This list shows in addition that this high chlorine-content is a distinguishing feature of the patina of Egyptian bronzes, as is only to be expected from the character of the Egyptian soil (vide pp. [1], [2] et seq.); in fact, although in most cases qualitatively only, I have proved the existence of chlorine in each Egyptian bronze without an exception. The destructive nature of chlorine is not often apparent in bronzes recently excavated, which usually show an apparently sound, dark green patina with a smooth surface, sometimes like malachite or azurite; personally I have not met with any bronze object from Egypt which could be said to have a patina deserving the name of “noble” patina. Not till some time, or it may be not till years after the objects have been placed in museums does the change become apparent, as has been so strikingly described by Mond and Cuboni (see page [27 ]). The varying amount of moisture in our atmosphere undoubtedly influences the spread of the patina, which, if the application of a preservative is delayed, gradually eats into the bronze. The adjoining figures (Fig. [9] to [12]) of the same bronze before and after the process of preservation show distinctly such ravages, whereby the surface has been in some places eroded to a depth of 2 to 3 mm. In other cases, especially hollow bronzes, the thin walls have been completely perforated. The explanation of these processes is found in the experimental work of Krefting[63], and also in the treatise by Berthelot, from which extracts have already been given. The theory enunciated by Mond and Cuboni, that the “wild” or spreading patina is due to the action of bacteria, cannot now be maintained, for not only do chemical reactions give an adequate explanation of the process, but these observers have failed to transplant the bacteria; nor were the experiments of Dr Stavenhagen, undertaken at our request, more successful. That certain bacteria are capable of attacking metal, as for example the metal lettering on books, is an established fact, while the universal distribution of bacteria will naturally lead to their presence upon bronzes and their patina. The application of heat checks chemical change by driving off the moisture, and therefore arrests the spread of a patina for some time, until by penetrating the oxidized layer the moisture and carbonic acid can again act upon the patina and the underlying metal. As has been already stated in the passage from Dingler’s “Polytechnic Journal” quoted above, I have observed the renewed formation of efflorescence upon a bronze statuette which had been thus sterilised. This, it may be urged, was a case of re-infection: it is, however, strange that Mond and Cuboni do not refer to chlorine as a component of the patina. The presence of chlorine may have been overlooked; it cannot well have been absent, for in every case of rodent patina I have found without exception chlorine in the bright green efflorescences, whatever may have been the original source of the bronze.
Fig. 9.
Bronze Pasht showing destructive patina.
Fig. 10.
The same after treatment (Finkener’s method).
Fig. 11.
Bronze Pasht showing destructive patina.