The blue colour obtained from woad plants is a mixture of blue indigo dye (or indigotin) and smaller quantities of other dyes including the reddish dye indirubin. Indigo and indirubin are insoluble in water, and most other common solvents, and do not dissolve with heat or vigorous stirring. The woad vat is the process by which the dye is traditionally made soluble.
Woad plants and other indigo yielding plants contain precursors of indigo dye, rather than indigo itself.
Woad plants contain two indigo (or indoxyl) precursors, indican (25%) and isatan B (75%) where indican is colourless, water-soluble, and stable, whilst isatan B is relatively unstable.
Hydrolysis of isatan B and indican releases the intermediate compound indoxyl from which indigo or indigotin is subsequently formed.
Transforming the precursors into indigo is usually now performed chemically as in the process described in Chemical Extraction, but traditionally this was carried out by bacteria through the couching of woad balls.
Steeping the woad leaves in warm water breaks down the waxy coating on the leaves releasing the precursors indican and isatan into the water. By adding soda ash you raise the pH making the solution alkaline which transforms the indican into molecules of indoxyl. When you whisk the solution you add oxygen which allows two molecules of indoxyl to join forming indigo. Isatan can also combine with indoxyl to form indirubin which is not as light fast as indigo and gives a red tinge to woad-dyed fabric. Indigo and indirubin are insoluble in water and they settle in the bottom of the container as a blue pigment.
There is almost no oxygen inside the woad ball and the bacteria Clostridium isatidis converts most of the precursors into indoxyl.
As mentioned above, indigo is not soluble in water. In order to make the blue powder soluble, and therefore, useful to the dyer, the woad pigment must be immersed in an alkaline solution, and reduced (i.e. the oxygen must be removed). This process is often called ‘making a woad vat’. Soda ash makes the solution alkaline and spectralite or sodium dithionite remove the oxygen. This transforms the indigo into leuco-indigotin (also called indigo white), a yellowish soluble dye. Leuco-indigotin is different from indoxyl; unlike indoxyl, leuco-indigotin is a double molecule; removing the oxygen from the indigo does not break down the bonding.
It is at this stage that you immerse the wool or cotton into the woad vat and the leuco-indigotin attaches itself to the fibre. When you take the fibre out of the vat, the oxygen in the air combines with the leuco-indigotin which then becomes indigo and fixes itself to the fibres. You see this happening as the colour changes from greenish yellow to blue.
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Last updated on 10 January 2022
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