Iron.

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. Figure4represents a fragmentwith 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 thesurface. 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.

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) wherethe 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].

Fig. 5.Pottery showing efflorescence of sodium nitrate.

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 (CO2) in solution, a ferrous bicarbonate being first formed; the bicarbonate is then converted into ferrous carbonate, which finally yields the hydrate with evolution ofcarbonic acid. This carbonic acid continues to attack further areas of metallic iron. Others[10]maintain that, while the formation of rustmayproceed as described, carbonic acid is not necessary, and that free oxygenalonecauses 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 hadbeen 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% Fe2O3, 8·6% H2O, thus resembling the “iron-black” of Lemery; on extraction with water the filtrate is found to be alkalineand 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 = FeCl2+ 2Na2Na + 2H2O = 2NaOH + H2.”

Fe + 2NaCl = FeCl2+ 2Na2Na + 2H2O = 2NaOH + H2.”

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. Thebehaviour 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:

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].

6FeCl2+ 3O = Fe2O3+ 2Fe2Cl6;2Fe2Cl6+ 2Fe = 6FeCl2.

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 conditionof 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 (Fe3P2O8.8H2O), 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 completeinsulation 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.

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. Bronzespreserved 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.”

(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. Bronzespreserved 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.

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.”

(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.

(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:

(α) Basic carbonate of copper.

(β) Basic carbonate and sulphide of copper.

(γ) Malachite (normal carbonate of copper), with occasional admixture of cuprous oxide and azurite (acid carbonate of copper) [Stolba].

(δ) 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:

(α) A green deposit with patches of blue.

(β) A blood-red mass.

(γ) A reddish coloured bronze.

(δ) Ordinary bronze unaltered in appearance.

The first in this category represents the ultimate stageof 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.

(β) 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.

(γ) The reddish colour seems to be due to the tin undergoing more alteration in the course of time than the copper.

(δ) 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:

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 cuproussulphide, Cu2S with 15% of tin. The third layer which, like the second, was incomplete, formed a fine black powder, and consisted of 59·8 Cu2S, 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:

Schuler calculates from these figures that the patina contains:

The analysis of the bronze itself was as follows:

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 bycopper 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:

They obtain the following formulae and composition for the patina of the three bronzes[49]:

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 speciesof 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]:

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