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Ferrous sulfate. Crystals of ferrous sulfate covered in a yellowish oxidised layer of basic ferric sulfate (mixed ferric sulfate and ferric oxide). Ferrous, or iron (ii), sulfate 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 iron (iii) sulfate Fe2(SO4)3 and iron (iii) oxide Fe2O3.
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