Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/598

576 576 DYEING tion only, for the colour fades rapidly in sunlight, and is easily washed out by soap and water. Substances similar to eosine, which have even still more recently appeared in trade, are called coccine and ncpaline; they yield beautiful but perishable red colours on wool and silk. Red colours on cotton. Turkey red. Cochineal, which is so suit able a colouring matter for wool, does not dye satisfactory colours upon vegetable fibres ; but from very remote times the Hindus have possessed a process for dyeing a brilliant and extremely per manent red upon cotton fabrics by means of madder. This process travelled westward through the Levant into Turkey and Greece, the date of its introduction into Western Europe going no further back than the middle of the 18th century, at which period Greek dyers were induced to settle in France and make known the methods in use for the production of this much desired colour. The name Turkey red, or Adrianople red, was applied to calico dyed with it at the time that such goods could be obtained only from the East, and it still retains the name. So much was the colour esteemed that in 1765 the French Government circulated a pamphlet describ ing the best known methods of dyeing it on yarns, and some years afterwards, the British Government paid a sum of money to a Frenchman named Papillon, for disclosing the whole process of obtaining it. The dyeing of Turkey red upon cloth and yam is now extensively carried on in Great Britain, and with great success. Turkey red is essentially a madder red with an aluminous basis, but differs from a common madder red by containing oil, and it is the fixing and combining of the oil with the fibre and the colour which constitutes its peculiarity. Divested of details the process of producing Turkey red may be divided into four stages : (1) the oiling of the cloth ; (2) mordanting with a salt of aluminium ; (3) dyeing with madder, or its equivalents garancin or alizarin ; and (4) the brightening of the dyed colour. The preparation of the cloth with oil is a process used in no other kind of dyeing ; of its utility there can be no doubt, but all the attempts of chemists to explain the rationale of its action have failed. There are many modifications of the method of applying the oil, but the older and more commonly used process is to mix the oil with a dilute solution of potash or soda ash, so as to diffuse it uniformly through the liquid, forming an emulsion ; the oil is not dissolved by the alkalies, nor is it supposed to combine with them, but is simply held in a state of excessively fine mechanical division. A low quality of olive oil is most generally used in Europe, that from Mogador, in the north of Africa, being very suitable. Certain kinds of oil do not answer for Turkey red, only those being suitable which, pro bably from containing free fatty acids or albuminous matters, readily form a milky emulsion with weak alkaline solutions ; other kinds are, however, in use in some places. The cloth to be dyed is steeped in the oily emulsion, wrung out, and dried in a warm stove ; this process is repeated six or eight times, -and the cloth is finally washed in weak alkali io remove from it all the oil not intimately united to the fibre. The result of this treatment, which is the most delicate and important in the Turkey red process, is that the cloth becomes impregnated with a fatty matter, which by the contact of alkalies and heated air has undergone some change from its original state, which is usually called an oxidation, but the nature of which is really unknown. The cloth now possesses a power of attraction for mordants and colouring matters greatly superior to untreated cloth ; and further, its physical condition is changed so that colours upon it are more transparent and more vivid than upon ordinary cotton. The cloth in this state is ready for mordanting, which is done by passing it through a bath of alum, partly neutralized with carbonate of soda or by chalk, or in a bath of acetate of alumina, the object being to obtain a regular deposition of the aluminous base upon the fibre ; the excess of mordant is carefully washed away from the cloth, which is now ready for dyeing. The dyeing is accomplished in the ordinary way, by keeping the cloth in continual motion in a vessel containing heated water and the dye stuff, which may be madder, garancin, or artificial alizarin. It is a very general practice to add a quantity of ox-blood to the water used in dyeing Turkey red. What purpose this fulfils is not known; its colouring matter cannot be supposed to be of any use; its albuminous constituents may have some useful action, but this seems very doubtful ; probably its addition is quite superfluous, and is retained from older times, when dyeing