PKODUCTION OF DYES 321 



That its use as a dye has been familiar for ages is indicated by 

 the fact that the blue cloths found on Egyptian mummies owe 

 their colour to this substance. Its introduction into Europe from 

 India appears to have occurred in the seventeenth century. 

 The only European plant which yields indigo blue is the woad 

 [I satis tinctoria, Nat. Ord. Cruciferce), which was formerly 

 cultivated to a considerable extent in the eastern counties of 

 England. Its use has much declined of late years, though a 

 small quantity is still grown for use in certain dye processes in 

 Yorkshire. 



The indigo plant is herbaceous and grows to a height of about 

 3 feet. The seed is sown in spring or autumn according to variety 

 and the nature of the soil. The plant is cut just before flowering 

 and is made into bundles which are placed in tanks or steeping 

 vats. The herb is then covered with boards, weighted with 

 stones, and water is added sufficient to cover it. A peculiar 

 fermentation ensues, which lasts from twelve to fifteen hours 

 according to the temperature of the air. From time to time a 

 small quantity of the liquid is taken from the bottom of the vat, 

 and when it exhibits the desired yellow colour the liquor is run 

 off into a separate tank, where it is agitated either by a paddle- 

 wheel or by workmen who stand in the liquid and beat it with 

 paddles. These successive stages in the process are shown in the 

 six illustrations (Figs. 108-113). 



In this process oxygen is absorbed from the air and indigo 

 appears as a greenish blue precipitate. This is allowed to settle 

 and is then boiled with water to prevent a second fermentation 

 which would spoil the product. The precipitate is strained off 

 on large canvas niters supported by bamboo canes, and the 

 nearly black mass is pressed, dried, and cut up into cubic cakes 

 (see Figs. 114 and 115). The indigo of commerce is a dark blue solid 

 which on being rubbed or pressed by any hard body presents 

 a bright copper coloured shining surface. It is insoluble in water 

 or alcohol, but dissolves in hot strong sulphuric acid, forming a 

 permanent blue liquid containing indigo-sulphonic acids, which 

 remain in solution when mixed with water and are used in 

 dyeing. 



The cultivation of the indigo plant in India has, down to 

 recent years, occupied a very large acreage of ground. It appears 

 from the Agricultural Statistics of India, published by the 

 Department of Revenue and Agriculture (Vol. I, 1904, pp. 2, 3, 



