The substance employed to give a black color to cloth, are red oxide of iron and tannin, also bichromate of potassa and logwood. These substances have a strong affinity for each other, and when combined assume a deep black color, not liable to be destroyed by the action of air or light.
Logwood is usually employed as an auxiliary because it communicates lustre, and adds considerably to the fulness of the black. It is the wood of a tree which is a native of several of the West India islands, and of that part of Mexico which surrounds the bay of Honduras. It yields its coloring matter to water. The decoction is at first a fine red, bordering on violet, but if left to itself it gradually assumes a black color. Acids give it a deep red color; alkalies, a deep violet, inclining to brown; sulphute of iron renders it as black as ink, and occasions a precipitate of the same color.
Cloth, before it receives a black color, is usually dyed blue; this renders the color much fuller and finer than it would otherwise be. If the cloth is coarse, the blue dye may be too expensive; in that case a brown color is given by means of walnut-peels.
_To Dye Woollens Black._
Wool is dyed black by the following process: It is boiled for 2 hours in a decoction of nut-galls and afterwards kept for 2 hours more in a bath composed of logwood and sulphate of iron, kept during the whole time at a scalding heat, but not boiling. During the operation it must be frequently exposed to the air, because the green oxide of iron of which the sulphate is composed must be converted into red oxide by absorbing oxygen before the cloth can require a proper color. The common proportions are 5 parts of galls, 5 of sulphate of iron and 30 of logwood for every 100 of cloth. A little acetate of copper is commonly added to the sulphate of iron, because it is thought to improve the color.
_To Dye Wool a Chrome Black._
Having cleaned the wool with soap and cream of tartar, take 4 oz. each of bichromate of potash and crude tartar to a copper of water, put in the merino, boil for 40 minutes, and, after cooling, immerse in a bath made from 4 oz. logwood chips with one-fourth of fustic chips to a copper of water.
_To Dye Silks Black._
Silk is dyed in nearly the same manner. It is capable of combining with a great deal of tan, the quantity given is varied at the pleasure of the artist, by allowing the silk to remain a longer or shorter time in the decoction.
_To Dye Cottons and Linens Black._
The cloth, previously dyed blue, is steeped for 24 hours in a decoction of nut-galls. A bath is prepared containing acetate of iron, formed by saturating acetic acid with sesquioxide of iron; into this bath the cloth is put in small quantities at a time, wrought with the hand for a quarter of an hour; then wrung out and aired again, wrought in a fresh quantity of the bath, and afterwards aired. These alternate processes are repeated till the color wanted is given; a decoction of alder bark is usually mixed with the liquor containing the nut-galls.
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