When one speaks of made in Italy, one cannot fail to include the ancient tradition of glass mosaic. The introduction of glass paste tesserae dates back to the Romans: the glass, obtained by solidifying silicon oxide, is opacified by a compound of lead and tin, and coloured by adding mineral salts. The tesserae can be coloured in a wide range of shades, while gold tesserae are obtained by inserting a gold leaf between two layers of glass: the lower one red or green, and the upper one transparent.