Indigo - chemistry

The chemical substances responsible for the blue colours in indigo and woad dyes are exactly the same, but are formed in different ways. In indigo, the naturally occurring precursor is a colourless water-soluble compound of indoxyl. Indoxyl is oxidized with oxygen from the air to produce indigotin, which is blue and insoluble.

Indigotin

A photograph of brick indigo from Kews Economic Botany Collection.
Image: Indigo dye can be dried and pressed to form brick indigo.

The blue of woad is also a result of indigotin. Here, a precursor called isatan-B hydrolyses to form the indoxyl leuco-indigo. Two molecules of this combine to form indigotin.

Both plants also contain indirubin and flavonols. Woad and indigo are called substantive dyes which do not need mordants to bind them to fabrics.

Synthetic indigo

In 1883 the German chemist Adolf Bayer discovered the chemical structure of indigo which then made it possible to produce the dye synthetically in the 1890s. His method is still used - this synthesises the indoxyl by fusion with sodium phenylglycinate in a mixture of caustic soda and sodamide. Indigo can be converted into other similar compounds but the only commercially important chemical reaction is the reduction of the soluble yellow indoxyl leuco-indigo which is the form in which it is applied to textile fibres from where it then reoxidizes to indigo and turns blue.