Caput Mortuum is a Latin term meaning 'death's head'. In alchemy, it signified a useless substance left over from a chemical operation such as sublimation. Alchemists represented this residue with a stylized human skull, a literal death's head. In its current limited usage, the caput mortuum represents decline and entropy. Alchemy is an early protoscientific practice combining elements of chemistry, physics, astrology, art, semiotics, metallurgy, medicine, mysticism, and religion. ... The thermodynamic entropy S, often simply called the entropy in the context of thermodynamics, is a measure of the amount of energy in a physical system that cannot be used to do work. ...

In art, Caput Mortuum is a dark purplish-red earth coloured pigment used in oil painting. It contains iron oxide primarily. The name for this pigment may have come from the alchemical usage, since iron oxide (rust) is the useless residue of oxydization. In biology, pigment is any material resulting in color in plant or animal cells which is the result of selective absorption. ... Girl With Red Dress, Oil on wood panel painting by Angel Botello (1983) Oil painting is done on surfaces with pigment ground into a medium of oil - especially in early modern Europe, linseed oil. ... Iron oxide pigment There are a number of iron oxides: Iron oxides Iron(II) oxide or ferrous oxide (FeO) The black-coloured powder in particular can cause explosions as it readily ignites. ...