INDIGOFERA 



Manufacture : 

 Dry Process 



THE INDIGO PLANT 



Native 

 Methods. 



Dry Leaf. 



Steeping. 



Adulteration. 



Advantages. 



Chemical 

 Changes. 



Analysis of 

 Indigo. 



Purport. 



European industry in India, the Natives should have quietly continued 

 their own methods of manufacture, oblivious of or indifferent to the numer- 

 ous patent processes and appliances brought out by their European rivals. 

 By the dry method the bundles of freshly cut plant, instead of being con- 

 veyed directly to the factory, are dried and the leaves separated from the 

 stem by beating. When kept dry the leaves turn in the course of a few- 

 weeks from green to a blue-grey colour. They are then subjected to 

 steeping and fermentation, when the indican present in the plant splits 

 through hydrolysis into indigotin and indiglucin. The fine green liquor 

 thus obtained is then drawn ofl to the beating vat, where the matured 

 indigo is precipitated and subsequently concentrated by boiling and com- 

 pressed into cakes and dried in the usual way. But for the adulteration so 

 largely resorted to by the Native manufacturers, it seems fairly certain the 

 dry-system indigo would be fully as valuable as the wet, and in some 

 respects have distinct advantages. This is exemplified by the fact that it 

 is preferred even in localities where the difficulty of drying becomes serious. 

 For a small manufacturer it has the great advantage of being attended to 

 at the most convenient time. 



The rationale of indigo manufacture may be briefly and pointedl} 

 told. Fermentation and hydrolysis of the indican found in the steeping 

 vat takes place. The indigotin produced is reduced by the indiglucin tc 

 hydrindigotin (indigo white), and this dissolves in the alkaline liquor. 

 Through the beating that follows the hydrindigotin is reoxidised, and -in- 

 digotin precipitated being insoluble in the alkaline liquid. The yield 

 about 0'2 per cent, of the weight of the plant. The addition of ammonia 

 to the vat is frequently practised, with the result that the yield is largel} 

 increased, as the formation of ammonia by the fermentative breaking 

 down of the indigo is thus hindered. The indigo obtained in this manner 

 varies greatly in quality. Its content of indigotin ranges from 20 to 9( 

 per cent., the average being 40 to 50 per cent. The remainder consists of 

 ash, 5 to 20 per cent. ; water, 2 to 8 per cent. ; indirubin, 2 to 4 per cent., 

 and various amounts of indigo-brown, indigo-gluten, and carbohydrates. 

 The value of the blue depends on its content of indigotin and indirubin. 

 [Of. Blount and Bloxam, Chem. for Engin. and Manuf., 320.] It is believec 

 the presence of indirubin gives the more pleasing result of the natural 

 compared with the synthetic dye. 



Mr. W. Popplewell Bloxam (who has devoted much attention to the 

 study of the chemistry of indigo) read a highly instructive and valuable 

 paper before the Society of Chemical Industry (Yorkshire Section) or 

 August 15, 1906, and still more recently a further paper before the Londoi 

 Section on November 30, 1907. The last mentioned is written by Bloxar 

 in collaboration with Dr. R. Gaunt and Mr. F. Thomas, and is specially 

 described as an analysis of indigo and of the dried leaves of I. arrectn anc 

 I. sumatrana. It is, therefore, desirable that readers anxious for infor- 

 mation of the nature indicated should consult the papers mentioned. But 

 it may be useful to give here the few concluding remarks in the first, since 

 these seem to give an indication of the direction and purport of Bloxam's 

 investigations : 



" From these results, it will be seen that if the percentage of indigotin 

 contained in the whole plant (ordinary Indian varieties) be taken, as seems 

 reasonable, at 0'6 per cent., then from consideration of the weights of 

 plant steeped and the indigotin recovered in the finished cake, the highest 



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