vaseline to ensure contact
with the glass plate upon which it rests. The calcium chloride should be
placed in an open glass vessel, beneath the bronze, but care must be taken
that they are not in actual contact.
[Illustration: Fig. 37.
Osiris showing cracking and destructive patina.]
[Illustration: Fig. 38.
Boeotian bridle with cracking patina.]
Immersion of bronzes in paraffin wax at 240°F. [115°-120°C.] gives results
which are as good, if not better, than those obtained by the use of
solutions.
Should efflorescences make their appearances upon bronzes which have been
impregnated, their further spread may often be successfully prevented
by smearing fish-glue on the parts affected. Fish-glue, however, has not
proved a satisfactory material for the complete impregnation or coating
of bronzes which are in the last stages of decay.
(B.) PRESERVATION BY REDUCTION. It has been previously explained (pp.
28 _et seq._) that the efflorescences upon bronze known as creeping or
malignant patina which may in time cause the complete destruction of
the metal are due to the action of sodium chloride. It is found upon all
Egyptian bronzes and upon those from some other localities.
The metal, especially the copper, is converted into the so-called basic
chloride. In the reduction processes an attempt is made to reduce these
compounds again to metal, while the chlorine thus liberated forms chemical
compounds, which may be subsequently washed out with water. There are two
methods which effect this reduction, viz., that of Finkener (Berlin) and
that of Krefting. The principle of both is electrolytic, and both bring
about the complete removal of the patina and the restoration of a clean
metallic surface.
To complete this portion of the subject a third method may be mentioned,
viz., reduction by heat in a stream of hydrogen. This method[142] is,
however, only applicable to small objects.
FINKENER'S METHOD. Care must be taken when examining the bronze that the
metallic-looking mixture of cuprous oxide with other copper compounds is
not mistaken for metallic copper. When it has been ascertained that the
bronze still has a good metallic core, and that any inlaid metals which
may be present rest on the metal itself and not upon a crust of oxide, a
platinum wire should be tightly wound round it. This should be connected
by an insulated copper wire to the zinc or negative pole of the first of
3 or 4 Daniell cells, or, better, of two accumulators arranged in series.
The object should then be immersed in a 2% aqueous solution of potassium
cyanide. In the same solution, as near as possible to the bronze without
actual contact, should be placed a piece of platinum foil connected first
by an emerging platinum wire, and then by an insulated copper wire to
the positive pole. The potassium cyanide completes the electric circuit
and electrolysis takes place, whereby the water is split up into its
constituents. The oxygen appears in small bubbles upon the platinum foil,
but the hydrogen does not immediately make its appearance at the other
pole, for, by combination with the chlorine and oxygen contained in the
bronzes, free hydrochloric acid and water are formed. The hydrochloric
acid in turn acts upon the potassium cyanide to form potassium chloride
and hydrocyanic acid, both of which substances are dissolved in the water
of the bath. The hydrocyanic acid can often be recognised in the room by
its characteristic smell of bitter almonds. The process may be expressed
by the following equations (neglecting the water produced by the oxygen
of the oxide, which is of no importance in the process):
CuCl_{2} + 2H = Cu + 2HCl,
HCl + KCN = KCl + HCN.
Although the chief portion of the potassium chloride and hydrocyanic acid
are dissolved in the bath, the remaining traces of these substances must
be removed by very carefully washing the bronze in water, after which it
should be dried, and if necessary finally subjected to impregnation.
Some further observations may be made in connection with the practical
application of this process.
Of course, other primary batteries may be used instead of the Daniell
cells, but these latter may be specially recommended for the ease with
which they can be procured and for the steadiness of their action.
Information concerning the method of filling and using them may be
obtained at any shop where they are sold. The copper wire and platinum
wire should not be too thin, but must be at least from 1 to 2 mm. in
thickness: they should be fastened together by binding-screws, and care
must be taken that both the wire ends and the screws have clean surfaces.
Glass vessels or glass cylinders are most suitable because the process
of reduction can be watched, but large objects will of course require
glazed earthenware baths. If wooden boxes are used they must be coated
inside with paraffin wax. The strength of the cyanide solution should be
2%. Having a large number of reductions to carry out, I keep a 20% stock
solution in a large bottle, one part of which is diluted with nine parts
of water when required for use. Potassium cyanide is, as is well known,
a strong poison, and care should therefore be taken to prevent access to
any sore or cut on the hands; this can be done by the use of india-rubber
finger stalls or gloves.
If the bronze object is neither too large nor too heavy it may be
suspended in the bath by looping the platinum wire over the edge of
the vessel. It is a convenient plan to use different coloured wires to
distinguish the negative and positive poles of the battery, but should
any doubt arise as to which wire should be connected with the bronze
or which with the platinum, the following test will readily decide the
question. Moisten a small piece of white filter paper with a drop of a
solution of potassium iodide[143], and touch the two conducting wires
with it simultaneously: a brown spot will be seen on the paper at the
point of contact with one of the wires; this is the positive wire, and
must therefore be connected with the platinum. If the current is passing
through the cyanide bath and the bronze, bubbles of gas will appear upon
the platinum foil, or the products of the decomposition of the potassium
cyanide may change the colour of the bath near the platinum to yellow or
brown, while at the same time cloudy streaks under the bronze will show
where the potassium chloride and hydrocyanic acid, resulting from the
reduction of the copper compounds, are meeting with the cyanide of the
bath. If the platinum wire is not firmly fixed round the bronze, hydrogen
may be formed upon it, and should this occur the wire should be drawn
tighter.
Whilst the reduction is going on it is advisable to renew the potassium
cyanide at least once, or even several times, if large and greatly
oxidized bronzes are under treatment, for otherwise all the potassium
cyanide may be consumed by the changes in progress; this can be
ascertained with certainty by a smell of chlorine. When the bath requires
renewal the bronze may be taken out with a pair of metal tongs, or if too
large, two strong copper wires should be passed underneath it, the ends
of which are wound round a strong glass rod or wooden stick. The bronze
should then be well rinsed or brushed with a soft brush before it is put
into the fresh bath.
Bronzes are frequently met with which are much deformed by an earthy or
sandy layer cemented by oxide. These incrustations can be partly removed
by a preliminary treatment with dilute hydrochloric acid, but the bronze
must be afterwards carefully rinsed with water or even steeped to prevent
unnecessary decomposition of the cyanide by the acid. Before reduction it
is useful to secure thorough penetration by placing the vessel containing
the solution and the bronze under a bell glass attached to an air pump,
as has been previously explained (p. 68).
During the process of reduction small whitish-green crystalline needles
often collect on the platinum foil, but although in large numbers they
are so minute that it has not been possible hitherto to determine their
composition; they seem to contain copper and cyanogen. After some time
the platinum becomes covered with a whitish-green or brownish deposit,
which should be removed by rinsing in water and brushing; if this should
not succeed the platinum must be dipped in hydrochloric acid, rinsed with
water, and rubbed with fine sand. The glass vessel may be cleaned in the
same way.
The reduction is complete when all the chlorine, previously combined with
the metal, has combined with the hydrogen produced by the electrolysis
of the water. There being no further chlorine with which the hydrogen
produced by the continued action of the current may unite, the completion
of the process is marked by the appearance of bubbles of that gas upon the
surface of the bronze. The bubbles which rise from beneath often mark out
the outlines of the object upon the surface of the bath.
Before the bronze is washed it should be placed in a fresh cyanide bath,
but of 1% strength only. For large and especially for thick objects, this
bath must be renewed several times, so as to allow the washing process to
begin in the bath itself whilst the current is still passing through it.
Care should also be taken that every side of the object in turn faces the
platinum foil for some time, for if one side remains turned toward the
platinum throughout the process, it will sometimes assume the red tint of
copper, while the rest of the bronze retains a somewhat dark colour.
When finally removed from the reducing bath, after the black metallic
powder has been thoroughly cleaned off with water and a soft brush, the
object should be suspended for a short time in water at the ordinary
temperature, or so fixed that there is a good depth of water beneath it;
it should then be washed in hot water. When the bronze is first placed in
water, whether hot or lukewarm, small bubbles of hydrogen will continue to
rise for some time, while at the same time a whitish, or sometimes grey,
gelatinous precipitate, consisting of a hydrated oxide of tin[144], will
often fall from it. The grey colour is caused by the admixture of small
particles of lead or copper.
At first I renew the water two or three times a day, then once in
twenty-four hours, and finally at longer intervals, using distilled
water throughout for small objects, but for larger specimens for the
final washings only. For the earlier washings at any rate I use warm
water. Cyanides as well as chlorides give a white precipitate with silver
nitrate; this reagent will therefore serve to indicate the progress of the
operation. If at the end of a fortnight in the case of small bronzes, or
in three to six weeks for large objects, the water shows no cloudiness, or
if upon the addition of yellow potassium chromate it instantly assumes a
red colour (p. 62), the steeping may be considered complete. Some Egyptian
bronzes, especially those which contain a large proportion of lead,
after steeping exhibit a whitish crystalline coating of lead carbonate or
small hemispherical groups of crystals scattered over the surface of the
metal, especially where the pores are large; when dry these can easily be
removed.
An extended experience points to the conclusion that bronzes should be
dried at once, and as quickly as possible. They should be wiped with soft
cloths and then dried in a drying chamber or upon glass or metal rings
on a stove. A simple form of drying chamber can be made with copper or
iron plate of sufficient thickness, with a loose lid provided with a
hole fitted with a cork, through which a thermometer passes. This can be
heated over a Bunsen burner, but the temperature should not exceed 230°F.
[110°C.]. Small objects may be freed from water by immersion in alcohol
for twenty-four hours before drying.
The completion of the process may be gauged by the yellowish or reddish
yellow colour which the bronzes should assume when they have been dried
and wiped with a cloth or brushed; brushes made of the finest steel
wire may be used for this purpose. A bright colour is but rarely seen on
bronzes which contain lead. Egyptian bronzes frequently contain as much as
20% of lead, and such bronzes have nearly always a dull-grey or blackish
appearance. A similar colour is seen on bronzes which contain no lead,
but which are very porous, and are in an advanced state of decomposition.
In such cases the finely divided particles of reduced metal are retained
upon the rough surface of the bronze, and as all metals, when sufficiently
finely divided, form a blackish powder without any metallic lustre, the
whole object then appears almost black. It is difficult, and in many
cases impossible, to remove this dust, especially that retained in the
pores. Metal dust is injurious to the lungs, and if recourse is had to
brushing, an efficient extractor for the removal of the dust-filled air is
required[145]; but brushing and the use of bellows in addition frequently
prove insufficient. Washing the objects with benzine is more effectual,
but a trustworthy method of giving the bronze a better appearance is to
place it into melted paraffin wax[146] at 250°to 285°F. [120° to 140°C.].
Yet the use of paraffin wax should be avoided if possible, for in spite
of the most careful washing blue efflorescences may sometimes appear
upon thick bronzes in the course of a year. If this should happen they
must be washed out at once, and the bronze can again be submitted to
the cyanide-reduction process. If however paraffin wax had been applied
an attempt would have to be made to remove it by immersing the bronze
in benzine or a mixture of ether and alcohol, or by heating, before the
reduction process could be repeated.
[Illustration: Fig. 39.
Bronze bull showing warty patina.]
[Illustration: Fig. 40.
The same after reduction by Finkener's method[147].]
There is no doubt that these bright-blue efflorescences are the
result of an incomplete reduction, which in many cases can scarcely be
remedied, for it is often impossible thoroughly to wash objects of great
thickness. Thin bronzes, bronze plate, and copper plate remain free from
efflorescences. Moreover, many bronzes, especially Egyptian ones, have a
hard, non-metallic core, which in the casting has been partly fused or at
least hard-burnt, and resists the effects of the washing.
[Illustration: Fig. 41.
Bronze axe-blade before treatment by Finkener's method (Aeg. 13203).]
[Illustration: Fig. 42.
The same side after treatment.]
[Illustration: Fig. 43.
Reverse side of axe-blade after treatment.]
It is occasionally found that a bronze cannot stand the process of
reduction, either because there is only a thin layer of metal over a stout
core, or because the metal is permeated with cuprous oxide, which when
tested with a file has a metallic appearance. The bronze must therefore
be continually watched whilst it is in the cyanide bath, and if necessary
should be taken out even before the reduction is complete. This should
be done if large pieces or large quantities of a powdery precipitate
fall from the bronze, or if it is found that a needle readily pierces
the oxidized layer. A specimen of this kind must be taken from the bath,
carefully steeped, dried, and impregnated[148].
It is not to be expected that bronzes which are in an advanced state of
decomposition (e.g. Figs. 9-12) can be so transformed by reduction as to
appear as they did when they left the artist's hand. For, although the
decomposed oxidized layer is now reduced to metal, this no longer forms a
coherent mass, but a loose powder, which, being deprived of its essential
constituents, chlorine, oxygen and carbonic acid, no longer retains its
coherency, but falls to the bottom. Only in the interior and in the pores
is the reduced metal retained.
