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.
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).
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.
But neither of them knew the mechanism which yielded the colour. Chemist and winner of the 1925 Nobel Prize in Chemistry Richard Adolf Zsigmondy was able to understand and explain that small colloids of gold were responsible for the red colour.
Legend holds that cranberry glass was first discovered when a noble tossed a gold coin into a mixture of molten glass. This legend is almost certainly not true, as the gold must be dissolved in aqua regia before being added to the molten glass.
This glass had a white enamel fired onto the glass in a design, usually of romantic variety.