Oil of Vitriol was made by roasting green vitriol (or a mix of vitriols) in air until it turned to a reddish powder called colcothar, then distilling that powder at strong, sustained heat in a retort; the vapors condensed into a heavy, oily, fuming liquid that etched glass, charred organic matter on contact, and released choking fumes, all properties that told chymists they had isolated something close to the vitriol's essential, corrosive spirit. Andalusian and later German and English chymists refined the method across the medieval and early modern periods, and by the seventeenth century oil of vitriol was being made in bulk by burning sulfur with saltpetre under a bell of water, the so called chamber process, a method that scaled dramatically once the Industrial Revolution needed sulfuric acid by the ton for bleaching, dyeing and, later, fertilizer manufacture.
In the workshop, oil of vitriol was prized as the strongest and most generally useful of the mineral acids: it dissolved metals other acids could not touch, it was mixed with common salt to drive off spirit of salt, and mixed with saltpetre it helped generate other strong acids and gases. Its extreme corrosiveness, capable of charring wood, cloth and flesh, was recorded with real caution by early modern writers, who handled it as one of the most dangerous substances in the laboratory.