In addition to the preservation of articles by the removal of the
injurious chlorine compounds (as is also the case with Blell's and with
Krefting's method for iron antiquities), the process may result in the
discovery of inlaid work, inscriptions or ornamentation, the presence of
which was not suspected. The accompanying illustrations (Figs. 39 and
40) show bronzes before and after the preservation process, while the
axe-blade shown in Figs. 41-43 illustrates equally clearly the advantages
which accrue from the treatment. Not less striking is the result of the
treatment in the case of the dagger-sheath shown in Figs. 44 and 45 by
which the design was discovered.
Reference may here be made to a case described elsewhere[149], in which
reduction proved that what had been thought a single bronze object
consisted in reality of two pieces which did not belong to each other, but
were fitted together by means of a bottle cork of modern date! In another
instance a bronze was found upon reduction to be brazed with a hard solder
containing zinc, which was thus quite inconsistent with the age ascribed
to the object.
[Illustration: Fig. 44.
Fig. 45.
Dagger sheath before and after treatment by Finkener's method.]
A short digression may be here made in order to discuss the question
whether the composition of the bronzes undergoes any alteration.
Three analyses[150] of Egyptian bronzes before and after reduction by
Finkener's method show that the change in composition is so slight as to
be immaterial. It is of course obvious that greater differences will be
seen in the results of the analyses before and after reduction of bronzes
which are in an advanced state of oxidation, for in this case chlorine,
oxygen, water, and carbonic acid constitute an appreciable proportion
of the total weight. But even in these cases the analysis made after the
reduction shows very slight variation from that of the original metal.
+-------------------------+---------------+----------------+
| Osiris | Osiris | Ibis |
|-------------------------+---------------+----------------|
| Before After | Before After | Before After |
| | Reduction | |
|Tin 2·16 2·27 | 4·30 4·21 | 8·66 8·46 |
|Copper 77·83 77·45 | 79·66 79·74 | 88·53 88·75 |
|Lead 19·23 19·86 | 15·51 15·58 | 1·69 1·95 |
|Iron 0·12 0·14 | 0·28 0·24 | 0·21 0·20 |
|Nickel &} | | |
|Cobalt } 0·29 0·24 | 0·20 0·17 | 0·30 0·29 |
|Arsenic 0·17 0·23 | 0·17 present| 0·32 present|
|Antimony trace trace | trace --- | 0·20 present|
--------------------------+---------------+----------------+
The two latter bronzes were tested qualitatively only for arsenic and
antimony, and when the three objects were washed the hydrated tin-oxide
described on p. 130 was only found in the case of the Ibis. In this
connection it should not be forgotten that slight differences in the
quantities may be due to errors in the analysis as well as to a want of
homogeneity in the alloy.
KREFTING'S METHOD. This method is similar to that used for the reduction
of iron (see page 108). The layer of oxidized material is removed in
several places by filing, hammering, or rubbing with emery cloth until
the metal is exposed. The object is then wrapped round with strips of
zinc, and placed in a 5% solution of caustic soda. The hydrochloric acid
produced in the process of reduction acts upon the soda to form sodium
chloride. Here too the greatest care must be taken that the steeping is
sufficient.
Personally I prefer Finkener's method, for potassium cyanide is more
easily washed out than soda, and also, although poisonous, is less
caustic.
Krefting's method however has proved of considerable success in some
cases, notably in the treatment of some 40-50,000 Roman copper coins
at the Berlin Museum. These were, with few exceptions, covered with a
crystalline layer resembling green malachite or blue azurite and were
quite illegible. Various unsatisfactory attempts were made to clean them
with ammonia, with warm and cold acids of different kinds, with acid
and iron nails, and by electric current both in an acid solution and
in a solution of potassium cyanide. The following method finally proved
satisfactory[151]:
Krefting's Method Applied to Oxidized Copper Coins.
"A thin plate of zinc with a bright metallic surface is perforated
with a brad-awl, having a diameter of from 2 to 5 mm., until
there are about 50 or 60 holes in each square metre. This is
placed with the sharp edges of the holes uppermost on a row of
glass rings (or crystallizing dishes will serve the purpose) 20
mm. in height resting upon the bottom of a large glass vessel.
The coins, which in this case were 20 mm. in diameter, were then
placed on the zinc plate, so that 7 or 8 of them occupy a space
of 1 square decimetre. Another similarly perforated plate is laid
upon them, and upon this more coins are arranged in the same way,
and so on until there are six or eight double layers. A perforated
zinc plate is then placed on the top with the sharp edges of the
holes turned downwards, and over this a few zinc plates which have
been previously used. The whole pile is surmounted with weights
or stones resting upon glass rings or inverted glass dishes in
order to press the sharp edges of the holes into the closest
possible contact with the coins. A 5% solution of caustic soda is
then poured over the whole, the immediate result of which is an
evolution of gas. The reduction of the coins is usually complete
in fifteen to eighteen hours, after which they should be well
washed. After several rinsings in cold water they are placed,
about 1000 at a time, in a large vessel fitted with a perforated
false bottom containing hot water, which should be renewed three
or four times every day. After four days the coins are wiped
with a cloth and thoroughly dried on a warm oven plate or in a
drying chamber at a temperature of about 212°F. [100°C.]. They
are then brushed with a bristle brush before a dust extractor, a
procedure rendered necessary by the fine metallic dust from the
coins, which then assume a light or dark brown colour such as is
seen on copper coins which are in actual circulation. The practice
of placing the coins whilst still wet into melted paraffin wax
at 260°F. [120°-130°C.], which gives a dark appearance even to
the brightest, has the disadvantage that the wax prevents the
use of sealing-wax for taking impressions, and is therefore not
recommended.
The reaction is analogous to that which occurs in the reduction
of iron. The copper of the coin forms in the alkaline solution
an electric couple with the zinc, and the hydrogen which forms at
the copper end reduces the copper compounds covering the coins to
metallic copper, and thereby loosens them, while the zinc oxide
which is simultaneously formed is dissolved in the soda solution.
In actual practice a part only of the zinc oxide is dissolved,
while the remainder forms a white coating on the zinc[152].
Experience shows that a 4-5% solution is the most suitable for
this method of reduction, which gives the most favourable results
when these details are followed. If for example the zinc plate is
laid immediately on the bottom of the glass trough, if the coins
are laid too close together on the plate, or if there are more
than 6 to 8 double layers in a trough, the process of reduction
is often incomplete, and it is then necessary to treat the coins
a second time. It is scarcely necessary to mention that larger
coins must be placed at proportionately greater distances from
each other.
The 40-50,000 coins which were thus treated had originally been
tinned, but the tin only remained at a few places. When the coins
were washed immediately after the reduction, this tin could still
be clearly distinguished, but on further washing, drying, and
brushing, it ceased to be visible on account of the dark colour
imparted to it by the finely powdered copper. In one or two cases
lead appeared on the surface of the coin, but was easily removed
by mechanical means."
[Illustration: Fig. 46.
Roman coins before treatment.]
[Illustration: Fig. 47.
Roman coins after treatment by Krefting's method.]
Cleaning Copper Coins by Melted Lead.
Although the results obtained by this method are less satisfactory than
those produced by the preceding, it has the advantage of simplicity[153].
"Using a pair of tongs, dip the coins one by one into melted
lead until the crackling, which begins at once, has ceased, which
occurs in from 3 to 10 seconds. The hand should be protected with
a glove from the spluttering molten lead. The coin is then thrown
into cold water, cleaned, and placed until the next day in hot
milk. It may be necessary to repeat the process when the coin has
become cold. By this method an olive colour is imparted to the
coin which many antiquaries prefer to dark brown, but personally
I prefer Krefting's method because it renders the inscription
and designs far more distinct. A coin which after the treatment
with melted lead has remained so covered with cupric oxide as to
be still illegible can seldom be improved by a repetition of the
treatment, whereas had the zinc treatment been applied in the
first instance the result would probably have been satisfactory.
This conclusion seems to be justified by the extremely small
percentage of coins which, in my experience, have remained
illegible after the treatment by electrical methods."
(C.) Preservation of Bronzes by the Exclusion of Air.
In those cases in which the advanced state of decomposition renders the
reduction process either inapplicable or at any rate inadvisable, or in
which the decay is not likely to be arrested by impregnation, a further
method of preservation remains, viz. the complete exclusion of air and
moisture.
If air is completely freed from moisture the oxygen can no longer act in
conjunction with the copper chloride upon the still intact metal (see page
29 _et seq._), and the condition of the bronze will consequently remain
unchanged.
A bronze, for example, which shows much decay should be placed after
impregnation under a hermetically sealed bell glass, and beneath or near
it should be placed some dehydrating agent, of which anhydrous calcium
chloride is the most suitable (see note, p. 123). To exclude the air
completely the bell glass should have a projecting ground edge, which
should be smeared with vaseline or grease and pressed firmly upon a
thick well polished glass plate. The dehydrating agent may be placed in a
glass vessel or dish in such a way as to be unseen, or it may be covered
with two or three thicknesses of dark gauze or with black cardboard
laid loosely over it. If an object is too large for a bell glass, or if
several objects are to be exhibited together, a square plate-glass case
with iron framework, made air-tight with putty, may be used as shown in
the illustration (Fig. 48). The lower part, containing calcium chloride,
is partitioned off by a perforated plate covered with black gauze[154]. A
hygrometer was placed behind the head, the indicator of which has remained
at zero since it was first fixed several years ago, and the bronze has
not hitherto shown any sign of change, although the inlaid gold is in
parts raised from the metal by a light-green oxychloride. The cost of
these cases is considerable, but for valuable objects this should not be
considered. In the place of calcium chloride, sticks or lumps of caustic
soda may be used with advantage, for this substance absorbs both moisture
and carbonic acid.
[Illustration: Fig. 48.
Method of mounting objects in air-tight cases.]
This method of preservation is of course applicable not only to decomposed
bronzes but to all valuable antiquities, whatever the material may be.
Appendix.
Methods of Bringing out Worn Lettering upon Coins.
These methods are founded upon the fact that the sunken areas of the coin
are, by the pressure of the die in stamping, rendered denser than the
raised portions, such as the inscription. The earliest method is that
published by Brewster, reported by Süpke[155]. The coins when cleaned
are placed upon red-hot iron, which causes the oxidation of the entire
surface of the coin. The thin film of oxides varies in colour according
to the duration and the intensity of the heat. The oxidation of the
letters of the inscription differs from that of the surrounding parts,
and is recognisable by a difference in colour. Drude[156], treating more
especially of silver coins, remarks that the inscription is rendered
legible by heating them to redness over a Bunsen-burner. It then,
according to "Prometheus[157]," when viewed in a dark room, appears dark
on a bright ground, especially if the coin has been previously polished
and then roughened again by slightly etching it with acid. In conclusion,
the method of Roux[158] may be quoted:
"The smooth-worn and polished coin is placed in a solution of
copper sulphate or of some other metallic salt, and suspended
between the electrodes of one or more cells of a battery (any
other form of continuous current will serve the purpose). If the
current is weak, the electrodes must be near to the coin. The
stronger the current the more rapidly the impression appears. On
the side which faces the anode or positive plate the impression
is metallic; on the other side, after gently wiping off the less
firmly adherent part of the oxide, the impression appears in grey
lines. These markings can be fixed by varnishing them with a thin
alcoholic solution of shellac. To render the impression legible
on both sides, the coin should be placed upon the four upturned
feet of an insulating stand. The larger the coin the deeper must
be the layer of solution above and below the coin. The depth below
should be equal to the radius of the coin.
This can perhaps be most conveniently carried out by placing that
electrode in immediate contact with the coin which upon immersion
in the solution becomes tarnished with the metal, i.e. the cathode
or negative pole. Other portions which it is not intended to treat
should be first covered with varnish.
The striking success of this method is due to the fact that that
portion of the metal which has been compressed by the stamp is
a better electrical conductor than the rest; no success could
therefore be expected from the use of this process for the
restoration of such objects as worn engraved copper-plates, etc."
(_j_) Silver.
Preservative treatment of silver is scarcely necessary (cp. pp. 49-52),
except in those cases in which the silver is alloyed with a large
percentage of copper, and which show efflorescences similar to those which
appear upon bronzes containing chlorine. Electrolytic reduction will be
found to be the most suitable method of treatment in such cases. To treat
silver coins they should be placed in contact with iron nails in lemon
juice. Instead of the citric acid, which is the active principle in this
process, other diluted acids and other metals, e.g. zinc, may be employed.
Flinders Petrie[159] has shown that the reduction can also be effected
by a weak solution of common salt. Silver chloride is soluble in ammonia,
and thin layers may be removed by the application of ammonia by means of
a soft brush. Thorough rinsing with pure water, drying with soft cloths,
and cautious warming are always essential.
