894 INDIGO 



for judging whether any error has been committed in the fomentation; which must 

 be corrected by the operation of beating. If the fermentation has been arrested too 

 soon, much froth rises in the beating, which must be allayed with a little oil, and then 

 a reddish tinge appears. If large round granulations are formed, the beating is con- 

 tinued, in order to see if they will grow smaller. If they become as small as fine san'd, 

 and if the water clears up, the indigo is allowed quietly to subside. Should the vat 

 have been over-fermented, a thick fat-looking crust covers the liquor, which does not 

 disappear by the introduction of a flask of oil. In such a case the beating must bo 

 moderated, and when the granulations become round, and begin to subside, and the 

 liquor clears up, the beating must be discontinued. When the fermentation has been 

 excessive, the froth or scum diffuses itself spontaneously into separate minute particles, 

 that move about the surface of the liquor. On the other hand, a rightly-fermented 

 vat is easy to work ; the froth, though abundant, vanishing whenever the granulations 

 make their appearance. The colour of the liquor, when drawn out of the steeper into 

 the beater, is bright green ; but as soon as the agglomeration of the indigo commences, 

 it assumes the colour of Madeira wine ; and speedily afterwards, in the course of 

 beating, a small round grain is formed, which falls down and leaves the water trans- 

 parent, when all the turbidity and froth vanish. 



The object of the beating is threefold : first, it tends to disengage a great quantity 

 of carbonic acid present in the fermented liquor; secondly, to give the newly-de- 

 veloped 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 those who manufacture the 

 superior qualities of indigo, avoid the use of lime, as it has a tendency to make the 

 indigo hard and red. In one side of the beating vessel 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 afterwards, 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 subsides, the upper plug is withdrawn to run off the supernatant liquor, and then the 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 affords, on being examined, an 

 indication of the success of both the processes. When the whole liquor has run 

 off, a labourer enters the vat, sweeps all the precipitate into one corner, and empties 

 the thinner part into a spout which leads into a cistern, 20 feet long, 3 feet wide, and 

 3 feet deep. When all this liquor is once collected, it is pumped through a bag 

 which retains the impurities into a boiler, placed at the side of the cistern and heated 

 to ebullition. The froth soon subsides, leaving an oily-looking film upon the liquor. 

 The indigo is by this process not only freed from the yellow extractive matter, but 

 its density and the intensity of its colour are increased. 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 20 feet long, 10 feet wide, 

 and 3 feet deep, having a false bottom, 2 feet under the top edge. The 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 : but 

 a piece of cotton cloth is generally preferred to wool, as the hairs which are detached 

 from the latter injure the quality of the indigo. 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 brass wire into 

 cubical pieces, measuring about 3 inches each way, which are dried in an airy house 

 upon shelves of wicker work. During the drying, a whitish efflorescence appears upon 

 the pieces, which must be carefully removed with a brush. In some places, j-nrti- 

 cularly on the coast of Coromandcl, 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. 



From some experiments it would appear that the gas disengaged during the middle 

 period of the fermentation is composed in 100 parts of 27'5 carbonic acid, 5'8 oxygen, 

 and 667 nitrogen ; and towards its conclusion, of 40'5 carbonic acid, 4'5 oxygen, and 

 65'0 nitrogen. Carburetted hydrogen does not seem to be disengaged. That the 

 liquor in the beating vat absorbs oxygen from the air in proportion as the indigo 

 becomes flocoulent and granular, has been ascertained by experiment, as well as that 

 sunshine accelerates the separation of the indigo-blue. Out of 1,000 parts of the 

 fermented liquor of specific gravity 1*003, the blue precipitate may constitute 076 

 of a part. Such a proportion upon the great scale is, however, above the average, 

 which is not more than 0'6. When lime-water in addi-l, an extractive matt.r j* 

 thrown down, which amounts to from 20 to d7 parts iu 1,000 of the liquor. It has a 



