1881.] on Indigo, and its Artificial Production. 589 



so that the price of the artificial (being 73s. per kikj.) is more than 

 twice that of the pure natural coh)ur. Hence competition with tlie 

 natural dye-stuff is not to be tliought of until tlio makers can reduce 

 the 2>rice of dry propiolic acid to 20s. per kilo., and also obtain a 

 theoretical yield from their acid. This may, or it may not, be some 

 day accomiilished, but at present it will not pay to produce indigo 

 from nitro-phenyl-propiolic acid. Nevertheless a large field lies open 

 in the immediate future for turning Baeyer's discovery to practical 

 account. It is well known that a great loss of colouring matter 

 occurs in all the i)rocesses now in use for eitlier dyeing or printing 

 with indigo. It has already been stated tliat a large percentage of 

 indigo is lost in the " cold vats " in the sediment. Another portion 

 is washed off and wasted after the numerous di2)pings, whilst in order 

 to produce a pattern much indigo must be destroyed before it has 

 entered into the fibre of the cloth. Moreover, the back of the piece 

 is uselessly loaded with colour. In the j)rocesses of printing with 

 indigo the losses are as great, or even greater, and, in addition, such 

 considerable difficulties are met with that only a few firms (Potter, 

 Grafton in Manchester, and Schlieper in Elberfeld) have been suc- 

 cessful in this process. But a still more imjiortant fact remains, that 

 no printing process exists in which indigo can be used in combination 

 with other colours in the ordinary way, or without requiring some 

 special mode of fixing after printing. Hence it is clear that the 

 weak j^oints of natural indigo lie in the absence of any good process 

 for utilising the whole of its colouring matter, and in the impossibility, 

 or at any rate, great difficulty of emidoying it in the ordinary madder 

 styles of calico printing. Such were the reasons which induced the 

 patentees to believe that although the artificial dye cannot be 

 made at a price to compete with natural indigo for use in the 

 ordinary dye-beck, it can even now be very largely used for styles 

 to which the ordinary dye-stuft' is inapplicable. 



To begin with, Baeyer emi)loyed (Patent 1177) grape sugar as a 

 reducing agent. The reduction in this case does not take place in the 

 cold, and even on long standing only small traces of indigo arc 

 formed, but if heated to 70"' or upwards the change takes place. 

 Unfortunately this production of indigo-blue is rapidly followed by 

 its reduction to indigo-white, and it is somewhat difficult in practice 

 to stop the reaction at the right moment. But "necessity is the 

 mother of invention," and Dr. Caro of Mannheim, to whom the speaker 

 is greatly indebted for much of the above information, found that 

 sodium xanthate is free from many of the objections inherent to the 

 glucose reduction process, inasmuch as the reaction then goes on in 

 the cold. Moreover, he finds that the red isomeride of indigo-blue, 

 Indirubin, which possesses a splendid red colour, also occurring 

 in natural indigo, but whose tinctorial power is less than that of 

 the blue, is produced in less quantity in this case than when 

 glucose is employed. On this cloth, alumina and iron mordants 

 may be printed, and this afterwards dyed in alizarin, &c., or this 



Vol. IX. (No. 73.) 2 s 



