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MINERAL See also:CoLouRs.—Those include Chrome Yellow, See also:Iron See also:Buff, Prussian See also:Blue and See also:Manganese See also: The colour is developed gradually as the temperature rises; it may be rendered brighter by the addition of stannous chloride. On wool and silk Prussian blue is very fast to light, but alkalis turn it brown (ferric oxide). Manganese brown or See also:bronze is applied in wool, silk and cotton dyeing. The See also:animal See also:fibres are readily dyed by boiling with a solution of potassium permanganate, which, being at first absorbed by the fibre, is readily reduced to insoluble brown manganic hydrate. Since See also:caustic potash is generated from the permanganate and is liable to See also:act detrimentally on the fibre, it is advisable to add some See also:magnesium sulphate to the permanganate See also:bath in See also:order to See also:counter-act this effect. Irritation furs are dyed in this manner on wool-See also:plush, the tips or other parts of the fibres being bleached by the application of sulphurous acid. Cotton is dyed by first impregnating it with a solution of manganous chloride, then dyeing and passing into a hot solution of caustic soda. There is thus precipitated on the fibre manganous hydrate, which by a short passage into a See also:cold dilute solution of bleaching powder is oxidized and converted into the brown manganic hydrate. This manganese bronze or brown colour is very susceptible to, and readily bleached by, reducing agents; hence when exposed to the See also:action of an See also:atmosphere in which See also:gas is freely burnt, the colour is liable to be discharged, especially where the fabric is most exposed. In other respects manganese bronze is a very fast colour. Dyeing on a large See also:Scale.—It is not possible to give here more than a See also:bare outline of the methods which are used on the large scale for dyeing textile fibres, yarns and fabrics. In principle, dyeing is effected by allowing an aqueous'. solution of the dye-stuff, with or without additions (alkalis, acids, salts, &c.), to act, usually at an elevated temperature, on the material to be dyed. During the See also:process it is necessary, in order to ensure the See also:uniform See also:distribution of the dyestuff in the material, that the latter should either be moved more or less continuously in the dye liquor or that the dye liquor should be circulated through the material. The former mode of operation is in See also:general use for hank, warp and piece dyeing, but for textile fibres in the loose See also:condition or in the See also:form of " slubbing," " sliver " or " cops " (see See also:SPINNING) the latter method has, in consequence of the introduction of improved machinery, come more and more into See also:vogue within See also:recent years.
Loose Material.—Cotton and wool are frequently dyed in the loose See also:state, i.e. before being subjected to any See also:mechanical treatment. The simplest method of effecting this is to treat the material in open vessels (boilers) which can be heated either by means of See also:steam or See also:direct See also:fire. • Since, however, a certain amount of felting or See also:matting of the fibres cannot be avoided, it is frequently found to be more advantageous to effect these treatments in specially constructed apparatus in which the dye liquors are circulated through the material.
See also:Yarn.—Yarn may be dyed either in the hank, in the warp or in the cop, i.e. in the form in which the yarn leaves the spinning See also:frame. The dyeing in the hank is carried out in rectangular dye-vats constructed of See also:wood or See also: Yarn in the warp is dyed in vats or " boxes " like that shown in fig. 2, through which it is caused to pass continuously. The warps to be dyed pass slowly up and down over the loose rollers in the first See also:box B, then through squeezing rollers S into the next, and the same thing occurs in the second (also third and See also:fourth in a four-box machine) box A, whence they are delivered through a second pair of squeezing rollers Sl into the See also:wagon W. The boxes may contain the same or different liquors, according to the nature of the dyestuff employed. Washing is done in the same machine, while drying is effected on a See also:cylinder drying machine like that shown in See also:figs. 8 and q of BLEACHING. Latterly, See also:machines have been introduced for dyeing warps on the See also:beam, the dye liquor being caused to circulate through the material, and the See also:system appears to be See also:meeting with considerable success. Large quantities of yarn, especially cotton, are now dyed in the cop. When the dyed yarn is to be used as weft the See also:main See also:advantage of this method is at once apparent, inasmuch as the labour, See also:time and See also:waste of material incurred by reeling into hanks and then winding back into the compact form so as to See also:fit into the See also:shuttle are avoided. On the other hand the number of fast dyestuffs suitable for cop dyeing is very limited. In the See also:original cop-dyeing machine constructed by Graemiger a thin tapering perforated metallic See also:tube is inserted in the hollow of