Those who know Latin will have deciphered ‘dead head’ (nothing to do with the Grateful Dead) – a term that originally came from alchemy, and applied to the useless residues that ensued from alchemical processes – in paintings of alchemists, the ‘dead head’ is often quite literally represented by a skull (thus putting paid to all those skulls as simple ‘memento mori’).
As a pigment, the name was applied to the ‘worthless remains’ resulting from the oxidization of iron, ergo iron oxide, ergo rust. The resulting colour is also known as Cardinal Purple, and was commonly used by historical painters to identify important personages – donors, for example – the colour purple being historically associated with royal personages. As a naturally occurring haematite, it was used in ancient wall painting – synthetic oxides produced after the nineteenth-century led to the creation of varying hues, including something called ‘Mars violet’ – which sounds more like science fiction than a naturally-occurring ancient earth pigment.
I should say that caput mortuum has also, historically, been applied to something called ‘mummy brown’, evidently used in the sixteenth and seventeenth centuries and derived from – grinding up mummies! That’s a story for another time –
Here it’s used by fifteenth-century Netherlandish painter Petrus Christus in the robes of a female donor: