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indigo (adjective, colour term; synonym: violet-blue) – the hue of the visible light spectrum located between violet and blue. The human eye typically perceives monochromatic light with a wavelength somewhere between 400 and 475 nm as indigo. The name for the colour refers to a violet-blue dye originally derived from the vascular plant Indigo (Isatis tinctoria L.). The dying process involves fermentation of plant material together with natural fibers or woven cloth. When fibers or cloth are removed from their fermentation bath, they are initially yellow or yellow-brown. Oxidation than turns the yellow colour into a violet-blue, the colour of indigo. The length of the fermentation bath and its concentration regulate how deep the staining will occur. Although a fairly labour-intensive process, indigo was easily cultivated and the dye therefore not as expensive as orchil or litmus (a purple dye). The isolation of anil, a colourless, oily substance from Isatis tinctoria was the first step towards artificial synthesis of a series of cheap blue dyes that soon were widely used for workman's clothes (blue jeans). Today, tar provides the raw material for the synthesis of anil and virtually no garment is stained with indigo anymore.

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