Alum is a commonly used mordant. The odors produced from processing woad were legendary. The leaves were crushed and made into balls which were then allowed to ferment. After fermentation, the product was allowed to dry and then refermented before use to make it soluble. In the Middle Ages, woad was a common article of commerce in Europe.
Indigo produces an intense deep blue color.
The leaves and branches of the plant are harvested, placed in a vat, covered with water, and permitted to ferment. The sludge of partially rotted plant material which settles to the bottom is collected and pressed into cakes. When dry, these produce a powder that makes a colorless solution. The color only develops when an item is dipped into the solution, removed and then exposed to air. See the diagram on page 535.