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.