Purple of Cassius

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.

This test was first observed and refined by a German physician and alchemist, Andreas Cassius (1600–1676) of Hamburg, in 1665.

[1] Richard Adolf Zsigmondy, who earned the 1926 Nobel Prize for chemistry, says that "Several of the red gold divisions prepared with formaldehyde as well as those reduced with phosphorus appeared perfectly clear in ordinary daylight (like good red wine).

In Thomas Graham’s dialysis, however, they behaved like colloidal suspensions: the gold particles did not pass through the parchment membrane.