Glass with multicoloured hues, translucid, with opaque veins, obtained by adding to the molten mass, say colourless transparent glass, a pigmented mixture based on different oxides (generally copper; iron, cobalt and tin) and metallic silver that also contains a reducing component (carbonor whatever); the mixture is partially blended with the molten mass and the whole is then mixed after some time.
The colouring effect is given both by the dissolution of the metallic oxides in the glass and by the formation of small colloidal particles of metallic silver and copper, smaller than the micro-crystals. This type of glass was already applied in Murallo in the XVI and XVII centuries but the use was later abandoned so much so that its formula was lost. Only in 1846 did the industrialist Lorenzo Radi, after a laborious seareh, manage to lay his hallds on it again and used it to produce articles, or blown in flat panes as complements of costly furniture.
The Venini & C. firm uses it, on the design of Napoleone Martinuzzi, to finish mouths and handles of large vases or for centre-table figures. In the early nineteenth century calcedonio is not used a great deal and this term has now come to mean a material consisting subslantially of a mixture of pastes that have already been melted in two or three colours. lt is now rare to come across true calcedonio, like that executed by the Cenedese firm in the 60s, purely for research.