Ferrous sulfate - (FeSO4) has been known as "copperas" since ancient times. In crystalline form it exists as blue-green heptahydrate. Ferrous sulfate was used in the manufacture of inks, particularly iron gall ink, which was used from the middle ages until the end of the 18th century. It is used as a mordant in wool dyeing and as "harewood" in marquetry and parquetry. Like all iron(II) salts, FeSO4 is a reducing agent. When exposed to air, it oxidizes to form a corrosive brown-yellow coating of basic ferric sulfate, which is an adduct of ferric sulfate and ferric oxide.