What It Actually Is

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.