Colcothar is the name given by the alchemists to the red powder (mainly red oxide of iron) which remains in the retorts when green vitriol or the sulphate of iron is calcined. It is used for polishing glass and the like, and is called crocus by artists.
Colcothar
Chambers's Encyclopaedia, Volume 3: Catarrh to Dion, p. 338
Source scan(s): p. 0349