Next, a little pure cold water, or weak lime-water, is sprinkled over the surface of the liquor, to hasten the settlement of the fecula, which occupies 3-4 hours. After this, the water ie drained away from the top, by means of plug-holes in the side of the vat. The precipitated fecula is then removed to a boiler. Here it is made to boil as promptly AS possible, and is kept boiling for 5-6 hours ; it is constantly stirred, and skimmed with a perforated ladle. After boiling, it is run off to a straining table, whore it stays for 12-15 hours to drain ; next it is pressed for about 12 hours, aud then cut, stamped, and placed to dry. The ordinary dimensions of a steeping-vat are 16 ft., by 14 ft., by 4} ft. deep ; this will contain about 100 mounds (8200 lb.) of plants, which may yield from 40 lb. downwards of indigo. The beating-vat is less deep.
Such are the methods of cultivation and manufacture most generally in use throughout India. In limited districts, however, some modifications are in vogue. On land Subject to inundation, the plants last only one year. South of the Ganges, the seed is sown at the beginning of the rains, and the plants remain on the ground for two years, thus giving a double crop, the second of which ie the larger aud better. In very strong land, a third crop ie sometimes secured. Occasionally, Sesame is sown on the samo ground, and harvested before the indigo is out. Small quantities of indigo are grown on peppy lands, and irrigated. The seed is sown in March-April, and the crop is gathered at the cud of tho rains, in time for an opium crop to be taken off the land. Indigo is sometimes manufactured by collecting the fecula, and dropping it in cakes to harden in the sun ; this is "gaud " indigo, of very inferior quality. The fceula is improved by boiling it in coppers, and pressing it into boxes. The production of the indigo blue is a result of the decompositiun of the colouring principle of the plant, which exists as a glucoeide. Plants grown on poor soils, and in dry climates, yield almost the whole of this glueoside to the ordinary process of steeping and beating, described above ; but plants raised on rich alluvial soil, and in damp heat, contain an amount of gluooside which cannot he utilized by the ordinary process. In order to prevent this waste, which causes the richest plants to give the least return, it ie necessary either to prolong the fermentation, and raise the heat to 35°-38° (95°-100° F.), or to add a solution of sugar or glucose to the vat-liquor. Olphcrt adopts the use of steam, to raise the temperature of the vat to 44° (111° F.), and thus obtains 25 por cent. more colouring matter.
The exports of indigo from British India, in 1878, were 120,605 cwt. About one-half of the total production comes from Behar and Bengal, especially from the districts of Tirhoot, Chum parun, and Sarun ; the best comes from Kishnagur, Jeseore, Moorshedabad, and Tirhoot. It is one of the most precarious of Indian crops, being very liable to the attacks of insects, and governed in a great measure by the seasons. The relative values of the various Indian brands in the London market are about as follows :—Bengal, fine violet, 8s. 6d.-Ss. 9d. a lb. ; good red violet, 8s. 4d. 8s. 6d.; mid. and ord. violet, 7s. 6d.-Ss. 3d.; mid. to good violet and copper, 7s. 3d. 7s. 10d.; mid. to fine, 6s. 10d.-7s. 9d. ; low and ord., 3s. 6d.-6s. 9d.; Kurpah, good to fine, 5s. 6d.-7s.; low to mid., 2s.-5s. 5d.; Oude, plantation, 4s. 6d.-6s. 6d.; native, 2s. 6d.-4s. 6d.; Madras, Vellore, 10d.; native, Is. 9d.-4s. 4d.