An excellent reducing agent for single coins, the characters of which are
rendered illegible by a layer of silver chloride, is molten potassium
cyanide, or a mixture of this substance with sodium or potassium
carbonate. In a short time the silver chloride is decomposed and removed
from the smooth surface of the coin. After boiling out with water,
steeping in alcohol, drying, and brushing with a soft brush, the coins
may be coated with zapon. Coins treated in this way appear to be less
brittle than those reduced by Krefting's method. More troublesome but
less dangerous, because potassium cyanide is not used, is the treatment of
silver coins with a fused mixture of potassium and sodium carbonates. In
this case the silver chloride is converted into silver carbonate, which
is then decomposed with 50% acetic acid. Further treatment by washing,
drying, and impregnation is carried out as previously described.
Silver which has become friable (p. 51) can be rendered more compact by
cautiously heating it to redness. It will however be advisable to entrust
heating and mechanical treatment of objects which are much bent to some
skilled silversmith, whose experience may prevent disaster. Silver objects
which are largely converted into friable chloride, especially if they
are much expanded, or if large portions have broken away in the process
of removing the chloride, will hardly bear any other treatment than that
of impregnation with gum-dammar solution or with paraffin wax. As silver
chloride is easily fused such articles should not be subjected to heat.
Earthy matter can often be removed with a neutral soap and warm water,
while calcareous accretions can be dissolved by a 2% solution of
hydrochloric acid. Silver which has been blackened by silver sulphide may
be laid in a warm 2% solution of potassium cyanide. All objects should be
subsequently well washed with warm water.
(_k_) Lead and Tin.
Objects of pure lead and pure tin are rare. If much oxidized they should
be washed with warm water, dried, and impregnated with a gum-dammar
solution or with paraffin wax (pp. 70 and 91). If in a good state of
preservation they may be freed from any earthy or calcareous coating
or from lead carbonate by the cautious use of very dilute nitric acid
followed by steeping in water.
Ceresole[160] cleans oxidized leaden seals with 10% acetic acid,
neutralises the acid with ammonia, and after five minutes in alcohol
coats them thinly with wax. The seals are preserved between glass dishes
(Petri dishes), the space between the dishes being filled with cement. I
employ Krefting's method for leaden medals, using either zinc and very
dilute sulphuric acid, or zinc dust and caustic soda. Occasionally the
zinc dust becomes firmly cemented by oxide to the surface of the lead,
and, if this is the case, great care must be used in removing it. The
washing process also requires care. A very efficacious method is to allow
a stream of warm distilled water, from which the dissolved air has been
driven off by boiling, to flow over the object in a porcelain dish. I now
omit any impregnation with paraffin wax, and instead recommend removal
of the water by alcohol, drying, and coating with zapon. To preserve the
specimens after treatment, more especially from the injurious action of
perspiration from the hands, they are placed between dishes of glass or
of celluloid[161].
(_l_) Gold.
Objects of pure gold usually need only be cleaned with soap and water and
a soft brush; lime may be removed by the application of a 2% solution of
hydrochloric acid. A coating of silver chloride occurring on gold which
contains a large percentage of silver may be removed by ammonia, or,
in certain cases, by the alternate use of dilute hydrochloric acid and
ammonia.
A layer of red ferric oxide (see p. 53) is of frequent occurrence upon
gold objects, and may be removed by warming the object in a stronger
solution of hydrochloric acid, but soft brushes will often serve the same
purpose. Pure gold being very soft, only the softest so-called "silver
brushes" should be used, and all pressure or bending should be avoided.
If friable the object should be carefully impregnated with a solution of
gum-dammar (p. 70).
(_m_) Glass and Enamel.
If covered with a film of dirt, or if when in a collection objects of
glass or enamel undergo any alteration, they should be washed or steeped
in lukewarm water. When dry they should be treated with pure olive oil
or poppy-seed oil, which may be diluted with benzine. The oil helps to
restore the lustre to the glass and to bring out the colour of the enamel.
When thus treated the objects should be carefully protected from dust.
A decomposition of ancient glass when deposited in a museum has been
hitherto only rarely observed, but allusion may be here made to the
so-called 'sweating' of glass which is a question of considerable
importance in Industrial-Art collections. In this case preservation is
insured by washing with distilled water, drying, and coating with zapon.
Further particulars may be obtained from the paper by Pazaurek[162].
II. Preservation of Organic Substances.
(_n_) Bones, Horns, Ivory.
Many curators dry carefully and impregnate them with a gum-dammar solution
or shellac; isinglass or glue are however preferable, for these aqueous
solutions may be used for the treatment of damp objects, which could
scarcely be dried without cracking. In order to permeate the object these
solutions must be very dilute, and are most advantageously applied at a
temperature of about 120°F. [50°C.]. The impregnation may also be effected
in rarefied air under a bell glass (p. 68). Friable bones and similar
objects which might fall to pieces in the solution during impregnation
should be bound with strips of gauze or with string before immersion; they
are easily removed when cold. To prevent the formation of mould a small
quantity of dissolved corrosive sublimate[163] is added to the glue, or
when dry after impregnation the objects may be covered with a solution
of shellac or resin. Impregnation is of very general application, and is
frequently used for the preservation of fossil and pleistocene bones.
(_o_) Leather.
At Copenhagen the method used to render leather soft and pliable is to
place it in train oil for an hour and then dry it with filter-paper.
Lanoline may also be used with success[164]. Poppy-seed oil in benzine
(p. 86) is said to produce good results, but the "Merkbuch" recommends
the preservation of leather in this condition in alcohol[165].
(_p_) Textile Fabrics, Hair.
Earth and soil may be removed by mechanical means, and, occasionally,
careful washing may be possible. The objects should be dried and
impregnated with a gum-dammar solution (p. 70), poppy-seed oil (p.
86), or a solution of india-rubber (p. 90), or they may be preserved
in alcohol (p. 159). Some textile fabrics in the Copenhagen Museum owe
their excellent state of preservation to Steffensen's treatment, i.e.
impregnation with a solution of india-rubber in turpentine with the
addition of bees'-wax.
The following account of the treatment of textile fabrics from the
Lake-Dwellings is due to Herr Heierli, of Zürich:
"The pieces as they were taken up were laid on the ground and thus
slowly allowed to dry in the air. They were then placed between
glass plates, the edges of which were pasted over with paper. Old
pieces which had been dry for a long time, and which had become
tender and friable, were laid on the ground and watered from time
to time until they were soaked through; they were then treated in
the manner already described."
Egyptian textile fabrics preserved between glass plates often deposit
a thin layer of salt on the glass, but this is easily wiped off (see
p. 155). It must first be ascertained by a previous trial in each case
whether the salt can be removed by steeping in water or in alcohol and
water.
Hair found in peat has always a dark-brown colour from impregnation with
peaty matter. The method proposed by Bille Gram[166] for restoring the
natural colour consists of repeated and alternate treatment with very
dilute alkali solution and acid at about 120°F. [50°C.]. When the liquid
ceases to show coloration the natural colour of the hair is restored.
(_q_) Feathers.
These do not require any treatment beyond protection against insects,
which is attained by immersion in an alcoholic solution of corrosive
sublimate, or by spraying with corrosive sublimate in either alcoholic or
aqueous solution. Of course the poisonous nature of corrosive sublimate
necessitates caution in its use and it should be always labelled as such.
The use of naphthalene is not always successful, and white scales of
naphthalene are apt to make their appearance; nor does finely powdered
pepper sprinkled on the feathers, either alone or mixed with finely
powdered alum, give satisfactory results.
(_r_) Papyrus.
The method of cleaning and preserving papyrus in use in the Egyptian
department of the Royal Museums at Berlin is as follows: Those pieces
which are folded together or rolled are carefully straightened, and, if
very friable, they are first placed between damp filter paper to render
them uniformly pliable. Dust and dirt are removed with soft paint-brushes,
crystals of salt which are often found[167] are picked off with forceps.
Any growths of fungus are carefully scraped off with a knife. The papyrus
thus prepared is then placed between two thick polished glass plates,
the two opposing surfaces of which are covered with a very thin layer
of vaseline. Air is frequently admitted to dry the papyrus, while the
pressure of the glass plates tends to smooth it out, and after it has been
so treated it is mounted between thin glass plates, the edges of which
are pasted over with paper covered with an oil paint.
A papyrus preserved between glass plates often shows round the edges
a whitish border about two millimetres in breadth, and on separation
the glass plates show a slight film of the same white material on the
surface which had been in contact with the papyrus. The formation of
this film, which consists chiefly of common salt and is easily wiped off,
may be prevented by previously washing the papyrus in distilled water, a
proceeding which experience has shown to be harmless. As the papyrus will
swim on the surface it should first be immersed in alcohol until soaked
through; the process of steeping is then quite simple. The thinness of
papyrus enables the steeping to be completed after 24 to 48 hours by two
changes of the water, and care must be taken lest a too prolonged steeping
should obliterate the lettering. The water assumes a yellowish or brown
tint and the papyrus becomes somewhat lighter in colour on drying. Papyrus
may also be preserved by zapon (see Appendix), but this method has no
advantage over that of mounting between glass plates.
(_s_) Wood.
To preserve adequately articles of moist wood (and they are generally in
this condition when first excavated), preliminary measures to prevent
their drying in the air must be taken immediately after they are dug
out of the earth. If found in water, as for instance articles from
pile-dwellings, they should be conveyed in water; moist objects should
be wrapped in several thicknesses of moist cloth, and the whole wrapped
in gutta-percha membrane, or in a layer of moist moss. The cracks which
arise in wooden objects which have become dried may frequently be closed
up by laying them in lukewarm water.
As the earliest attempts at preservation were probably made upon wooden
objects there is scarcely a collection in which a number of methods are
not employed. One exception only is known to me, and here, after a plaster
of Paris cast has been taken, the object is simply allowed to shrink.
The methods proposed and carried out are so different and so numerous,
especially as regards the liquid used for impregnation, and in such
variety, that it is only necessary to deal with the most important. These
may be divided into two classes, viz. dry and wet.
(1) Dry Preservation of Wood.
Moist or wet objects are placed in thin size or in a solution of isinglass
till they are impregnated, after which they are dried gradually in a
shady place. A solution of shellac, or varnish diluted with petroleum or
benzine, is then put on with a brush.
Sometimes the objects are placed directly into a mixture of varnish and
petroleum, or they are impregnated with melted paraffin. The former is
preferable as a means of impregnation if there are cracks or holes, for
the superfluous solution readily drips from the wood when it is taken out,
while paraffin sets too soon to drain out of the cracks, and thus imparts
an unnatural white appearance to the wood. Owing to the large size of the
vessels which would be otherwise required, paraffin is only useful for
small or medium-sized objects, but when making use of varnish one end of
a large object[168] may be placed in the mixture while the solution is
repeatedly poured over the object. After two or three days the opposite
end should be placed in the solution. By repeating this process every part
of the object will soon be thoroughly impregnated.
Objects of still greater size, such as a Viking's ship, can only be
preserved by painting the surface. In such cases it is advisable to begin
with dilute varnish so as to allow the impregnating solution to penetrate
as deeply as possible into the material, instead of merely forming a skin.
A solution of waterglass has in one instance been used for the
preservation of a large boat, but the result is not satisfactory.
LEINER'S METHOD[169]. The wooden articles are laid in glycerine mixed
with a small percentage of carbolic acid. The length of time during which
they remain in the glycerine depends upon their size. When taken out they
are lightly wiped and preserved without further treatment. If a growth of
mould should occur it may be washed off.
Objects thus treated retain their moist condition and should therefore be
very carefully protected from dust.
SPEERSCHNEIDER'S METHOD[170] (cp. p. 91). Small specimens are heated for
two hours in a mixture of
8 parts of rape-seed oil,
1 part of bees'-wax,
1 part of pine resin, and
2 parts of benzene.
Larger objects require a proportionately longer heating, but the mixture
must not be allowed to actually boil. The moisture rises as steam and
causes the solution to bubble. The bubbling however continues after the
moisture has been driven off; great care must therefore be taken that
the heating is not so prolonged as to cause the object to shrink. The
highly inflammable nature of the mixture renders great caution necessary,
and should it ignite, a lid, which should always be in readiness, should
be put on the vessel. After impregnation the objects are wrapped in
blotting-paper and laid in ashes for four days to prevent the access of
air. The aim is doubtless to insure thorough absorption of the superfluous
liquid which remains upon the object, which exposure to air would prevent
by causing the mixture to set too rapidly. The same mixture can be used
repeatedly, but each time two-thirds of the original quantity of benzene
must be added.
