Literally meaning "Death's Head," the Caput Mortuum (also known as nigredo) is an alchemical term for a useless substance left over after an alchemical working. In artwork and in notes, it was represented by a stylized skull. This evolved into a circle containing three dots forming a triangle with one point down.
A form of iron oxide (rust) derived from manufacturing sulfuric acid produced a purple pigment used in paints. Since rust was a "useless" substance left over after an (al)chemical working, this pigment was known as caput mortuum or cardinal purple. The meaning of the name also resulted in it being an alternative name for Egyptian brown or mummy brown, as it was originally made from ground up mummies! When artists learned of the source of this color (in the 1800s) they stopped using it.