Caput mortuum
Caput mortuum is a Latin term used in alchemy to signify 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, and crocus veneris.