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.