Indigo dye

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11(9)17-13(15)14-16(20)10-6-2-4

390–392 °C

decomposes

Indigo dye is an organic compound with a distinctive blue color (see indigo). Historically, indigo was extracted from plants, and this process was important economically because blue dyes were once rare. Nearly all indigo produced today — several thousand tons each year — is synthetic. It is the blue of blue jeans.

Contents

Uses

The primary use for indigo is as a dye for cotton yarn, which is mainly for the production of denim cloth for blue jeans. On average, a pair of blue jean trousers requires 3 – 12 g of indigo. Small amounts are used for dyeing wool and silk.

Indigo carmine, or indigotine, is an indigo derivative which is also used as a colorant. Approximately 20M kilograms are produced annually, again mainly for blue jeans.[1] It is also used as a food colorant, and is listed in the USA as FD&C Blue No. 2, and in the European Union as E Number E132.

Natural indigo

Relevant plant sources

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. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as Indigofera sumatrana). A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia (Japanese: リュウキュウアイ. Chinese: 馬藍/山藍). In Central and South America the two species Indigofera suffruticosa (Añil) and Indigofera arrecta (Natal indigo) were the most important. In temperate climates indigo can also be obtained from woad (Isatis tinctoria) and dyer's knotweed (Polygonum tinctorum), although the Indigofera species yield more dye.

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