A sign in alchemy for caput mortuum, the skull, death's head, the epitomy of decline and
decay, nigredo. Further data in the entry below.
This sign was also used in eighteenth century chemistry for residue, remainder, residuum.
Caput mortuum was also used to mean crocus metallorum, that is some brown-red metal compounds like crocus Martis, iron oxide, and crocus Veneris, copper oxidule.