Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/599

577 DYEING 577 The wood vat. To a course of lectures upon dyeing, recently delivered l&amp;gt;y Mr Jarmain before the Society of Arts, we are indebted for the substance of the following account of the woad vat used by the Yorkshire dyers. The materials employed are indigo, woad, madd er, bran, and lime. For this process as for every other in which it is employed, the indigo must be reduced to the finest possible powder. It is generally ground mixed with water, in closed revolving cast- iron cylinders containing iron rollers or balls, for several days, or until the slime or pulp formed contains no visible particles of the dye-stuff. The proportions of materials employed are : Lincolnshire woad 5 cwt. Whcaten bran 18 lb. Slaked lime in dry powder 2- Madder 2 Indigo 24 , The woad is first placed in the dyeing vat nearly filled with Avater, which is heated to between 1 40 and 1 50 F. ; after some hours (required to soften the woad), the bran, madder, and indigo are added, and half of the whole quantity of lime. In a few hours, if all is right, signs of commotion produced by fermentation will be visible, the liquid will become greenish, and a blue scum will be visible on the surface; a piece of wool is put in as a test, and if in a short time it becomes dyed blue the process is proceeding well ; a little more lime is added, but at intervals, so as not to check the progressing fermenta tion, and, if it should become necessary, the vat is heated up by steam to its original temperature; on the third day the vat should be ready for dyeing. Such a vat as this requires skilful management to control the fermentation ; without lime the reduced indigo would not be dissolved; with too much lime the fermentation would be stopped. The woad acts as an easily fermentable matter, and furnishes a portion of blue colour ; the bran also no doubt is useful, on account of the ease with which it begins and promotes fermen tation; the madder is probably of no use at all, its employment being still continued from an old unfounded notion&quot; that it gives some of its red colouring matter to the indigo-dyed goods, for the small amount of saccharine matter present in 2| lb of madder cannot be held of any importance in the presence of 5 cwt. of woad. A woad vat, when ready for dyeing, consists of a certain depth of a tolerably clear solution of white indigo in lime, and a somewhat voluminous semi-solid mass at the bottom, consisting of the bulk of the woad, the excess of the lime, the insoluble part of the madder, and the impurities always present in indigo. To keep the cloth to be dyed from contact with the muddy bottoms an iron hoop, of the internal diameter of the vat, covered with a network of open meshes is lowered into it and secured at a safe distance from the bottom. The pieces to be dyed, after being well cleansed, are placed in the liquor, and kept in constant movement to insure full access of the colour to all parts. The time required to dye, varying from 20 min utes to two hours, will depend upon the fineness and weight of the cloth, and upon the depth of colour required; if the goods require it, they are dyed a second time. In moving the pieces about, they must not be brought above the surface of the liquid, for the oxygen of the air -would restore the dissolved white indigo to its blue in soluble state. When the pieces are found to be sufficiently impreg nated with the dye, they are withdrawn from the vat ; at the moment of leaving the dyeing liquor they are seen to be of a yellowish colour, which almost instantly changes into a bright green, then darker green, and finally becomes blue through the absorption of oxygen by the white indigo. Loose wool or yarn is dyed by inclosing it in an open and movable network bag. The vat above described can of course dye only a limited quantity of material, becoming after every operation poorer in indigo; but it is not necessary to re-set a vat. The strength of its contents is kept up by constant additions of indigo, lime, and bran; no more woad is added, the quantity used at first being sufficient for about its own weight of indigo. Bran and molasses vat. Another kind of indigo dye vat, very extensively used on the Continent, and highly spoken of by practical men, is prepared as follows. A vat 6 feet in diameter and 7 feet deep is filled with water warmed to 130 F. ; then 4J lb of ground indigo, 34 lt&amp;gt; crystals of soda (or instead 161b soda ash) and 67 lb of bran, and twelve hours afterwards 2 lb slacked lime, are added ; in 24 hours the indigo should commence to be dissolved, and a test strip of stuff plunged in the liquid should be speedily dyed, but some hours longer and the gradual addition of 18 or 20 lb more of lirne are required to bringthe liquor into its best condition. In this vat, as in the woad vat, the lime controls the fermentation of the bran, and has to be added with care. With each pound of indigo added to ieplace what has been removed from the vat during a day s dyeing A lb of molasses and Jib crystals of soda and 3 or 4 lb lime must be used. By daily replenishing the vat it can be used continually for four or five months ; at the expiration of that time the bottoms must be removed; the supernatant liquor containing indigo in solu tion may be used instead of water for setting a fresh vat. This vat is said to have quite supplanted the old woad and madder vat, molasses