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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:

(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 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.

(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:

	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 (CO2)		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, 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:

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%	H2SnO3
34·55%	CuCO3, CuH2O2
4·51%	(PbCO3)2PbH2O2.

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.
CuCO3, 2CuO2H2	85·83	CuCO3, 3CuO2H2	95·11	56·08
2PbCO3, PbO2H2	13·01		4·49	24·62
SnO3H2	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: