Alchemy and chemistry
In alchemy, Caput Mortuum (alternately called nigredo) signified a useless substance left over from a chemical operation such as sublimation and the epitome of decline and decay; alchemists represented this residue with a stylized human skull, a literal death's head.
The symbol shown on this page was also used in 18th century chemistry to mean residue, remainder, or residuum. Caput mortuum was also sometimes used to mean crocus metallorum, i.e. brownish-red metallic compounds such as crocus martis (ferrous sulphate), and crocus veneris (copper oxidule).
Caput mortuum (variously spelled caput mortum or caput mortem), also known as Cardinal purple, is the name given to a purple variety of haematite iron oxide pigment, used in oil paints and paper dyes. It was a very popular colour for painting the robes of religious figures and important personages (e.g. art patrons).
The name for this pigment may have come from the alchemical usage, since iron oxide (rust) is the useless residue of oxidization. It was originally a byproduct of sulfuric acid manufacture during the 17th & 18th centuries, and was possibly an early form of the copperas process used for the manufacture of Venetian red and copperas red.
Caput mortuum is also sometimes used as an alternative name for mummy brown (alternatively, Egyptian brown), a pigment that was originally made in the 16th and 17th centuries from ground-up mummies, and whose use was discontinued in the 19th century when artists became aware of its ingredients.
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