Oil of Vitriol

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.

Oil of Vitriol is sulfuric acid, H2SO4, in a concentrated form, one of the cleanest and least contested identifications among all the named alchemical liquors; John Emsley and other historians of chemistry treat it as effectively synonymous with its modern name once the color qualified vitriols themselves are understood, since oil of vitriol is simply what a vitriol's acid spirit becomes when fully concentrated by heat. Its recognition as a distinct, powerful acid, rather than merely one variety of generic corrosive water, is usually credited to the medieval Latin alchemical tradition and to writers working in the orbit of Jabir ibn Hayyan's school, though the exact chain of transmission from Arabic sources to the European recipe that survives is debated by historians and cannot be traced to a single named discoverer with confidence.

What is not in doubt is the substance's importance afterward: sulfuric acid manufacture became, from the eighteenth century onward, a benchmark by which historians of industry measure a nation's chemical capacity, precisely because the alchemical process of distilling vitriol scaled so cleanly into an industrial one. Later economic historians have gone so far as to track a nation's industrial development partly by its annual sulfuric acid output, a modern echo of how central this one distilled vitriol became to chemistry far beyond the workshop that first isolated it.