A
mordant is a substance used to set
dyes on fabrics or tissue sections 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. The term mordant comes from the present participle of French
mordre, "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. The resulting coordination complex of dye and ion is
colloidal and can be either
acidic or
alkaline.