170 DYEING 



duced by means of mordants which ai-o termed adjective. Amongst this class of dyes 

 and dye-drugs stands pre-eminent indigo-blue. Indigo in its natural stall' is entirely 

 insoluble in water, and is of a deep blue colour. The composition of this blue indigo 

 is represented as C ltf H 5 N0 2 (C h H 5 KTO). But it is found capable of parting with a 

 portion of the oxygen, and by so doing, losing entirely its blue colour; and in this 

 deoxidised condition it is sohiblein alkaline lyes and lime-water; this colourless com- 

 pound is termed indigogene. The opinion of Liebig upon the constitution of this 

 substance was, that indigo contained a salt radical, which he termed A-nyle, c<>; 

 of C 16 H 5 N (C H H 5 H). He considered that indigogeno or white indigo is the hydratcd 

 protoxide of this radical, and that blue indigo is the peroxide, represented thus 



Salt radical, anyle, C I6 H 5 N. Indigogene, C 16 H 5 NO. HO. Blue indigo, C 10 H 5 NO*. 



Advantage is taken of this property of indigo, of parting with its oxygen and becom- 

 ing soluble, to apply it to dyeing, and it is effected by the following means, when 

 for tho purpose of dyeing vegetable stuff, as cotton ; from the circumstance of these 

 operations being done cold, tho method is termed tho cold vat, which is made up as 

 follows : The indigo is reduced to an impalpable pulp, by being ground in water to 

 the consistence of thick cream. This is put into a suitable vessel filled with water, 

 along with a quantity of copperas, and newly-slaked lime, and the whole well mixed 

 by stirring. After a short time tho indigo is deoxidised and rendered soluble by a 

 portion of the lime which is added in excess, the reaction being represented thus : 



1. Indigo, composed of 6116 ' -Djoing Solution 



f" Protoxide of Iron . -Zg^ss* Peroxide of Iron. 



2. Copperas . . 



(^Sulphuric Acid . 



fLimo . . . . -/- ^"""^r^ Sulphate of Lime. 



3. Lime . . .s Lime . . . .- / ^^ Sulphate of Lime. 



I, Lime . . . / 



The peroxide of iron and sulphate of limo are precipitated to the bottom, and the iiidi- 

 gogene and lime form a solution of a straw colour, with dark veins through it. 



The operation of dyeing by this solution is simply immersion, technically, dipping. 

 The stuff by immersion imbibes the solution, and when taken out and exposed to tho 

 air, the indigogene upon and within the fibre rapidly takes oxygen from tho atmo- 

 sphere, and becomes indigo-blue, thus forming a permanent dye, without any necessary 

 attraction between the indigo and the stuff. 



The indigo vat for wool and silk is made up with indigo pulp, potash, madder, and 

 bran. In this vat the extracts of madder and bran perform the deoxidising functions 

 of the copperas in the cold vat, by undergoing a species of fermentation. 



Pastel and woad, either alone or with the addition of a little indigo, are also used for 

 the dyeing of wool and silk stuff, the dcoxidation being effected by the addition of 

 bran, madder and weld. In dyeing with these vats, the liquor is made warm, and they 

 require much skill and experience to manage, in consequence of their complexity. 

 being always liable to go out of condition, as the dyeing goes on, by the extraction of 

 the indigogene and the modification of the fermentable matter employed to deoxidise 

 tho indigo to supply that loss. Tho alkaline solvent also undergoes change, so there 

 must bo successive additions of indigo and alkali ; tho principal attention of the dyer 

 is the maintaining the proper relation of these matters, as too much or too littlo of 

 either is injurious. 



Sulphate of indigo forms an intense blue solution, unaffected also by monlants. 

 Vegetable stuffs dipped in this retain no dye, for tho washing-off the acid in order to 

 preserve the fibre removes the colour; but animal fibre, such as woollen and silk, 

 becomes dyed ; a portion of the bluo remains upon tho stuff after washing-off the acid, 

 being retained by capillary attraction. This dye is termed Saxon blur,, but it has very 

 little of the permanence of indigo or vat blue, although it is also a substantive colour. 



Another truly substantive colour is that dyed by carthamus or safflowiT, 1-ut tho 

 fixation of this dyo upon the stuff -liters from any of those referred to. Like indigo, 

 it has no affinity for any base or substance capable of forming a mordant ; its 

 is an alkali, but in this dissolved state it does not form a dyo. The mode of proceed- 

 ing in dyeing with carthamus is, first, to extract tho dyo from the vegetable in wliirli 

 it is found, by soda or potash, which is afterwards neutralised by an acid previous to 

 dyeing, which renders the colour insoluble, but in so fine a state of division that no 

 precipitation can bo seen for some time and tho stuff immersed in this imbibed the 



