copperasn.

1. A name given from early times to the protosulphates of copper, iron, and zinc (distinguished as blue, green, and white copperas respectively); etymologically it belonged properly to the copper salt; but in English use, when undistinguished by attribute or context, it has always been most commonly, and is now exclusively, applied to green copperas, the proto-sulphate of iron or ferrous sulphate (Fe SO₄), also called green vitriol, used in dyeing, tanning, and making ink.The extension of the name beyond its etymological meaning is anterior to its appearance in English, and indeed inherited from Greek χάλκανθον, the description of which by Dioscorides gives prominence to blue vitriol, while its use as shoemaker's ink implies green vitriol; the same is true of Pliny's account of chalcanthum. It is probably that, at all times, the occurrence of composite salts containing a variable proportion of copper and iron, as well as the failure to distinguish between copper and iron pyrites, contributed to the confusion. It has to be remembered also that from the medieval point of view ‘copperas’ was a species, occurring in various colours, the difference of composition being only vaguely apprehended, and that the phenomenon of the dissolving of iron by a solution of green copperas, with deposition of its copper, was explained as conversion of iron into copper by the mediation of the ‘copperas’, which changed its colour from blue to green in the process.