vitreous paste with a dark base, red when transparent, with multicoloured veins, imitating a variety of natural chalcedony, zoned agate. Invented at Murano in the mid-15th century, its difficult preparation requires the addition of various metallic compounds to the glass mixture, in specific ways and at specific intervals. The secret of its manufacture, lost between the end of the 18th century and the first half of the 19th, was rediscovered by Lorenzo Radi, during the last century