Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/763

garancin, and both natural and artificial alizarin, but the pieces previously to being dyed have to undergo a long series of operations, which consist in passing them suc cessively in olive oil and carbonate of soda, and hang ing them in the air between the processes. They are then passed into a weak solution of red mordant, and afterwards of gall-nuts or sumach, well washed, and dyed in madder. When this has been effected, the colours are brightened by being boiled under pressure in a solution of soap and chloride of tin. On cloth so prepared certain discharge mixtures, principally tartaric acid properly thickened, are printed, and the pieces are passed through a solution of chloride of lime which removes the red, leaving a white pattern on a red ground. If a mineral colour or mordant is printed with the discharge it is left on the cloth in place of the discharged Turkey red, and thus various shades are produced in the brilliant red ground. In bandanna printing the Turkey red calico is folded between metallic plates, which are perforated with designs, and so arranged that each figure of the design corresponds through the pile of prints so folded. The whole is then submitted to pressure, and a chlorine liquor is forced by pressure to percolate through the mass, which destroys the red colour in all those parts where the perforated plates allow the

bleaching liquor to circulate.

Although madder and its various derivatives are the principal dye colours, there are various others which may be and sometimes are so treated. Of these the most important is logwood, the wood of Hceni itoxylon campech- ianuin, which, although chiefly used as an oxidation colour, also yields with alumina and iron mordants black and sombre slate tints, which, however, do not possess the peculiar fastness of madders. Sapanwood and peachwood are also used as dye colours, the mordant and method of dyeing being the same as for madder or garancin ; tha cloth, however, does not receive the same treatment after dyeing, and does not require it, because these colours are much more easily removed from the parts of the cloth which are destitute of mordant. Beautiful reds and pinks are produced by means of cochineal ; but this dye-stuff is chiefly used as a steam colour and for mousselines de laine. The mordant in the case of calicoes is either alumina or oxides of tin, and the method of proceeding is similar to that already described for madder and garancin colours. Quercitron bark (Quercns tinctoria), and flavin a prepara tion from it, fustic, the wood of Madura tinctoria, and Persian berries, the fruit of Rhamnus infectorms, are all used as dye colours, chiefly for the production of various shades of yellow. In the paper above quoted, Mr C. Dreyfus states that mahogany has lately been brought out as a colour-giving substance, that it gives with the tin and alumina mordants very bright and fast shades of brown, much more brilliant than those made from catechu, and that he has dyed some very good specimens with Spanish mahogany.

Oxidation Colours.

Under this head is included a class of tinctorial substances which attach themselves to cotton fibre without the intervention of any mordant, but which for the development and fixation of their colour must undergo a process of oridation after printing in the machine. The oxidation may be induced either by exposure of the pieces to atmospheric influences, by passing them through a solution containing an oxidizing agent, or by printing with the material some chemical substance which on exposure to heat gives off oxygen. The materials principally treated in this manner are indigo, catechu, aniline black, and certain blacks obtained from logwood. The processes adopted for the printing of indigo and aniline black the two most important styles under this head will make the practical application of the oxidation principle clear.

Indigotin, the colouring principle of indigo, is a substance obtainable from several other plants besides the species of Indigofera, from which it is commercially prepared. It is a body altogether insoluble in water, alcohol, ether, oils, or dilute acids or alkalies ; but in presence of a variety of substances it takes up an additional equivalent of hydrogen, and thus is converted into white indigo, a colourless substance soluble in solutions of alkalies and alkaline earths. The change is thus represented.

Imligotin Hydrogen White Indigo 2(C 8 H.NO) + 2H = C 1G H 12 N 2 0. 2 .

White indigo is a most unstable compound, taking up oxygen with great facility either from the air or from certain solutions, and thus becoming retransformed into blue indigo. Advantage is taken of these circumstances in printing indigo colours ; the colour is hydrogenized and dissolved, in which condition it is applied to calico, and on exposure of the pieces so dyed to the influence of oxygen the blue colour is both developed and fixed in the fibre. The following are the principal styles in practice:—

Indigo-blue dips.—This fine blue colour is produced in the old copperas vat method by putting in a vat hold ing 2000 gals, of water GO Ib of finely ground indigo, to which is gradually added 120 Ib of green copperas, or sulphate of protoxide of iron, together with 180 Ib of slaked lime. Owing to the lime removing the sulphuric acid from the salt of iron, the protoxide of that metal is liberated, and by its affinity for oxygen it decomposes the water, liberating hydrogen, which in its nascent condition reacts on the blue indigo, and thus transforms it into white indigo, which is soluble in the excess of lime employed in the operation. A zinc vat of recent introduc tion is now much more generally adopted than the above, its advantages being that the indigo is much more quickly converted, and by avoiding the abundant precipitate of sulphate of ]ini3 a better class of work is produced. The zinc vat is prepared by adding to the 2000 gals, of water 20 Ib of ground indigo, 30 Ib of iron filings, 30 Ib of finely powdered zinc, and 35 Ib of lime. The powdered zinc in presence of the lime decomposes the water, giving off hydrogen, which is taken up by the indigo, which then as white indigo dissolves in the lime.

By whichever process prepared, the dye-vat being ready, a piece of calico is hooked on a wooden frame and well stretched out ; it is then dipped into the vat for fifteen minutes, taken out, and left exposed to the air for five minutes. The piece of calico, which is white when it comes out- of the vat, gradually becomes green and then blue, owing to the oxygen of the air oxidizing the white indigo, and transforming it into blue, which is insoluble in water and fixed on the calico. The number of successive dips that the piece undergoes varies according to the various shades of blue which the printer requires. The pieces, after having been passed into a weak solution of sulphuric acid or " sours," which fixes the indigo thoroughly, only require to be well washed and dried.

To produce the well-known style of print which con

sists in a blue ground and white design, it is necessary to print a resist, pass the pieces into a vat containing lime, and then dye them in the above indigo vat. The prin cipal resist used is the blue resist, a mixture of sulphate, acetate, and sometimes nitrate of copper, and the solution is thickened with British gum, or calcined flummery, together with pipe-clay for the block, and flour for the machine printing. When the cloth on which this paste has been printed is dipped into an indigo vat, the indigo is

oxidized before ife reaches the surface of the cloth. After