being preferable on the score of cheapness and also of solubility. The soda not being necessary for the solution of the indigo could be omitted in the setting of the vat, but it is reckoned useful in assist ing the wool to take the dye; for the wool, however well it be bleached, is said to retain some greasy matters that yield to the soda, which thus enables the indigo to give fuller and faster colours than when lime alone is used. It is to be observed that the two vats just described are what are distinguished as warm vats, &quot; being made and worked at a tempera ture considerably above that of the air, a condition held necessary for dyeing wool and some kinds of heavy cotton goods. For ordinary cotton dyeing the vats are used cold or at the ordinary temperature of the air, and are prepared in quite a different manner. Copperas and lime vat. A strong copperas and linie vat is com posed as follows : 900 gallons of water. 60 tb green copperas 36 lb ground indigo. 80 to 90 tb dry slaked lime. These materials are well mixedtogetherandrakedupafrintervals for say 24 hours, when the vat is ready for use. The lime decomposes the salt, liberating ferrous oxide, which acts upon the indigo, con verting it into white indigo, which dissolves in the lime water. In large establishments for dyeing calico blue, it is usual to have a series of such vats in a row ; the pieces to be dyed are tightly stretched on a frame and dipped in the liquid for from seven to ten minutes, after which they are believed to be as fully saturated as possible ; the frames are next raised into the air, and in a few minutes the blue colour becomes developed ; the same process is then repeated until the required depth of colour is obtained. By printing certain resisting compositions on the cloth previous to the dipping, white figures can be obtained upon a blue ground, producing what is known as the navy-blue style of print, formerly much worn by the lower classes in England. By combining suitable mordants with the resisting composition, not only white, but orange, yellow, and green coloured figures can be obtained upon the blue ground; but the production of these is rather a branch of calico printing than of dyeing proper. Although this kind of vat is most generally used for the lighter qualities of calicoes, it can also be applied to such woollen goods as merinoes, which are not very closely woven, and also to silks. Hydrosulphite of soda vat. In 1871 Sclmtzenberger and Lalande introduced a new reducing agent applicable to indigo dyeing, the so-called hydrosulphite of soda, obtained by acting upon acid sul phite of soda with metallic zinc. It possesses the most energetic deoxidizing powers, and in the presence of alkalies almost immedi ately reduces and dissolves indigo. It has been applied both in dye ing and in printing indigo colours, but cannot be said to have succeeded in displacing the older kinds of vats, having the disad vantage of costing much more without producing any apparent improvement in the colour yielded. By preparing a very strong indigo vat, and thickening the fluid with gum, it is possible to print indigo blue colours in designs, but the many difficulties attending the process have very much restricted its application. The colour yielded by indigo, though far from brilliant, is ex traordinarily permanent, and is much used for articles intended to withstand much wear and rough usage, and also as a basis for the best quality of black upon fine woollen cloth. Sulphate, of indigo. When indigo is acted upon by concentrated sulphuric acid it forms a solution of the so-called sulphate or extract of indigo, which, though possessing an intensely blue colour, cannot by any means be made to furnish the original dye. This preparation of indigo is applied only in wool and silk dyeing ; it gives blues which are tolerably bright, but possess none of the stability of those obtained from real indigo. For vegetable fibre it has no affinity whatever either with or without mordants. Prussian blue. This, perhaps the earliest of artificial dye-stuffs, was accidentally discovered in 1710, though not used in dyeing for some time afterwards. The simplest method of employingit consists in first impregnating the material to be dyed with peroxide of iron, and then passing it into a solution of yellow prussiate of potash acidified slightly with sulphuric acid. Prussian blue upon silks is thus dyed. The most convenient way of obtaining a deposit of the oxide of iron consists in soaking the silk in a somewhat strong solution of the ordinary dyers nitrate of iron; in the course of two or three hours a certain quantity of the oxide is found to be intimately combined with the silk ; the excess of nitrate is then washed away and the silk worked in the acidified prussiate bath, when it imme diately assumes a light azure shade; by repeating the treatment several times any depth of colour may be obtained. Calico can be dyed in the same way, but both for that and for silk it is usual to add to the iron solution a small quantity of salt of tin, which is useful in giving a purplish tone to the blue and preventing the production of a disagreeable greenish tinge. A deep colour cannot in this way be satisfactorily given to woollen, for which a treatment is adopted depending upon a decomposi tion of the prussiate by means of heat and acids. For dyeing say 110 lb of merino the following proportions and methods may VTL 73