Mordant

Most natural dyes do not bond directly to wool or cotton without a mordant. The mordant, typically a metallic salt such as alum, iron, copper, or chrome, creates a chemical bridge between the dye molecule and the fibre. The same dye with different mordants can produce completely different colours: madder root with alum gives red; with iron, it gives dark brown or black; with chrome, it gives a burnt orange.

Traditional Moroccan natural dye practice used locally available mordants: alum (potassium aluminium sulphate), iron-rich water or iron filings, tannins from oak galls or pomegranate rind (which act as both a mordant and a dye source), and occasionally chrome (in later periods, from synthetic sources).

Mordanting skill was and is a critical part of the dyer's knowledge, controlling colour and lightfastness through the choice and application of mordant.