INDIGO (Sp. indigo, Indic°, from Lat. in dicum, from Gk. 11,5m6v, indikon, indigo, from *Ivola, India, India ). A coloring matter first employed as a dyestuff in India, whence it was brought by traders to the Mediterranean coun tries. Europeans were not acquainted with its vegetable origin until the time when its im portation assumed considerable dimensions. after the circumnavigation of Africa; and as the method of dyeing with woad did not present the indigo in substance. the virtual identity of the two was not suspected. Of late years. artificial indigo has been produced from coal-tar products, and is seriously threatening the existence of the indigo plantations. For the history of artificial indigo, see COAL-TAR COLORS.
of the natural indigo of commerce is ob tained from species of Indigofera, of the family Leguminosle. The genus embraces about 400 spe cies. widely distributed throughout tropical and subtropical regions. The best-known and most widely cultivated species. as well as the ones which supply nearly all the indigo found in the markets. are Indigofera anil, a native of tropical America. and Indigofera tinctoria. the original home of which is not positively known. although it is said to occur wild in the Bombay Presi dency. Indigofera argentea, a native of Arabia and parts of Africa, is a source of much of this valuable dye. Indigo plants are extensively cul tivated in the East and West Indies (especially in Bengal). in Central America. parts of Eu rope, and Africa, etc. The plants are shrubby, attaining a height of about six feet, have pinnately compound leaves, and usually pink or purple flowers. The pods of /ndigoRro anti are sickle-shaped, short and compressed. while those of Indigoftra linctorin are straight. cylindrical, and many-seeded. When cultivated the indigo plant a rich, friable soil, N't ell watered, but not to wet. Tilt' are sown in drills about a foot apart, and the plants are cut when beginning to flower. in the tropics this can lie done at frequent intervals, and four or five crop; a year obtained. After tutting. the crop is handled in various way,. fermented in tanks. as described and the indigo extracted. Although the plants are perennial, the greatest yields are obtained front annual plantings. in addition to the species mentioned above, at least half a dozen other specie- of Indigofera are known to produce indigo of good character. it is also produced by species of Sophora, Baptisia, Amondia. Tephrosia. and Galega. all belonging to the order Leguminos:e. Baptisia tinetoria and Amorpha fruticosa are known as 'false in (lino' in the United States, where they are widely distributed. Plants of other families produce indigo, as lsatis tinctoria, a ernciferons plant which was cultivated in Europe during the Mil die Ages and is still planted in Southern Franey (see Wo.?1) Wrightia, and :Cerium, which be long to the family Apocynacerc; Narsdenia tine toria and Gyinnema tingens. of the natural order Aselepiadaee•: Polygonum tinetorium. of the order Pol•gonace:e; Strobilanthes flae•hlifolius, of the order Acanthace:r; Spilanthes tinctoria (Composit:e), and Seabiosa succisa (Dips:l e:tenet. as well as many others, representing
widely separated orders of plants.
Neither the indigo plant nor Isatis tinetoria contains the dyestuff (indigotinl ready formed. hut rather a colorless glneoside. indieane, whieh breaks up, by fermentative processes, into indi gotin. and a glucose sugar called indiglu•in. It is a curious fact that indicant. is a normal constituent of human urine, and beeontes very plentiful in certain diseases. Natural indigo is prepared for the market in the following fashion: The plants are cut down ju•t before reaching the flowering stage. and are thrown into vats. where they are steeped in water and allowed to ferment for 12 to 15 hours. practically out of contact with air. This produces (lie soluble indigo white. which is taken up by the water: the liquid is then drawn off into 'beating-vats,' where it is violently agitated by machinery. in order to promote contact with the air, so that the indigo white is oxidized to the insoluble in digo Mite, which forms a thick on the surface and then sinks to the bottom as a bluish land. After settling, the clear liquid is drawn off. and the indigo blue is cone•ted, squeezed between cloth.. and dried in the air. Tt is sold in irregular lumps. which differ in tint, size, and according to the various localities in \Odell they are produced. The amount of available dyestuff ranges from 20 to 90 per cent. of the commercial product. sonic of the impurities hoing neeidental. although intentional adulteration is common enough. This irregu larity of composition in the natural product is one of the causes that have advanced the manu facture of artificial indigo. which is always chem ically pure. There are a large number of patented processes for the preparation of synthetic indi pain, eltielly held by the I:erman color-factories, and largely based upon the researches of Adolf Itaeyer, who showed. about ISsu, what the exact clientieal constitution of this substance is, and indicated the general principles for its prepara tion from aniline derivatives. At present. indigo is manufautured on the largest scale by the llenniann process. Plithalic acid, front naphtha It 111., is eonverted into anthranilie (ortho-amido benzoic) acid by way of the plithalimide, and the ant hranilie acid is transformed, by treat meta with ehlo•acetie acid and fusion with sodium hydroxide, into indoxyl, which readily oxidizes to indigo. According to the United States Census pato. three million pounds, or (me-quarter of the world', supply of indigo, are annually pro duced by synthesis. Pure italigotin, whether ex tracted front natural indigo or prepared artifi cially, is crystalline. with a coppery lustre and a very characteristic odor: when crushed, it fonts a blue powder. and when fixed upon the fibre it constitutes a remarkably fast and bril liant 'navy-blue' dye. A few chemical compounds, analogous to indigo. and of slightly different shades, have been produced: but they are as yet of very slight importance. :Mention may also be made of the so-called indiruhin, a red coloring matter of insignificant tinetorial value, which is found in natural indigo.