Most fabrics dyed with plant dyes need to be treated with a
mordant to produce a light and water-fast colour but woad can be
used without a mordant. Traditionally a fermentation vat was
used to dye with woad, often started with stale urine but madder
roots can also be used to start the vat. The use of both madder
and urine are discussed under the Fermentation method, below. 1.
Fermentation Method The fermentation removes the oxygen from the
vat making the woad soluble. Wool or fabric is dipped into the
vat, left for a few minutes, and then removed. At first the wool
is pale yellow, but with exposure to air, the colour gradually
turns to green and then to blue. Repeated dippings create a
darker and darker blue. 2. Chemical Method Nowadays, chemicals
such as spectralite are often used to remove the oxygen from the
vat. It can be difficult, however, to achieve a dark blue when
using a Spectralite vat with silk, as the chemical removes the
colour almost as fast as it is deposited. It is, however, easier
to get a