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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