INDIGO. 



357 



wall of the vat. The fecula at the bottom is then removed to the 

 boiler. It is brought to the boiling point as quickly as possible, and 

 kept there for five or six hours. While boiling, it is stirred to keep 

 the indigo from burning, and skimmed with a perforated ladle. When 

 sufficiently boiled, it is run off to the straining table, where it remains 

 twelve or fifteen hours draining. It is then taken to the presses and 

 gradually pressed. This process takes twelve hours. It is then 

 ready to be taken out, cut, stamped, and laid in the drying house to 

 dry. 



A good sized steeping vat is 16 feet by 14 feet, by 4^ feet in depth. 

 The beating vat is somewhat shallower. Two hundred maunds of the 

 plant (16,400 lbs.) do very well to yield one maund of indigo (82 lbs.). 

 A vat of the above size holds about 100 maunds of plants. The plant 

 sown in June is cut three months afterwards and manufactured. A 

 second crop will be taken from it in the following August. This 

 cutting produces the largest quantity and best quality. 



In the manufacture of indigo the ordinary processes of fermentation, 

 of drawing off the liquor, of beating and of collecting the fecula, or 

 preci|)itate of indigo from the liquor, and pressing, are generally well 

 known and are followed with but trifling variations in different pro- 

 vinces and manufactories in India. 



The main points appear to be the watching the soaking plants, so 

 as to be able to tap off the infused liquor exactly at the right point of 

 fermentation, and next, to beat the liquor in the second vat long 

 enough. Knowledge of these things can only be acquired by careful 

 observation and long experience. 



The indigo of commerce is the result of the action, by atmospheric 

 oxygen, on the liquor drawn from a vat in which the plants have been 

 decomposed in water, the oxidation producing an insoluble granulation 

 of particles, commonly known as Indigo fecula, which is found de- 

 posited at the bottom of the vat. The indigo blue is derived from a 

 substance similar to the Indican of woad, that exists in the plant as 

 a glucoside compound, and which is dissolved during the steeping 

 process. 



Mr. Paul Michea, by Indian patents dated December 20, 1875, and 

 November 12, 1876, adopts some improvements in the processes, and 

 thus utilises the whole of the natural alkalies of the plant. He intro- 

 duces solutions of sugar or glucose in the steeping vat along with the 

 water, at a higher degree of temperature (95° to 100° Fahr.) and a 

 longer fermentation, and thus increases the production of indigo blue. 



Similar results are also obtained by replacing the effects of a 

 higher temperature, or a prolonged fermentation, by an artificial 

 supply of alkalies, principally ammonia. It is necessary to remark, 

 that the glucoside juice in the plant varies considerably under the 

 difference of latitude and the various countries where indigo is grown, 

 and also according to seasons. 



It is only when the quantity of indican is deficient, as in plants 

 grown in a poor soil and under a dry climate, that the ordinary manu- 

 facturing process can utilise the whole or nearly the whole of the 

 indican for its transformation into indigo blue ; but, on the contrary, 

 plants grown in a rich alluvial soil and under a damp hot climate 

 will contain an abundance of that glucoside juice which the present 



