Scientific American 14, 23.12.1848
Among dyers and color makrs, the Bengal indigo is highest prized. it is far superior to any other kind. The Guitamala or South American is the next in quality, and then the various grades of Spanish float, &c. The best Bengal sells for $2 per pound and it is a great source of revenue to the British Government. As this is at present the most valuable of all the dye drugs, selling far more than c och ineal, the United States must consume more and more of it, as we increase in manufactures. The cultivation of the indigo plant should therefore arrest the attention of our Southern planters, as there can be no doubt of an open and ready sale at all times, if the quality is good. We say this because some may say that "there is not a good market for what is now made in the States," that which is raised in Louisiana and S. Carolina. But the reason of the American indigo being unsaleable in the market, is owin to its inferior quality. it is far easier to work, as it is called, a good than a bad quality of indigo. in making the sulphate of indigo, the inferior requires more sulphric acid than the superior quality, while it dows not yield one fourth the amount of coloring matter, and the labor to use them both is the same. it is therefore of the utmost consequence to pay attention and particular attention to the quality. Bengal exports more than eight million of pounds every year and the quality has been, steadily increasing. indigo succeeds best near the tropics, where the mean temperature reaches 75° and 80° Fahrenheit. The soild should be light and rich. Sow in April 12 lbs. to the acre, in drills 15 to 20 inches apart. Moisture is requisite but undrained soil should be avoided - to be kept free from weeds and grass and thinned by hoeing. Cut with a reaping hook near the ground, when about the flower, or so soon as the lower leaves begin to turn: this period will be in July in South Carolina. A second crop is cut at the end of August, and a third in Guatimala and india. The first crop is the best. The excellence of indigo depends upon the brightness of the season - wet weather produces large plants, but a small quantity of coloring matter.
The culture is very precarious, both as regards the growth of the plant from year to year and the quantity and quality of the drug, even in the same season. Good indigo is known by its lightness or small specific gravity, indicating the absence of earthly impurities - by not readily parting with its coloring matter when a mass is drawn over a white surface; but above all, by the purity of the color itself.
In the Delta of the Ganges, where the best and largest quantity of indigo is produced, the plant lasts for only a single season, being destroyed by the periodical inundations; but in the dry central and western provinces, one or two rattoon crops are obtained.
In South Carolina the following method is employed to extract the indigo from the plant, which answers well enough for domestic purposes, but it is time that greater attention was paid to the manufacture of a better article.
When the underbearers begin to dry, they are cut down and put into a barrel filled with rain water with boards and weights placed on them to keep them under water.
When bubbles begin to form on the top and the water begins to look of a reddish color, it is soaked enough, and must be taken out, taking care to wring and squeeze the leaves well, so as to obtain all the strength of the plant; it must then be churned (which may be done by means of a tolerable open basket, with a handle to raise it up and down) until the liquor is quite in a foam. To ascertain whether it is done enough a spoonful is taken out on a plate, and a small quantity of very strong lye put into it.
If the liquor curdles, it is a sign that it is churned enough, when potash lye of considerable strength is added by small quantities and the churning continued until it is all sufficiently curdled; care must be taken not to put in too much lye, as that will spoil it. When it curdles freely with the lye, it must be sprinkled well over the top with oil, which immediately causes the foam to subside, after which it must stand till the indigo settles to the bottom of the barrel. This may be discovered by the appearance of water, which must be let off gradually by boring holes first near the top, and afterwards lower, as it continues t o settle. When the water is all let off, and nothing remains but the mud, it is taken and put into a funnel bag, and hung up to drip, afterwards spreading it to dry on large dishes. None of the foam, which is the strength of the weed, should escape.