In Bengal indigo cultivators revolted against exploitative working conditions created by European merchants and planters in what became known as the Indigo revolt in 1859. The Romans latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo. The association of India with indigo is reflected in the Greek word for the dye, indikón ( Ἰνδικόν, Indian). India was a primary supplier of indigo to Europe as early as the Greco-Roman era.
It was a luxury item imported to the Mediterranean from India by Arab merchants. The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. Indigo was most probably imported from India. In Mesopotamia, a neo-Babylonian cuneiform tablet of the seventh century BC gives a recipe for the dyeing of wool, where lapis-colored wool ( uqnatu) is produced by repeated immersion and airing of the cloth. The preparation of indigo dye is practised in college laboratory classes according to the original Baeyer-Drewsen route. This material readily decarboxylates to give indoxyl, which oxidizes in air to form indigo. The process is easier than the Pfleger method, but the precursors are more expensive. It involves heating N-(2-carboxyphenyl)glycine to 200 ☌ (392 ☏) in an inert atmosphere with sodium hydroxide. An alternative and also viable route to indigo is credited to Heumann in 1897. Variations of this method are still in use today. This highly sensitive melt produces indoxyl, which is subsequently oxidized in air to form indigo. In this process, N-phenylglycine is treated with a molten mixture of sodium hydroxide, potassium hydroxide, and sodamide. The first commercially practical route of producing indigo is credited to Pfleger in 1901. Johannes Pfleger and Karl Heumann eventually came up with industrial mass production synthesis. This route is highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but proved impractical for industrial-scale synthesis. It involves an aldol condensation of o-nitrobenzaldehyde with acetone, followed by cyclization and oxidative dimerization to indigo. The Baeyer-Drewson indigo synthesis dates back to 1882. Given its economic importance, indigo has been prepared by many methods. Light exposure during part of the dyeing process can convert the dibromoindigo into indigo, resulting in blue hues known as royal blue, hyacinth purple, or tekhelet. Natural sources of indigo also include mollusks the Murex genus of sea snails produces a mixture of indigo and 6,6′-dibromoindigo (red), which together produce a range of purple hues known as Tyrian purple. The powder was then mixed with various other substances to produce different shades of blue and purple. They precipitate from the fermented leaf solution when mixed with a strong base such as lye, pressed into cakes, dried, and powdered. The leaves were soaked in water and fermented to convert the glycoside indican present in the plant to indigotin. Indican was obtained from the processing of the plant's leaves, which contain as much as 0.2–0.8% of this compound. Oxidation by exposure to air converts indoxyl to indigotin, the insoluble blue chemical that is the endpoint of indigo dye. Indican readily hydrolyzes to release β- D- glucose and indoxyl. The precursor to indigo is indican, a colorless, water-soluble derivative of the amino acid tryptophan.
Several plants contain indigo, which, when exposed to an oxidising source such as atmospheric oxygen, reacts to produce indigo dye however, the relatively low concentrations of indigo in these plants make them difficult to work with, with the color more easily tainted by other dye substances also present in these plants, typically leading to a greenish tinge. In Europe, Isatis tinctoria, commonly known as woad, was used for dyeing fabrics blue, containing the same dyeing compounds as indigo, also referred to as indigo. In Central and South America, the species grown is Indigofera suffruticosa, also known as anil, and in India, an important species was Indigofera arrecta, Natal indigo. Until the introduction of Indigofera species from the south, Polygonum tinctorum (dyer's knotweed) was the most important blue dyestuff in East Asia however, the crop produced less dyestuff than the average crop of indigo, and was quickly surpassed in favour of the more economical Indigofera tinctoria plant. A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia. The primary commercial indigo species in Asia was true indigo ( Indigofera tinctoria, also known as I. A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus Indigofera, which are native to the tropics, notably the Indian Subcontinent.
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