each cop. The cops are then attached to a perforated disk (which See also:con- stitutes the lid of a chamber or box) by inserting the protruding ends of the tubes into the perforations. The chamber is now immersed in the dye-bath and the hot liquor is See also:drawn through the cops by means of a centrifugal See also:pump and returned continuously to the dye-bath. This principle, which is known as the skewer or spindle system, is the one on which most See also:modern cop-dyeing machines are based. In the so-called " compact " system of cop dyeing the cops are packed as closely as possible in a box, the See also:top and bottom (or the two opposite sides) of which are perforated, the interstices between the cops being filled up with loose cotton, ground See also:cork or See also:sand. The dye liquor is then drawn by suction or forced by pressure through the box, thus permeating and dyeing the cops. Pieces.—See also:Plain shades are usually dyed in the piece, this being the most economical and at the same time the most expeditious means of obtaining the de- sired effect. The dyeing of piece goods may be effected by See also:running them through the dye liquor either at full breadth or in rope form. The machine in most com- mon use for the first method is the See also:Lancashire " jigger," which is See also:simple in principle and is shown in See also:section in fig. 3. It consists essentially of a dye-vessel constructed of wood or See also:cast iron and containing loose See also:guide rollers, r and r, at the top and bottom. By coupling up the the pieces which are batched on A are drawn through the dye liquor and rolled on to B. A See also:band See also:brake (not shown in the figure) applied to the See also:axis of A gives the pieces the required amount of tension in passing through the dye-bath. As soon as the whole of the pieces have passed through in this way from A to B, the machine is reversed, and See also:roller A draws them back again through the bath in a similar way on to roller A. This alternating process goes on until the dyeing is finished, when the goods are washed off, squeezed and dried. The jigger is especially useful in cotton piece dyeing, one See also:great advantage being that it is suited for what is known as a " short, bath," i.e. a bath containing a minimum amount of dye liquor, this being of great importance in the application of dyestuffs which do not exhaust well, like the direct colours and the sulphide colours. The See also:padding machine is similar in principle to the jigger, the pieces running over loose guide rollers through the See also:mordant or dye solution contained in a trough of suitable shape and See also:size, but on leaving the machine they pass through a pair of squeezing rollers which uniformly See also:express the excess of liquor and cause it to be returned to the bath. The padding machine is used more for preparing (mordanting, &c.) than for dyeing. For the dyeing of pieces in rope form a so-called "dye-See also:beck " is used, which is a machine of larger dimensions than the jigger. Across the dye-bath is attached a winch W (see fig. 4), by means of which the pieces, sewn together at the ends so as to form an end- less band, are caused to circulate through the machine, being drawn up on the front See also:side of the machine and allowed to drop back into the dye liquor on the other. This form of machine is particularly suited for the mordanting and dyeing of heavy goods. Washing off may be done in the same machine. The drying of piece goods is done on steam-heated cylinders like those used for the drying of bleached goods (see BLEACHING). The operations which precede dyeing vary according to the material to be dyed and the effects which it is desired to produce. Loose wool, woollen and worsted yarn and piece goods of the same material are almost invariably scoured (see BLEACHING) before dyeing in order to remove the oily or greasy impurities which would otherwise interfere with the penetration of the dye solution. Silk is subjected to the process of discharging or boiling off (see BLEACHING) in order to remove the silk See also:gum or sericine. Cotton which is to be dyed in dark shades does not require any preparatory treatment, but for light or very See also:bright shades it is bleached before dyeing. Wool and silk are seldom bleached before dyeing. Cotton, wool and See also:union (cotton warp and worsted weft) fabrics are frequently singed (see BLEACHING) before dyeing. Worsted yarn, especially two-See also:fold yarn, is very liable to curl and become entangled when scoured, and in order to avoid this it is necessary to stretch and " set " it. To this end it is stretched tight on a specially constructed frame, placed in boiling water, and then cooled. Similarly, union fabrics are liable to " See also:cockle " when wetted, and although this defect may be put right in See also:finishing, spots of water or raindrops will give an uneven See also:appearance of a permanent See also:character to the goods, To avoid this, the pieces are subjected previous to dyeing to the so-called " crabbing " process, in which they are drawn under great tension through boiling water and See also:wound on to perforated hollow cylinders. Steam is then blown through the goods and they are allowed to cool.
