Religiously traumatic and militarily ominous, the fall of Constantinople in 1453 also created a crisis for the European textile industry. The problem, in a word, was alum.
To securely attach to fibers, most dyes need chemical help: a mineral salt known as a mordant, from the Latin word mordere, meaning to bite. Allowed to saturate the material before dyeing, the mordant bonds with the fibers and provides a bridge to bond with and fix the dye.
Alum, a potassium or aluminum sulfate, is the most important mordant. It "is no less necessary to dyers of wool and woolen-cloth than bread is to humankind," wrote Vannoccio Biringuccio in his 1540 book De la Pirotechnia.
By the Middle Ages, alum mining, production, and trade were big businesses—the first international chemical industry. The typical alum operation mined alunite, a mineral found in volcanic areas, then heated the rocks in a kiln and repeatedly poured water over them until they formed a paste. The paste was then boiled and decanted to get rid of insoluble compounds, resulting in a saturated solution that crystallized into purified alum, writes Virginia Postrel.