indigo, an important and valuable vat dyestuff, obtained until about 1900 entirely from plants of the genera Indigofera and Isatis. Indigo was known to the ancients of Asia, Egypt, Greece, Rome, Britain, and Peru. It is used in the United States mainly for dyeing cotton for work clothes; for a long time it was used to produce heavy (navy blue) shades on wool.
The naturally occurring precursor of indigo is indican, a colourless, water-soluble substance that is easily hydrolyzed to glucose and indoxyl; the latter is converted to indigo by mild oxidation, such as exposure to air.
The chemical structure of indigo was announced in 1883 by Adolf von Baeyer; a commercially feasible manufacturing process was in use by the late 1890s. The method, still in use throughout the world, consists of a synthesis of indoxyl by fusion of sodium phenylglycinate in a mixture of caustic soda and sodamide.
Indigo can be converted to numerous simpler compounds, but the only chemical reaction of practical importance is its reduction to the soluble yellow leucoindigo, in which form it is applied to textile fibres and reoxidized to indigo.
Tyrian purple, a dyestuff of great importance in antiquity, was obtained from a secretion of a sea snail (Murex brandaris) common in the Mediterranean. Its structure is very similar to that of indigo. It has never been produced synthetically on a commercial basis.