Chromic Oxide: Potassium Dichromate Ignited with Sulphur

Chromic oxide, Cr2O3, is the green oxide of chromium: a pigment of great permanence, and the oxide that aluminium reduces to chromium metal. Thorpe (1912) gives Lassaigne's way: potassium dichromate with half its weight of flowers of sulphur, ignited in a crucible, and the mass digested with water. Dichromate is a carcinogen and the spare sulphur burns to sulphur dioxide: respirator, gloves, and work outdoors. The picture: chromium(III) oxide (Wikimedia Commons).