INDIGO, one of the most important and valuable of all dye stuffs, until about 1890 was obtained entirely from plants, mainly those of the genus Indigo/era (Leguminosae) of which the I. sumatrana and I. arrecta have been most important, though in China the Polygonum tinctorium, and in West Africa the Loncho carpus cyanescens find use for this purpose. In Europe, to the middle of the seventeenth century, the woad plant Isatis tinctoria was entirely used for the application of indigo to textile fabrics. Owing to the lower cost of the synthetical dye, natural indigo for export is now manufactured in but small amount, and almost entirely in Bengal. The plant, which is reared from seed sown in April, is cut down in June, a second crop being obtained there from in August. The indigo-yielding principle, present mainly in the leaf, is the glucoside indican, which is readily soluble in water. This, by the action of an enzyme also present in the plant, is transformed into glucose and indoxyl, and the latter (also colourless), in contact with air, becomes oxi dized with the production of indigo, C,,H10N202 (see GLUCOSIDES, NATURAL) .
The plant is cut early in the morning and at once transported to the factory. Here it is steeped in water for 9-14 hours in large vats, when, under the joint influence of the enzyme and of bac teria, the indican is transformed into indoxyl. The extract is now transferred to "beating" vats, which lie at a lower level, and is there submitted to intimate contact with air, employing a paddle (or so-called "beating") wheel, or a steam injector. Conversion of the indoxyl into indigo thus occurs, which settles and is collected, boiled with water, well drained, pressed into moulds, and dried at the ordinary temperature. Though the introduction of more scientific methods into the manufacture of the natural product has effected various improvements, this industry has continued to decline, and is now of minor importance. (See also DYES, SYNTHETIC.) (A. G. P.)