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 DYEING In the case of the coal-tar colours we are, for the most part, well acquainted with their chemical constitution, ClassHlca- an<^ accordance with this knowledge the tion of chemist has arranged them in the following colouring groups :—1. Nitro Colours. 2. Azo Colours, inmatters. cluding Amido-azo, Oxy-azo, Tetrazo, and Polyazo Colours. 3. Hydrazone Colours. 4. Oxy-quinone Colours, including Quinone-oxime Colours. 5. Diphenyl methane and Triphenylmethane Colours, including Itosaniline, Rosolic acid, and Phthaleine Colours. 6. Quinoneimide Colours, including Indamine, Indophenol, Thiazime, Thiazone, Oxazime, Oxazone, Azine, Induline, Quinoxaline, and Fluorindine Colours. 7. Aniline Black. 8. Quinoline and Acridine Colours. 9. Thiazol Colours. 10. Oxy-ketone, Xanthone, Flavone, and Cumarine Colours. 11. Indigo. 12. Colours of unknown constitution. This arrangement of the colouring matters in natural chemical groups is well suited for the requirements of the chemist, but another classification is that based on the mode of their application in dyeing. This is much simpler than the previous one, and being better adapted for the practical purposes of the dyer, as well as for explaining the various methods of dyeing, it is preferred for this article. According to this arrangement colouring matters are classified under the following groups:—1. Acid Colours. 2. Basic Colours. 3. Direct Colours. 4. Developed Colours. 5. Mordant Colours. 6. Miscellaneous Colours. 7. Mineral Colours. It is well to state that there is no sharp line of division between some of these groups, for many colours are applicable by more than one method, and might quite well be placed in two, or even three, of the above groups. This may be due either to the kind of fibre to which the colouring matter is to be applied, or to certain details in the chemical constitution of the latter which give it a twofold character. Acid. Colours.—These dyestuffs are so called because they dye the animal fibres wool and silk in an acid hath ; they do not dye cotton. From a chemical point of view the colouring matters themselves are of an acid character, this being due to the presence in the molecule of nitro (XOa) or sulphonic acid (HS03) gi-oups. According to their origin and constitution they may be distinguished as nitro compounds, sulphonated azo compounds, and sulphonated basic colours. The Acid Colours are usually sold in the form of their alkali salts, as variously coloured powders soluble in water. For the alkali salts in neutral or alkaline solution wool and silk have little or no affinity, but dyeing rapidly occurs if the solution is acidified with sulphuric acid whereby the colour-acid is liberated. This addition of acid, however, is necessary not only to set free the colour-acid of the dyestuff, but also to alter partially the chemical composition of the fibre, and thus render it capable of uniting more readily with the free colour-acid. It has been shown, namely, that if wool is boiled with dilute sulphuric acid, and then thoroughly washed with boiling water till free from acid, it acquires the property of being dyed with Acid Colours even in neutral solution. By this treatment a portion of the wool substance is converted into so-called lanuginic acid, which has a strong attraction for the colour-acid of the dyestuff’, with which it forms an insoluble coloured compound. For dyeing wool, the general rule is to charge the dyebath with the amount of dyestuff necessary to give the required colour, say from ^ to 2 or 6 per cent, on the weight of wool employed, along with 10 per cent, sodium sulphate (Glauber s salt) and 4 per cent, sulphuric acid (1'84° sp. gr.). The woollen material is then introduced and continually handled or moved about in the solution, while the temperature of the latter is gradually raised to the boiling-point in the course of f to 1 hour ; after boiling for I to J hour longer, the operation is complete, and the material is washed and dried. In practice, modifications of this normal process may be introduced, in order to ensure the dyeing of an even colour, i.e., free from such irregularities as cloudiness, streaks, &c., which may be 16 fv, .^leColour ffua,bty of the material or to special of the Acid employed. Materials of athe firm, close properties texture, also the existence of a strong affinity between fibre and colouring matter, are not conducive to the dyeing of even colours, or to a satisfactory penetration of the material. Some Acid Colours dye e r 'X J colours without any difficulty ; others, however, do not. The addition of sodium sulphate to the dyebath acts beneficially by causing less sulphuric acid to be attracted by the wool, so that its

