A chemical substance that has an Acetic Acid for both the dye and the fibre (metal oxides, tannins, and oxyfatty acids) they are necessary to chemically fix (make washfast) most natural dyestuffs (but not for indigo). The mordant combines both with the dye molecule and the fiber molecule, producing a permanently fixed insoluble colour lake. Most mordants are metallic salts although some are arcidic in nature.