The scouring should be carefully per formed, bacause the wool is thereby bet ter fitted to receive the dye. In this pro cess the volatile alkali of the urine unites with the grease, and forms a kind of soap, soluble in water.
When wool is dyed in the fleece, its filaments, being separate, absorb a larger quantity of the colouring matter ; for the same reason woollen yarn takes up more than cloth ; but cloths themselves vary considerably in this respect, according to their degree of fineness, or the closeness of their texture. The wool dyed in the fleece is chiefly intended to form cloths of mixed colours.
For most colours wool requires to be prepared by being boiled with saline sub stances, principally with alum and tartar. For some dyes wool does not require this preparation, and then it must be well washed in warm water, and wrung out, or left to drain.
The asperity of the surface of the fila ments of wool, and their disposition to ac quire a progressive motion towards their roots, form an obstacle to the spinning of wool, which is removed by impregnating it with oil. This oil must be discharged previous to the stuff, formed of the wool, being dyed. For this purpose it is carri ed to the fulling mill, where it is beaten with large beetlesin a trough of water, in which a particular kind of clay has been diffused ; that, uniting with the oil, ten. ders it soluble in the water.
Of Silk.
Silk is naturally coated with a sub stance, which has been considered as a gum, to which it owes its stiffness and elasticity ; that which is most commonly met with contains, besides, a yellow co. louring matter. Most of the purposes for which silk is employed require that both these substances should be remoy'. ed, which is effected by scouring it wit"( soap.' The scouring ought not to be so plete for silks which are to be dyed, ad fbr those which are intended to white, and a difference ought to be 'viscid.' according to the colour the silk shoolsi leave.
This difference consists in the quantity' of soap employed ; for common colours' the silk is boiled for three or four hour' 7 in a solution of twenty pounds of soap for every hundred of silk, taking care to fill up the kettle from time to time that there may be always a sufficient proportion of fluid. The quantity of
soap is increased for those silks which are to be dyed blue, and more especially for those which are to be scarlet, cherry colour, &c. because for these colours the ground must be whiter than for such as are less delicate.
When silk is intended to be employed white, it undergoes three operations : First, the hanks of silk are kept in a so lution of thirty poilnds of soap to the hun dredweight of silk, which ought to be very hot, but not boiling. When the im mersed part of the hanks is freed from the gum, they are turned upon the skein sticks, that the parts not before immers ed may undergo the same operation; they are then taken out of the kettle, and wrung out according as the operation is completed.
In the second operation, the silk is put into bags of coarse cloth, five and twenty or thirty pounds in each bag, which is called a boiling bag. In these bags it is' boiled for an hour and a half in a bath of soap, prepared like the former, but with less soap, taking care to keep the bags constantly stirred, that those which touch the bottom of the kettle may not receive too much heat.
The third operation is intended prin cipally to give the silk a slight cast, to make the white more pleasing ; front which it derives different names, such as China white, silver white, azure white, or thread white. For this purpose a so lution of soap is prepared, the proper strength of which is determined by its mode of frothing when agitated ; for the China white, which should have a slight tiuge of red, a small quantity of anotta is added, and the silk is shaken over in it till it has acquired the desired shade. To the other whites more or less of a blue tinge is given, by adding a little blue to the solution of soap.
The preparation of silk with alum is necessary in all cases, for without it the greatest part of the colours applied would possess neither beauty nor dura bility. For this operation forty or fifty pounds of Roman alum, previously dis solved in warm water, is mixed with about forty or fifty pails of water.