An excellent description of the processes employed at Manchester, England, in preparing and working the copperas, or cold vat, is given in Ure’s “Dictionary of Manufactures.” “The ingredients necessary for setting the vat are copperas, newly slacked quicklime, and water. Various proportions of these ingredients are employed, as, for instance: 1 part by weight of indigo (dry), 3 parts of copperas, and 4 of lime; or, 1 of indigo, 2¹⁄₂₃ of copperas, and 3 of lime; or, 8 of indigo, 14 of copperas, and 20 of lime; or, 1 of indigo, ¾ of copperas, and 20 of lime; or 1 of indigo, 4 of copperas, and 1 of lime. The sulphate of iron should be as free as possible, from red oxide of iron, as well as sulphate of copper, which reoxidize the reduced indigo-blue. The vat, having been filled with water to near the top, the materials are introduced, and the whole, after being well stirred several times, is left to stand for about twelve hours. The chemical action which takes place is very simple. The protoxide of iron, which is set at liberty by the lime, reduces the indigo-blue; and the indigo, which is then dissolved by the excess of lime, forming a solution, which, on being examined in a glass, appears perfectly transparent and of a pure yellow color, and becomes covered, whenever it comes in contact with the air, with a copper-colored pellicle of regenerated indigo-blue. The sediment at the bottom of the vat consists of sulphate of lime, peroxide of iron, and the insoluble impurities of the indigo, such as indigo-brown in combination with lime, as well as sand, clay, &c. If an excess of lime is present, a little reduced indigo-blue will also be found in the sediment in combination with lime. The dyeing process itself is very simple. The vat having been allowed to settle, the goods are plunged into the clear liquor, and, after being moved about in it for some time, are taken out, allowed to drain, and exposed to the action of the atmosphere. While in the liquid, the fabric attracts a portion of the reduced indigo-blue. On now removing it from the liquid, it appears green, but soon becomes blue on exposure to the air, in consequence of the oxidation of the reduced indigo-blue. On again plunging it into the vat, the deoxidizing action of the vat does not again remove the indigo-blue which has been deposited within and around the vegetable or animal fibre, but, on the contrary, a fresh portion of the reduced indigo-blue is attracted, which, on removal from the liquid, is again oxidized like the first, and the color thus becomes a shade darker. By repeating this process several times the requisite depth of color is attained. This effect cannot, in any case, be produced by one immersion in the vat, however strong it may be. The beauty of the color is increased by finally passing the goods through diluted sulphuric or muriatic acid, which removes the adhering lime and oxide of iron. After being used for some time, the vat should be refreshed