The Preservation of Antiquities: A Handbook for Curators. Friedrich Rathgen
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 | |