Mordant

A mordant is a mineral that fixes a natural dye to the wool and shifts its final color. Without it, many plant dyes would simply wash out; with it, they bind to the fiber and hold for generations.

The mordant is also a color tool. The same madder root, with a different mordant, can give a clear red, a rust or a brownish rose. Much of the dyer's skill lies in this control of mordant, heat and water, the same control that produces the soft tonal shifts of abrash.