Blue Vitriol was gathered from the blue efflorescence on weathered copper ores and mine workings and was also made deliberately by exposing copper to air, moisture and, where available, the sulfurous fumes of roasting ore, then leaching and crystallizing the liquor into its characteristic deep blue, glassy crystals. Cyprus and the Rio Tinto district of Spain were major historical sources, worked since antiquity; the vitriol traveled as a valuable trade good under names that advertised its origin, Cyprian vitriol or Roman vitriol among them. Dyers used it to fix certain colors, engravers used it in etching baths to bite copper plates, and it entered pharmacy as a caustic and emetic, applied with considerable caution given its toxicity in any real dose.
A striking laboratory demonstration, recorded from antiquity onward, was cementation: an iron nail or blade left standing in a strong solution of blue vitriol emerges coated in a film of native copper, the iron having dissolved in its place. Early chymists took this as proof that one metal could transmute into another, since iron seemed to become copper before their eyes; the truer explanation, that dissolved copper ions and metallic iron were simply exchanging places, was not available until oxidation and reduction were understood as a transfer of charge rather than a change of elemental identity. Blue vitriol was also, alongside the other vitriols, a standard source of vitriolic spirit under distillation.