Silver.

“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, CuCl2, 4H2O. 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 = 4CuO4CuO + 2NaCl + CO2+ 4H2O = 3CuO, CuCl2, 4H2O + Na2CO3.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 sodiumchloride is formed. The remaining portion of copper is converted into cuprous oxide:3CuO, CuCl2, 4H2O + 4Cu + 2NaCl = Cu2Cl2, 2NaCl + 3Cu2O + 4H2O.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:3Cu2Cl2+ 3O + 4H2O = 3CuO, CuCl2, 4H2O + 2CuCl2.The copper chloride which remains, if in contact with air and copper or even cuprous oxide, is similarly converted into oxychloride:CuCl2+ 3Cu + 3O + 4H2O = 3CuO, CuCl2, 4H2O.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.”

4Cu + 4O = 4CuO4CuO + 2NaCl + CO2+ 4H2O = 3CuO, CuCl2, 4H2O + Na2CO3.

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 sodiumchloride is formed. The remaining portion of copper is converted into cuprous oxide:

3CuO, CuCl2, 4H2O + 4Cu + 2NaCl = Cu2Cl2, 2NaCl + 3Cu2O + 4H2O.

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:

3Cu2Cl2+ 3O + 4H2O = 3CuO, CuCl2, 4H2O + 2CuCl2.

The copper chloride which remains, if in contact with air and copper or even cuprous oxide, is similarly converted into oxychloride:

CuCl2+ 3Cu + 3O + 4H2O = 3CuO, CuCl2, 4H2O.

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 asozone; 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 itsformation. 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.

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 itsformation. 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, becomesomewhat 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 page24), and by Arche and Hassack (see page27). 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 originalcontours 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 bebright 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 (CuCO3, Cu(OH)2) and azurite (2CuCO3, 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 page142may 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 ofcuprous 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.

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 page17)[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 Figures41-43). 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.

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. Figure8shows an Etruscan mirror covered with a patina which generally results in the progressive destruction of the bronze[60].

Fig. 8.Etruscan mirror showing warty patina.

The following series of quantitative determinations ofchlorine obtained from the examination of bronzes in the Berlin Museums, shows conclusively the destructive influence of chlorine in the production of patina:

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 page40), 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.1,2et 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, darkgreen 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 inmuseums does the change become apparent, as has been so strikingly described by Mond and Cuboni (see page27). 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 intothe bronze. The adjoining figures (Fig.9to12) 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 completelyperforated. 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.

Fig. 9.Bronze Pasht showing destructive patina.

Fig. 10.The same after treatment (Finkener’s method).

Fig. 11.Bronze Pasht showing destructive patina.

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 beento 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.

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 ofcarbonic 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.

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, Ag2Cl; 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]thelayer 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, Ag2Cl 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 asmall quantity of gold; the inner layer, which was soft, grey and powdery, had the following composition:

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:

Beneath this layer was a very thin, almost black, brittle layer of silver subchloride:

Between the metal and the latter layer was a smallquantity 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 + CO2= 2AgCl, nNaCl + Na2CO3.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.”

“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 + CO2= 2AgCl, nNaCl + Na2CO3.

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.”

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.

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 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 beremoved 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.

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.

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 numerousand 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 ironobjects, 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.

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.

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 wherepossible 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 (CaCO3), 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 and1⁄2metre 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 appearancewhich 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 oxidationin 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 + AgNO3= AgCl + NaNO3.

The white precipitate is the silver chloride, whilst thesodium 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.

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