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DYEING &c., as in the case of cotton or silk, it still has the “lakes,” which vary in colour according to the metallic oxide or oleate, ofsilicate, combining with the colouring matter in the dyebath to salt employed. The most stable lakes are those in which the power the coloured “ lake ” or dye on the material. colouring matter is combined with two metallic oxides, a sesqui- form dyeing operation consists in working the mordanted material oxide and a monoxide—c.g., alumina and lime. In applying colour- in The a solution of the necessary colouring matter, the dyebath being ing matters of this class the object of the dyer is to precipitate gradually raised to the boiling-point. With many colouring and fix these coloured lakes upon and within the fibre, for which pur- matters, e.g., with Alizarin, it is necessary to add a small perpose two operations are necessary, namely, mordanting and dyeing. centage of calcium to the dyebath, and also acetic acid if The mordanting operation aims at fixing upon the fibre the wTool is being dyed. acetate In wool-dyeing, also, the mordanting operation necessary metallic oxide or insoluble basic salt, which is called the may follow that of dyeing instead of preceding it, in which case mordant, although the term is also applied to the original metallic boiling of the wool with dyestuff is termed “ stuffing,” and the salt employed. In the subsequent dyeing operation the mordanted the material is boiled with a solution of the colouring matter, during subsequent developing of the colour by applying the mordant is “saddening,” because this method has in the past been which the metallic oxide attracts and chemically combines with termed carried out with iron and copper mordants, which give dull the colouring matter, producing the coloured lake in situ on the usually fibre, which thus becomes dyed. The mode of applying the mor- or sad colours. The method of “stuffing and saddening” may, dants varies according to the nature of the fibre and the metallic however, be carried out with other mordants, even for the prosalt employed, the chief mordants at present in use being salts duction of bright colours, and it is now frequently employed with alizarin dyestuffs for the production of pale shades which of chromium, aluminium, tin, copper, and iron. The method certain of mordanting wool depends upon its property of decomposing require to be very even and regular in colour. There is still metallic salts, and fixing upon itself an insoluble metallic^ com- another method of applying Mordant Colours in wool-dyeing, in pound, when boiled in their solutions. This decomposition is which the dyestuff and "the mordant are applied simultaneously facilitated by the heating and by the dilution of the solution, but from the beginning; it is known as the “single-bath method.” it is chiefly due to the action of the fibre itself. The exact nature It is only successful, however, in the case of certain colouring of the substance fixed upon the fibre has not in all cases been matters and mordants, to some of which reference will be made in determined ; probably it is a compound of the metallic oxide with the following paragraphs. The Natural Mordant Colours.—It is interesting to note the wool-substance itself, which has the character of an amidoacid. The mordant most largely employed for wool is bichromate that nearly all the natural or vegetable dyestuffs employed belong of potash, since, besides being simply applied, and leaving the to the class of Mordant Colours, the most important of these wool with a soft feel, it yields with the various mordant-dyestuffs being included in the following list:—Madder, Cochineal, Peacha large variety of fast colours. The wool is boiled for 1 to 1^ wood, Sapanwood, Limawood, Camwood, Barvjood, Sanderswood, hourDin a solution containing 2 to 3 per cent, bichromate of Old Fustic, Young Fustic, Quercitron Bark, Persian Berries, Weld, potash on the weight of the wool employed. During this operation Logwood. Madder consists of the dried ground roots of Buhia tinctorum, the wool at first attracts chromic acid, which is gradually reduced to chromium chromate, so that the mordanted fibre has finally a a plant of Indian origin. Formerly cultivated largely in France pale olive-yellow tint. In the dyebath, under the influence of a and Holland, it was long one of the most important dyestuffs portion of the dyestuff, further complete reduction to chromic employed, chiefly in the production of Turkey-red and in calicohydrate occurs before it combines with the colouring matter. Not printing, also in wool-dyeing. With the different mordants it unfrequently certain so-called “assistants” are employed in small yields very distinct colours, all fast to light and soap, namely, amount along with the bichromate of potash—e.g., sulphuric acid, red with aluminium, orange with tin, reddish brown with cream of tartar, tartaric acid, lactic acid, &c. The use of the chromium, purple and black with iron. Madder contains two organic acids here mentioned ensures the complete reduction of closely allied colouring matters, namely, purpurin and alizarin. the chromic acid on the wool to chromic hydrate already in the The latter, which is by far the most important, is now prepared mordant bath, and the pale greenish mordanted wool is better artificially from the coal-tar product anthracene, and