HERBST'S METHOD[171]. The moist objects are boiled in a saturated solution
of alum for two hours (hot water dissolves about 3-1/2 times its weight of
alum), but if they are of some thickness the time must be proportionately
longer. They are then taken out, and when the alum in crystallizing has
made them more or less firm, the crystals adhering to the surface are
washed off with warm water.
When thoroughly dry the wood is brushed over with hot linseed oil, which
operation is repeated until no more oil is absorbed. A final thin coating
of varnish or shellac is then given. According to Steffensen, the method
followed at Copenhagen is to lay the objects in warm thin size for a
quarter of an hour after impregnation with alum. This alum-method is there
used for objects of oak, although the "Merkbuch" (p. 60) states that only
the varnish-petroleum mixture should be used for impregnating this class
of object.
(2) Preservation of Wooden Objects in Liquids.
The expense entailed by this method renders it applicable only to articles
of small size.
The preservation of small objects in a flat vessel, the bottom of which
is covered with glycerine, has the disadvantage that glycerine extracts
organic substances and thus assumes a brown colour. If glycerine is
used the object should undergo a thorough preliminary steeping, and
the glycerine should be renewed until it remains colourless. Closed
cylinders filled with glycerine or a mixture of glycerine and water are
not convenient because wood nearly always floats in the liquid. This may
be remedied however by the addition of alcohol.
JENNER'S METHOD[172]. When the objects have been thoroughly cleaned with
water, pure alcohol, diluted with water until the specific gravity at
54°F. [12·5°C.] reaches 0·96, is poured over them. After six or eight
weeks the alcohol is poured off and replaced by fresh alcohol of the same
specific gravity. This alcohol is examined in a year's time, and should
always show a specific gravity of 0·96. The alcohol which has been poured
off may be filtered, and if necessary decolourized by animal charcoal;
when the specific gravity has been again raised to 0·96, by the addition
of fresh alcohol, it may be used again.
The same process is applicable to textile fabrics, yarn, and leather.
Protection against Wood-worms, etc.
All the methods mentioned above will destroy insects and their larvae.
In cases in which it is either impossible or undesirable to use immersion
or external application, as for instance in the treatment of objects of
dry wood, the larvae may be destroyed by dropping petroleum, an aqueous
solution of potassium arsenite, or corrosive sublimate, into the various
small openings. This will also help to prevent further attacks.
If solutions are not applied insects may be destroyed by the vapour
of carbon bi-sulphide or of crude benzene. These liquids, which are
sufficiently volatile at the ordinary temperature, should be placed,
together with the objects to be treated, in a closed box.
I have used a similar method for the destruction of wood-worms in Egyptian
coffins. The coffin is placed in a large wooden box lined with tin plate.
The lid, also lined with tin, is provided with projecting edges, to which
strips of felt are glued. The weight of the lid by compressing the felt
is sufficient to render the box air-tight. Six or eight glass vessels
containing crude benzene are placed at the bottom of the chest and of the
coffin itself. It need scarcely be added that the box must not be opened
near a fire or light, as the vapour forms an explosive mixture with air;
it is in fact advisable to have no light or fire in the room.
Insects can also be killed by naphthalene vapour, but as naphthalene
is insufficiently volatile at ordinary temperatures, the method above
described is more convenient[173].
Preservation and Cleaning of Coloured Wooden Objects.
For objects of this kind materials should not be used which, like varnish,
tend to darken and so to damage the colours. Gum-dammar solution (page
70) answers the purpose, but colourless collodion is better. Colours which
are soluble in water (as is frequently the case with wooden objects from
Egypt) cannot of course be cleaned with water, but benzine may be applied
by means of soft cloths or brushes. Resinous or pitch-like substances may
often be removed from coloured objects by turpentine mixed with benzine
or ether.
A method of cleaning gilded or brightly coloured ecclesiastical figures
which is used in the Breslau Museum is the application of a mixture of
copaiba balsam and ammonia. This method is similar to that used to clean
paintings[174], the action of the solution being that of a mild soap.
Antiquities which were originally uncoloured, but which have been
subsequently painted, may be cleared of paint by means of a solution of
caustic soda in water or alcohol.
(_t_) Amber.
After the mechanical removal of any adherent earth and dust, the specimen
should be rubbed carefully backwards and forwards between the fingers
covered with a soft woollen glove. Particles of soil should be picked out
of any holes and indentations by using a strong horse hair[175]. It is
then preserved by impregnation with a solution of shellac, poppy-seed oil,
or isinglass (pp. 70 and 86).
The following particulars of the method used in Messrs Stantien and
Becker's collection of amber have been supplied by Prof. Klebs:
"Amber is preserved best in distilled water: I add a very small
quantity of glycerine and a still smaller amount of alcohol. A
proportion of alcohol greater than 1% is injurious to the amber.
A thick layer of gelatine containing glycerine is an excellent
medium for the preservation of large objects if they are kept free
from dust. This layer should be washed off and renewed every few
years."
The Care of Antiquities after Preservative Treatment.
In addition to the protection from dust afforded by closed glass cases, it
is also important to protect objects from the action of direct sunlight,
especially during the summer months. There is, for instance, no doubt
that the decay of bronzes, even of those with a patina which is apparently
sound, is hastened by the great variations of temperature, caused by the
rays of the sun falling directly upon them. Similarly objects which have
been preserved by the application of solutions of resin or varnish should
be protected from the direct access of sunlight, for the sudden warming
may easily cause cracks. Nor should antiquities be kept near the heating
apparatus. There is another precaution, to which too little attention
has been paid, viz. the protection of objects as far as possible from
even diffused daylight. Although no investigations upon the extent of
the injurious action of light have as yet been published, light is not
without influence upon the outward appearance, and therefore also upon the
material condition of antiquities of organic origin. But even inorganic
objects, such as pigments, glass, enamel, amber, etc., are affected by
light; it is therefore certainly advisable to protect antiquities of all
kinds from light during the time in which they are not exhibited to the
public.
The public is effectually prevented from fingering antiquities which are
enclosed in glass cases, but it may be well to remind those who have to
handle them in the course of their duties that contact with the bare hand
can only be harmful, even though fingering is understood to be beneficial
to modern bronzes by inducing the formation of patina. The bright surface
of metallic iron which results from treatment by Blell's or by Krefting's
method, especially if there is a thin coating of paraffin, should not
be touched at all with the bare hand, but only with a cloth or a glove.
Bronzes, whether intact or restored, and iron objects, should never be in
direct contact with those which show efflorescences.
The usual custom is to attach labels of painted cardboard or metal by
means of thin metal wire. The tendency to rust makes iron wire unsuitable,
especially for objects containing salt, which are quickly affected; thus
light coloured earthenware may soon be covered with spots of rust. Copper
wire and nickel wire are liable to be similarly attacked. Many years
ago it was noticed[176] in the Ethnological Museum at Berlin that nickel
wire when in contact with silver objects which were covered with silver
chloride was destroyed by the formation of a deliquescent green nickel
salt. Silver or platinum wire forms the most suitable means of attachment,
but if the expense of these is too great, copper or nickel wire may be
used, except in the cases mentioned above.
Small objects of any kind, which one still frequently finds kept in
open cases, are better preserved in upright glass cylinders with glass
stoppers, or in cheaper glass tubes, one end of which is fused and the
other closed with a cork.
Conclusion.
The methods of preservation which have been described in the preceding
pages may be thus tabulated and summarised:
Methods. Application.
Steeping in water, drying Limestone,
and impregnation. Earthenware,
Iron, much corroded.
Direct impregnation. (1) Unbaked earthenware, etc.,
(2) Bronze objects with little or
no metallic core, or showing a
cracked or warty surface,
(3) Objects of wood and of other
organic substances.
Removal of compounds of oxygen
or chlorine
(_a_) by chemical process, Iron objects in a good metallic
condition,
(_b_) by electrolytic process. (1) Iron objects with a sound
metallic core,
(2) Bronze objects with a sound
metallic core.
Mounted thoroughly dry and Valuable bronzes in an advanced
hermetically sealed. state of decomposition.
There will be no difficulty in the choice of methods for limestone or
earthenware, whether kiln-dried or sun-dried, for a simple experiment will
prove whether steeping is likely to cause injury or disintegration.
The methods are themselves simple and inexpensive. For organic substances
the chief question is the choice of the most suitable medium for
impregnation.
Iron and bronze present some difficulty, although the use of a file will
readily show whether reduction is feasible.
The simplicity of the apparatus required for Krefting's method gives it an
advantage over other methods, at any rate for iron objects. Objection has
been taken to the methods of reduction, because they give to the objects
thus treated an appearance to which the public are not accustomed. It may
be safely asserted however that this appearance more truly represents the
object when in actual use, than the oxidized and rust-covered specimens
to which we are accustomed in antiquarian collections. To those who value
an antique object for the crust that covers it, all methods of restoration
must be objectionable. Such persons ought to object to the removal of the
incrustations which hide the cuneiform inscriptions on clay tablets. On
the other hand, those who regard these methods with approval should go a
step further and confide their collections to experienced hands for some
form of treatment which may bring to light inscriptions and inlaid work
which will greatly enhance their value.
To spread the knowledge of these methods and to invite the co-operation of
others is the aim of this book. As to the best method to be used in each
particular case it is unnecessary to lay down any hard and fast rule, for
this can only be learned by observation and experience.
APPENDIX A.
METHOD OF TAKING SQUEEZES OF INSCRIPTIONS, ETC.
For this purpose a proper brush is required with strong bristles,
closely set as in a scrubbing brush; the brush should have a firmly fixed
handle, preferably slightly curving upwards to save the knuckles from
being bruised upon the stone. A so-called "silver brush" will serve the
purpose. The paper should be stout and stiff enough to resist the blows
of the brush without tearing. An admirable paper, which possesses these
qualities, is specially prepared for the purpose by the O.W. Company,
100, Great Russell Street, London, W. As a substitute for the specially
prepared paper stout packing paper may be used with satisfactory results.
The stone should be tilted if possible at an angle of about 45°, and the
surface bearing the inscription should be well washed or carefully scraped
free of dirt and foreign matter and should be rendered thoroughly wet. A
piece of the special paper of suitable size should be soaked in water for
a minute or more. It should then be carefully applied to the surface of
the stone in such a way as to prevent air-bubbles. This may be assisted
by gently smoothing it with the hand or back of the brush. When close
adhesion has been secured, and all air-bubbles removed (this can sometimes
be done by pricking through the paper with a pin), the paper should be
sharply beaten with the brush, the blows being delivered from the wrist
and not from the shoulder until it begins to show a fluffy appearance. It
should then be peeled off and allowed to dry, after which it may be rolled
or folded without danger of injury to the embossed inscription.
Should the paper tear, another piece soaked as before may be placed on
the top and beaten until it becomes incorporated with the first. If the
letters are large and deep, or if the surface is much cracked, two or more
sheets superimposed should be used. In the case of large inscriptions it
is advisable to take impressions by sections, care being taken that each
sheet slightly overlaps the preceding one to prevent the possible omission
of some of the letters.
It is also useful to take at the same time a pen or pencil copy of the
inscription, for a comparison of the copy and the squeeze will often
prevent errors in deciphering. The squeezes can be very well deciphered
by artificial light, while doubtful letters may sometimes become clear on
holding up the sheet to the light. The reverse side of the squeeze, upon
which the inscription stands in relief, may afford great assistance when
read by the aid of a mirror. A photograph of the squeeze will often reveal
more than a photograph of the inscription itself.
The method is described by S. Reinach in his "Traité d'Épigraphie Grecque"
(Introduction, p. XX.), where he also refers to Hübner, "Ueber mechanische
Copien von Inschriften," 1871.
APPENDIX B.
ZAPON.
Further particulars may be given of the new preparation known as Zapon.
This substance is now made on a large scale, and can be obtained from the
British Xylonite Co., Brantham Works, Manningtree (Xylonite lacquer F.
6631). The following excerpt is from a short communication in "Prometheus"
(XV. 1904, pp. 485 and 499), which deals with the preservation of wax
seals and of glass.
Zapon, the invention of Crane, of Shorthills, U.S.A., has been used
for 20 years past for the protection of metals from oxidation and the
action of sulphuretted hydrogen. Although the products of the various
manufacturing firms differ in composition, zapon is essentially a solution
of nitro-cellulose in various solvents. The nitrated cellulose, i.e.
gun-cotton (pyroxyline), is generally, with the addition of camphor,
dissolved in a mixture of amyl acetate (hence the peardrop-like smell)
to which distillation products of petroleum, etc., are added. It comes
into the market as a faintly yellow, slightly oily liquid. Its use as a
preservative depends upon the fact that the evaporation of the solvent
leaves behind it a fine transparent coating of gun-cotton (pyroxyline).
