In preparing indigo from dried leaves, the ripe plant being cropped is to be dried in sunshine from 9 o'clock in the morning till 4 in the after noon during two days, and thrashed to separate the sterns from the leaves, which are then stored up in magazines till a sufficient quantity be collected for manufacturing operations. The newly-dried leaves must be free from spots, and friable between the fingers. When kept dry, the leaves undergo, in the course of four week, a material change, their beautiful green tint turn ing into a pale blue grey, previous to which the leaves afford no indigo by maceration in water, but subsequently a large quantity. Afterwards the product becomes less considerable. The dried leaves are infused in the macerated steeping vat, with six times their bulk of water, and allowed to macerate for two hours, with continual stirring, till all the floating leaves sink. The fine green liquor is then drawn off into the beater vat ; for if it stood longer in the steeper, some of the indigo would settle among the leaves and be lost. Hot water, as employed by some manufacturers, is not necessary. The process with dry leaves possesses these advantages, that a provision of the plant may be made at the most suitable times, inde pendently of the vicissitudes of the weather, the indigo may be uniformly made, and the fermen tation of the fresh leaves, often capricious in its course, is superseded by a much shorter period of simple maceration.
Dr. Walker mentions, in his Account of the Productions of Humamkunda in the Dekhan, that only one species, Indigofera tinctoria, is there used for the preparation of indigo, and it is collected in the rains, when the dye is commonly made. A strong decoction is made of the plant, —leaves, flowers, pods, and twigs being all indis criminately thrust into a pot ; when this is hot an infusion of Eugenia jambolana (rose-apple tree) is added, the indigo is immediately precipitated, and, the superincumbent water being drawn off, is dried in the sun.
The native plan of mounting the indigo vat merits attention. A potash ley is prepared from the ashes of the Euphorbia tiruculli (milk bush hedge) and lime, by mixing them together and then filtering ; in this ley seeds of the Trigonella fcenum-grecum and Cassia tora are boiled, and, the liquor being strained, is poured into the water drawn off after the precipitation of the indigo, and the indigo itself is then put in, and some more potash ley is added.
In three or four hours the fermentation is per fected, and the vat filled for the purposes of the dyer. The theory of this vat is very obvious : extractive matter derived from the liquor in which the indigo was first boiled, with the sugar, starch, and mucilage of the two leguminous seeds, cause a fermentation by which the indigo is rendered soluble in the alkaline solution. The process is more simple than that usually followed by dyers in Europe, and is in perfect accordance with every rule of practical chemistry. There is no super fluity and no waste, and on the whole it is a most favourable specimen of native ingenuity and skill. The indigo plant is cultivated in China to a great extent, as well as a species of polygonum, from the leaves of which a colour is produced which nearly equals the blue obtained from indigo.
With the Javanese, who of all the Malayan race have certainly made the highest progress in all the useful arts, there is a specific term for dyeing or tinting,—madall ; but the Malays express it only by Chalup, the word for dipping. Yet tho only generic words which either of them possesses for colour, are the Sanskrit Varna and the Portuguese Tints. Their colours are usually sombre,—little varied, but generally fast. Blues are always pro duced from indigo, yielded for the most part by the Indigofera tinctoria, as in other parts of India, but in Sumatra occasionally from the Marsdenia tinctoria, a plant of the natural order of the Asclepiacere. Kulaf or vasma, in the Panjab, is the pounded dried leaves of the indigo plant, used principally as a hair-dye after the previous application of henna (Lawsonia inermis). The powdered leaf of Indigofera anil is used in the cure of hepatitis.—Capper's Three Presidencies; Powell's handbook; Mr. Rohde's MSS.; Simmonds ; Royle's Hint. Botany; Annals of Indian Adminis tration, 1870; Bonynge, America ; Tomlinson's Diet.; Sirr's China; Cal. Cat. Ex., 1862; Carnegy.