Making It and Using It

Common Salt reached the alchemical workshop by the same routes it reached every kitchen in Europe, mined as rock salt from deposits such as those at Wieliczka in Poland, or evaporated from seawater and inland brine springs in shallow pans, a trade so old and so essential that salt taxes and salt roads shaped medieval European economics and politics well beyond chemistry. In the laboratory it was valued less for its own properties than for what it could be turned into: distilled together with oil of vitriol, common salt yields spirit of salt, the strong acid known today as hydrochloric acid, one of the most useful reagents in the whole alchemical repertoire. It also served as a flux in metalworking and as a preservative and mordant outside the laboratory proper.

Because it was so common and so cheap, common salt attracted less alchemical mystique than the vitriols or the named preparations of the metals, and writers who wanted to discuss salt as a philosophical category more often meant the Paracelsian principle than the substance on the table, a distinction not every reader kept straight then and one that still causes confusion in casual accounts of alchemy today. Common salt was, nonetheless, studied seriously as a representative of the whole class of neutral salts, crystals that were neither acid nor alkali, and its cubic crystal habit was noted by observers centuries before crystallography could explain why salts crystallize in characteristic geometric shapes.