INDIGO, ('Gr. Indikon, Indian), a most important vegetable dyestuff, yielding a beau tiful blue and very durable dye, the basis also of the best black dye in woolen cloths. It has been used in India from a very early peried, and was imported thence by the ancient Greeks and Ron cans, but was lost to Europe during great part of the middle ages —although the cultivation of • the plant and preparation of the dye were described by Marco Polo in the 13th c.—until reintroduced by the Dutch about the middle of the 16th century. Its use in England, France, and Saxony was then for a considerable time pre vented by a strong prejudice against it, arising from the difficulty experienced in fixing the color. Since this has been overcome the cultivation of plants producing indigo, long confined to India, has extended to many other tropical and subtropical countries, as Egypt, the West Indies, Mexico, Brazil, etc. These plants generally belong to the genus indigofera, of the natural order iegyminoscs, sub-order papili'onacecs. The keel of the corolla is furnished on both sides with an awl-shaped spur. The species of this genus number at least 150, and are natives of almost all tropical akd subtropical countries. Of these, 1. tinctoria is the species most generally cultivated in India. It is a half shrubby plant, 2 to 3 ft. high, with pinnate leaves, which have five or six pair of long obovate, dull, bluish-green leaflets, anti racemes of axillary pale red flowers.
The province of Tint:levelly produces a great quantity of indigo. Bengal produces, on an average, about nine millions of pounds annually, The sum which Europe annually pays for indigo is estimated at eight or ten millions of pounds sterling.
Indigo is, however, obtained from plants of other genera, particularly from TV)-ightia tinctoria (natural order apoeynacece), East Indies; baptisia tinctoria (natural order legu• minosce), North America, which yields indigo. of a pale color and very inferior quality; tephrosia tinctoria (natural order leguminosce), Malabar; and T Apollinea, Egypt and Nubia; marsdenia tinetoria (natural order aselepiadaeece), in Sylhet; and polygonum tine Corium and P. chinense (natural order polygonacete), China and Japan. tinctoria is a large shrub, indigenous to great part of India and to Ceylon, yielding Indigo of the *finest quality, and is recommended by Dr. Roxburgh for cultivation, as dependent than the common indigo plants on rain and irrigation. It grows' very freely, and throws out shoots rapidly on their 'being cut away.—In times when East Indian indigo was not known, or was brought to Europe only in small quantity, the same dyestuff was obtained from woad (q.v.).—A coarse kind of indigo, called bastard indigo, was also at one time
made in North America from the young shoots of amorpha ecernlea.
The Manufaeture and Applications of indigo plant, in its general appear ance, is not unlike the lucerne of our fields. The seed is sown in drills about 10 in. apart, and soon makes its appearance above ground, when it requires incessant care tc keep the weeds down, which otherwise would soon choke so tender a crop. In about •three months the plants begin to flower, and are then cut clown, but soon shoot up again, and yield a second cutting, sometimes a third, the same year. Formerly, indigo was carefully dried after being cut, and even fire-heat was sometimes used for the purpose, but now—at-least in India—the practice is abandoned, and it is found in every respect better to use the plait whilst fresh and green. The first process is to pack a large vat full of the freshly cut indigo; heavy wooden beams are placed on the top to press it and fix it doIvi; and water is then let into the vat, enough just to cover it. Being left •in this state for from 10 to 12 honrs. fermentation is set up, and much gas disengaged, the water becoming a light-green color. The green liquor is then run off into the second vat, which is placed below the level of the first, in which, whilst the fermentation pro cess is being repeated upon a fresh supply in the first vat, it is violently agitated by being beaten with poles: this causes the grain, as it is called, to separate, and the green matter suspended in the liquor becomes blue and granular. When this operation is sufficiently advanced the contents of the vat are allowed to settle, and the sediment is then run into the third vat, which is below the level of the second; from which it is pumped into a boiler. The boiler is slightly heated, and then allowed to stand for a few hours, during which time the indigo settles down, and as much clear water as • possible is drawn off from above it. The boiler is then again heated, and this time up to the boiling-point; after which its contents are allowed to run on to a frame of wood, lined with " cloth " sheeting, where they remain to drain till about the consistence of very thick cream, when they are removed, and subjected to very heavy screw pressure; and when as hard and dry as ordinary soap, are cut by brass wre on a frame into cubes about 3 in. square; and these are laid out, so as not to touch each other, on the shelves of the drying house. Finally the cakes are cleaned, one by one, and tightly packed in boxes for the market.