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Indigo chemistry

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Wild Colours natural dyes > indigo dye > indigo chemistry

Chemistry of Indigo Dye

1) Indigo – A Blue Dye
2) Dye Precursors in the Indigo Plant
3) Transforming indigo precursors into indigotin
4) The Indigo Vat - Making Indigo soluble
5) Different shades of blue

1) Indigo – A Blue Dye
The blue colour obtained from indigo plants is indigo dye (or indigotin as it is often called) together with small amounts of impurities. Indigo is insoluble in water, and most other common solvents, and will not dissolve with heat or stirring. The indigo vat is the traditional process by which the dye is made soluble.

Indigo plants contain precursors of indigo dye, rather than indigo itself.

Indigo chemical structure2) Dye Precursors in the Indigo Plant
The main precursor in indigo plants is indican which is colourless, water-soluble, and stable, as well as being un-reactive to light and warm temperature.

3) Transforming indigo precursors into indigotin
Steeping the indigo leaves in warm water breaks down the waxy coating on the leaves releasing indican into the water. Adding soda ash raises the pH making the solution alkaline and transforming indican into molecules of indoxyl. When you whisk the solution, you add oxygen which allows two molecules of indoxyl to join forming indigotin or indigo pigment. This pigment is then insoluble in water and it settles on the bottom of the container.

4) The Indigo Vat - Making Indigo pigment soluble
To make the blue powder soluble, and therefore, useful to the dyer, indigo pigment must be immersed in an alkaline solution and reduced (that is the oxygen must be removed), a process called ‘making an indigo 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.

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It is at this stage that you immerse the wool or cotton in the indigo vat and leuco-indigotin attaches itself to the fibre. When you take the fibre out of the vat, leuco-indigotin combines with oxygen in the air to become indigo and fixes itself to the fibres. You see this happening as the colour changes from greenish yellow to blue.

5) Different shades of blue
Natural indigo contains from 20% to 70% indigotin and may also contain indirubin, indigo gluten, indigo brown and mineral matter which add shades and subtlety to the colours obtained. There is a noticeable difference between the hues of blue obtained with couched woad, and the blues from indigo and Japanese indigo plants.

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Last updated on 30 January 2014
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