Manufacturer and builder 2, 1881
This well-known vegetable dyestuff is derived from a great number of plants which belong to the same family as our beans and peas. They grow in tropical countries, and are found native in Asia, Africa and America. There are in all about two hundred species of these plants, of which only a portion furnish the indigo of commerce. The species cultivated in India, when full grown, is a shrub from two to five feet in h[e]ight, with leaves and flowers somewhat like our sweet pea. The seed is sown in drills about a foot apart, during the rainy season, and the young plants are kept free from weeds for a few months, when they are ready for cutting. They are not allowed to come into bloom, as this would injure the quality of the dye. They are, therefore, cut just before flowering and tied up in large bundles, and at once taken to the factory. If kept even for a short time in these bundles, a sort of fermentation takes place which destroys the indigo. As soon as the plants reach the factory, they are thrown into a vat and strongly pressed down by means of a crossbar and lever; the vat is then filled with water and the mass is allowed to steep for then or twelve hours, according to the state of the weather and the skill of the planter. This steeping process must be carefully watched, as, if contunued too long, the indigo will be damaged or "burst," and if not continued long enough the whole of the indigo will not be extracted. At the proper time the liquor is drawn off into another vat, where it is beaten and stirred with poles until granulation takes place, which occurs in from one-half to three hours. When the grains are properly formed, a few pailfuls of cold water or lime water are added to hasten the deposit. The lime water is said to injure the quality, as it precipitates foreign matter along with the indigo. After the deposit has settled the water is drawn off, and this deposit is removed to a copper boiler, where it is retained until it begins to ferment. It is then placed on a cloth-covered frame and allowed to remain until the water is drained off. The residue is then placed in proper frames and strong strongly pressed; it is next removed and cut into cakes of a proper size, which are removed to the drying-house. After drying, in some districts the indigo is sent to market in this form; but in the interior of Bengal it is loosely packed in boxes with hemp between the layers. It here undergoes a sort of sweating, and is then removed to the drying-house, and when thoroughly dry is repacked and sent to market. Another method of procuring the indigo is by gathering the leaves and drying them in the sun; after enough have been accumulated, they are put in the vat and treated as above described.
Asiatic indigo is brought from several parts of India, and from Java and Manila. The best Bengal indigo shipped fro mCalcutta is the superfine or light blue, in cubical cakes, so light as to float on water, friable, soft, of clean fracture, and of a beautiful copper color when rubbed with the nail. Of the Benga and Java there are many varieties in the market, and the amount of indigo blue present in these varies from forty to eighty per cent. Other kinds have from 10 to 37 per cent. Indigo is insoluble in water, but is easily dissolved by sulphuric acid without injurin its colouring qualities; it is then freely soluble in water, and may thus be used for the purposes of dyeing. Saxon blue is simply a solution of this in water, and is used for laundry purposes. The best quality of indigo will float upon water, and when rubbed with the finger-nail will give a glossy and purplish read streak; when the streak is dull and furrows on each side, the quality is poor. Indigo is replaced in dyeing operations by the aniline colors, of which alarge amount is now used. The increase of these or other dyes in Europe has been greater than in this country, as the indigo consumed here has increased to quite an extent since 1875. AS the crop is about the same each year, the conclusion must be that less has been used in Europe. The total importations of all kinds of indigo into the United States during the fiscal year ending June 30, 1880, were 2,625,240 pounds, agains 1,611,812 pounds for the previous year; 1,831,494 pounds in 1878, 1,504,783 in 1877, 999,139 in 1876, and 885,752 in 1875.