Woad has been grown for centuries in Great Britain and prepared for market as a mordant or fixer for indigo dye. Its use in dyeing requires much skill, and its preparation is a lengthy process.

woad offered blue and yellow was derived from weld. Plants were soaked in hot water and wool was immersed in the dye bath. Stale urine may have been used as a mordant to fix dyes. Diodorus Siculus ...