Silk is scoured by means of boiling in soap and water, whereby it is freed from a sort of varnish if intended to be very white, it is bleached by humid sulphurous acid. Wool is first washed in running water to separate its coarser impurities ; it is then deprived of its yolk (a species of animal soap secreted from the skin of the sheep) either by the action of ammoniacal urine, by soap and water, or by a weak lye of carbonate of soda. It receives its final bleaching by the fumes of burning sulphur, or by aqueous sulphurous acid.
Tinctorial colours are either simple or com pound. The simple are black, brown or dun, blue, yellow, and red ; the compound are gray, purple, green, orange, and others. Gall-nuts, pyrolignite of iron, logwood, copperas, and verdigris, are the chief materials for producing black. Walnuts, sumach, madder, cochineal, cudbear, acetate of iron, catechu, Brazil wood, arnotto, are all employed in producing brown. Indigo, Prussian blue, and woad, are employed for blue. Fustic, Persian berries, quercitron, turmeric, and weld, for yellow. Cudhear, Brazil wood, cochineal, kermes, lac, logwood, madder, safflower, for red ; and various com pounds for purple, green, orange, &c.
The black dye for hats is communicated by logwood, copperas, and verdigris, mixed in certain proportions in the bath. The ordinary proportions used by the English black dyers for 100 pounds of cloth, previously treated in the indigo vat, are about 5 pounds of copperas, as much nut-galls bruised, and 30 pounds of logwood. They first gall the cloth, and then pass it through the decoction of logwood in which the copperas has been dissolved. A finish of weld is often given after fulling.
Silk is dyed black in two methods, accord ing to the market for which it is made. When sold by weight, it is an object with the dyer to load it with as much colouring or other matter as possible. When silk is sold by superficial measure, on the other hand, it be comes the dyer's object to give it a black colour with as little weight of materials as possible. Hence the distinction well known in trade of heavy and light silks, The silk dyers keep up from year to year a black vat, often of very complex composition. The es sential constituents of the vat are sulphate of iron and gum ; but many vegetable matters, as well as filings of iron, are usually added.
The infusion of walnuts, as used by the continental dyers, affords very agreeable and permanent brown tints without any mordant, while it preserves the downy softness of the wool, and requires but a simple and economi cal process in applying it. Sumach is usually employed in this country to dye fawns, and some browns ; but more beautiful browns may be given to woollen stuffs by boiling them first with one-fourth their weight of alum and some tartar and copperas ; washing, and afterwards dyeing them in a madder bath. The shade of colour depends upon the proportion which the copperas bears to the alum. The finest browns are produced by boiling each pound of the wool with two ounces of alum, dyeing it in a cochineal bath, and then transferring it into a bath containing a little cochineal darkened with acetate of iron.
Silk may receive a ground of arnotto, and then be dyed in a bath of logwood or Brazil wood, whereby a fine brown tint is obtained. Catecbu is used for giving a bronze and brown to cotton goods.
Additional details_ are given under the names of the colours and of the dye-materials, such as ARNOTTO, COCHINEAL, INDIGO, MAD DER, &c.
The dye-materials imported from foreign countries which find a place among the Customs Returns prepared by the Board of Trade are chiefly the following :—Cochineal, fustic, gum arabic, gum Senegal, gum animi, gum copal, gum tragacanth, indigo, lac dye, shell lac, logwood, madder, Nicaragua wood, safflower, sumach, smalts, valonia, yellow berries, and zaffre. Of these nineteen sub stances, about 4000 cwts. were imported in 1848.
There are few manufacturing arts in which the English may more profitably study the productions of the French than dyeing. The French chemists have studied the production of colour more than those of our own country ; and the French dyers have availed them. selves of every aid which science has been able to afford. It is one of the good features in the Exhibition which is about to take place, that the dyes of various nations will fairly be brought into comparison. If the French are really our superiors in this art, it will be just to acknowledge, graceful to commend, and profitable to imitate their skill.