Colours of Roman Glass
It is evident that the Romans had at their command a full gamut of colours, both transparent and opaque, obtained from iron, copper, manganese, and antimony—the same metals, in fact, as the Egyptians made use of. But their deep transparent blue they probably obtained, in most cases, from cobalt, a metal unknown to the latter people.[[26]] There was one great deficiency, however, in their palette. They were never able to obtain a transparent red. The ruby red derived from copper or from gold was known to the early mediæval alchemists, but no undoubted instance of the use of this valuable colour has been observed in glass of the classical period.[[27]] The nearest approach to a transparent red is to be found in the honey and brown-red tints resembling the sard and the hyacinth; colours such as these are derived chiefly from iron, and may pass, on the one hand, into a pale yellow, and on the other into various shades of olive-green. The opaque red glass containing a large percentage of the basic oxide of copper and also some oxide of tin,[[28]] was much admired by the Romans; it was probably the vitrum hæmatinon of Pliny. In the Gréau collection is a head of Neptune in this material, of considerable artistic merit; to this head the oxidation of the surface has given the appearance of a finely patinated bronze.