INDIGO, the name of a genus of plants, and of the blue coloring matter obtained from them. The indigo plants are tall herbs of the pea family, forming the genus Indigofera, of which there are several color-yielding species in various warm parts of the world. The one yielding the indigo of commerce, and formerly extensively cultivated, is I. tinctoria, which is native to India, grows five feet high and has pinnate leaves. .The coloring matter most abounds in the leaves, and especially at the time of flowering, and that is the time when the crop is gathered by cutting down the plant, and making immediate use of the green stems or foliage, or by drying them for subsequent treat ment. This coloring matter is a chemical sub stance called indican, the glucoside of indoxyl, which is converted by oxidation into indigo. Until the discovery of the sea-route to India the only blue vegetable dye available in Europe was that derived from the woad (q.v.), which was limited and costly; this dye-substance was therefore regarded as one of the most valuable of new commodities and a large capital was soon invested in its cultivation in India, Cey lon, China and other where a profitable industry continued until after the middle of the century. The indigo is obtained by mac erating the leaves and stems in vats for several hours. Fermentation arises and the water be comes clear yellow. It is then run off into a lower basin, where it is subjected to incessant agitation and gradually turns green, whereupon the indigo begins to form in flakes and settle. The residuum is then thoroughly boiled, filtered through linen, •molded into small cakes, and dried. The best quality comes from Bengal and eastern India. Indigo plantations were made with more or less success in Brazil, Cen tral America and Mexico; and one of the fore most inducements held out to settlers in the southern colonies, from Maryland to Louisiana, was the probability of its successful cultivation there. The experiments never yielded results of much importance, partly because crops of tobacco, cotton and foodstuffs were more profit able. Since the discovery of cheap methods of forming blue dyes from coal-tar the cultiva tion of indigo has declined greatly, but still supplies a steady demand from cloth-dyers who wish an imperishable blue of certain tints.
The•wild indigo of the United States is any of several species of a closely related genus Baptisia, which flourishes especially in the Southern States. The best known is the yel low-flowered false indigo (B. tinctoria), or in digo brown, from which country people obtain a blue dye and a domestic medicine.
Indigo Before it can be employed in dyeing, the indigo must be brought into solu tion; anti as indigo itself is insoluble, it must be first transformed into a soluble substance, so that it can penetrate the pores of the cloth, where it is subsequently again restored to the i form of indigo. To bring the indigo into solu tion it is ground up to a soft paste with water, after which it is thrown into vats along with ferrous sulphate, slacked lime and water. The ferrous sulphate reacts with the lime to form calcium sulphate and ferrous oxide, the latter being immediately oxidized at the expense of part of the oxygen of the indigo, which in its turn is reduced to a substance called indigo white. This dissolves in the presence of excess of lime, and the fabric to be dyed is dipped into the vat after the liquid in it is clear. On removing the fabric the indigo-white which has penetrated its pores is reoxidized by the air to indigo-blue; and by repeating this treat ment a shade of blue of any desired depth may be obtained. The dyed fabric is finally passed through dilute acid to remove any adhering lime or ferric oxide. Indigo appears to exist in the plant in the form of a glucoside known as lindican,° which has the formula CoallasNION, and to be developed from this glucoside in the course of the fermentation by the action of a special bacillus, which closely resembles the bacillus of pneumonia. Indigo is now made artificially, the total production of synthetic indigo being probably about one-fourth of the world's consumption. Although artificial in digo-blue appears to have the same chemical formula (Cia-LiNs02) as the natural product, and to be identical with it in every way, it is more expensive than the natural product at the present time. If it could be made more cheaply it would work as great a revolution in dyeing as did the introduction of alizarin in the place of madder. See Com. TAR Comas and the references there given.