Indigo culture extends very widely beyond India. In Cochin China, the plant is cultivated on light alluvial soils, of upper Tertiary age, where floods are of very rare occurrence. The native mode of preparing the dye is very rude; but several Frenchmen possess factories for the purpose, at Saigon and Cambodia. Small quantities of indigo arc produced in Siam. Various plants (mentioned above) arc cultivated extensively in all the provinces of China, for the production of the valuable dye ; Canton exported 547 piculs (of 1331 lb.) in 1878. Japan possesses several large factories for preparing indigo from the native Polygonum tinctorium. The plants, 2-3 ft. high, are cut into three parts, the uppermost being the most valuable. The best dye is made from the leaves alone, which, after a few hours' exposure to air and sun, are placed in straw bags. They are after wards removed from the bags, and moistened with water, which must be proportioned with the greatest exactitude. They are then spread upon, and covered by, mats, for a few days, after which the sprinkling is repeated. The process continues for about 80 days, the moistening being renewed about 25 times for the best leaves, and 9 for the inferior. After this fermentation, the leaves are
pounded in wooden mortars for two consecutive days, by which they are reduced to a pulp ; this is then formed into balls of dark-blue colour. The central provinces of Java yield large quantities of indigo, which are exported to Holland, and thence widely distributed. The indigo prepared by the natives is of an indifferent quality, in a semi-fluid state, and contains much quicklime ; but that prepared by Europeans is of a very superior quality. An inferior variety, having smaller seeds, and being of quicker growth, is usually planted as a second crop on land where one rice crop has been raised. In these situations, the plant rises to the height of about 34 ft. It is then cut, and the cuttings are repeated three, or even four, times, till the ground is again required for the annual rice crop. But the superior plant, when cultivated on a naturally rich soil, not impoverished by a previous heavy crop, attains a height of 5 ft., and grows with the greatest luxuriance. The plants intended for seed are raised in favoured spots, on the ridges of rice-fields in the neigh bourhood of the villages, and the seed of one district is frequently exchanged for that of another. That of the rich mountainous districts, being esteemed of best quality, is occasionally introduced into the lowlands, and is thought necessary to prevent that degeneration which would be the consequence of cultivating for a long time the same plant upon the same soil. The climate, soil, and state of society of Java seem to offer peculiar advantages for the extensive cultivation of this plant. The periodical droughts and inundations of the Bengal provinces are unknown in Java, where the plant, in favoured situations, may be cultivated nearly throughout the whole year, and where it would be secure of a prolonged period of that kind of weather, necessary for the cutting. The dye is prepared in a liquid state by the natives, by infusing the leaves with a quantity of lime ; in this state, it forms by far the principal dye-stuff of the country. The indigos prepared in Java by Sayers' process are of unusually high and constant quality. They contain an average of 70i per cent. of indigotine, and a minimum of 65-66 per cent. ; and an average of 2.77 per cent. of ash. Ordinary commercial indigos seldom attain 65-66 per cent. of indigotine ; and their ash averages about 14 per cent. The exports from Java, in 1878, were to Holland, 867,973 lb. ; Italy, 19,496 lb. ; Port Said for orders, 26,957 lb. ; Singapore, 107,594 lb. The Philippines produce considerable quantities of indigo, the best coining from Luzon. The plants suffer from locusts and storms, but the cultivation is very profitable. The yield of indigotine is large, but the preparation is conducted in such a primitive manner that the value of the product is much deteriorated. The exports from Manilla, in 1878, were 151,500 lb., valued at 10,605/. ; in 1877, 395,000 lb. were shipped. In many parts of Africa, as Sierra Leone, Liberia, Abeokuta, the Niger valley, Natal, Cape Colony, Tunis, and the Soudan, species of indigo plants are found in a wild state, and from them the natives prepare an inferior dye-stuff. In some of the S. States of America, notably S. Carolina, indigo culture has been attended with more or less success. The method of preparation pursued here varies but very slightly from the ordinary Indian process, almost the only important modification being the addition of a little oil to the liquor in the beating vat, when the fermentation becomes too violent. The preoipitated fecula is placed in coarse linen bags, and hung up to drain. The drying is finished by turning it out of the bags upon a floor of porous timber, and working it up. It is frequently exposed to the sun for short periods at morning and evening, and is then placed in boxes or frames, to cure till it is fit for the market. Several of the Central American States have figured conspicuously as indigo producers. The dye is precipitated in the beating vat by the sap contained in the bark of Tihuilate (Yonidium), Platanillo (Myrosma Indica), or Cuaja tints. The fecula is left during the night ; and, on the following day, is boiled, filtered, pressed, and sun-dried, The London market values of the different grades, known here as " Guatemala," are as follows :— Flores, 7s. 2d.-7s. 3d. a lb. ; Sobres, 5s, 9d.-7s. id.; Cortes, 5s. 8d.-6s. 8d. ; low and lean, 4s. ld. 5s. 6d. In most districts, the cultivation is declining, partly owing to the carelessness exhibited in the preparation of the dye.