Page:Archaeological Journal, Volume 8.djvu/455

ANTIENT BRITISH AND ROMAN DEAD.S. llb'i bility of its solution under ordinary circumstances, wc can readily explain why leadless glass should be employed in chemical manipulation. Now, in respect to hardness and freedom from decomjio- sition, I have long fancied that I could remark a difference in British and many Roman beads. I have before me examples of the latter, which are of a light bluish-green colour, nmch corroded, and powdered over with a white substance. In these the colouring matter is still the same, namely, Copper ; but the white powder, on analysis, proves to be Lead. Now Lead, under the combined agencies of atmospheric causes and carbonic acid, becomes converted into a carbonate of that base, and hence the greater amount of corrosion and decomposition observable in these examples of Roman or leaded beads, when compared with the British or leadless ones. With respect to the colouring matter, it is now known that Copper, in the form of protoxide, was used by the antients to impart a variety of tints to their glass, the variations of which the same substance is capable being produced by the difference of combination and manipulation ; hence yellow, ruby,^ green, and blues, of various shades, can be obtained from the same basis. It is a curious fact that Sir Humphry Davy did not find Copper in any specimen of antient blue glass, but always Cobalt. In a jDaper, " On the Colours used in Painting by the Antients," he sa^^s, — " I have examined many pastes and glasses which contain Oxide of Copper ; they are all bluish- green, or of an opaque watery blue. The transparent blue vessels which are found with vases in Magna Grcecia, are tinged with cobalt ; and on analysing different antient trans- parent blue glasses, which Mr. ][illigan was so good as to give me, I found cobalt in all of them." And further he remarks, — " I have examined some Egyptian pastes, which are all tinged blue and green with Coi)per ; but, though I have made experiments on nine different specimens of antient Greek and Roman transparent blue glass, I have not found Copper in any, but cobalt in all of them." In all the examples of Roman blue glass which I have obtained from Corinium, inclusive of the example under consideration, being antient glass of the British period, the ' See Analysis of Ruby glass, in Illustrations of Roman Remains of (.'nrinium, p. ho.