Notes Upon Indigo

by John L. Hayes

The plant called nil, cut down to the root and gathered up in packages, is worked up the same evening. The package is formed from the product of a space of land embraced by a chain about three yards long. The value of the first material changes with the value of the soil. Thus, one soil produces a plant which has many stems and few leaves, while another gives many leaves and few stems. The richness in coloring material depends upon the quantity of leaves, but varies also with an equal weight of leaves with atmospheric influences. Thus regular dealers in the article observe a marked difference in the quality of indigo in different seasons.

M. A. Koechlin Schwartz has recently published some interesting notes upon the preparation of indigo in Lower Bengal. In that country, which furnishes excellent indigo, the factory includes, besides filters, presses, a steam-engine, drying apparatus, and reservoir of water, two lines of vats, arranged one above the other, from fifteen to twenty in each line. These vats are built up with bricks, and covered with a strong coat of solid and well made stucco. They are square, about six yards on a side, and about a yard deep. The back row is about a yard above the front one. The plant is fermented in the vats of the upper row; when the operation of fermentation is terminated, a faucet is opened, and the liquid is run into the lower vat. The water of the Ganges, which is relatively pure, and thus well suited for this work, is brought into basins of deposition, where it becomes clarified, and is distributed by a common canal to the vats of the upper row. The plants, cut in the morning and bound up into packages, come to the factory after midday, and are thrown into the vat in the evening. A vat contains one hundred packages carefully arranged, one beside the other; heavy timbers are placed upon the plants, which are pressed down by means of large wedges. It is necessary that the plants should be pressed together very compactly, as without this the fermentation does not take place to advantage. At nightfall the water is introduced into the vats, and fills them so as completely to submerge the plants. The fermentation is more or less prolonged according to the temperature. Its duration varies from nine to fourteen hours. The workmen judge as to the procedure of the operation by withdrawing a little of the liquid in the lower vat. If it is of a clear pale yellow when withdrawn, it will furnish a product less abundant but more pure than if of a deep gold color.

At the moment of its issue from the fermenting vat the liquid is of a yellow color, more or less deep. The liquid is allowed to remain undisturbed for a brief period, when twelve naked men, armed with long bamboos, enter the vat to beat the water while it is still warm. During this time the upper vat is emptied and cleaned out for the succeeding operation. One vat requires seventeen workpeople (twelve men and five women). They thrash the water for two or three hours. The liquid passes by little and little to a pale green, and the indigo is found on suspension in the form of small floccules. The liquor is suffered to remain undisturbed for half an hour; it is then gradually decanted by opening, one after the other, the discharging holes placed at different heights. The water returns to the river, and the precipitate, under the form of a thin bouille, is turned into a reservoir. This bouille is pumped up into a vessel, and made to boil for a moment to prevent a second fermentation, which would injure the quality of the product, by turning it black. It is suffered to rest about twenty hours, and the next morning it is again subjected to boiling, the ebullition being kept up three or four hours. The boiling deposit is then turned off upon a large filter, through which the water drips. This filter is composed of a vat constructed of masonry, covered with stucco, about eighteen feet long by six feet wide and three feet deep. This is covered with bamboos, upon which is a grating of smaller reeds, and above by a stout strained cloth. There remains upon the cloth a thick paste, of a deep blue and nearly black color. The water which is run into the vat deposits some indigo which has pressed through the filter. This is decanted after being allowed to rest, and the turbid liquid is boiled the next day with the fresh indigo.

The paste of the filter is introduced into some small boxes of wood, pierced with holes, and provided above and below with a strong cotton cloth. The whole is again covered with a piece of stuff, and then with a covering of wood, pierced with small holes, and it is placed under a press, the force being gradually applied, so as to cause the water to run out as much as possible. There is withdrawn from the box a cake of the size of a cake of Marseilles soap. The water squeezed out flows back into the filtering vat, to be boiled again with the fresh indigo. The drying of the cakes ought to be done very slowly.

The dry-house is a large building of masonry, quite high, and pierced with many openings, provided with narrow blinds, to prevent the direct light of the sun from penetrating into the interior. Care is taken also to surround the dry-house with large shade-trees. The cakes take from three to four days to dry, after which they are packed in small boxes and carried to Calcutta, the great market of Bengal.

The details above given apply to the factories managed by European planters. The natives operate in nearly the same manner, but with less care, and consequently their products are inferior. The average product of indigo in Lower Bengal is stated at 4,000,000 kilograms, or 8,840,000 pounds per year. The most remarkable fact to be noticed in these operations is, that the blue principle is developed by chemical action from certain absolutely colorless principles existing in the plant. The theory of the change effected is still somewhat in doubt, because no chemist has studied the fresh plant, and observed upon the spot the phases of the operation of the production of indigo on a large scale. But the most accepted theory is that derived from the researches of Dr. Schunck, upon the isatis or woad-plant, which produces indigotine in a much less degree than the true indigo plants; viz., that the indigo exists in the plants combined with sugar, forming a glucoside, to which he gives the name indican. This compound, under the influence of fermentation in the manufacturing process, is supposed to be unfolded into indigo and sugar.

Without dwelling upon this question, which is beyond our province, we observe that the plants of the genus indigofera are used for the production of commercial indigo, on account of the greater richness in the coloring principle. Other plants, which furnish the same coloring principle, indigotine, are more frequently used directly in dyeing to furnish the blue principle than they are for the production of indigo.

The most important of these plants, although there are others, such as the Polygonum tinctorium and the Nerium tinctorium, is the Isatis tinctoria, which produces pastel, or woad. This, plant belongs to the family of cruciferæ, and is a biennial. It is represented in the accompanying figure.

The leaves which surround the stem are collected in May or June of the second year, when they begin to turn yellow. The wasted and dried leaves are sometimes used directly for dyeing, but more generally the leaves, after being cut and dried, are carried to a mill, and then ground to a paste, after which it is formed into a mass or heap, and being covered to protect it from rain, is left to undergo a partial fermentation for about a fortnight. The heap is then well mixed and formed into balls, which are exposed to the sun and wind to dry, and thereby prevent the putrefaction which would otherwise take place. Being afterwards collected in heaps, these balls again ferment, become hot, and emit the odor of ammonia, which Hume tells us, in the History of England, gave such offence to Queen Elizabeth that she issued an edict to prohibit the cultivation of this plant. After the heat has continued for some time, these balls fall into a dry powder, in which form the woad is usually sold to the dyer. The best French woad comes from Provence, Languedoc, and Normandy. In Germany, the pastel of Thuringia is used almost exclusively; the packages have the trade-mark of three towers, with the numbers 4, 5. In this country, owing probably to the prejudices of practical dyers, who have generally come from England, the Lancashire woad is almost exclusively used. The very little imported of late years, ranging from two thousand to twelve thousand dollars annually in value, is used for mixing with indigo in the so-called woad vat, to be hereafter described.