Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/596

574 574 DYEING owing to decomposition of the mordanting salts, a true dyeing taking place. PKACTICAL DYEING PROCESSES. By the foregoing preliminary observations the reader will have been prepared to comprehend the rationale of the practical processes of dyeing. In order to give a fairly comprehensive account of these, it has been found con venient to take the colours in the old arrangement of simple and compound colours. Red, blue, and yellow are supposed to be simple or primitive colours ; the methods of obtaining these being given, then follow the colours from mixtures of two of the elementary colours, as green from yellow and blue, orange from red and yellow, and purple from red and blue. The colours not included in the above, and in the dyer s philosophy made by mixing the three elementary colours, red, blue, and yellow, in different proportions namely, the browns, greys, and chocolates, and black will be conveniently treated of after those sup posed to result from the mixture of two of the primary colours. This arrangement, though perfectly arbitrary, is both convenient and consistent as far as regards dyeing ; for though modern discoveries in optics may show that pure blue and yellow do not make green, and may in other respects disturb the older ideas concerning primitive and secondary colours, yet the dyer has sufficient justification for retaining the old system, because he can show that his blue and yellow always make green, and that the proper mixture of the so-called simple colours produces a compound shade which can be calculated beforehand from the propor tion of the respective colours employed. Red Colours. The most important of the red colours produced by dyeing are obtained from cochineal and from madder, the former being used for woollen and the latter for cotton goods. They are both old colours, and have arrived at their present excellence by slow degrees ; they are deep and brilliant, and, as far as regards permanency, hold the highest position among all dyed colours. The processes employed are instructive as illustrating the diversity of treatment required by different fibres and colouring matters. Red upon wool from cochineal. Let it be assumed that the shade of red required is fine scarlet, such as is worn by officers of the British army, and that the woollen cloth is of finest quality. The cloth first requires purifying from all the adventitious substances which it has acquired in the process of manufacture, in order to prevent irregularity and unerenness in the shade of colour ; this is done by methods described in the article BLEACHING. The only materials required to produce a fast scarlet upon wool are oxide of tin and the colouring matter of cochineal, but it requires much practical skill to bring them into contact properly. After the cloth is cleaned, and while it is still wet from its last washing, it is mor danted by boiling it in a solution of a salt of tin with or without cream of tartar. The parts of the boilers not in actual contact with the fire are frequently constructed of pure block tin, or at least all parts out of water should be of this metal, or else protected by wood, or the dyeing vessel should be made entirely of wood and heated by steam pipes ; for if the cloth containing the acid solution of tin comes in contact with a copper or brass surface it acquires a stain which afterwards dyes up an impure colour. What ta&quot;kes place in the course of boiling is that eventually a certain portion of tin, probably in the state of stannic oxide, becomes fixed upon or within thi! fibres of the wool, and this in a perfectly uniform manner. The tin not in intimate combination with the wool, or held merely by capillary attraction, is washed off by water before the cloth isbrought into contact with the colouring matter. The mordanted cloth is now brought into a boiler containing finely ground cochineal diffused through a sufficient quantity of water, to which it is usual to add some more tin mordant and tartar ; the cloth is turned continually to prevent folds or creases from interfering with the free access of the dye to all parts of it. The contents of the boiler are heated to the boiling point, and in half an hour or so the liquid becomes nearly colourless, and the cloth is found dyed of a bright red. The above may suffice to furnish a general view of the procedure usually followed, and to illustrate the principles involved with regard to numerous other dyes besides cochineal. To give the general reader a further idea of certain operations practised in the use of that colour (and the description applies more or less to others), the following particulars may be noted. The tin mordant used for scarlet on wool. It is now 200 yenrs since the discovery was made of the use of tin with cochineal for dyeing scarlet ; it might be thought that by this time the exact kind and quantity of tin solution to be used would have been settled ; there exists, however, the greatest diversity upon this point among practical dyers. The