Mordant
A substance, often a metal salt, that helps a dye grip the fibre and can change its colour.
Also: dye fixative
Many natural dyes will not stay on cloth by themselves. A mordant forms a bridge between dye and fibre. Alum, a salt of aluminium, is the most common and gives clear, bright colours. Iron darkens and dulls them; the same dye may give red with alum and purple or black with iron. Indian block printers use this to print several colours from one dye bath, by stamping different mordants on the cloth first. Plant tannins, such as those from myrobalan fruit, are also used. The word comes from the Latin for to bite.