Blue Vitriol

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.

Blue Vitriol is identified without serious dispute as copper(II) sulfate pentahydrate, CuSO4.5H2O, the same salt sold today as an agricultural fungicide and a school chemistry demonstration piece; its natural mineral form is called chalcanthite. As with the other named vitriols, the color qualifier is what carries the modern identification, since the bare word vitriol could denote any sulfate salt of a heavy metal, and historians of chemistry including Lawrence Principe caution against assuming a specific compound wherever an early text uses the unqualified term.

The cementation reaction that so impressed alchemical observers is now explained by electrochemistry: iron is more easily oxidized than copper, so it gives up electrons to the copper ions in solution, going into solution itself as iron sulfate while metallic copper plates out. That the same experiment could be read either as a genuine metallic transmutation or as a straightforward displacement reaction, depending on the theoretical frame brought to it, is a clear case of a shared observation supporting two entirely different explanations, one alchemical and one modern, without either side having gotten the raw phenomenon wrong.