Zapon for preservative purposes must have a neutral reaction, and must not
under any circumstances redden litmus paper. Its use in this connection is
due to Schill, who also recognised its suitability for other materials,
as, for example, for plaster casts, the treatment of which is eminently
simple, for it consists in dipping small casts, or in painting larger
ones with a soft brush. It is advisable to begin at the top and apply
it from above downwards, using a clean dry cloth to wipe off any excess
of the fluid which collects in the deeper parts of the cast. If zapon
containing about 4% of gun-cotton is used, the coating left on drying
is scarcely visible; with a 5% solution a certain degree of polish
results. Casts treated with zapon are less easily damaged by dust than
those untreated, and may be cleaned with soap and water without injury
to their surface, provided that a soft brush is used, but brushes which
are stiff enough to injure the zapon coating will damage the contours of
the statue. It should only be used for objects kept under cover, for rain
and wide variations of temperature will attack them almost as readily
as untreated casts. It can be used with equal success for antiquities
of stone, clay, baked or unbaked, or for plaster after the soluble salts
have been thoroughly removed by steeping, for if this has not been done
the salts will soon crystallize out and loosen the protective coating.
For objects which are free from salts impregnation with zapon possesses
the advantage that it renders them less liable to damage from handling
or dust, whilst the appearance is scarcely altered, if at all. This
applies also to antiquities of metal, for unless the injurious chlorine
compounds are removed by simple steeping, or reduction and subsequent
steeping, treatment with zapon is useless. To bronzes, which in spite
of mechanical cleaning show a somewhat unpleasant grey non-metallic
appearance, zapon often imparts a distinct metallic lustre. To enhance
this lustre by a second vigorous application is not recommended, for this
gives the impression of a varnish. To protect articles of silver from the
blackening influence of sulphuretted hydrogen, zapon is very useful, but
does not afford absolute protection unless it has been thickly applied. In
collections of armour much use may be found for this material. The objects
are dipped and then placed in a drying oven at 105°F. [40°C.] to secure
rapid drying and uniform distribution. The amyl acetate or other solvent
is best conducted away, as it evaporates, into a flue or into the open,
although the vapours can hardly be considered dangerous to health.
The following references will afford some information on the use of zapon
in the preservation of Archives:
E. Schill, "Anleitung zur Erhaltung und Ausbesserung von Handschriften
durch Zapon-Imprägnierung," Dresden, 1899.
O. Posse, "Handschriften Konservirung," Dresden, 1899.
G. Sello, Das Zapon in der Archivpraxis ("Korrespondenzblatt des
Gesamtvereins der deutschen Geschichts- und Alterthumsvereine," L., 1902,
p. 195).
Schoengen, Over hat Zapon ("Nederl. Archivenblad," 1902, 1903, Nos. 1 and
3).
J. Perl, Das Archiv-Zapon ("Korrespondenzblatt," LII., 1904, pp. 119 and
435).
G. Sello, Die bei der Zaponverwendung in der Archivpraxis gemachten
Erfahrungen ("Korrespondenzblatt," LII., 1904, p. 439).
INDEX.
Acetic acid 150
Air-pump 68, 95, 129
Akermann 9
Alabaster 74
Alcohol 87, 93, 95, 132, 155, 161, 162
Jenner's method 159
preservation of leather 153
removal of oil by 87
Algae 10, 60, 76, 85
Alum 154, 158
Amber 55, 162
Ammonia
action upon bronze 18, 25, 31, 121
action upon silver 148, 150
Analyses
bronzes 22-27, 47, 138
iron rust 12
patina 23, 39
salt crystals 155
silver 51
silver chloride 51
Appelgren on application of Krefting's method 108, 115
Arche 27
Archives, preservation of, by zapon 170
Armour, iron, treatment of 105, 169
Assyrian tablets, treatment of 78, 81
Atacamite 21, 22, 26, 29
Azurite 17, 21, 36, 139
Bacteria, influence of, on iron 10
influence of, on bronze 28, 46
Barium chloride 77
Barium nitrate test 61, 77, 86
Barth 155
Bassett, analysis of bronze 25
Bees'-wax 90, 97, 102, 106, 153, 158
Bell-glass, use of 68, 95, 129, 152
Belmontyl oil 86
Benzene 91, 158, 160
Benzine 70, 86, 90, 95, 96, 106, 131, 161
Berthelot 28, 52
Bibra
on patina 20
on silver 52
Bille Gram 154
Bischoff 41
Blell 13, 91
Blell's method (iron) 102
Bolle 161
Bones, influence of contact with 14
preservation of 55, 152
Book-bindings, treatment of 152
Brewster's method (coins) 146
Bronze 15, 120
action of ammonia upon 18, 25, 31, 131
analyses of 23-27
analyses after reduction 138
Fellenberg's classification of 15
cleaning of 120
drying of 123, 131
impregnation of 122
incrustations, removal of 121
glue, treatment by 121, 125
hammers 120
heating 121
hydrochloric acid 121
inlaid metals upon 122
Springer's method 121
lead in bronze 24, 130
preservation of, by exclusion of air 144
preservation of, by carbolic acid 120
reduction of 125
Finkener's method 125
Krefting's method 139
Krefting's method (coins) 140
Villenoisy's classification of 32
Brushes, wire 107, 115, 131, 141
"silver brushes" 151, 166
Bucholz, experiment 18
Bunsen 54
Calcium chloride 123, 146
Carbolic acid 120, 157
Carbonic acid, influence of, on iron 8, 9
influence of, on bronze 31
Carbon bisulphide 90, 160
Caries of bronzes 26
Casts (plaster), zapon for 169
Caustic soda 110, 115, 119, 139, 140, 161
Celluloid 150
Cement for pottery 87
for stone 88
Ceresole, treatment of lead 150
Chevreul 21
Chlamydothrix 10
Chlorine, destructive action of 12, 26, 41, 46, 100, 121
estimation of 76
Church, analysis of silver 50
Citric acid 148
Clay, baked 4,81
Clay tablets, treatment of 78
Clay vases, sodium sulphate in 6
treatment of 80
Coffey, analysis of Irish celts 26
Coins, treatment of 139
Brewster's method 146
Krefting's method 140
Roux' method 147
treatment of, by melted lead 143
Collodion 71, 91, 161
Copaiba balsam 161
Copal varnish 90
Copper 15, 49, 120
absence of "edel-patina" 49
carbonate 21, 33
chloride 17, 21, 52
oxychloride 17, 21, 29, 41
preservation of 122
stannate 31, 33
sulphide 21, 24, 33
"Copper crystals" of bronzes 18
Corrosive sublimate 152, 154, 160
Covelline 23
Crenothrix 10, 60
Crum Brown 7
Cuboni 27, 46, 48
Cuneiform tablets 6, 78
Cupric oxide 18, 22, 33, 37
Cuprite 18
Cuprous oxide 17, 21, 24, 28, 37, 41, 52
Cyanogen 129
Damp, influence of 29, 43, 47, 49
Daniell cells 126
Davy 17
Dechend's apparatus 73
Dextrin 88
Dowris find, analysis of bronze from 26
Drude 147
Dunstan, on rusting 9
Dust extractor 131, 141
Dust, protection from 162
Earthenware
baked, impregnation of 78
baked, steeping of 74
unbaked, impregnation of 81
unbaked, treatment of 81
with colouring 79
"Edelrost" 14
Edel-patina 49, 120
Egypt, soil of 1
dry climate of 2, 56
Egyptian bronzes 130, 134, 138
absence of "edel-patina" in 42
high chlorine-content in 42
lead in 24, 33, 130
Egyptian coffins 160
Egyptian coloured objects 161
Egyptian textile fabrics 154
Egyptian ostraca 4, 57
efflorescences upon 4
treatment of 75
Ekhoff's method (iron) 96
Electric current a cause of rust 9
Electric muffle furnace 84
Electrolysis 111, 126, 147, 148
Elster 23
Enamel 151
Ephesus, bronze from 25
Ether 74, 90, 133, 161
Fat 106, 107
Fayence 6, 86
Feathers 154
Fellenberg 15
Finkener's method (bronze) 125
Fire-clay 84, 88
Fire-clay dust cement 88
Fish-glue 86, 88
Flinders Petrie 1, 71, 87, 119
on impregnation of earthenware 81
on reduction of silver 148
Fluates 71
Forge scale 8, 14, 104
Formalin 60, 152
Friedel 46
Gallionella 10
Gelatine 162
Glass, changes in 54, 151
Glue 87, 88, 120, 151
Glycerine 157, 159, 162
Gold, changes in 53
treatment of 150
existence of, in silver objects 50
ferric oxide upon 53, 151
Granite 87
Gum arabic 88
Gum dammar 70, 95, 101, 149, 151,
161
Gypsum 74, 81
Haidinger, on patina 21
Hair 55, 153
Hammers 120
Hartwich's method (iron) 117
Hassack, on patina 27
Heat-recorders 84
Herbst's method (wood) 158
Hierli, on textile fabrics 153
Hildesheim silver-find 49, 51
Horn, changes in 55
treatment of 151
Horn-silver 49, 51
Hünefeld 17
Hydrated ferric oxide 60
Hydrated tin oxide 130, 138, 139
Hydrocyanic acid 126
Hydrochloric acid 78, 84, 85, 87,
111, 116, 121, 126, 129, 149, 150
Hydrogen, reduction of bronzes in
(Finkener's method) 22, 125
Hydrogen, reduction of iron in
(Hartwich's method) 117
Impregnation media
bronze and iron 90
limestone 70
Incrustations, bronzes 37, 120, 128
earthenware 78
limestone 74
silver 149
India-rubber solution 90, 153
Ink upon ostraca 75
Inlaid metals 100, 136
Inscriptions, "squeezes" of 166
Insects, attacks of 154, 160, 161
Iridescence of glass 54
Iron 7, 89
absence of antiquities of iron in Egypt 14
acid bath for 103
alcohol, steeping in 93, 95
bone-ash, influence of 14
chain-mail 105
ferric chloride 12, 93
ferric hydroxide 7, 93
ferric oxide 14
ferric oxide on gold 53, 151
ferroso-ferric oxide 8, 14, 101
ferrous chloride 13, 92
ferrous oxide 14
ferrous phosphate 14
ferrous sulphide 60
heating of iron 92, 99, 106
impregnation of iron 93
impregnation media for iron 90
inlaid work upon iron 100
linseed-oil for iron 91
medieval iron objects 85
methods of treatment:
Blell's method 102
Ekhoff's method 96
Hartwich's method 99, 117
Jacobi's method 99
Krause's method 92
Krefting's method 108
Steffensen's method 102
Straberger's method 97
"Passivity" of iron 119
Reduction by heat 99
Reduction by potassium cyanide 118
Reduction by potassium sulphocyanide 119
Rusting, see "rust"
Irvine 8
Isinglass 90, 98, 100, 151, 156, 162
Ivory 55, 151
Jackson 10
Jacobi 14
Jacobi's method (iron) 99
Jenner's method (wood) 159
Karabacek 1
Kessler's fluate 71, 72, 74
Kisa 143
Klebs, on amber 162
Krause's method (inlaid iron) 101
Krefting, analysis of rust 12
experiments of 10, 45
method for bronze 139, 140
method for iron 101, 108, 118
method for lead 150
theory of rust 8
Kröhnke 9, 48
Lanoline 152
Lead, changes in 31, 53
treatment of 149
treatment of coins with 143
Lead carbonate 53, 150
Lead oxide 33, 90
Lead stannate 31
Leaden objects, Ceresole on 150
Leather 152
preservation of, at Salzburg 56
Lechat 30
Lecythoi, treatment of 80
Leiner's method (wood) 157
Lemery 10
Lepsius 67
Light, influence of 10, 60, 162
Lime, incrustations of 74
Limestone 57
action of salts upon 2
changes in 2
drying of 67
dust, removal of 58
impregnation of 51, 68
impregnation media for 70
steeping 58, 66
disadvantages of 67
test of progress of 62
Linseed oil 71, 90, 91, 97, 99
Linseed varnish 90, 91, 93, 99
Magnesium sulphate 5
Malachite 17, 18, 21, 36, 139
Marble 72, 74
Medieval iron objects 119
Mêdûm, analysis of bronze from 26
"Métaux malades" 29
Meten chamber
description of 2
limestone blocks from 56, 67
impregnation of 73
steeping of 59, 64
table of results of steeping 65
Milbauer 119
Mitzopulos, on patina 24
on silver 51
Mond, on patina 27, 46, 48
Moody, on rusting of iron 9
Moulds, attacks of 72, 157
Muffle furnace 84
Mycene, copper alloys from 24
silver from 51
Naphthalene 154, 161
