Fig. 12.
The same after treatment (Finkener’s method[64]).
Nor am I able to endorse the statement of Friedel[65] that a spreading patina is characterised by a peculiar and disagreeable smell, although some oxidized bronzes have a distinct smell which it is not easy to describe.
The presence of chlorine is particularly dangerous to those bronzes which consist of a casing of metal of variable thickness around a core of sandy clay, the object of which has been to economize metal. These constitute an important class amongst Egyptian bronzes. The chlorine often exists in the core as sodium chloride, and can thus attack the metal from both sides. Moreover, the structure of many Egyptian statuettes of a later period is very porous and spongy, and thus presents a large surface to destructive agencies. On sawing through the support of an Osiris[66] numerous small bright spots were found, upon examination with a lens, to be small pores filled with a salt solution. A few days later the action of the carbonic acid had begun, and the bright spots of moisture were represented by small green patches. The following figures show the absorption of moisture and of carbonic acid by this specimen and by another Osiris from the Egyptian collection.
These figures show that in the first case the absorption of carbonic acid, oxygen, and water proceeded at first slowly, but more rapidly after three months, as was evidenced also by the appearance of marked efflorescence on the oxidized surfaces. The Osiris, which was more highly oxidized, showed a more rapid increase in weight from the first. The increased action after the heating was also manifest externally, for at the end of a fortnight the bright green efflorescences had made their appearance. In this case therefore the heating recommended by Mond and Cuboni, so far from proving beneficial, actually induced a more rapid decay.
The patina layer, as Schuler has also observed, often contains a greater proportion of tin than does the alloy; a result which is manifestly due to the solution and removal of the copper salts by the subsoil water. The bright efflorescences of an Egyptian statue of Buto[67] contained 10·49% of tin, while the percentage in the metal itself was only 7·66. In certain circumstances it may even result that an object which was originally composed of bronze is represented only by tin oxide[ [68]. The small proportion, and occasionally the complete absence, of copper is the result of the action of ammonia which may arise from the decomposition of dead bodies and of carbonic acid, both of which agents, with the help of oxygen, attack the buried bronzes, and, dissolving the copper compounds by the subsoil water, leave only the insoluble tin oxide.
Upon the whole the foregoing remarks upon bronzes are equally applicable to objects of copper, which however appear to possess a greater power of resistance to the destructive action of carbonic acid and moisture, even where salt is present. This is probably due to the fact that the absence of tin and lead precludes any interaction between the compounds of these metals and those of copper. Copper objects with a sound so-called “noble” patina apparently do not occur.