has almost adapted for dyeing with colours which are susceptible to oxidation— entirely superseded madder. Cochineal is the dried scale-insect Coccus-cacti, which lives on e.g., Alizarin Blue. For special purposes chromium fluoride, chrome alum, &c., are employed. Alum or aluminium sulphate (8 per certain of the cactus plants of Mexico and elsewhere. The rearing cent.), along with acid potassium tartrate (cream of tartar) (7 per of cochineal was once a large and important industry, and although cent.), is used for brighter colours—e.g., reds, yellows, &c. The still pursued, it has seriously declined, in consequence of the disobject of the tartar is to retard the mordanting process and ensure covery of the Azo Scarlets derived from coal-tar. The colouring the penetration of the wool by the mordant, by preventing super- matter of cochineal, carminic acid, is believed by chemists to be a ficial precipitation through the action of ammonia liberated from derivative of naphthalene, but its artificial production has not yet the wool; it ensures the ultimate production of clear, bright, full been accomplished. Cochineal dyes a purple colour with chromium colours. For still brighter colours, notably yellow and red, mordant, crimson with aluminium, scarlet with tin, and grey er stannous chloride was at one time largely employed, now it is slate with iron. Its chief employment is for the purpose of dyeing used less frequently ; and the same may be said of copper and crimson, and more especially scarlet, on wool. Crimson is dyed ferrous sulphate, which were used for dark colours. Silk may be by mordanting the wool with alum and tartar and dyeing in a often mordanted in the same manner as wool, but as a rule it is separate bath with ground cochineal. Scarlet on wool is obtained treated like cotton. The silk is steeped for several hours in by the single-bath method, namely, by dyeing the wool with a cold neutral or basic solutions of chromium chloride, alum, mixture of stannous chloride (or nitrate of tin), oxalic acid, and ferric sulphate, &c., then rinsed in water slightly, and passed cochineal. It is usual to add also a small amount of the yellow into a cold dilute solution of silicate of soda, in order to fix dyestuff Flavine in order to obtain a yellower shade of scarlet. the mordants on the fibre as insoluble silicates. Cotton does not, The cochineal colours are very fast to light, but somewhat susceptlike wool and silk, possess the property of decomposing metallic ible to the action of alkalis. Peachwood, Sapanwood, and Limawood are usually referred salts, hence the methods of mordanting this fibre are more complex, and vary according to the metallic salts and colouring matters to as the “soluble red-woods,” because of the solubility in water employed, as well as the particular effects to be obtained. One of the colouring principle they contain. They consist of the method is to impregnate the cotton with a solution of so-called ground wood of various species of Ccesalpinia found in Central “ sulphated oil ” or “ Turkey-red oil ” ; the oil-prepared material is America, the East Indies, and Peru. They all yield more or less then dried and passed into a cold solution of some metallic salt— similar colours with the different mordants—claret-brown with e.g., aluminium acetate, basic chromium chloride, &c. The mor- chromium, red with aluminium, bright red with tin, dark slate dant is thus fixed on the fibre as a metallic oleate, and after a with iron. Owing to the fugitive character of all the colours to passage through water containing a little chalk or silicate of soda light, these dye woods are now comparatively little employed in to remove acidity, and a final rinsing, the cotton is ready for dyeing, being replaced by the Alizarin Colours. All these dyedyeing. Another method of mordanting cotton is to fix the woods seem to contain the same colouring principle, namely, metallic salt on the fibre as a tannate instead of an oleate. This is brazilin, which, either before or during its union with the effected by first steeping the cotton in a cold solution of tannic acid mordant, is converted by oxidation into the colouring matter or in a cold decoction of some tannin matter, e.g., sumach, in which brazilein. The chemical constitution of this substance has reoperation the cotton attracts a considerable amount of tannic cently been definitely established, and it appears to be a member acid ; after squeezing, the material is steeped for an hour or more of the 7-pyrone group of colouring matters, to which several of in a’solution of the metallic salt, and finally washed. The mor- the natural yellow dyestuffs also belong. Camwood, Barwood, and Sanderswood represent the sodants employed in this case are various—e.g., basic aluminium or ferric sulphate, basic chromium chloride, stannic chloride (cotton called “insoluble red-woods,” their colouring principles being spirits), &c. There are other methods of mordanting cotton besides sparingly soluble even in boiling water. They are obtained iioni those mentioned, but the main object in all cases is to fix an certain species of Pterocarpus and Baphia, large trees growing m insoluble metallic compound on the fibre. It is interesting to note the interior of West Africa. Their_ general dyeing properties are similar, a claret-brown being obtained with chromium mordant, a brownish red with aluminium, a brighter red with tin, and
