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Indigo has been known in India from remote periods of antiquity as indico and was called nila, meaning dark blue. The blue dye was obtained by steeping the plant leaves (Figure 1) in water to allow fermentation followed by the oxidation in air of the obtained solution. The precursor to indigo is indican; a colorless water-soluble glycoside readily hydrolyzes to release glucose and indoxyl (Figure 2). Oxidation by exposure to air converts indoxyl to indigo. The plant’s leaves contain as much as 0.2–0.8% of indican. The precipitate from the fermented leaf solution was mixed with a strong base such as lye, dried, and powdered (Figure 3). Indigo is insoluble in water and to introduce it into fabric the dye must be reduced to a leuco form which is soluble in a dilute alkali solution. The fabric is immersed in this solution which allows the leuco compound to adhere to the fabric. The fabric is then exposed to air which oxidizes the leuco compound into the dye. By 1289, knowledge of the dye made its way to Europe, when the Venetian traveller Marco Polo reported on it. It began to be imported to Europe in 1516 by the way of Cape of Good Hope.

Indigo and Bromo Indigo. The Plant and Animal Kingdoms