III. Observations on the changes which have taken place in some antient alloys of copper. By John Davy, M. D. F. R. S. In a letter to Sir Humphry Davy, Bart. Pres. R. S

Abstract

Permit me through you to lay before the Royal Society, the results of some experiments and observations on the incrustations of certain antient alloys of copper, which I trust may not he undeserving of notice, whether considered in connexion with the arts of Antient Greece, or in relation to the slow play of chemical affinities acting during a long period of time. I shall commence with the most interesting object that I have examined since I have been in the Mediterranean, for which I am indebted to His Excellency Sir Frederick Adam, His Majesty’s Lord High Commissioner in the Ionian islands. This is a bronze helmet of the antique Grecian form, recently found in a shallow part of the sea, between the citadel of Corfu, and the village of Castrades. Both internally and externally it is partially encrusted with shells, and a deposit of carbonate of lime. The surface of the helmet generally, both under the incrustation, and where freed from it, is of a variegated colour, mottled with spots of green, dirty white, and red. On minute inspection, the green and red patches exhibit a crystalline structure, and the red very distinctly; and on examining them with a lens, they were found to be formed of octohedrons of the red oxide of copper, intermixed with crystals of the same form of metallic copper. These crystals are most distinct at the surface; they even adhere to the calcareous crust, and may be detached with it, pre­senting well-formed facets towards the part from which they have been separated. Chemical examination shows that they are actually composed of metallic copper, without the least tarnish, and of the pure ruby protoxide of copper; and that the green rust consists principally of carbonate and sub-muriate of copper, and the dirty white chiefly of oxide of tin. The mineralizing process (if I may be allowed the expression) which has produced these new combinations, in general has penetrated very little into the substance of the helmet. The incrustation and rust removed, the metal is found bright beneath; in some places considerably corroded, in others very slightly, It proves on analysis to be copper alloyed with 18.5 per cent, of tin. Its colour is that of our common brass, and it possesses a considerable degree of flexibility.