Mordant

Mordant is a substance used to fix dyes to fibers or to intensify stains in microscopy and histology. In dyeing, a mordant forms a coordination complex between dye molecules and the substrate, increasing the affinity of the dye for the fiber and improving wash-fastness and light-fastness. In histology, mordants are additives that allow dyes to adhere more firmly to tissue components, producing more durable stains.

Common mordants include metallic salts such as aluminum salts (often used as alum, potassium aluminum sulfate), iron(III) salts, tin, copper, and chromium salts. Organic mordants include tannins and other polyphenols that can form metal-dye complexes. Alum and related aluminum mordants are among the most widely used in traditional natural-dye processes; mordants may be used alone or in combination (dual mordants) to influence color and fastness.

Mechanistically, mordants act by introducing metal ions that coordinate with dye molecules, forming insoluble complexes that attach to the substrate. For protein fibers (wool, silk), the mordant can form ionic or covalent linkages that anchor the dye; for cellulose fibers and in microscopy, the metal-dye complex binds to tissue components and enhances contrast.

History and safety: The technique dates back to antiquity with natural dyes and later medieval and early modern dyeing. While mordants enable bright, durable colors, some metal mordants can be toxic or environmentally harmful; modern practice emphasizes safer alternatives and careful disposal of metal-containing effluents.