thumb|200 px|[[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.]]
thumb|A French [[Indienne, a printed or painted textile in the manner of Indian productions, which used mordants to fix the dyes]]
A mordant or dye fixative<!-- more modern parlance? Now redirects here ---> is a substance used to set (i.e., bind) dyes on fabrics. It does this by forming a coordination complex with the dye, which then attaches to the fabric (or tissue). 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, they have been largely displaced in industry by substantive 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, and one example is chromium (III). The resulting coordination complex of dye and ion is colloidal and can be either acidic or alkaline.
Common dye mordants
Mordants include tannic acid, oxalic acid, alum, chrome alum, sodium chloride, and certain salts of aluminium, chromium, copper, iron, iodine, potassium, sodium, tungsten, lead, and tin.
Iodine is often referred to as a mordant in Gram stains, but is in fact a trapping agent.
Residual iron mordant can damage or fade fabric, producing "dye rot".
Dye lake
The dye lake is an insoluble complex formed upon combining the dye and mordant, which then attaches to the substrate. Mordants increase the fastness of the dye since the higher molecular weight dye is now bonded to the fibre.