was less understood than at present. When the dyeing is completed the colour is a full and deep but dull red, which requires brightening. The brightening operations consist in removing brownish matters from the dye by boiling in soap and alkalies. To give a still more brilliant colour, the goods are boiled for several hours in a closed copper boiler with a mixture of salt of tin with the soap used in the last process of brightening, occasionally under a pressure greater than that of the atmosphere, in order to obtain a temperature some degrees higher than 212 F. In many processes of Turkey red dyeing, the cloth is treated with decoction of gall-nuts, or with sumach, after the preparation with oil and before the mordanting ; this enables it more easily to absorb and fix the aluminous mordant, but it is not essential, and is most generally omitted. No allusion has been made to a number of excrementitious and other animal matters, which the old dyers used in the oiling process, such as sheep-dung, cow-dung, ox-bile, &c. ; they can be dispensed with, and were employed probably from caprice and ignorance. Barwood red. An imitation of Turkey red is obtained from barwood ; it is much inferior both in beauty and stability to the real colour, but the ease with which it can be dyed, and the less costly nature of the materials employed, enable it to be sold at a much lower price, and for some purposes it is largely used. Bar- wood is one of the red dye-stufl s of which the colouring matter is very slightly soluble in water ; it is used in a state of fine powder. The cotton to be dyed is impregnated with a tin mordant by any of the means known to dyers, and then boiled with the dye-stuff ; the colouring matter as it dissolves is fixed by the mordant, and the process is continued until the required shade is obtained. This wood, and a similar material called camwood, are also employed in woollen dyeing to give brownish reds, and to dye a &quot; bottom &quot; or foundation for indigo blue colours, by which some economy in indigo is effected, and a peculiar bloom on the blue is produced. The class of woods represented by Brazil wood, do not yield good reds upon cotton. Blue Colours. The most important of the blue colouring matters is indigo. This may be said indeed to be the most im portant of all colouring matters, both as regards the large quantity and monetary value of what is produced and sold, and the permanence and solidity of the dyed colours which it yields. The indigo dye is a manufactured article, prepared in the place of growth of the plant which produces it. The indigo plant could itself be used for dyeing, but from 200 to 250 Ib of it would be required to produce the effect of a single pound of the prepared indigo. In England, and many other countries possessing a temperate climate, the species Isatis tinctoria, or woad, has been cultivated, and has been used from time imme morial for dyeing blue. Its comparative poverty in colour ing matter has caused it long since to be disused by dyers as a source of colour ; it is, however, employed by them in the preparation of their indigo vats, but rather as a convenient material to induce fermentation than as a dye. Indigo is distinguished from nearly all other colouring matters by its complete insolubility per se in water and other ordinary solvents. It dissolves to a very slight extent in heated aniline, petroleum, and acetic acid, which upon cooling redeposit it ; the only real solvent for it is anhy drous acetic acid mixed with a little sulphuric acid, from which water precipitates it unchanged, but this solvent is inapplicable in dyeing. But solubility is an essential condition for dyeing, and means have been found to obtain satisfactory solutions of indigo by circuitous methods which involve the temporary destruction of its blue colour and a change in its chemical composition. By various deoxidizing agents, indigo blue can be changed into a white substance, indigo white, which dissolves with facility in all alkaline liquids, forming a colourless or slightly yellow solution. On exposure to the air or other sources of oxygen, the solution yields the insoluble blue indigo, and permanently dyes any fibre which has been saturated with it. This is the only case in which such a method of dyeing is applicable, and on that account it possesses much interest. We shall now proceed to describe some of the practical methods in use for indigo dyeing, Fermentation process. The oldest of these, and one naturally suggested by the method employed in preparing the dye-stuff, is the process of fermentation in contact with lime, or sometimes soda or potash. During this process, gaseous or liquid substances are formed, which have the power of reducing indigo from the blue to the white which is greater for woollen than for all other kinds of cloth.