Making It and Using It

Alum was mined as a naturally occurring mineral salt, and also manufactured from aluminous shale or clay by roasting and leaching, across major production centers in the ancient and medieval Mediterranean world; the discovery and control of rich alum deposits at Tolfa in the Papal States in 1462 was significant enough to be treated as a matter of state and papal finance, since Christian Europe had previously depended heavily on alum imported from Ottoman controlled territories. Its overwhelming practical importance was as a mordant in dyeing: cloth pretreated with a solution of alum bonds far more strongly and permanently to many natural dyes than untreated cloth does, a chemistry not understood in modern terms until far later but exploited with real skill by dyers across the ancient and medieval world, whose finished colors depended directly on getting the alum treatment right.

Beyond dyeing, alum served as a tanning agent for leather, an astringent styptic in medicine to stop minor bleeding, and, in the alchemical laboratory specifically, as one of several acid salts distilled together with saltpetre and vitriol to help produce aqua fortis, its aluminium sulfate component contributing to the acid generating reaction alongside the more commonly cited vitriol and saltpetre. Its clean, colorless crystals and reliable, well understood behavior made it one of the more thoroughly practical, unmysterious substances in the whole alchemical repertoire.