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.