| Indigo | 1 | part. |
| Crystallized sulphate of iron | 3 | „ |
| Freshly slacked lime | 3 | „ |
Others, he says, use more lime than copperas, as in the following proportion:—
| Indigo | 2 | parts. |
| Sulphate of iron | 5.5 | „ |
| Quicklime | 6.5 | „ |
M. de Kæppelin, who is especially familiar with the cotton dyeing in Mulhouse, describes the ordinary vats for cotton dyeing as bound with iron, and placed on the level of the ground. They hold from 3,000 to 4,000 litres (1,055 gallons) of liquid. In preparing them the dyer fills them about three-quarters full of water, and pours in a milk of lime, prepared with 45 kilograms (100 lbs.) of freshly slacked lime; a fine liquid paste having been previously made from 15 kilograms (33 lbs.) ground in water. This is added to the lime in the vat by portions, the liquid in the vat being stirred up by a rake after each portion of the indigo paste has been added. The indigo becomes dissolved in about twenty-four hours, when the vat can be used. After describing the manner in which the frame, or champignon, containing the goods to be dyed is arranged and immersed in the vat, this author continues: “It will be understood that the vat is composed according to the degree of intensity of the color which is sought to be obtained, and that hues more or less deep may be obtained by means of more or fewer repeated immersions of the fabric to be dyed. After each immersion the champignon is lifted out of the vat, and the fabrics are left to ungreen themselves by contact with the air. (It must be observed that, although soluble indigo is called white, because it is without color when carefully prepared in the laboratory, the goods, when first taken from the ordinary vat, are of a green color.) Exposed to the air, the soluble indigo is precipitated in the state of blue indigo upon the fibres of the tissue. This oxidation, or dehydryzation, may be hastened by plunging the tissue into a vat containing a solution, very much diluted with water, chloride of lime, bichromate of potash, or sulphuric acid. The first two act as oxidizing agents; the last facilitates the restoring of the blue indigo by depriving it of the lime which is in excess in the solution of indigo which the tissue has imbibed from the vat.”
He adds further: “To facilitate the formation of blue indigo in the interior of the fabrics, the stuff to be dyed may be previously impregnated by a saline solution, which has the property of precipitating the white indigo from the alkaline solution, and of fixing itself more rapidly upon the tissue. Oxide of copper and oxide of manganese possess these properties in a high degree, and are used in many establishments to hasten the dyeing process, and produce an economy in raw materials. The pieces of cloth are placed in a solution of sulphate of copper, in the proportion of 15 to 20 grams to the litre (2.11 pints), and lightly thickened with starch. The fabrics, thus impregnated by a kind of mordant, before receiving the blue dye are first passed through a weak bath of milk of lime, which fixes the oxide of copper upon the tissue. The blues thus obtained are more intense, and have a peculiar lustre. This process is used in Austria and Germany, where cotton fabrics are printed on both sides of the tissue.”
Coming to the English authorities, Dr. Grace Calvert, in his recent lectures before the Society of Arts, speaking of the cold vat for dyeing cotton, says: “The oldest, and still most generally employed method of preparing cold vats, consists of putting into a vat containing about 2000 gallons of water 60 lbs. of indigo, very finely powdered, 180 lbs. of slacked lime, and 120 lbs. of sulphate of protoxide of iron, or green vitriol (free from any trace of copper salt), the two latter substances being added from time to time. The greater part of the lime used unites with the sulphuric acid of the iron salt, to produce sulphate of lime or gypsum; and the liberated protoxide of iron removes the oxygen from the indigo, becoming converted into saline oxide, whilst the reduced indigo dissolves in the excess of lime employed.”
He adds the following facts, which may be of practical value:—
“Messrs. R. Schloesser & Co., of Manchester, have introduced within the last year or two a marked improvement, in the preparation of cold vats, which removes the great objections of the bulky precipitate of sulphate of lime, the formation of an oxide of iron, and the loss of indigo by its combination with the oxide of iron. The bath remaining much more fluid, the pieces are less apt to be spotted, and a better class of work is produced. To carry out their process, they add to the ordinary 2,000 gallon vat 20 lbs. of ground indigo, 30 lbs. of iron borings, 30 lbs. of their remarkable powdered zinc, and 35 lbs. of quicklime; the whole is stirred up from time to time, for twenty-four hours, when it is ready for use. If the bath is not considered sufficiently strong, a little more lime and zinc are introduced. The chemical theory of the process is, that the zinc, under the influence of the lime, decomposes the water, combining with its oxygen, and the hydrogen thus liberated removes oxygen from the indigo which then dissolves in the lime.”
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 or fed with copperas and lime, upon which occasion the sediment must first be stirred up, and then allowed to settle again, so as to leave the liquor clear. The indigo-blue, however, is in course of time gradually removed, and by degrees the vat becomes capable of dyeing only pale shades of blue. When the color produced by it is only very faint, it is no longer worth while using it, and the contents are then thrown away. In dyeing cotton with indigo, it seems to be essential that the reduced indigo-blue should be in contact with lime. If potash or soda are used in its place, it is impossible to obtain dark shades of blue.”