Woad--Sarah's Natural Colour 

Chemistry - changing woad juice to indigo

The precursors in woad -- Isatan A, Isatan B and Indican, amoung others, must be precipitated out of the leaves through chemical or enzymatic action.  In the making of woad balls this happens when the enzymes on the leaves are macerated into the juice of the plant, which contains the precursors. In modern extraction methods the waxy coating on the outside of the leaves is broken with heat and the precursors are released into a solvent of water.   This precursor rich liquid is then oxygenated at a high pH in order to chemically transform the precursors into indigo.   Chemically, the isatans and indican precursors form molecules of indoxyl. Two indoxyl molecules join to form indigotin (the main molecule in indigo). Free radicals of isatan can also join to form indirubin, an isomer of indigotin, and its red component. The formation of indirubin is discouraged when indigo pigment is desired, since it decreases the purity of the product.  However, indirubin is being examined for its medicinal properties and it may be important to know how it is formed from indigo precursors.