Purple of Cassius is a purple pigment formed by the reaction of gold salts with tin(II) chloride. It has been used to impart glass with a red coloration (see cranberry glass), as well as to determine the presence of gold as a chemical test.
Generally, the preparation of this material involves gold being dissolved in aqua regia, then reacted with a solution of tin(II) chloride. The tin(II) chloride reduces the chloroauric acid from the dissolution of gold in aqua regia to a colloid of elemental gold supported on tin dioxide to give a purple precipitate or coloration.
When used as a test, the intensity of the color correlates with the concentration of gold present. This test was first observed and refined by the German Physician and Alchemist, Andreas Cassius (1600–1676) of Hamburg in 1666.