- a substance used in dyeing to fix the coloring matter, especially a metallic compound, as an oxide or hydroxide, that combines with the organic dye and forms an insoluble colored compound or lake in the fiber.
late 15c., "caustic" (of words, speech), from M.Fr. mordant, lit. "biting," prp. of mordre "to bite," from L. mordere "to bite or sting" (see smart (v.)). Related: Mordantly. The noun sense in dyeing is first recorded 1791; the adj. in this sense is from 1902.
"Sulfuric acid is a relatively recent manufactured chemical. Prior to this, the important analogous chemical substances were the sulfate salts of iron, copper, and aluminum, known to the ancients as the vitriols... They were used as mordants in the dyeing industry. In order for natural dyes to be fixed in the cloth—and not be washed out during the next rainy day—it is necessary to treat the cloth with a mordant. The mordants widely used in dyeing were solutions of the vitriols."
- David Rickard, "The Many Faces of Fool's Gold", May-June 2016 American Scientist (http://www.americanscientist.org/issues/feature/2016/3/the-many-faces-of-fools-gold/1)