With respect to the question of colour, we meet with two kinds of substances in nature, those which possess colour and those which do not. Why this difference? The physicist
says the former are bodies which reflect all the coloured dyeory of dyeing.
rays of the spectrum composing See also: The latter are bodies which absorb some of the spectrum rays only, reflecting the See also:remainder, and these together produce the impression of colour. A See also:black substance is one which absorbs all the spectrum rays. The fundamental See also:reason, however, of this difference of action on the See also:part of substances towards light remains still unknown. All substances which possess colour are not necessarily dyestuffs, and the question may .be again asked, Why? It is a remarkable circumstance that most of the dyestuffs at See also:present employed occur among the so-called aromatic or benzene compounds derived from coal-tar, and a careful study of these has furnished a general explanation of the point in question, which briefly is, that the dyeing See also:property of a substance depends upon its chemical constitution. Speaking generally, those colouring matters which have the simplest constitution are yellow, and as the molecular See also:weight increases their colour passes into orange, red, See also:violet and blue. In recent years chemists have begun to regard the constitution of nearly all dyestuffs as similar to that of Quinone, and some even believe that all coloured organic compounds have a quinonoid structure. According to O. N. Witt, a colourless See also:hydrocarbon, e.g. benzene, becomes coloured by the introduction of one or more See also:special See also:groups of atoms, which he terms the colour-bearing or chromophorous groups, e.g. NO2, – N:N –, &c. Benzene, for example, is colourless, whereas nitro-benzene and See also:azo-benzene are yellow. Such compounds containing chromophorous groups are termed chromogens, because, although not dyestuffs themselves, they are capable of generating such by the further introduction of See also:salt-forming atomic groups, e.g. OH, See also:NH2. These Witt terms auxochromous groups. In this way the chromogen tri-See also:nitrobenzene, C6H3(NO2)3, becomes the dyestuff tri-nitro-phenol (picric acid), C6H2(NO2)3(OH), and the chromogen azo-benzene, C6HS•N N•See also:C6H5, is changed into the dyestuff amido-azo-benzene (Fast Yellow), C6H5•N : N•CSH4(NH2). These two dyestuffs are typical of a large number which possess either an acid or a basic character according as they contain hydroxyl (OH) or amido (NH2) groups, and correspond to the Acid Colours and Basic Colours to which reference has already been made. Other important atomic groups which frequently occur, in addition to the above, are the carboxyl (COOH) and the sulphonic acid (HSO3) groups; these either increase the solubility of the colouring See also:matter or assist in causing it to be attracted by the fibre, &c. In many cases the See also:free colour-acid or free colour-See also:base has little colour, this being only developed in the salt. The free base rosaniline, for example, is colourless, whereas the salt See also:magenta (rosaniline hydrochloride) has a deep See also:crimson colour in solution. The free acid See also:Alizarin is orange, while its alumina-salt is bright red. It may be here stated that the scientific See also:classification of colouring matters into Nitro-colours, Azo-colours, &c., already alluded to, is based on their chemical constitution, or the chromophorous groups they contain, whereas the classification according to their mode of application is dependent upon the character and arrangement of the auxochromous groups. The question of the mordant-dyeing property of certain colouring matters containing (OH) and (COOH) groups has already been explained under the See also:head of Artificial Mordant Colours. The See also:peculiar property characteristic of dyestuffs, as distinguished from See also:mere colouring matters, namely, that of being readily attracted by the textile fibres, notably the animal fibres, appears then to be due to their more or less marked acid or basic character. Intimately connected with this is the fact that these fibres also exhibit partly basic and partly acid characters, due to the presence of carboxyl and amido groups. The behaviour of magenta is typical of the Basic Colours. As already indicated, rosaniline, the base of magenta, is colourless, and only becomes coloured by its union with an acid, and yet wool and silk can be as readily dyed with the colourless rosaniline (base) as