Ferrous Sulfate - Iron (II) Sulfate - FeSO4 - Crocus Martis

Crocus martis is calcined ferrous sulfate, iron (II) sulfate (FeSO₄), historically also called calcined copperas. It is a fine reddish-brown to purplish iron powder, and it has been used as an iron colorant far longer than the synthetic oxides that largely replaced it.

Potters use it in glazes and slips for warm reds, maroons and browns, and for the speckled, variegated surfaces that come from residual sulfate breaking down during firing rather than the flat uniformity of red iron oxide. It also serves as an artist's pigment and as a general laboratory iron source.