The purple pigment that is used in colouring molten glass and porcelain known as Purple of Cassius discovered in the 17th century was found by Richard Zsigmondy in 1897 to consist of nano gold with stannic oxide. 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 dissolved gold to elemental gold supported on tin dioxide to give a purple precipitate which is thermally stable and is used as a pigment to give purple colour to glass and porcelain.