Natterer, analysis of Ephesian statuette 25
Neufeld 10
Nickel wire for labels 163
Nile mud, objects of 87
Nitric acid, action upon iron 119
Oak, objects of 159
Oil colours, removal of 87
Oleate of lead 90
Olive oil 90, 151
Olshausen _vii_, 11, 48, 55
Organic substances
changes in 54
influence of, on patina 31
treatment of 151
Ostraca, _see_ Egypt
Oxygen, influence of 54
Paint, removal of 87, 161
Papyrus 154
crystals from 155
treatment of 155
zapon useful for 156
Paraffin wax 71, 85, 91, 95, 97, 115, 118, 125, 132, 141, 149, 157
Patina
composition of 16
heat, action of upon 28, 122
varieties of
black 33
blue 32
cracking 122
creeping or malignant 28, 125
"edel-patina" 34, 120
green 16
warty 38
Peat, influence of
bronzes 15, 33
iron 13
organic substances 55
hair 154
Pepper (for feathers) 154
Petroleum 93, 96, 97, 157
Petroleum ether 71
Phosphoric acid 14
Picht 17
Pile-dwellings
antiquities from 53, 156
horn and wool 55
textile fabrics 153
Plaster of Paris 88, 156
Pole paper 127
Poppy-seed oil mixture 70, 86, 151, 162
Potassium
arsenite 160
carbonate 148, 149
chromate 62, 130
cyanide 118, 126, 148, 149
iodide 126
sulphate 1
sulphocyanide 119
Priwoznik, on patina 23
Quicklime 96, 115
Reduction
bronze 125
iron 99, 118, 119
Rein, on Japanese bronze 36
Reinach, on inscriptions 167
Resin 91, 158, 162
Resin, removal of 74, 161
Reuss, on patina 17
Rhousopulos
treatment of bronze 111
treatment of earthenware 80
treatment of iron 111
treatment of marble and limestone 72
Rice water 71, 72
Rogna, variety of patina 26
Roux, method of cleaning coins 147
Rubber solution 90, 153
Rust
bacteria, a cause of 10
"edel-rost" 14, 111, 118
removal by acid 103
removal by sodium sulphide 119
theories of causation 7
Salt crystals, analysis of 155
Salzer 93, 95
Sandstone, treatment of 87
Schertel 51
Schill 168
Schliemann 26
Schuler, analysis of patina 24, 48
Schulz, analysis of bronze 138
Seger cones 82
Setlik 110
Shellac 70, 86, 88, 90, 147, 162
Silver, analysis of 50, 51
changes in 49, 149
electrolytic treatment of 148
preservation of 148
"Silver brushes" 151, 166
Silver chloride 49, 51, 52, 148
Silver nitrate solution 61, 76, 78, 93, 130
Silver subchloride 49, 51
Silver sulphide 50, 149, 168
Simon 8
Size 71, 72, 86, 90, 156
Soda 87, 93, 97, 98, 102, 106, 115
Sodium chloride 1, 4, 5, 10, 47, 56, 61, 85, 93, 148
Sodium nitrate 6
Sodium sulphide 119
Speerschneider's method 91, 158
Spennrath 8
Spray apparatus 73
Springer's method 120
Squeezes of inscriptions 166
Stapff 8
Stavenhagen 46
Stearine 71, 92
Stearine glaze 152
Steel wire brushes 107
Steeping
baked clay 85
bronze 130
earthenware 74
iron 92
limestone 59
Steffensen's method (iron) 102
textile fabrics 153
wood 159
Stolba 21
Stone cement 88
Straberger 162
Straberger's method (iron) 97
Stucco 87
Sulphates, test for 61, 77, 86
Sulphur, influence of 31
Sulphuretted hydrogen 24, 31, 61, 169
Sulphuric acid 13, 102, 107, 116, 123
Sunlight, _see_ Light
Süpke 146
Syndeticon 87
Tannic acid, influence of 13
Tapioca water 71, 72
Terra-cotta 74
Terreil 23
Test for progress of steeping 61
Textile fabrics 153
Tin, changes in 31, 53
treatment of 149
Tin oxide 22, 37, 41, 53
Tin, proportion of, in patina 26, 48
Titration 5, 61
test in connection with 61
Tolomei 8
Train oil 152
Turpentine 70, 90, 153, 161
Unbaked clay 81
Varnish-benzine mixture 71, 74, 75, 78, 81, 85, 87
Vaseline 90
Verdigris 16
Villenoisy, on patina 30
classification of 32
Vivianite 14
Volney 1
Voss _v_, 163
Wagner 10
"Wall-saltpetre" 7
Warrington 51
Water-bath 94
Waterglass 71, 72, 88, 90, 157
Watkin's heat-recorder 84
White of egg solution 86, 153
Wibel 16, 17
Wire for labels 163
Wood, treatment of
Herbst's method 158
Jenner's method 159
Leiner's method 157
Speerschneider's method 158
coloured wood 161
large objects, varnish for 157
Wood-worms 160
Wool, changes in 55
preservation of 153
Yarn 160
Yorkshire, analysis of bronze from 26
iron objects from 113, 114
Zapon 71, 81, 118, 132, 149, 150, 156, 168
Zinc 31, 110, 116, 138, 139, 140, 150
Zinc oxide 31, 111, 141
Zylonite lacquer, _see_ Zapon
CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS.
FOOTNOTES:
[1] "Lexikon d. gesamten Technik," Vol. I. p. 257. O. Lueger.
[2] "Merkbuch." The excavation and preservation of Antiquities, 2nd
edition, Berlin, 1894.
[3] "Mittheilungen aus der Sammlung der Papyrus Erzherzog Rainer." Vol.
I. p. 118. See also Flinders Petrie, Archaeological Journal, Vol. XLV.
1888, p. 88.
[4] Aeg. 105. [This and similar notes have reference to the catalogue of
the Egyptian (Aeg.) or Antiquarian (Ant.) sections of the Berlin Royal
Museums.] The limestone blocks were brought from the Mastaba of Meten,
at Abusir near Memphis, explored by Lepsius in 1846. Meten was one of
the chief officials under King Snefru, B.C. 2800. The inscriptions
relate to his possessions and official career, while the pictorial
representations depict hunting scenes and the offering of the gifts for
the dead. The statue of Meten was found in the grave and is now in the
Egyptian department (No. 1106) of the Royal Museum. Comp. "Ausführliches
Verzeichniss der aegyptischen Alterthümer," Berlin, 1899.
[5] Aeg. P. 4730
[6] Aeg. V. A. 2846.
[7] Aeg. P. 4739.
[8] It may be here mentioned that, as is well known to chemists, the
efflorescences which often go by the name of "wall-saltpetre," in most
cases do not contain any saltpetre, but consist of sodium sulphate.
[9] Crum Brown, "Chem. Centralblatt," 1890, I. p. 212; E. Simon, "Ueber
Rostbildung u. Eisenanstriche," p. 4.
[10] J. Spennrath, "Verhandlungen d. Vereins zur Beförd. d.
Gewerbefleisses," 1895, p. 245.
[11] "Christiania Videnskabs-Selskabs Forhandlinger" for 1892, No. 16, p.
8.
[12] "Chemische Zeitung. Repetitorium," 1895, p. 289.
[13] "Chem. Centralblatt," 1895, I. p. 441.
[14] Id., 1891, I. p. 860.
[15] "Berg- und Hüttenmännische Zeitung," 1882, p. 469.
[16] [It may not be out of place here to give the main conclusions, drawn
from a long series of experiments by Prof. W. R. Dunstan ("Proc. Chem.
Soc.," XIX. 150, 1903).
(_a_) Pure iron is not oxidised in the presence of gases and water-vapour
only, but for the appearance of rust the presence of water in the liquid
state is necessary.
(_b_) The reagents which prevent the rusting of iron are those in
the presence of which hydrogen peroxide is decomposed, and which are
consequently inimical to its formation: among such reagents the following
are given--sodium chloride, sodium sulphate, ferrous sulphate and
potassium nitrate.
(_c_) The action of H_{2}O_{2} on metallic iron leads to the production
of red basic ferric hydroxide, which is identical with ordinary rust.
The composition of rust may therefore be represented by the formula
Fe_{2}O_{2}(OH)_{2}, the reaction being represented by the equations:
Fe + O_{2} + H_{2}O = FeO + H_{2}O_{2}.
2FeO + H_{2}O_{2} = Fe_{2}O_{2}(OH)_{2}.
These views are however combated by Moody ("Proc. Chem. Soc.," XIX. 157
and 239) who concludes that aerial rusting must be regarded as a change
involving the interaction of iron and carbonic acid and the subsequent
formation of rust by oxidation of the ferrous salt.
He also states that those salts which do not combine with and which are
not decomposed by CO_{2} have no retarding influence on the formation of
rust, e.g. sodium chloride, sodium sulphate, etc.
On the other hand substances which absorb and combine with carbonic
oxide (e.g. sodium carbonate or hydroxide, ammonium carbonate, calcium
hydroxide), or which are decomposed by carbonic acid (potassium and sodium
nitrites), inhibit rusting, which may therefore be regarded as a change
involving the interaction of iron and acid and the subsequent formation
of rust by the oxidation of the ferrous salt.
O. Kröhnke ("Wochensch. Brauerei," XVII. 233) gives the following
equations:
Fe + 2CO_{2} + H_{2}O = Fe(HCO_{3})_{2} + H_{2}.
2Fe(HCO_{3})_{2} + O + H_{2}O = 2Fe(OH)_{2} + 4CO_{2}.
Comp. also Dammer, "Handbuch der anorg. Chem.," Vols. III. and IV.
(supplement).
Considerable attention has also been directed to the influence of bacteria
upon iron. Thus the growth of Crenothrix may cause much trouble in
waterworks, vide "Centralblatt für Bakterien und Parasitenkunde," II. 12,
681. A variety, Chlamydothrix (Gallionella) ferruginea (Mig.) appears to
play an important part in the formation of rust (comp. Zopf, "Crenothrix
polyspora die Ursache der Berliner Wasser-Calamität," Berlin, 1879. De
Vries, "Unter. der Crenothrix Commission," Rotterdam, 1887 and 1890).
Neufeld ("Chem. Centralblatt," 1904, I. 1621--abstracted from "Zeitschrift
für Untersuchung der Nährungs- und Genussmittel," VII. 478) gives
particulars of three varieties: Crenothrix polyspora, which separates
iron; Cr. ochracea, which separates aluminium and some iron; and Cr.
manganifera, which separates manganese.
Jackson ("Journal of Society of Chemical Industry," 1902, p. 681) gives
micro-photographs of these varieties. Microscopically the masses of
Crenothrix are seen enclosed in a gelatinous sheath, in which is imbedded
the precipitated metallic hydrate. It is anaërobic and its action is
favoured by absence of light. In the absence of dissolved oxygen, the
bacillus appears to take its iron from the pipes. Cr. polyspora is found
however ("Zeitschrift für analytische Chemie," XLII. 590) to separate the
iron not from the ferrous carbonate (FeCO_{3}) but from iron organically
combined. See also Winogradsky, Ueber Eisenbakterien, "Bot. Zeit.," 1888,
and "Chem. Centralbl." 1904, II. 1332. Transl.]
[17] Wagner in Dingler's "Polyt. Journal," CCXVIII. p. 70. Axel Krefting
in the above-quoted "Forhandlinger," p. 4.
[18] Olshausen often noticed on freshly excavated antiquities of various
kinds a peculiar smell of resin or gum, especially after treatment with
hydrochloric acid ("Verhandl. d. Berl. anthropol. Ges.," 1884, p. 520).
It may be supposed that this odour is due to the traces of hydrocarbons
present.
[19] Termed by E. Friedel "Dunstperlen." "Eintheilungsplan d. Märk.
Prov.-Museums," p. 9.
[20] "Om Konserviring af Jordfundne Jernsager," in "Aarsberetning fra
Foreningen till norske Fortidsmindesmaerkers Bevaring," 1892, p. 52.
[21] Krause, "Verhandl. d. Berl. anthropol. Ges.," 1882, p. 533.
[22] Private communication.
[23] "Sitzungsberichte der Alterthumsgesellschaft Prussia," 1881-2, p. 9.
[24] Merkbuch, Alterthümer aufzugraben und aufzubewahren, 2nd edition, p.
71.
[25] [This fact was noticed by Sir Thomas Browne, 1658, cp.
"Hydriotaphia," cap. iii. Transl.]
[26] In this work the Patina on antiquities only is considered; with that
on modern bronzes we are not concerned.
[27] "Mittheilungen d. naturforsch. Gesell. in Bern," 1865, p. 12.
[28] "Mitth. d. naturforsch. Gesellschaft in Bern," 1860, p. 69.
[29] "Jahrbuch für Mineralogie," 1860, p. 813.
[30] Malachite, CuCO_{3}, Cu(OH)_{2}.
[31] Azurite, 2CuCO_{3}, Cu(OH)_{2}.
[32] "Jahrbuch für Mineralogie," 1865, p. 400.
[33] The extract which here follows is in part verbatim.
[34] "Annalen der Chemie u. Pharmacie," 1853, Vol. LXXXV. p. 253.