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action is moderated; the dyeing therefore proceeds more slowly and regularly, and a more equal distribution and better absorption of the colouring matter takes place. Other devices to obtain even colours are : the use of old dye-liquors, a diminished amount of acid, the employment of weaker acids, e.g., acetic acid or ammonium acetate, and the entering of the material at a low temperature. In the application of so-called Alkali Blue the process of dyeing in an acid bath is impossible, owing to the insolubility of the colour-acid in an acid solution. Wool and silk, however, possess an affinity for the alkali salt of the colouring matter in neutral or alkaline solution, hence these fibres are dyed with the addition of about 5 per cent, borax ; the material acquires only a pale colour, that of the alkali salt, in this dyebath, but by passing the washed material into a cold or tepid dilute solution of sulphuric acid a full bright blue colour is developed, due to the liberation of the colouracid within the fibre. In the case of other Acid Colours, e.g., Chromotrope, Chrome Brown, Chromogen, Alizarin Yellow, &c., the dyeing in an acid bath is followed by a treatment with a boiling solution of bichromate of potash, alum, or chromium fluoride, whereby the colouring matter on the fibre is changed into insoluble oxidation products or colour-lakes. This operation of developing or fixing the colour is effected either in the same bath at the end of the dyeing operation, or in a separate bath. See also Artificial Mordant Colours. When dyeing with certain Acid Colours, e.g., Eosine, Phloxine, and other allied bright pink colouring matters derived from resorcinol, the use of sulphuric acid as an assistant must be avoided, since the colours would thereby be rendered paler and duller, and only acetic acid must be employed. The properties of the dyes obtained with the Acid Colours are extremely varied. Many are fugitive to light; on the other hand, many are satisfactorily fast, some even being very fast in this respect. As a rule, they do not withstand the operations of milling and scouring very well, hence Acid Colours are generally unsuitable for tweed yarns, or for loose wool. They are largely employed, however, in dyeing other varieties of woollen yarn, silk yarn, union fabrics, dress materials, leather, &c. Previous to the discovery of the coal-tar colours, very few Acid Colours were known, the most important one being Indigo Extract. Prussian Blue as applied to wool may also be regarded as belonging to this class, also the purple dyestuff known as Orchil or Cudbear. The following list includes some of the more important Acid Colours now in use, arranged according to the colour they yield in dyeing Red.—Wool Scarlet, Brilliant Scarlet, Erythrine, Brilliant Crocein, Violamine G, Scarlet 3 R, Crystal Scarlet, New Coccine, Chromotrope 2 R, Azo Acid Magenta, Victoria Scarlet, Xylidine Scarlet, Palatine Scarlet, Biebrich Scarlet, Pyrotine, Orchil Red, Milling Red, Azo Carmine, Acid Magenta, Fast Acid Violet A 2 R, Naphthylamine Red, Fast Red, Claret Red, Eosine, Erythrosine, Rose Bengal, Phloxine, Cyanosine. Orange.—Diphenylamine Orange, Methyl Orange, Naphthol Orange, Crocein Orange, Brilliant Orange, Orange G, Orange N, Mandarin G R. Yellow.—Vicrio, acid, Naphthol Yellow S, Fast Yellow, Brilliant Yellow S, Azoflavine, Metanil Yellow, Resorcine Yellow, Tartrazine, Quinoline Yellow, Milling Yellow, Azo Yellow, Victoria Yellow, Brilliant Yellow S. Green.—Acid Green, Guinea Green, Fast Green, Patent Green. Blue.—Alkali Blue, Soluble Blue, Opal Blue, Hoechst New Blue, Patent Blue, Ketone Blue, Cyanine, Thiocarmine, Fast Blue, Induline, Violamine 3 B, Azo Acid Blue, Wool Blue, Indigo Extract. Violet.—Acid Violet, Red Violet, Regina Violet, Formyl Violet, Violamine B, Fast Violet. Brown.—Fast Brown, Naphthylamine Brown, Acid Brown, Resorcine Brown, Azo Brown, Chrome Brown, Chromogen. Black.—Naphthol Black, Azo Black, Wool Black, Naphthylamine Black, Jet Black, Anthracite Black, Victoria Black, Azo Acid Black, Brilliant Black. Basic Colours.—These colouring matters are the salts of organic colour-bases, their name being derived from the fact that their dyeing power entirely resides in the basic part of the salt. In the free state the bases are colourless and insoluble, but in combination with acids they form salts which are coloured and for the most part soluble in water. They are usually sold in the form of powder or crystals, the latter exhibiting frequently a beautiful metallic lustre. Wool and silk are dyed in a neutral bath, i.e., without any addition, the material not requiring any previous preparation. During the dyeing operation the animal fibres appear to play the part of an acid, for they decompose the- colouring matter and unite with the colour-base to form an insoluble coloured salt or lake, while the acid of the colouring matter is liberated and remains in solution. Although, as a rule, a neutral dyebath is employed in dyeing wool, a slight addition (2 per cent.) of soap is sometimes made in order to give a brighter colour, while in other cases, e.g., with Victoria Blue, the dyebath must of necessity be