Mordants are used in combination with natural dyes to make the colours lightfast and washfast. Some Mordants, like alum, prepare the cloth to receive the dye and do not add their own colour, others, like myrobalan, tannin and iron, will impart a colour to the cloth and can be used to significantly alter the colour of a dye. It is the combination of dyes and mordants that will yield a rich spectrum of colours.
Indigo is alone among natural dyes in that it requires no mordant.
