912 INDIGO 



in the liquor by means of hooks for about 20 or 30 minutes. It is then taken out 

 and well wrung. It now appears green, but on being unfolded and exposed to the air 

 rapidly becomes blue. When the vat contains an excess of lime the cloth has a 

 dark green colour when taken out. It is then passed through hot water and dipped 

 again, if a darker shade is required. When woollen flocks are to be dyed, they are 

 placed in a net made of cord, which is suspended by hooks at the side of the vat. 

 They are then transferred to a stronger net and wrung out by several men. In dye- 

 ing flocks a more active fermentation of the vat is required than with cloth. 



The process of dyeing by moans of sulphate of indigo is quite different from indigo 

 dyeing in the vat. This process was discovered by Earth, at Grossenhayn in Saxony, 

 about the year 1740, and the colour produced by it is hence called Saxon blue. The 

 method of purifying sulphate of indigo, by immersing wool in the solution of crude 

 indigo in oil of vitriol, previously diluted with water, has been described above. The 

 process of making sulphate of indigo or extract of indigo, as it is called, as now prac- 

 tised on the large scale, is as follows : 1 Ib. of indigo is mixed with from 8 to 9 Ibs. 

 of oil of vitriol, and the mixture is left to stand for some hours in a room, the tem- 

 perature of which is 90 Fahr. It is then diluted with water, and filtered through 

 paper. There is left on the filter a dirty olive-coloured residue, which is used for 

 some purposes by woollen dyers. By now adding common salt to the liquid, a blue 

 precipitate of sulphate of indigo is produced, which is collected on a filter, and 

 washed with a solution of salt in order to remove the excess of acid. No neutra- 

 lisation with alkali is required when this plan is pursued. The blue produced on 

 wool and silk by means of sulphate of indigo is very fugitive, and is now seldom 

 required, its place having been in a great measure taken by the blue from prussiate of 

 potash. The chief use of sulphate of indigo is for dyeing compound colours, such as 

 green, olive, grey, &c. E.S. 



On a new Method of Dyeing and Printing by means of Indigo. By reason of its 

 insolubility, alike in neutral and in alkaline solvents, the colouring matter of indigo 

 cannot be fixed upon any textile fibre until it has been reduced, i.e., converted into 

 white indigo, which is soluble in alkalis and solutions of the alkaline earths. The 

 energetic reducing properties of hydrosulphito of soda, and its almost instantaneous 

 action upon indigo, which it converts into white indigo, in presence of an alkaline 

 solution even at ordinary temperatures, have induced Messrs. Schutzenberger and 

 Lalande to examine the practical employment of this salt in the various applications 

 of indigo in the arts of dyeing and printing. 



The indigo vats most generally used in modern times are the sulphate of iron (green 

 copperas) vat for vegetable fibres, and the fermenting vat for wool-dyeing. The main 

 defect of the copperas vat is the presence of a bulky sediment of oxide of iron and of 

 sulphate of lime, which reqiiires to subside before the clear portion of the liquid can 

 be used for dyeing. The fermentation vat is difficult to work, and is subject to 

 accidents, or morbid changes, which sometimes in the course of a few hours involve 

 the entire loss of the indigo which they contain. (Such accidents are not unfrequontly 

 due to the malice of some workman, and are, of course, a kind of rattening.) The 

 hydrosulphite vat, which the authors propose in lieu of the present methods, both for 

 animal and vegetable fibres, is ' set ' as follows : Bisulphite of soda, marking 30 to 

 35 Baume (1'26 to T30 specific gravity), is put in a covered cask filled up to the 

 surface with coils of sheet zinc, or granulated zinc. This arrangement serves to in- 

 crease the points of contact between the liquid and the metal. After standing for about 

 an hour, the liquid is drawn off into milk of lime, which precipitates the salts of zinc. 

 The whole is well stirred, and the clear liquid is separated either by filtration and 

 pressure or by decantation, water having been previously added. During all these 

 operations air should bo as far as possible excluded. If the hydrosulphite of soda thus 

 obtained is mixed with ground indigo, and the amounts of lime or soda needful to 

 dissolve the reduced indigo, we immediately obtain a yellowish solution, which contains 

 no insoluble matter except the earthy matters present in the indigo. By this process 

 1 kilogramme of indigo may be reduced and dissolved in such a concentrated state that 

 the liquid does not exceed 10 to 15 litres. In dyeing the beck is filled with water 

 a suitable amount of reduced indigo added, and the operation is performed in the cold 

 for cotton and at a hand-heat for wool. The dye-liquid being clear for its entire depth, 

 the dyeing process can be conducted without loss of time. The excess of hydrosulphite 

 present constantly reduces the scum of oxidised indigo which forms on the surface of 

 the bath, and successive quantities of the concentrated solution of indigo are added 

 from time to time as they are required. By moans of this facility of keeping the vat 

 at any degree of strength required, any shade may be produced with the least possible 

 time and trouble. As regards cotton-dyeing the new process is distinguished for its 

 ease and rapidity. In wool-dyeing all risk of spoiling the indigo is avoided. Shades 

 are produced at once brighter and more solid than with the old vats, and it is easy to 