two salts of tin met with in commerce, de signated by chemists stannous and stannic chlorides, have received various names from dyers. Crystallized stannous chloride is generally known as &amp;lt;: tin crystal;&quot; the solution of the same as muriate of tin. A single muriate and a double muriate of tin are also distinguished, the difference being in the degree of concentration ; but in some parts of the country double muriate of tin is the name given to a solution of stannic chloride, elsewhere called bichloride of tin, and a good deal of confusion is sometimes caused by the various uses of the trivial names of the solution of tin. Experi ence teaches the dyer that there are scarcely two dye-works in the world in exactly the same condition with regard to either water and air, or apparatus, or quality of materials, and that the nature and quantities of drugs, mordants, and dye-stuffs used, and the duration and temperatures of the operations which secure admirable results in one place are altogether unsuitable in another. It .is, however, clear that by far the greater part of the variations introduced by practical dyers are not really founded upon necessity. Thus although the best colours can be obtained by the use of simple tin solutions manu factured on the large scale, in nine cases out of ten the operative dyer of scarlet insists upon preparing his own solution, and pretends that he employs special methods and preparations without which it would nover be lit to use ; and hence a countless number of tin solutions are in use. Tin spirits. The solution of tin used by dyers for the scarlet and for many other colours upon wool, silks, and cotton, are commonly called spirits, or &quot;tin spirits,&quot; a name which is very old, and appears to have originated in the use of nitric and hydrochloric acids to dis solve the tin, which ac;ds were formerly, and are even at present, called spirits of nitre and spirits of salts. One solution which is a favourite, from the ease with which its metal goes to the wool, is the so-called nitrate of tin (sometimes called &quot; bowl spirits,&quot; from being prepared in an earthenware bowl) made by dissolving thin metallic tin in moderately strong nitric acid. This is an operation requiring great care and some experience to prevent the formation of insoluble metastannic acid; the tin is added by small portions and gradually, so that the acid does not become hot; the solution takes place quietly, inodorous nitrous oxide is evolved, and ammonia is formed. If the tin be added too rapidly to the acid, red fumes of nitric oxide are evolved, the liquid boils up, becomes thick from separation of metastannic acid, and is utterly useless as a mordant. This so-called nitrate of tin is a veiy unstable compound, decomposing spontaneously in a few days, so that it has to be prepared just as it is wanted; it is theie- fore not an article of commerce. The other very numerous &quot;tin spirits&quot; may be said to be solutions of tin in a mixture of nitric and hydrochloric acids ; but the latter acid is sometimes replaced by the chlorides of sodium and ammonium, the resulting mordant being essentially a stannic chloride mixed with stannous chloride. Closely woven and loose woollen fabrics, sush as yarn and flannel, require different tin mordants, as some mordants are more quickly decom posed than others. The result of using an easily decomposable mordant such as the nitrate of tin upon closely woven cloth would be the formation of a deposit upon the external fibres of the wool, the interior of the cloth being unaffected. For such cloth, therefore, a tin spirit which is only slowly decomposed, such as the muriate alone or mixed with tartar, must be chosen, so as to allow of a tolerably thorough saturation of the cloth before the breaking up of the mordant during the boiling. Here it may be observed that good, thick, and finely woven cloth which is dyed in the piece, that is, after weaving, is hardly ever completely dyed through ; this can easily be shown by cutting through the cloth with a sharp knife, when the interior will be seen sometimes nearly white and generally much paler than the exterior ; hence the preference which is given to cloth made from yarn dyed before weaving, the colours of which do not fade so readily as those of piece-dyed goods. Imperfection in the dyeing of the latter can by care, however, be reduced to a minimum, and in dark goods is hardly discernible. Use of tartar along with tin mordant. The &quot;tartar&quot; of the dyer is a more or less impure form of the cream of tartar of the shops, or the acid potassium tartrate of chemists. It is in very general use for wool dyeing, and when employed with dye-stuffs plays the part of an acid, and could in fact be replaced by an acid ; in other cases, when used in mordanting, it no doubt acts as a salt, contributing to neutralize the strong mineral acids of the mordant, and rendering them more ready to decompose in the presence of the cloth. In a particular receipt for dyeing scarlet the proportions of niatoials are