What It Actually Is

White Vitriol is zinc sulfate heptahydrate, ZnSO4.7H2O, structurally almost identical to green vitriol (iron) and blue vitriol (copper), the three forming a natural family of hydrated divalent metal sulfates that early modern chemists began to recognize as a family well before they understood why the pattern held. The identification is secure and uncontested for the color qualified name; historians of chemistry such as Lawrence Principe treat the qualified vitriols, unlike the bare unqualified word, as reliably mapping onto a single modern compound once the color is specified.

What is more historically interesting than the chemistry is the metal underneath it: zinc's slow recognition as a distinct element, rather than a variant or impurity of other metals, meant that white vitriol was chemically characterized and used in commerce for centuries before anyone could have named the metal it actually contained, a reminder that alchemical and early chemical classification tracked substances by observable behavior long before it could ground that behavior in elemental composition. The parallel with calamine, another zinc material whose recognition as a zinc substance lagged well behind its practical use, is not a coincidence: zinc's late isolation as a distinct metal is exactly why both white vitriol and calamine kept descriptive, place based or color based names for so long instead of a name built on the metal itself.