Green Vitriol

Green Vitriol formed naturally wherever iron pyrite weathered in damp air, a slow oxidation that early chymists reproduced on purpose by stacking crushed pyrite or old iron in open heaps and letting rain and air do the work over weeks or months. The resulting liquor was boiled down and left to crystallize into pale green, translucent rhomboid crystals with a metallic, puckering taste, sold across Europe under names like copperas or sal martis, the salt of Mars, since iron was Mars's metal. Workshops used it by the barrel. Tanners and dyers needed it to fix black dyes and to make ink; the standard iron gall ink of medieval and early modern Europe was made by steeping oak galls, rich in tannic acid, in a solution of green vitriol, the tannin and iron combining into the durable black or blue-black pigment found in manuscripts and account books for over a thousand years.

In the laboratory proper, green vitriol was one of the three vitriols regularly distilled for its spirit, a strongly acid liquor driven off by heating the dried salt in a retort until it glowed red. That spirit, and the oily residue left behind, were what later chymists learned to call oil of vitriol. Georgius Agricola and other sixteenth century writers on mining and metals described its extraction from mine waters in detail, and it remained an important commercial chemical, made at industrial scale, well into the age of modern chemistry.

Modern chemistry identifies Green Vitriol cleanly as iron(II) sulfate heptahydrate, FeSO4.7H2O, one of a family of hydrated sulfate salts that lose water in stages when heated and that are collectively called melanterite in their natural mineral form. The identification was never seriously contested; unlike many alchemical names, vitriol of a specified color mapped onto a definite modern compound once the concept of a metal cation combined with a fixed sulfate anion was understood in the eighteenth century, chiefly through the work of Torbern Bergman and other Swedish and French chemists who systematized salt nomenclature. What took longer to sort out was the word vitriol itself, which unqualified could mean any of several sulfate salts; Lawrence Principe and other historians of chemistry note that early modern authors were not always careful to specify which vitriol they meant, and undifferentiated mentions of vitriol in older texts cannot always be assigned to iron with confidence. The color qualifier, green for iron, blue for copper, white for zinc, is the detail that actually resolves the identification, and where a text omits it modern readers are working from probability rather than certainty.

The ferrous, iron(II), state is also chemically unstable in air, which is why old green vitriol crystals develop a rusty brown crust of oxidized iron(III) sulfate on standing, a transformation early chymists observed without the electron-transfer chemistry to explain it.