indigo

/in"di goh'/, n., pl. indigos, indigoes, adj.

n.

1. a blue dye obtained from various plants, esp. of the genus Indigofera, or manufactured synthetically.

2. See indigo blue (def. 2).

3. any of numerous hairy plants belonging to the genus Indigofera, of the legume family, having pinnate leaves and clusters of usually red or purple flowers.

4. a color ranging from a deep violet blue to a dark, grayish blue.

adj.

5. Also called indigo-blue, indigotic. of the color indigo.

[1545-55; < Sp or Pg, var. of ÍNDICO < L indicum < Gk indikón, n. use of neut. of Indikós INDIC]

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Blue vat dye, obtained until about 1900 entirely from some species of the indigo plant.

Extraction of the dye was important to the economy of colonial America and remained so in India until the early 20th century. Synthetic indigo has replaced the natural dye; it is reduced chemically to the soluble yellow compound leucoindigo, in which form it is applied to textile fibres and reoxidized to indigo (see oxidation-reduction).

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▪ dye

 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.

▪ plant genus

      in botany, any shrub or herb of the genus Indigofera within the pea family (Fabaceae). Most species occur in warm climates and are generally silky or hairy. The leaves are usually divided into smaller leaflets. The small rose, purple, or white flowers are borne in spikes or clusters. The fruit is a pod, usually with a thin partition between the seeds.

      Some species, particularly I. sumatrana and I. arrecta, were once an important source of indigo dye. The cultivation of indigo plants and the extraction of the dyestuff were an important industry in India up to the beginning of the 20th century. Synthetic indigo, developed about that time, gradually replaced natural indigo as a dyestuff.

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