with the magenta (salt). The explanation is that the base rosaniline has See also:united with the fibre, which here plays the part of an acid, to form a coloured salt. It has also been proved that in dyeing the animal fibres with magenta (rosaniline hydrochloride), the fibre unites with the rosaniline only, and liberates the hydrochloric acid. Further, magenta will not dye cotton unless the fibre is previously prepared, e.g. with the mordant tannic acid, with which the base rosaniline unites to form an insoluble salt. In dyeing wool it is the fibre itself which acts as the mordant. In the See also:case of the Acid Colours the explanation is similar. In many of these the free colour-acid has quite a different colour from that of the See also:alkali-salt, and yet on dyeing wool or silk with the free colour-add, the fibre exhibits the colour of the alkali-salt and not of the colour-acid. In this case the fibre evidently plays the part of a base. Another fact in favour of the view that the union between fibre and colouring matter is of a chemical nature, is that by altering the chemical constitution of the fibre its dyeing properties are also altered; oxycellulose and nitrocellulose, for example, have a greater attraction for Basic Colours than See also:cellulose. Such facts and considerations as these have helped to establish the view that in the case of dyeing animal fibres with many colouring matters the operation is a chemical process, and not merely a mechanical absorption of the dyestuff. A similar explanation does not suffice, however, in the case of dyeing cotton with the Direct Colours. These are attracted by cotton from their solutions as alkali salts, apparently without decomposition. The See also:affinity existing between the fibre and colouring matter is somewhat feeble, for the latter can be removed from the dyed fibre by merely boiling with water. The See also:depth of colour obtained in dyeing varies with the concentration of the colour solution, or with the amount of some neutral salt, e.g. sodium chloride, added as an assistant to the dye-bath; moreover, the dye-bath is not exhausted. The colouring matter is submitted to the action of two forces, the solvent See also:power of the water and the affinity of the fibre, and divides itself between the fibre and the water. After dyeing for some time, a state of See also:equilibrium is attained in which the colouring matter is divided between the fibre and the water in a given ratio, and prolonged dyeing does not intensify the dyed colour. Some investigators hold the view that in some cases the fibres exert a purely See also:physical attraction towards colouring matters, and that the latter are held in an unchanged state by the fibre. The phenomenon is regarded as one of purely mechanical See also:surface-attraction, and is compared with that exercised by animal See also:char-coal when employed in See also:decolourizing a solution of some colouring matter. Some consider such direct dyeing as mere See also:diffusion ofthe colouring matter into the fibre, and others that the colouring matter is in a state of " solid solution " in the fibre, similar to the solution of a metallic oxide in coloured See also:glass. According to this latter view, the cause of the dyeing of textile fibres is similar to the attraction or solvent action exerted by See also:ether when it with-draws colouring matter from an aqueous solution by agitation. Latterly the view has been advanced that dyeing is due to precipitation of the colloid dyestuffs by the colloid substance of the fibre. In the case of colours which are dyed on mordants, the question is merely transferred to the nature of the attraction which exists between the fibre and the mordant, for it has been conclusively established that the union between the colouring matter and the mordant is essentially chemical in character. From our present knowledge it will be seen that we are unable to give a final See also:answer to the question of whether the dyeing process is to be regarded as a chemical or a mechanical process. There are arguments and facts which favour both views; but in the case of wool and silk dyeing, the prevailing See also:opinion in most cases is in favour of the chemical theory, whereas in cotton-dyeing, the mechanical theory is widely accepted. Probably no single theory can explain satisfactorily the fundamental cause of attraction in all cases of dyeing, and further investigation is needed to answer fully this very difficult and abstruse question. The poisonous nature or otherwise of the coal-tar dyes has been frequently