[35] Von Bibra, "Die Bronzen und Kupferlegirungen," p. 206 _et seq._
[36] See the very full quotation from Wibel's work given above.
[37] I have not been able to find anything on this point in the literature
of the subject.
[38] Atacamite, CuCl_{2}, 3Cu(OH)_{2}.
[39] Cf. "Comptes rendus," 1856, XLIII. p. 735.
[40] "Journal f. praktische Chemie," XCIV. 1865, p. 314.
[41] "Verhandl. d. Ver. z. Beförd. d. Gewerbf.," 1869, p. 182.
[42] Dingler, "Polyt. Journal," 1878, Vol. CCIV. p. 483.
[43] "Berg- u. Hüttenmännische Zeitung," Vol. XXXVII. 1878, p. 329.
[44] Dingler, "Polyt. Journal," 1879, Vol. CCXXXII. p. 333.
[45] [Several other analyses of bronzes from various sources have been
recently published. Thus Natterer ("Monatshefte für Chemie," XXI. 256,
1900; also "Chem. Centralblatt," 1900, I. 1262) examined a corroded bronze
statuette from Ephesus. The bronze contained:--Tin 6·09%, lead 4·87%, and
copper 89·64%, with traces of zinc.
Bassett ("Proc. Chem. Soc.," 1903, XIX. 95) gives an analysis of the base
of an Egyptian statuette, found in the Nile Delta, probably dating from
200-100 B.C. The base was hollow but filled with lead, and was covered
with a thick green coating, which in parts entirely replaced the original
metal.
Table I.
Cu 50·65%
Pb 6·74%
Sn 2·94%
Fe 0·15%
Ni, Mn, etc. 0·11%
Cl 15·71%
SiO_{2} (as sand) 1·14%
H_{2}O 11·07%
(NH_{4}) 0·11%
-------
88·62%
Table II.
CuCl_{2} 29·34%
CuO 46·10%
H_{2}O 11·07%
SnO_{2} 3·73%
PbO 7·26%
Fe_{2}O_{3} 0·22%
NiO, etc. 0·14%
SiO_{2} 1·14%
(NH_{4})Cl 0·32%
-------
99·32%
In the second column the chlorine has been calculated as copper chloride,
the remaining copper and other metals as oxides.
Traces of calcium were also found, but the amount of sodium was so small
that it could only be detected by the flame test. If all the copper
had been present as basic chloride (atacamite CuCl_{2}, 3CuO, 3H_{2}O),
this would require 26·84% CuCl_{2}, 47·57% CuO, and 10·78% H_{2}O. It
would therefore appear that the substance produced by corrosion is less
basic than atacamite, and that ammonium chloride may have played a more
important part than sodium chloride in the formation of copper chloride,
for in this case the sodium was only found in amount too small for
estimation.
An analysis of the earliest piece of bronze known, i.e. that from Mêdûm,
Egypt (3700 B.C.), gives 8·4% of tin (inner core 9·1%) to 89·8 of copper
with a small quantity of arsenic.
An analysis of a celt from the Dowris find (King's County, Ireland, 1825)
gave copper, 85·23; tin 13·11; lead 1·14, with traces of sulphur and
carbon. The waste material from the same place yielded 89% copper, 11%
tin, with traces only of lead, iron and silver.
On the other hand an early bronze celt (Butterwick, E.R., Yorks.) showed
a smaller quantity of tin--10·74%, compared with 87·97% of copper. (Guide
to Bronze Age Antiquities, British Museum.)
Mr George Coffey has also published ("Brit. Assoc. Reports," 1899, p. 873)
a tabulated series of analyses of Irish celts which proved to be composed
of practically pure copper. Transl.]
[46] Schliemann, "Ilios," pp. 527 and 571.
[47] "Verhandl. d. Berl. anthropol. Ges.," 1882, p. 537.
[48] Dingler, "Polyt. Journal," 1884, Vol. CCLIII. p. 514.
[49] It should be observed that the change in the proportions according
to Schuler (see page 24) is only true of the analysis, Column III. In
Columns I and II the amount of metallic copper of the patina is indeed
smaller, but so is also that of lead and especially of tin. But this may
be due to a faulty method in the determination of tin noticed by Olshausen
(v. "Verhandl. d. Berl. anthropol. Ges.," 1897, p. 349). The different
proportion of the copper compounds in the patina should also be noted,
Schuler giving carbonate and hydrate in the proportion 1:1, Arche and
Hassack once as 1:2, and again as the result of two analyses as 1:3.
[50] "Atti della Reale Accademia dei Lincei," 1893, p. 498.
[51] "Étude sur les métaux découverts dans les fouilles de Dahchour" in
"Fouilles à Dahchour." March-June, 1894, p. 131 _et seq._ J. de Morgan.
See also "Comptes rendus," 1894, I. 118, p. 768.
[52] "Revue archéologique," v. 28, 1896, pp. 67 and 202. In the
publication for the second half of the same year Lechat maintains the
assertion (comp. Elster, p. 23) that in many cases the antique patina is
due to the artist.
[53] Ant. Misc. 7382.
[54] J. J. Rein, "Japan," Vol. II. p. 528.
[55] Ant. Misc. 8579.
[56] According to Graham-Otto, "Lehrbuch der Chemie," Vol. III. p. 849,
cuprous oxide is decomposed by dilute acids which contain oxygen; the
cupric oxide is dissolved and metallic copper remains.
[57] Buto, Aeg. 11867.
[58] Ant. Fr. 29.
[59] [The extraordinary deformation produced by this type of patina may be
judged from the fact that the features in this instance were so obscured
that the nature of the specimen was not recognised and it had accordingly
been mounted upside down. Transl.]
[60] Ant. Fr. 53.
[61] Ant. Fr. 53.
[62] Bischoff, "Das Kupfer und seine Legirung," p. 43. Layard,
"Discoveries in the ruins of Nineveh and Babylon," p. 191. Fellenberg,
"Mittheilungen d. naturforsch. Ges. in Bern," 1860, p. 68.
[63] "Christiania Videnskabs-Selskabs Forhandlinger" for 1892, 16, p. 5.
[64] [Having been broken off and soldered, the base was not subjected to
treatment. Transl.]
[65] E. Friedel, "Eintheilungsplan des märk. Provinzialmuseums," 1882, p.
20.
[66] Aeg. 2348.
[67] Aeg. 13787.
[68] Olshausen, "Verhandl. d. Berl. anthropol. Ges.," 1884, p. 532, and
1897, pp. 346-7. Kröhnke, "Chem. Untersuchungen an vorgeschichtl. Bronzen
Schleswig-Holsteins," p. 41. See also quotation from Schuler, p. 25.
[69] [This celebrated hoard was found Oct. 9, 1868, on the Galgenberg,
near Hildesheim (Hanover), 10 feet below the surface. It consisted of
more than 60 pieces, including plates, dishes, tripods, etc., the most
notable being a crater, 15-1/2 inches in height, ornamented with graceful
scroll-work, and a cylix with an Athene in high relief. The workmanship
points to a date not later than the first century A.D. Cp. Wieseler, "Der
Hildesheimer Silberfund," Bonn, 1869. Darcel, "Trésor de Hildesheim,"
1870. Transl.]
[70] Compare also the analysis by Schertel, p. 51.
[71] "Polytechn. Centralblatt," 1871, p. 916.
[72] "Polytechn. Centralblatt," 1871, p. 917.
[73] "Berg- u. Hüttenmänn. Zeitung," 1878, No. 37, p. 327.
[74] Dingler, "Polyt. Journal," 1871, Vol. CCI. p. 52.
[75] v. E. v. Bibra, "Ueber alte Eisen- u. Silberfunde," p. 74.
[76] Morgan, "Fouilles à Dahchour," p. 135.
[77] "Verhandl. d. Berl. anthropol. Ges.," 1897, p. 348.
[78] Krause, "Verhandl. d. Berl. anthropol. Ges.," 1883, p. 360.
[79] Muspratt's "Chemistry," Vol. III. p. 1389.
[80] "Verhandl. d. Berl. anthropol. Ges.," 1889, pp. 243 and 244.
[81] Id. 1892, p. 449.
[82] Id. 1897, p. 347.
[83] At that time obtained from the Stralau waterworks.
[84] This was not the well-known Crenothrix only. Cp. "Polytechn.
Centralblatt," 1891-92, p. 195. (See footnote, p. 10.)
[85] It has been found that the formation of this layer of slime may be
avoided by the use of tubs which are lined with sheet zinc. The addition
of 10-20 cubic centimetres of formalin [40% solution of formaldehyde] to
every hundred gallons of water also prevents or restrains the formation of
slime. It is not necessary to add formalin each time the water is changed.
[86] Since chlorine compounds (especially common salt) form the
predominating substance in the soluble salts contained in limestones
their removal may be considered a proof that other salts (e.g. sulphates)
have also been removed. Hence it is sufficient to prove the disappearance
of chlorine. In the rare cases in which sulphates only are present,
a test similar to that mentioned on p. 77, applied to clay objects,
should be used. If the water used for soaking salt-containing limestone,
earthenware, etc., gives no precipitate, or only turbidity with the silver
solution, the determination of chlorine by titration is not applicable.
[87] Though some other kinds of burette may be easier to use, that here
recommended (that of Gay-Lussac) is the most convenient for reasons into
which we need not enter. The following precautions should be observed:
where the burette is not closed by a cork, let a few drops out first
to wash away crystals of silver nitrate which may have formed at the
mouth. The silver solution should be kept in well stoppered bottles. When
filling the burette a glass funnel should be used, so that the cork used
for closing the burette is not wetted with the silver solution. Before
reading off wait until the level of the fluid is constant, in order that
any solution on the sides of the glass tube may have time to run down.
[88] See note, p. 61.
[89] I have found that the amount of chlorine is smaller in winter than
in summer. In the summer of 1894, 100 c.c. tap-water from the Stralau
Waterworks often required 0·8 c.c. silver solution: but at that time
stronger disinfectants were used on account of the cholera, and this may
have caused the increase of chlorine; for since then, and even at the
present time (winter 1898), 100 c.c. tap-water requires 0·5 to 0·6 c.c.
silver solution.
[90] It may be here observed that objects of limestone or of earthenware
may be numbered or marked at the back in black iron ink, which does not
disappear even after prolonged steeping in water.
[91] Lepsius, "Denkmäler aus Aegypten und Aethiopien."
[92] It is scarcely necessary to add that any other form of air-pump may
be used.
[93] A powerful pressure of water [in combination with a well-acting pump]
may cause the fluid to evaporate with such rapidity as to produce bubbles,
but these bubbles are easily distinguished by their size from the minute
bubbles of air. To avoid this ebullition, the air should not be pumped
out too rapidly.
[94] "Merkbuch," p. 62.
[95] Flinders Petrie, "Archaeological Journal," Part 45, 1888, p. 88.
[96] Zapon: for further information and references see Appendix.
[97] "Chemische Zeitschrift," ii. 1903, p. 203.
[98] For particulars of the composition and action of Kessler's Fluates
(salts of Hydrofluosilicic acid) see H. Hauenstein, "Die kessler'schen
Fluate" (2nd ed., Berlin, 1895). The depot is "H. Hauenstein, Berlin N.
Reinickendorferstrasse, 2b."
[99] Ger. Patent, No. 31032. The apparatus was one of those used in the
moulding room of the Royal Museums for the impregnation of plaster moulds
and casts.
[100] In applying the above test it is advisable to add one or two drops
of nitric acid before the addition of the barium salt. In this case, too,
should any other than distilled water be used for steeping, a preliminary
examination should be made to determine the presence or absence of
sulphates.
[101] [Pure hydrochloric acid is usually sold in two strengths.
Concentrated acid has a strength of about 32%, whilst the "diluted
hydrochloric acid" of the Pharmacopoeia is about 10%. The former should
therefore be diluted with about 15, the latter with about 4 volumes of
water. Transl.]
[102] "Chemische Zeitschrift," II. 1903, p. 761.
[103] [Lecythoi: slender narrow-necked painted vessels which
were frequently burnt or buried with the dead; cp. Aristophanes,
"Ecclesiazusae," 996:
#hos tois nekroisi zôgraphei tas lêkythous.# Transl.]
[104] The method of washing objects of unbaked clay suggested by Flinders
Petrie in the "Archaeological Journal" (XLV. 1888) is in my experience
impracticable.
[105] [Muffle furnaces may be obtained from Messrs Fletcher, Russell and
Co., Warrington. If electricity is available, the electric muffle may be
used. These may be obtained from Messrs A. Gallenkamp and Co., 19, Sun
Street, Finsbury Square, London. The Heat-recorders supplied by H. Watkin,
225, Waterloo Road, Burslem, will be found very convenient in place
of Seger's cones, which may be obtained from Messrs Brady and Martin,
Northumberland Road, Newcastle-on-Tyne. Transl.]
[106] "Merkbuch," p. 78.
