Copperas

Copperas, also known as green vitriol or ferrous sulfate, is a chemical compound with the formula FeSO4. It is an iron salt that commonly appears as a blue-green crystalline solid. Copperas is soluble in water, forming an acidic solution. It occurs naturally as the mineral melanterite.

Historically, copperas was an important industrial chemical. It was widely used in the production of inks, particularly iron gall ink, which was a staple writing medium for centuries. It also found applications in dyeing, acting as a mordant to fix colors to fabrics. In medicine, it was sometimes used as an astringent and for treating iron deficiency anemia, though its use in modern medicine is limited.

The production of copperas historically involved the slow oxidation of iron pyrite (fool's gold) in the presence of air and water. This process, known as "bog iron" formation, could occur naturally in certain environments. Industrial production methods were developed to obtain larger quantities more efficiently. Today, while still produced, its applications have largely been replaced by more modern or safer alternatives. However, it still sees use in water treatment as a flocculant and in agricultural applications as a soil amendment to correct iron deficiencies.