caput mortum

a) An inorganic pigment of brownish-red or purple to brownish-purple colour. Its chemical composition is iron oxide with traces of manganese, Fe2O3. It is produced by burning ferrous sulphate or from the residue left over in the process of obtaining sulphuric acid from pyritic ores; b) A brownish-red or purple underpainting under the pigments, also called morello.