[107] [A paraffin prepared from Burmese petroleum. Transl.]
[108] Flinders Petrie, "Archaeological Journal," XLV. 1888, p. 89.
[109] "Merkbuch," p. 79.
[110] [It is important to avoid confusion and mistakes arising from the
similarity of the terms benzine and benzene.
Benzene (Benzol) is the specific coal-tar product which has the formula
C_{6}H_{6}.
Benzine (Benzin) is a light-boiling petroleum distillate, lighter than
lamp oil, and with a varying boiling-point. It consists of a number of
saturated hydrocarbons of the methane series. It is also called benzoline
or petroleum naphtha. Transl.]
[111] Appelgren, Finskt Museum, 1895, p. 56.
[112] A communication from Herr Schjerning, Copenhagen.
[113] Speerschneider, "Antiquarisk Tidsskrift," 1858-60, p. 178.
[114] Blell, "Sitzungsberichte der Prussia," 1881-82, p. 24.
[115] Krefting, "Aarsb. fra foreningen t. norske fortidsmindesm. bevar."
1892, p. 54.
[116] Salzer, "Chem. Zeitung," XI. 1887, p. 574.
[117] Probably first recommended by Salzer, "Chemiker Zeitung," XI. 1885,
p. 574.
[118] "Kongl. Vitterhets Historie och Antiqvitets Akademiens Månadsblad,"
1885, p. 134.
[119] The addition of the lime is not advisable, comp. p. 93.
[120] "Chemiker Zeitung," XI. 1885, p. 605.
[121] "Zeitschrift f. Ethnologie," XXXIII. 1902, p. 431, and XXXIV. 1903,
p. 791.
[122] "Sitzungsberichte," 1881-82, p. 10 _et seq._, and 16 _et seq._
[123] In this acid treatment bare hands may be used, but care must be
taken to avoid splashing clothes or linen, which will cause red or yellow
spots. These are best removed by the immediate application of ammonia,
but the yellow spots can only be removed by oxalic acid.
[124] [Germ. "Hammerschlag." The iron scales which chip off from heated
iron at a forge or blacksmith's shop. Transl.]
[125] No. 17, 1897, p. 333 _et seq._
[126] A number of modifications in the metals employed and the composition
of the bath have been suggested. Setlik ("Chemiker Zeitung," XXVII. 1903,
p. 454) imbeds iron objects which have a very weak core in a zinc-wire
basket immersed in a magma of zinc dust and caustic soda. Personally I
should prefer not to attempt a reduction process in such cases, but should
rely rather upon mechanical removal of the rust, soaking and impregnation.
For the treatment of bronzes this observer prefers the Finkener method
(q. v.) and suggests caustic soda, sodium chloride, or ammonia chloride,
instead of potassium cyanide as the electrolyte. Rhousopulos ("Chemische
Zeitschrift," II. 1903, pp. 202 and 364) uses zinc and hydrochloric acid,
and when dry gives to the bronze a coating of wax. I should deprecate
the use of either of these substances, the hydrochloric acid because of
the difficulty of completely removing it by steeping and the danger of
subsequent decomposition of the bronze, the wax because the contained
fatty acids may act upon the metal.
[127] [The period required for complete reduction is, in our experience,
often considerably longer. We have sometimes found an 8% soda solution
more satisfactory. Transl.]
[128] Krefting's method affords an excellent illustration of the
truth of my remarks in the preface that the literature upon these
preservation-methods is very scattered and in consequence has been
hitherto but little studied. In 1892, when visiting the Museum at
Christiania, I had the opportunity of examining some iron objects which
had been treated by Krefting's method. I then obtained his address, and
Herr Krefting kindly communicated his method to me by letter, and in the
following year forwarded a reprint of his article referred to above. In
1887 he had described his method to Appelgren by letter, but at that
time he treated the object after reduction, washing, and drying, by
impregnating it with a paraffin-petroleum solution. In 1897 Appelgren
published this method, with drying and impregnation, in ignorance of
Krefting's publication in 1893.
[129] The bottle should not be closed by a glass stopper, but by a rubber
bung, or by a cork coated with paraffin or wrapped round with parchment.
Soda solution attacks glass, and especially ground glass; thus the stopper
may become so firmly fixed into the neck of the bottle as to render its
removal impossible.
[130] [Excavated by Dr Thurnam, 1848 (vide "Archaeolog. Journal," Vol.
VI. p. 27). Transl.]
[131] "Chemiker Zeitung," XI. 1887, p. 605.
[132] Ant. Fr. 1154 _a_.
[133] I should now use paraffin for impregnation. (Author's note.)
[134] [Great caution must be used to prevent inhalation of the gas, which
is extremely poisonous. Transl.]
[135] Instead of potassium cyanide, I have made experimental use of the
much more readily fusible potassium sulphocyanide. This converts the iron
compounds into iron sulphide, which is easily got rid of. The sulphide
which still adheres to the iron and imparts a not unpleasing blackish
colour to the object appears to be stable.
The rest of the treatment is similar to that above described. Having so
far only experimented with a few specimens I am not yet in a position
to offer any judgment as to the practicability of the process. (Author,
1904.) [For practical objections to this method, which we do not consider
satisfactory owing to the instability of the products resulting from the
reaction, and the difficulty in removing them by the subsequent washing,
see Milbauer, "Zeit. f. anorg. Chem." XLII. 1904, p. 433 ("Journ. Chem.
Soc." Abstracts, i. 121), where it is stated that treatment of Fe_{2}O_{3}
at 400°C. leads to the formation of K_{2}Fe_{2}S_{4}. Transl.]
[136] Stolba, "Chemiker Zeitung," XX. 1896, Repertorium, p. 240. [Sodium
sulphide has a very deleterious action upon the skin and fingernails which
should be protected when using this substance. Transl.]
[137] Flinders Petrie, "Archaeological Journal," Vol. XLV. 1888, p. 88.
[138] Cp. Mugdan, "Zeit. Elektrochem." 1903, ix. p. 442.
[139] [A method used by the explorers of the Palestine Exploration
Committee for the preservation of much decayed bronzes, as, for instance,
those from wells and cisterns, is to place them immediately they are
discovered into a strong solution (1 in 10) of carbolic acid. Transl.]
[140] "Merkbuch," p. 68.
[141] Instead of calcium chloride strong sulphuric acid may be used for
dehydration, but the dry chloride is more simple, and less dangerous. If
kept in corked bottles, the corks should be covered with paraffin wax to
prevent access of moisture.
[142] First put forward by Chevreul (see pp. 22 and 117).
[143] [The so-called 'pole paper' is supplied by most dealers in
electrical apparatus. Transl.]
[144] In such a case the hydrated oxide of tin is either present as
such in the oxidized bronze, or it is a product of the reduction which
has been prevented from falling to the bottom by the high sp. gr. of
the potassium cyanide solution. It is also possible that finely divided
tin in the reduced bronze may decompose the warm water into oxygen and
hydrogen, forming a hydrated oxide of tin. Such a reaction would account
for the formation of hydrogen. The hydrogen might at the same time remain
occluded until allowed to separate by the cessation of the current and
the temperature of the water.
[145] Such, for instance, as is obtained by attaching a suitable nozzle to
a fall pipe where there is sufficient water-pressure; v. e.g. "Polytechn.
Centralblatt," 1891-92, p. 199.
[146] I now consider it a better plan not to employ the method of coating
with paraffin wax. I thoroughly soak and then dry the reduced bronze with
a cloth, and place it in 96% alcohol. This must be renewed after a time,
and for large bronzes a third or even a fourth renewal is advisable. The
bronze is again wiped and introduced into a drying oven raised to about
212°F. [100°C.]. The unsightly grey colour and rough surface may be much
improved by using a brush of the finest wire or very fine emery cloth. The
object is finally impregnated with zapon (see Appendix). (Author's note,
1904.)
[147] [The base was not treated owing to the advanced destruction of the
metal. Transl.]
[148] In only about 2% of the bronzes treated in the laboratory of the
Royal Museums at Berlin has it been found necessary to interrupt the
reduction.
[149] "Polytechn. Centralblatt," 1891-92, p. 200.
[150] These analyses were made by Schulz in the Laboratory of the Royal
Museums.
[151] I quote here the greater part of an article published in Dingler's
"Polytechn. Journal," 1896, Vol. CCCI. p. 44. The reduction of about 7000
Danish copper coins, undertaken while the above was in the press, gave
similarly good results.
[152] The zinc, which in the course of the reduction process may become
coated with a thin layer of metallic copper, may be used again. It should
be first put through dilute sulphuric acid (in the proportion of 1:2),
then washed, rubbed with a steel wire brush and again washed. But it must
in such cases be used again while still wet, for if allowed to dry it
becomes coated with a layer of oxide and requires to be re-polished.
[153] "Publications de la société pour la recherche et la conservation des
monuments historiques dans le grandduché de Luxembourg," Vol. X. As I have
been unable to consult the original, I have here inserted a communication
sent to me by Dr Kisa of Cologne. I have tried this method on a few coins.
[154] In another instance--that of a Minotaur group [Ant. Misc. 7382]--the
calcium chloride is contained in four shallow glass troughs which are
placed round the marble pedestal of the bronze and are loosely covered
with a black card.
[155] Grote, "Blätter für Münzkunde," 1835, I. No. 31, VI.
[156] "Zeitschrift für Numismatik," Vol. XVII. 1890, p. 100.
[157] "Prometheus," VIII. 1897, p. 351. A report on the other methods is
here given.
[158] "Zeitschrift für Numismatik," Vol. XX. 1897, p. 325.
[159] "Archaeological Journal," XLV. 1888, p. 87.
[160] "Riv. Ital. Numism." 1903, p. 31; also "Chemiker Zeitung," XXVII.
1903, p. 825.
[161] [In this connection however it must be remembered that celluloid
gives off traces of acid for a long time. This may possibly involve risk
of injury to certain specimens. Transl.]
[162] "Mittheilungen des Nordböhmischen Gewerbe Museum," 1903, p. 104.
[163] [A few drops of formalin will serve the same purpose. Transl.]
[164] [Lanoline, especially if applied warm, retains the flexibility of
the leather very satisfactorily. It may be here mentioned that the leather
of old book-bindings may be preserved by the application, by means of
a soft brush, of a mixture of white wax with a small quantity of white
vaseline and paraffin wax, brought to a pasty consistency with benzine or
turpentine.
The 'Stearine Glaze' used for the same purpose is made by boiling one part
of caustic soda with eight parts of stearic acid and 50 parts of water
till dissolved, then mixing another 150 parts of cold water and stirring
until the whole sets to a jelly.
Either of these media should be applied very thinly and then polished with
a brush or flannel. If the cover is very bad, considerable improvement
is effected by repeated applications of the stearine glaze so as to fill
up the damaged surface of the leather. In some cases the addition of
some dye such as logwood, or one or other of the acid coal-tar dyes, is
advantageous.
Lanoline, or wool fat, i.e. lanoline without the water, is useful, but
gives a dull surface to the leather.
In some cases a thin coating of diluted white of egg, to which a few drops
of clove oil, or some other essential oil, has been added as an antiseptic
is beneficial. Transl.]
[165] On preservation in alcohol see p. 159 under the heading 'Wood.'
[166] "Aarb. for nordisk Oldkynd. og Historie," 1891, p. 112.
[167] According to an analysis published by L. v. Barth in an account of
the collection of papyri belonging to the Archduke Rainer, Part I. p. 120,
the salt crystals, after removal of the insoluble constituents, consisted
of:
Potassium sulphate 0·8%
Potassium and sodium chlorides 92·0%
Calcium sulphate 4·6%
Magnesium chloride 2·8%
Organic substances 0·2%
[168] "Merkbuch," p. 60.
[169] Communicated by Herr Leiner of Constance.
[170] "Antiquarisk Tidsskrift," 1858-60, p. 176.
[171] Id., p. 174.
[172] Olshausen, "Verh. der Berl. anthropol. Ges." 1885, p. 242, an oral
communication from Herr v. Jenner.
[173] Attention may be drawn to a paper which (Dec. 1904) will shortly be
published by the Imperial Commission for Monuments of Art and History in
Vienna. At a meeting in Vienna a paper was given by Bolle on the animal
enemies of paper, leather, and wood, and their destruction by means of
carbon bisulphide. Carbon bisulphide is an infallible poison and has
no effect upon colours when used in a perfectly dry state. This may be
carried out by a preliminary displacement of the air by carbonic acid
which is readily obtained in the liquid form in cylinders. Benzine would
probably act equally well, but would require a longer time for its action.
References to other methods such as the employment of a vacuum or of heat
will be found in the same publication.
[174] Keim, "Technische Mittheilungen für Malerei," 1888, p. 4.
[175] Communication from Herr Straberger of Linz on the Danube.
[176] Communication by Dr Voss.
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