Copperas (Er. couperose), a metallic salt, known also as green vitriol. It is a hydrous ferrous sulphate or protosulphate of iron, and is represented by the formula FO,SO3,7HO. It forms pale green transparent prismatic crystals, which are very soluble in water. When exposed to a gentle heat they lose their water of crystallization and are converted into a whitish pulverulent mass. Copperas slowly absorbs oxygen from the air, with the formation of a basic sulphate of peroxide of iron, of a rusty reddish yellow color; this change takes place more rapidly when it is dissolved in water. Copperas has a peculiar metallic styptic taste, which belongs to the other pro-tosalts of iron. It is largely used in the arts for dyeing black, and in the manufacture of ordinary black writing ink. It is also employed in photography as a reducing agent, and in many other chemical processes. Pure copperas may be obtained by dissolving metallic iron in dilute sulphuric acid; hydrogen gas is evolved, and a green solution is obtained, from which crystals are got by concentration and cooling. In this way considerable quantities of copperas are manufactured from the refuse sulphuric acid which has been employed in the refining of petroleum.
The chief supply of copperas is however obtained from the oxidation of the native sulphides of iron, some of which spontaneously absorb oxygen from moist air, and become covered with an efflorescence of copperas. In the case of other varieties of pyrites, which are less readily oxidable, they are first subjected to a partial calcination, by which a part of the combined sulphur is expelled, and then piled up in heaps, which are frequently moistened and lixiviated With water from time to time. The solutions thus obtained are concentrated by evaporation over a fire in lead or iron tanks, and then led into crystallizing vats, where the copperas separates on cooling in large masses of fine green crystals. The process of oxidizing a large quantity of pyrites is very slow, and requires many months or even years for its completion. Large quantities of copperas are manufactured by this method at Stratford in Vermont.