This dye is, without doubt, the oldest in use; the Greeks and Romans a knowledge of its uses from India, l•bere its employment has been very general for a great length of time. Much obscurity Inv( Ives indigo and its early use, in consequence of the variation in its name; for instance, the Tamools of India call the plant arerie and the dye itself neelum • in Sanskrit, the plant is viskashodanie, and the dye nila and whence the aril of the Portuguese. The Malays call the dye taroom, and the Arabs, neei, Commercially speaking, indigo may be said to be the produce of India and Central America, as these are the only localities which supply the recognized form of the article. In India, tlfe chief sesit of the indigo manufacture, Bengal is the most important district. The total quantity received in Great Britain in 1876 was upwards of 88,000 cwts.—a vast quantity, when it is borne in mind with what difficulty it is cultivated and manufactured. When pure, indigo has a rich, dark-blue color, almost purple; it is in small cubes or parts of cubes, and its fracture shows a tendency to break up into square pieces, and. indicates cracks in its substance, often filled up with a Min of whitish efflorescence, prob ably the lime used in precipitating it. It has neither taste nor smell, and its specific gravity is about 1.50; if rubbed with any hard substance, it gives a streak with a bright coppery luster. The varieties recognized in commerce are—lst, Bengal, which, from the care taken in its preparation, and the large scale on which it is made in that district, is the best; and its various gradations of quality, ten in number, varying from 9s. to 5s. per lb., are always kept distinct. In other sorts they are usually much mixed. 2d, Madras and Kurpah; 3d, Oude; 4th, Manila; 5th, Java; and 6th, South America. The last is packed in serons or cases of dried ox-skin, and its qualities are as follows: 1st, fibres; 2d, sobres, and 3d, tortes; all the others are in wooden chests, con taining about 250 lbs. each.
Few materials are of greater importance to the dyer than indigo, and none require the exercise of more care and skill in using. Being insoluble in water, it requires the action of other solvents to render it capable of penetrating the fibers of the materials to be dyed. The method generally employed is the following: The indigo is broken into small lumps,.
i and these are soaked in hot water, and left for at least 48 hours, in order that the moisture may soak through and soften them; which they are put into the indigo-mill, which is a levigating machine, consisting of a vessel in which a roller is made to work by machinery, so as to rub down the indigo, mixed with plenty of water, to a very fine paste. This is a tedious operation; therefore, in large establishments there are usually numerous mills in the grinding-room. When sufficiently ground. the paste is removed to the dyeing-vat, where to one part of indigo is added one part of lime and three-fourths of sulphate a copper; these are well mixed with sufficient water to fill the vat, and the dyer then proceeds to dye either cotton, linen, or silk goods. See DYEING. After being dyed, the goods are • dipped into a bath of diluted sulphuric or hydrochloric acid, which gives brightness and purity to the color; they are then finished by washing in a stream of pure water, and drying.
Green indigo, called lodetto by the Chinese, is a substance resembling indigo, which is obtained from a tree called it is highly valued by the Chinese artists as a pigment, and also gives a beautiful permanent green color to cotton and silk cloths; it is, however,. so costly, that it never can,- unless differently prepared, be used as a dyeing material. The fact that the Chinese dye cotton cloths with it, is accounted for by the nature of the process of preparing the lo-kno, which is this: A well-macerated decoction of the bark of the horn-bi tree is largely diluted with water mixed with a little lime; pieces of cotton cloth are then dipped into the vat, and taken out and exposed to the sun, which changes them to a bright green; they are then placed in perfectly clean water, and agitated until the water has removed all the free coloring matter; this water is then evaporated, and the small sediment left -is the lo-kao. It is the cotton cloths thus used that are soid
as green-dyed goods. It is said that a similar dye stuff is obtained from another tree c lied and although this, as made by the natives, is much too costly to use in Euro pean dyeing, yet probably, if better means of obtaining it can be pointed out, it may become an import ant- article of• commerce.
Chemistry of IrodigO. —The plants which yield indigo present no indication. when rowing, that they contain any ehromogen, or matter capable of yielding pigment, nor is it definitely known in what form the indigo exists in the vegetable tissues.
The indigo of commerce is by no means a homogeneous body. Its essential and most important constituent is indigotin, or indigo blue, but it likewise contains indigo brown, indigo red, and other ingredients.
Indigo blue, or indigotin is obtained from commercial indigo by extracting the ingredients with which it is mixed by acetic acid, alkalies; and boiling alcohol. it occurs either as a dark-blue amorphous powder, or in purple, crystalline scales, with a metallic luster. It is devoid of smell and taste, and is insoluble in water, alcohol, ether, dilute acids, and alkalies. When carefully heated, it may be sublimed without decom position. Among the products of its destructive distillation are hydrocyanate and cal bonate of ammonia, aniline, etc. Indigo blue dissolves without any evolution of gas in strong sulphuric acid, forming a blue solution of sulphindigotic acid, which is extensively used for dyeing cloth, under the name of Saxony blue.
Under the action of reducing agents. such as alkaline fluids containing sulphate of iron, or a mixture of grape-sugar, alcohol, and strong soda lye, indigo blue becomes converted into indigo white or reduced indigo, which forms a yellow solution in alkaline fluids, but which, on free exposure to the air, absorbs oxygen, and is reconverted into indigo blue. Indeed, this is the best method of obtaining the latter in a state of purity from commercial indigo, of which it should form about 50 per cent.
Indigo blue occurs in small quantity in the urine of man, the horse, and the cow, and occasionally in the milk of the cow, when these fluids have been exposed for some time to the action of the air; but Schunck obtained it from the urine in so many cases (in the urine of 39 persons out of 40), that indican (or the cromogen-yielding indigo blue) must be regarded as a normal urinary constituent. See M. Schunck's paper in The Memoirs of the Literary and Philosophical Society of Manchester, 1857, vol. siv., or Day's Chemistry in its Relations to Physiology and Medicine, 1860, pp. Indigo white or reduced indigo. in a state of purity, occurs in white flakes, which are devoid of taste or smell, are perfectly neutral, and arc insoluble in water, but di.>olve in alcohol, ether, and alkaline solutions. Its composition is represented by the formula C.H6NO2, and as it only differs from indigo blue, in containing one more equivalent of H, it may be considered as the hydride of the latter. If yarn or woven goods be immersed in an alkaline solution of this substance till they are thoroughly saturated, and are then exposed to the air, indigo blue is formed within the fibers of the tissue. The blue dye thus obtained is very intense and permanent. From its property of becoming blue ou exposure to the air, indigo white is a sensitive test for the presence of free oxygen.
Many compounds of great chemical interest have been derived from indigo blue. It was from indigo that aniline (now so largely employed in the production of the pig ments known as mauve and magenta) was first obtained.