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:
“By the terms ‘rogna’ or ‘caries’ of bronze, archaeologists designate a peculiar change, to which ancient bronzes, as statues, coins, vases, etc. are sometimes liable when preserved in museums. This consists in a species of efflorescence of light green colour at one or more points upon the surface, which spreads by degrees, like oil over a sheet of paper, destroying the surface and converting the interior of the bronze into an amorphous whitish-green powder. The rapidity with which this destruction proceeds varies much according to circumstances which are not yet sufficiently known. Sometimes the destructive spot grows so slowly that it is hardly perceptible even after some months; sometimes it grows very rapidly, numerous spots form, spread, and unite, until in a few months an ancient coin may be entirely destroyed. In this way antiquities which are valuable for their history, or for their workmanship, are sometimes more or less injured by this development of patina, which archaeologists regard as a plague in their collections.”
Mond and Cuboni believe that the growths above described are caused by Bacteria. Although they have not succeeded in producing the appearances of spreading patina by transference of cultures of bacteria to intact bronzes they think that their observations sufficiently support this supposition, which they believe is further strengthened by the fact that bronzes exposed for a quarter of an hour to a temperature of 300°F. (150°C.), whereby any bacteria would be killed, showed no further change after a period of six months. The following is an extract from an article by Berthelot[51]: