Synthesis of Indigo and “Vat Dyeing”

Indigo is a dark blue/purple dye that has been known since antiquity. Egyptian mummies have been found wrapped in cloth dyed with indigo. The natural source is a leguminous plant which is ground up to produce a juice which contains a colorless, water soluble precursor of the dye. If the juice is fermented, a colorless liquid is recovered. If this liquid is allowed to stand in contact with air, perhaps after soaking some fabric with it, a chemical reaction (oxidation) takes place, and a dark blue pigment (indigo) precipitates.From James R. McKee and Murray Zanger, Journal of Chemical Education 68, A242-244 (1991)
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