Purple of Cassius

Permanent purple pigment historically used to make ruby glass and to create vitreous ceramic glazes used in painting, porcelain, and coloring enamels. It is synthesized by precipitating fine gold particles onto tin oxide particles. Adding potassium carbonate to the solution deepens the purple hue. The pigment is named for Andreas Cassius who first made it in 1676. It was largely replaced in the 19th century by the less costly equivalent cobalt violet.