Indigo - history

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chemical synthesis
industrial synthesis
    This dye has been used since Neolithic times in Europe. It was highly prized for its colour and light fastness. Until the end of the 19th century, the sole source was from plants, woad (Isatis tinctoria) and Dyer's Knotweed (Polygonum tinctorum) in temperate climates and Indigofera species in the tropics. Woad was widely grown in Europe, making some regions, especially Toulouse (France) and Erfurt (Germany), very wealthy until the end of the 16th century. After that time, it was used to make a woad vat for dyeing with indigo from India.

How the dye was produced in India ...
The cut plant is tied into bundles, which are then packed into the fermenting vats and covered with clear fresh water. The vats, which are usually made of brick lined with cement, have an area of about 400 square feet and are 3 feet deep, are arranged in two rows, the tops of the bottom or "beating vats" being generally on a level with the bottoms of the fermenting vats. The indigo plant is allowed to steep till the rapid fermentation, which quickly sets in, has almost ceased, the time required being from 10-15 hours. The liquor, which varies from a pale straw colour to a golden-yellow, is then run into the beaters, where it is agitated either by men entering the vats and beating with oars, or by machinery. The colour of the liquid becomes green, then blue, and, finally, the indigo separates out as flakes, and is precipitated to the bottom of the vats. The indigo is allowed to thoroughly settle, when the supernatant liquid is drawn off. The pulpy mass of indigo is then boiled with water for some hours to remove impurities, filtered through thick woollen or coarse canvas bags, then pressed to remove as much of the moisture as possible, after which it is cut into cubes and finally air-dried.
Nature 1 November 1900 
Indigofera tinctoria
Indigofera tinctoria

For more about the history and much more, see Kew Gardens

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