Mordant
A metal salt or other substance that helps a natural dye bind to the fibre and stay fast.
Also: fixative
Most natural dyes will not hold on cloth by themselves. A mordant is a substance, usually a metal salt, that joins the dye to the fibre and makes the colour resist washing and light. Alum, iron and sometimes tin or copper salts are common. The mordant also changes the colour: the same madder root can give red with alum and a dark brown or purple with iron. In block printing, the mordant is often printed on the cloth first, and the cloth is then put in the dye bath, so that colour appears only where the mordant went.