discussed, and the popular opinion, no doubt dating from the time when magenta and its derivatives were contaminated with See also:arsenic, seems to be that they are con for the most part really poisonous, and ought to be ciuson. avoided for colouring materials worn next the skin, for articles of See also:food, &c. It is satisfactory to know that most of the colours are not poisonous, but some few are—namely, Picric acid, See also:Victoria Orange, Aurantia, Coralline, Metanil Yellow, Orange II. and See also:Safranine. Many coal-tar colours have, indeed, been recommended as See also:antiseptics or as medicinal remedies, e.g. Methyl Violet, Auramine and Methylene Blue, because of their special physiological action. In See also:histology and See also:bacteriology many coal-tar colours have rendered excellent service in staining microscopic preparations, and have enabled the investigator to detect See also:differences of structure, &c., previously unsuspected. In See also:photography many of the more fugitive colouring matters, e.g. Cyanine, Eosine, See also:Quinoline Red, &c., are employed in the manufacture of ortho-See also:chromatic plates, by means of which the colours of natural See also:objects can be photographed in the same degrees of light and shade as they appear to the See also:eye—blue, for example, appearing a darker See also:grey, yellow, a lighter grey, in the printed photograph. Since the See also:year 1856, in which the first coal-tar colour, See also:mauve, was discovered, the See also:art of dyeing has made enormous advances, mainly in consequence of the continued introduction of coal-tar colours having the most varied properties and suitable for nearly every requirement. The old See also:idea that the See also:vegetable dyestuffs are See also:superior in fastness to light is gradually being given up, and, if one may See also:judge from the past, it seems evident that in the future there will come a time when all our dyestuffs will be prepared by artificial means. AuTxoxrrIas.—Macquer, Hellot and le Pileur d'Apligny, The Art of Dyeing Wool, Silk and Cotton (See also:London, 1789) ; See also:Bancroft, See also:Philosophy of Permanent Colours (2 vols., London, 1813) ; Berthollet-Ure, Elements of the Art of Dyeing (2 vois., London, 1824) ; See also:Chevreul, Recherches chimiques sur la teinture (See also:Paris, 1835–1861) ; O'See also:Neill, The See also:Chemistry of See also:Calico See also:Printing, Dyeing and Bleaching (See also:Manchester, 186o) ; See also:Dictionary of Calico Printing and Dyeing (London, 1862) ; See also:Schutzenberger, Trite See also:des matieres colorantes (2 vols., Paris, 1867) ; Bolley, See also:Die Spinnfasern and die See also:im Pflanzen- and Thierkorper vorkommenden Farbstoffe (1867) ; See also:Crookes, A See also:Practical Handbook of Dyeing and Calico-Printing (London, 1874) ; Jarmain, Wool-Dyeing (1876) ; rO'Neill, Textile Colourist (4 vols., Manchester, 1876) ; See also:Calvert, Dyeing and Calico Printing (Manchester, 1876); Moyret, Traite de la teinture des soies (See also:Lyon, 1877); O'Neill, The Practice and Principles of Calico Printing, Bleaching and Dyeing (Manchester, 1878) ; See also:Girardin, Matieres textiles et matieres tinctoriales (Paris, 188o)'; See also:Hummel, The Dyeing of Textile Fabrics (London, 1885) ; Sansone, Dyeing (Manchester, 1888) ; Witt, Chemische Technologie der Gespinnstfasern (See also:Brunswick, 1888) ; Benedikt and Knecht, The Chemistry of the Coal-Tar Colours (London, 1889) ; See also:Hurst, Silk Dyeing, Printing and Finishing (London, 1892) ; Noelting and Lehne, Anilinschwarz (See also:Berlin, 1892); Knecht, Rawson and Loewenthal, See also:Manual of Dyeing (London, 19o8); Steinbeck, Bleichen and Farben der Seide and Halbseide (Berlin, 1895) ; See also:Gardner, Wool-Dyeing (Manchester, 1896) ; Rawson, Gardner and Laycock, A Dictionary of Dyes, Mordants, &c. (London, 1901); See also:Gros-Renaud, See also:Les Mordants en teinture et en impression (Paris, 1898) ; Georgievics, The Chemical Technology of Textile Fabrics (London, 1902) ; See also:Paterson, The See also:Science of Colour Mixing (London, 1900); Paterson, Colour Matching on Textiles (London, 1901) ; See also:Beech, The Dyeing of Cotton Fabrics (London, 1901) ; Beech, The Dyeing of Woollen Fabrics (London, 19o2); The See also:Journal of the Society of Dyers and Colourists (See also:Bradford, 1885–1908) and the publications of the colour manufacturers. (J. J. H.; E. Additional information and CommentsThere are no comments yet for this article.
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