NATURAL INDIGO 59 



only eight and a half hours are required. It is considered that 

 a proper fermentation does not take place below 90° F., so 

 some factories have a steam-pipe to heat the water if it is not 

 up to the desired temperature. In Behar, however, this 

 temperature is usually reached or exceeded during the mahai 

 season without the aid of artificial heat. 



During the steeping a complicated series of changes occurs. 

 These are induced by bacterial and enzymic actions, and the 

 net result is that, after a brisk evolution of gases, nitrogen 

 followed by hydrogen and carbon dioxide, the glucoside indi- 

 can is hydrolysed and its important constituent, indoxyl, is 

 liberated from the leaf and passes into the steeping water, to 

 which it imparts a greenish fluorescence. The steeped plant, 

 or seet, is used as a manure for cereal and other non-leguminous 

 plants, as it is specially rich in readily available forms of 

 nitrogen. 



When the steeping is at an end the liquor is run off into 

 the beating-vat, where it is immediately beaten for an hour to 

 an hour and a half. From time to time samples are removed 

 and tested. When all the indoxyl has been oxidised the 

 beating is stopped, and the indigo blue (indigotin) is allowed 

 to settle. Under normal conditions this requires two or three 

 hours. When, however, settling is not good, a serious loss of 

 indigo occurs, as the seet water retains a minute percentage 

 of the finely divided dye, which imparts to it a greenish tinge. 

 This condition is known as " green vat." 



The precipitated indigo tends to collect in the lower end 

 of the sloping beating-vat, the seet-water is drained off through 

 plugs in the side, and the watery precipitate is passed through 

 strainers into a well, from which it is pumped up to a tank. It 

 is again strained and passed to the boiling-tank. 



The boiling of the watery precipitate, containing about 0*5 

 per cent, of indigo, is necessary in order to prevent a destructive 

 type of fermentation and to promote good filtration. The 

 liquid is heated by steam to 160° F., allowed to settle, the 

 supernatant liquid drained off, and more water is added. With 

 this dilute sulphuric acid is mixed, and the liquid is again 

 heated to 160° F. After settling, draining, and renewing the 

 water the liquid is heated almost to boiling. The clear liquid 

 is again run off and the watery indigo, or mal, is run on to 

 filtering-tables across which stout cloths are stretched. The 

 mal thus concentrated contains 8-10 per cent, of indigo and 

 is carried to presses and subjected to pressure for several hours. 

 As a result a slab about 3 inches thick is produced. These 

 slabs are cut into small cakes and allowed to dry in special 

 sheds for several months. During this time bacterial action 

 results in the liberation of ammonia and moulds grow on the 



