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