Vinegar

Vinegar was made, as it still is, by exposing wine or ale to air and allowing it to sour over days or weeks, a transformation early observers attributed to a further, spoiling fermentation acting on the alcohol, distinct from the first fermentation that had turned grape juice or grain mash into wine or ale in the first place. Distillers found that vinegar could itself be distilled to concentrate its sourness, yielding distilled vinegar, a purer and notably stronger acid than the ordinary kitchen liquid, used in the workshop wherever a mild acid was called for and the strong mineral acids would have been too destructive. Its most important alchemical role was as a solvent and reagent for making verdigris, the green copper corrosion pigment, produced by exposing copper plates or shavings to vinegar fumes over an extended period until a green crust formed on the metal's surface.

Vinegar also served in medicine as a mild astringent and preservative, and it was used across many recipes, alchemical and culinary alike, as a gentle acid for dissolving lime, chalk and certain metal oxides where a stronger acid would have overreacted or destroyed the material being worked. Its comparative mildness, rather than any dramatic property, was exactly what made it useful.

Vinegar is identified as dilute acetic acid, CH3COOH, dissolved in water alongside the residue of whatever wine or ale it was made from, an identification with essentially no historical ambiguity since the substance has been continuously produced and used, under the same name, from antiquity to the present. Where the alchemical account differs from the modern one is in mechanism rather than identity: early modern writers described vinegar's formation as a further fermentation of wine, a natural continuation of the same process that had made the wine alcoholic in the first place, whereas modern microbiology recognizes it as a distinct biochemical process, the oxidation of ethanol to acetic acid carried out by acetic acid bacteria rather than by the yeast responsible for alcoholic fermentation proper.

The alchemical framing was not wrong about the observable sequence, wine left in air does turn to vinegar, but it collapsed two biologically distinct transformations, fermentation and bacterial oxidation, into a single continuous process, a simplification the underlying chemistry does not support. The distinction matters beyond pedantry: vinegar making and wine making require different, incompatible conditions, wine fermentation wants to exclude air while vinegar production needs it, a practical difference vintners and vinegar makers managed skillfully long before anyone could explain why the two processes needed opposite handling.