The object of the beating is three-fold; first, it tends to disengage a great quantity of carbonic acid present in the liquor; secondly, to give the newly-developed indigo its requisite dose of oxygen by the most extensive exposure of its particles to the atmosphere; thirdly, to agglomerate the indigo in distinct flocks or granulations. In order to hasten the precipitation, lime water is occasionally added to the fermented liquor in the progress of beating, but it is not indispensable, and has been supposed capable of deteriorating the indigo. In the front of the beater a beam is fixed upright, in which three or more holes are pierced, a few inches in diameter. These are closed with plugs during the beating, but two or three hours after it, as the indigo subsides, the upper plug is withdrawn to run off the supernatant liquor, and then the lower plugs in succession. The state of this liquor being examined, affords an indication of the success of both the processes. When the whole liquor is run off, a laborer enters the vat, sweeps all the precipitate into one corner, and enters the thinner part into a spout which leads into a cistern, alongside of a boiler, twenty feet long, three feet wide, and three feet deep. When all this liquor is once collected, it is pumped through a bag, for retaining the impurities, into the boiler, and heated to ebullition. The froth soon subsides, and shows an oily looking film on the liquor. The indigo is by this process not only freed from the yellow extractive matter, but is enriched in the intensity of its color, and increased in weight. From the boiler the mixture is run, after two or three hours, into a general receiver called the dripping vat, or table, which, for a factory of twelve pairs of preparation vats, is twenty feet long, ten feet wide, and three feet deep, having a false bottom two feet under the top edge. This cistern stands in a basin of masonry (made water-tight with Chunam, hydraulic cement), the bottom of which slopes to one end, in order to facilitate the drainage. A thick woollen web is stretched along the bottom of the inner vessel, to act as a filter. As long as the liquor passes through turbid, it is pumped back into the receiver; whenever it runs clear, the receiver is covered with another piece of cloth to exclude the dust, and allowed to drain at its leisure. Next morning the drained magma is put into a strong bag, and squeezed in a press. The indigo is then carefully taken out of the bag, and cut with a brass wire into bits, about three inches cube, which are dried in an airy house, upon shelves of wicker work. During the drying a whitish effloresence comes upon the pieces, which must be carefully removed with a brush. In some places, particularly on the coast of Coromandel, the dried indigo lumps are allowed to effloresce in a cask for some time, and when they become hard they are wiped and packed for exportation.
2. Indigo from dried leaves.—The ripe plant being cropped, is to be dried in sunshine from nine o'clock in the morning till four in the afternoon, during two days, and threshed to separate the stems from the leaves, which are then stored up in magazines till a sufficient quantity he collected for manufacturing operations. The newly dried leaves must be free from spots, and friable between the fingers. When kept dry, the leaves undergo, in the course of four weeks, a material change, their beautiful green tint turning into a pale blue-grey, previous to which the leaves afford no indigo by maceration in water, but subsequently a large quantity. Afterwards the product becomes less considerable.
The following process is pursued to extract indigo from the dried leaves:—They are infused in the steeping vat with six times their bulk of water, and allowed to macerate for two hours, with continual stirring, till all the floating leaves sink. The fine green liquor is then drawn off into the beater vat, for if it stood longer in the steeper, some of the indigo would settle among the leaves and be lost. Hot water, as employed by some manufacturers, is not necessary. The process with dry leaves possesses this advantage, that a provision of the plant may be made at the most suitable times, independently of the vicissitudes of the weather, and the indigo may be uniformly made; and, moreover, that the fermentation of the fresh leaves, often capricious in its course, is superseded by a much shorter period of simple maceration.
PRODUCTION OF INDIGO IN INDIA.
| maunds. | |
| 1840 | 120,000 |
| 1841 | 162,318 |
| 1842 | 79,000 |
| 1843 | 143,207 |
| 1844 | 127,862 |
| 1845 | 127,862 |
| 1846 | 101,328 |
| 1847 | 110,000 |
| 1848 | 126,565 |
| 1849 | 126,000 |
Average of the ten years 126,744 maunds.
The yield from the different districts in 1849, was nearly as follows:—
| maunds. | |
| Bengal | 84,500 |
| Tirhoot | 24,500 |
| Benares | 9,500 |
| Oude | 6,500 |
| 125,000 |
In 1790 the general object of cultivation in Mauritius was indigo, of which from four to five crops a year were procured. One person sent to Europe 30,000 lbs., in 1789, of very superior quality.