Page:The American Cyclopædia (1879) Volume IX.djvu/262

 250 INDIGO cies, several of which yield the indigo of com- merce. I. tinctoria is the one most cultivated in the East, and /. anil is the most employed in America ; these two species are found natu- ralized in the southern states as remains of former cultivation. /. Caroliniana and /. lep- tosepala are indigenous species from North Carolina southward, and are said to be useful in domestic dyeing. Indigo is also yielded by a few other genera of leguminosae. These plants contain the coloring principle in their leaves, in the form of a coiorless substance which is brought out and rendered apparent by its oxidation as the leaves dry, or is devel- oped by submitting green leaves to a process of fermentation and oxidation. The /. tinc- toria is cultivated both in the East and West Indies. It is a shrub, though sometimes culti- vated as an annual, 4 to 6 ft. high, with pinnate leaves and rose-colored papilionaceous flow- ers. The seeds are sown in March and April in a light soil, and harrowed in. Weeds are removed, but after a few showers the plants cover the ground, keeping out all other vege- tation. Before they have reached their full height the plants should be cut, always early in the morning, and carried the same day to the factory. Here they are laid at once in a stone cistern 20 ft. square and 3 ft. deep. Hurdles are placed upon them, and heavy beams are laid across these and secured to the sides of the vat, the object being to keep the plants down when they swell. Water is then admitted so as to cover the plants. Fer- mentation soon commences, and may be allowed to go on for 10 to 14 hours, according to the condition of the plants, the temperature, and the weather. The liquor is in commotion as if boiling ; frothy bubbles rise to the surface, and their color, first white, becomes grayish blue and then deep purple, and finally a copper- colored scum covers the surface. When the agitation subsides the liquor is drawn oft into a lower vat; and the beams and hurdles being removed from the upper one, the steeped plants are taken out to be dried for fuel, and the vat is prepared for another charge. Several men enter the lower cistern and beat up the liquid with their hands or with paddles till the color- ing matter begins to app*ear in small atoms. This may require an hour and a half. The ap- pearance of a precipitate as fine as small sand, leaving the water clear, indicates favorable progress ; the beating is then discontinued, and the vat is left a few hours for the indigo to subside. The liquor is then run off from an upper vent, and after this the indigo from a lower one ; or the latter is sometimes left in part as the water is drained away, and is then gathered up by a person entering the vat. Lime and gum have been employed to hasten the precipitation, but their use is considered objectionable from their supposed injurious effect upon the quality of the indigo. The pulpy precipitate is next freed by standing in another cistern from more of the water mixed with it, and is then passed through a strainer into a boiler, in which it is heated to ebullition, and by some kept boiling for five or six hours. Being freed from scum, it is drawn off into a vat, from which, after subsiding, more water is taken off the top, and the rest is removed to the dripping vat, a wooden case having its floor perforated with holes and covered with a woollen cloth. The liquor passes through this filter, and the operation is completed by subjecting the residue to the action of a press, forming it into a cake, which is cut by a wire into 64 square blocks. These are laid out upon hurdles to dry in the shade, and left for sev- eral days or weeks in the drying house before packing. By the other method the leaves sep- arated from the stems are dried in the sun, and then stored. When a large quantity is collected they are infused with six times their bulk of water, and stirred for two hours till the leaves all sink. The liquor is then drawn off, beaten, and further treated as in the pro- cess already described. The Asiatic commer- cial indigo is brought from the several port? of India, and from Java and Manila. It differs much in quality and in shades of color. The best Bengal indigo shipped from Calcutta is the superfine or light blue, in cubical cakes, so light as to float upon water, friable, soft, of clean fracture, and of beautiful copper color when rubbed with the nail. Other qualities are of shades of violet, red, and copper color. The African indigoes from Egypt and Senegal are fine blues, but generally contaminated with earthy matters. The best American qualities, as some of those from Guatemala and Caracas, are equal to the best Bengal. These countries furnish a considerable portion of the indigo of commerce. The southern portion of the Uni- ted States exported annually in the early part of the present century about 184,000 Ibs. of indigo, worth 62 cts. per Ib. Up to the time of the civil war it was cultivated in Flor- ida and South Carolina, where the yield was about 60 Ibs. to the acre, and the crop required attention from July to October. In 1871 the United States imported 1,994,752 Ibs., about equal portions coming from Bengal and from Central and South America. A very superior quality is now produced at Bogota. The color- ing matter of indigo, called pure indigo or indigo blue, usually constitutes nearly 60 per cent, of the commercial article, which may be obtained by dissolving out what is soluble in boiling water, then that which alcohol will remove, and finally what hydrochloric acid will take up. The residue is pure indigo and any silica that may be present. Various methods are adopt- ed by different chemists for determining more exactly the proportions of indigo blue in samples of indigo. Some reduce the coloring matter by deoxidizing agents to indigo white, which is supposed to have been its original condition in the plants, and then precipitating and collecting this. Thus Dr. Dana dissolves the indigo by boiling in caustic soda with cau-