Cold vats.—The copperas or common blue vat is so named because the indigo is re duced by means of the protoxide of iron. This salt should therefore be as free as possible from the red oxide, and espe cially from any sulphate of copper, which would re-oxidize the indigo. The neces sary ingredients are : copperas (green sulphate of iron), newly slaked quick lime, finely ground indigo, and water ; to which sometimes a little potash or soda is added, with a proportional diminution of the lime. The operation is conducted in the following way : the indigo, well triturated with water or an alkaline ley, must be mixed with hot water in the preparation vat, then the requisite quan tity of lime is added, after which the so Intion of copperas must be poured in with stirring. Of this preparation vat, such a portion as may be wanted is laded into the dyeing vat. For one pound of indigo three pounds of copperas are ta ken, and four pounds of lime (or 1 of indigo, 2i of copperas, and 3 of lime). If the copperas be partially peroxidized, somewhat more of it must beused.
A vat ee-taining a considerable excess of lime is called a sharp vat, and is not well adapted for dyeing. A soft vat, on the contrary, is that which contains too much copperas. In this ease the preci pitate is apt to rise, and to prevent uni formity of tint in the dyed goods. The sediment of the copperas vat consists of sulphate of lime, oxide of iron, lime with indigo brown, and lime with indigo blue, when too much quicklime has been em ployed. The clear, dark wine yellow fluid contains indigo blue in a reduced state, and indigo red, both combined with lime and with the gluten of indigo dissolved. After using it for some time the vat should be refreshed or fed with copperas and lime, upon which occasion the sediment must first be stirred up, and then allowed time to settle again and become clear. For obtaining a series of blue tints, a series of vats of different strengths is required.
Linen and cotton yarn, before being dyed, should be boiled with a weak alka line ley, then put upon frames or tied up in hanks, and after removing the froth from the vat, plunged into and moved gently through it. For pale-blues, an old, nearly exhausted vat is used ; but for deep ones, a fresh, nearly saturated vat. Cloth is stretched upon a proper square dipping-frame made of wood, or preferably of iron, furnished with sharp hooks or points of attachment. These frames are suspended by cords over a pulley, and thus immersed and lifted out alterf...tely at proper intervals. In the course of 8 or 10 minutes, the cloth is sufficiently saturated with the solution of indigo, after which it is raised and suspended so as to drain into the vat. The number of clippings determines the depth of the shade ; after the last, the goods are allowed to dry, taken off the frame, plunged into a sour bath of very dilute sulphuric or muriatic acid, to re move the adhering lime, and then well rinsed in running water.
The mode of making the China blue dye has been described under Cameo PRECTING.
A blue dye may likewise be given by a solution of indigo in sulphuric acid. This process was discovered by Barth, at Grossenhayn, in Saxony, about the year 1740, and is hence called the Saxon blue dye. The chemical nature of this pro cess has been already fully explained. If the smoking sulphuric acid be employed, from 4 to 5 parts are sufficient for 1 of indigo; but if oil of vitriol, from 7 to 8 parts. The acid is to be poured into an earthenware pan, which in summer must be placed in a tub of cold water, to pre vent it getting hot, and the indigo, in fine power, is to be added, with careful stirring, in small successive portions. If it becomes heated, a part of the indigo is decomposed, with the disengagement of sulphurous acid gas, and green is produced. Whenever all the indigo has been dissolved, the vessel must he cover ed up, allowed to stand for 48 hours, and then diluted with twice its weight of clear river water.
The acidulated mass has a black color, is opaque, thick, attracts water from the air, and is called indigo composition, or chemic-blue. It must be prepared before hand, and kept in store. In this solu tion, besides the cerulin, there are also indigo-red, indigo-brown, and gluten, by which admixture the pure blue of the dye is rendered foul, assuming a brown or a green cast. To remove these con taminations, wool is had .recourse to. This is plunged into the indigo previ ously diffused through a considerable body of water, brought to a boiling heat in a copper kettle, and then allowed to macerate as it cools for 24 hours. The wool takes a dark-blue dye by absorbing, the indigo-blue sulphate and hyposul phite, while at the same time the liquor becomes greenish-blue ; and if the wool be left longer immersed, it becomes of a dirty-yellow. It must therefore be taken out, drained, washed in running water till this runs off colorless, and without an acid taste. It must next be put into dcopper full of water, containing one or two per cent. of carbonate of potash, soda, or ammonia (to about one-third the weight of the indigo), and subjected to a boiling heat for a quarter of an hour. The blue salts forsake the wool, leaving it of a dirty red-brown, and dye the wa ter blue. The wool is in fact dyed with the indigo-red, which is hardly soluble in alkali. The blue liquor may now be employed as a fine dye, possesshd of su perior tone and lustre. It is called dis tilled blue and soluble blue. Sulphuric acid throws down from it the small quan tity of indigo-red which had been held in solution by the alkali.