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Indigo
Several different plants produce the dye indigo that is used to dye fabric blue. Plants of the genus Indigofera produce the largest quantities of the pigment and grow best in the tropics. Commercially available indigo powder comes from Indigofera tinctoria. Different species of Indigofera grow in Asia and in Central and South America. Polygonum (Japanese indigo) and Isatis (woad) are, however, better for temperate countries and a different genus (Lonchocarpus) grows well in Africa.
Indigo does not require that the fibre be mordanted beforehand and is easier to use on cotton than most other natural dyes.
Indigo is not, however, soluble in water, making it a challenging dye to use. It needs to be made soluble in a vat where oxygen has been removed either by fermentation or with a chemical. The fibre is dipped in the vat, and the soluble indigo combines with the fibre. When the fibre is exposed to the oxygen in the air, the indigo reverts to its insoluble blue form.
Natural indigo is expensive to produce, 20 tonnes of indigo leaves produce only 45 kilos of pigment. And therefore when the much cheaper synthetic indigo was discovered in the early 1900s, it quickly superseded natural indigo for commercial dyeing.
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