Mordant

A mordant or dye fixative is a substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to the fabric (or tissue).[1] It may be used for dyeing fabrics or for intensifying stains in cell or tissue preparations. Although mordants are still used, especially by small batch dyers, it has been largely displaced in industry by directs.[2]

Mordant red 19 is a typical mordant dye. Like many mordant dyes, it features the azo group (RN=NR) and various sites for chelating to metal cations.
A French Indienne, a printed or painted textile in the manner of Indian productions, which used mordants to fix the dyes

The term mordant comes from the Latin mordere, "to bite". In the past, it was thought that a mordant helped the dye bite onto the fiber so that it would hold fast during washing. A mordant is often a polyvalent metal ion, often chromium(III).[3] The resulting coordination complex of dye and ion is colloidal and can be either acidic or alkaline.