mordant

Latin mordere (to bite, to gnaw) gave Old French mordre and the present participle mordant (biting). In medieval and early modern textile practice, mordants were metallic salts — alum, iron, copper, tin — that formed chemical bonds between the dye molecule and the fiber. Without a mordant, most natural dyes would wash out: the color would not fix. The mordant bit into both, holding them together. Different mordants produced different colors from the same dye: madder root with alum mordant gives a bright red; madder with iron mordant gives a deep purple-brown.

Alum (potassium aluminum sulfate) was the most important mordant in European textile production from the medieval period onward. The main source of alum in medieval Europe was the Papal States — the alum mines at Tolfa, discovered in 1462, gave Pope Pius II a near-monopoly on European alum supply. The Medici bank managed the Tolfa alum trade. Flemish and Italian wool-dyers depended absolutely on the Pope's alum; the alum trade funded papal projects and shaped diplomacy across the continent.

The Ottoman Turks controlled Anatolian alum sources, and the Ottoman-papal alum conflict was a significant economic dimension of 15th-century geopolitics. When the Tolfa mines were discovered by a Genoese entrepreneur, Bartolomeo de Buti, it was seen as a divine provision against the Ottoman threat — the Pope's alum would fund the Crusade against Constantinople. The mordant, the small mineral salt that made color stick, shaped papal finance and Christian-Ottoman relations.

The word mordant also entered aesthetic criticism: a mordant wit is a biting one, sharp and corrosive. The metaphor was available from the start — the biting mineral that fixes color is the same concept as the biting remark that fixes an impression. The mordant in the dye vat and the mordant in the sentence both work by the same chemistry: they make things stick.