Cranberry glass

Cranberry glass or 'Gold Ruby' glass is a red glass made by adding gold salts or colloidal gold to molten glass. Tin, in the form of stannous chloride, is sometimes added in tiny amounts as a reducing agent. The glass is used primarily in expensive decorations.

Production

Cranberry glass is made in craft production rather than in large quantities, due to the high cost of the gold. The gold chloride is made by dissolving gold in a solution of nitric acid and hydrochloric acid (aqua regia). The glass is typically hand blown or molded. The finished, hardened glass is a type of colloid, a solid phase (gold) dispersed inside another solid phase (glass).

History

The origins of cranberry glass making are unknown, but many historians believe a form of this glass was first made in the late Roman Empire. This is evidence by the British Museum's collection Lycurgus Cup, a 4th-century Roman glass cage cup made of a dichroic glass, which shows a different colour depending on whether or not light is passing through it; red (gold salts) when lit from behind and green (silver salts) when lit from in front.