BLACK DYE 359 



which has already served for the black of boiled silk. For this purpose, silk, in its 

 native yellow colour, is made choice of. It should be remarked, that -when it is de- 

 sired to preserve a portion of the gum of the silk, which is afterwards made flexible, 

 the galling is given with the hot bath of gall-nuts in the ordinary manner. But here, 

 where the whole gum of the silk, and its concomitant elasticity, are to be preserved, 

 the galling is made cold. If the infusion of galls be weak, the silk is left in it for 

 several days. 



Silk thus prepared and washed takes very easily the black dye, and the rinsing in 

 a little water, to which sulphate of iron may be added, is sufficient. The dye is 

 made cold ; but, according to the greater or less strength of the rinsings, it requires 

 more or less time. Occasionally three or four days are necessary ; after which it 

 is washed, it is beetled once or twice, and it is then dried without wringing, to avoid 

 softening. 



Any of these processes will produce a black without the goods being previously dyed 

 blue, but generally when such common blacks, as they are technically termed, are dyed, 

 more of the dye drugs are required, and also a little modification in the operations. 

 Sometimes they are ' bottomed ' or ' rooted,' by first working them in a decoction of 

 walnut-husks, and then dyed as above ; or, a good black may be dyed without any 

 previous rooting, by working 1 cwt. of the stuff, for an hour, at a heat of 190, in 6 

 Ibs. of camwood : 6 Ibs. of copperas are then added, and the stuff worked for another 

 hour ; the fire is then withdrawn from the boiler, and the stuff allowed to remain in 

 the liquor for 10 or 12 hours. It is washed from this, and worked in a second bath 

 with 60 Ibs. of logwood for an hour and a half, then add 3 Ibs. of copperas, and after 

 another hour's working, it is washed. 



Bichromate of potash is also used for dyeing blacks upon wool. A very good colour 

 may be dyed direct by working, for 2 hours, 1 cwt. of the stuff in a solution of 5 Ibs. of 

 bichromate, 4 Ibs. of alum, and 3 Ibs. of fustic, then exposing it for an hour and wash- 

 ing well. It is again wrought for 2 hours, in a second bath, made up with 45 Ibs. of 

 logwood, 3 Ibs. of barwood or camwood, and 3 Ibs. of fustic ; then adding 3 Ibs. of 

 copperas, and after half an hour's longer working, the dye is finished. A much 

 cheaper blue black than that produced by previously dyeing the stuff in the indigo vat, 

 is obtained by using a Prussian blue, and then proceeding as directed above. 



Haw silk may be more quickly dyed by shaking it round the rods in the cold bath 

 after the galling, airing it, and repeating these manipulations several times, after 

 which it is washed and dried. 



Macquer describes a more simple process for the black by which velvet is dyed at 

 Genoa : and he says that this process, rendered still simpler, has had complete suc- 

 cess at Tours. The following is his description. 



For 1 cwt. (50 kilogrammes) silk, 22 Ibs. (11 kilogrammes) of Aleppo galls, in 

 powder, are boiled for an hour in a sufficient quantity of water. The bath is allowed 

 to settle till the galls have fallen to the bottom of the boiler, from which they are 

 withdrawn ; after which 32 Ibs. of copperas are introduced, and 22 Ibs. of country 

 gum, put into a kind of two-handled colander, pierced everywhere with holes. This 

 kettle is suspended by two rods in the boiler, so as not to reach the bottom. The 

 gum is left to dissolve for about an hour, stirring it from time to time. If, after this 

 time, some gum remains in the kettle, it is a proof that the bath, which contains two 

 hogsheads, has taken as much of it as is necessary. If, on the contrary, the whole 

 gum is dissolved, from 1 to 4 Ibs. more may be added. This colander is left constantly 

 suspended in the boiler, from which it is removed only when the dyeing is going on ; 

 and afterwards it is replaced. During all these operations the boiler must be kept 

 hot, but without boiling. The galling of the silk is performed with one-third of 

 Aleppo galls. The silk is left in it for six hours the first time, then for twelve hours. 

 The rest, secundum artem. 



Lewis states that he has repeated this process in the small way ; and that, by adding 

 sulphate of irop progressively, and repeating the immersion of the silk a great number 

 of times, he eventually obtained a fine black. 



Astringents differ from one another as to the quantity of the principle which enters 

 into combination with the oxide of iron. Hence, the proportion of the sulphate, or 

 of any other salt of iron, and that of the astringents, should vary according to the 

 astringents made use of, and according to their respective quantities. Gall-nut is the 

 substance which contains most of the astringent principle ; sumach, which seems 

 second to it in this respect, throws down (decomposes), however, only half as much 

 sulphate of iron. 



The most suitable proportion of sulphate of iron appears to be that which corresponds 

 to the quantity of the astringent matter, so that the whole iron precipitablo by the 

 astringent may be thrown down, and the whole astringent principle may be taken 

 up in combination with the iron. As it is not possible, however, to arrive at such 



