From our present knowledge it will be seen that we are unable to give a final 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 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 arsenic, seems to be that they are Conclusion. for the most part really poisonous, and ought to be avoided for colouring materials worn next the skin, for articles of food, &c. It is satisfactory to know that most of the colours are not poisonous, but some few are—namely, Picric acid, Victoria Orange, Aurantia, Coralline, Metanil Yellow, Orange II. and Safranine. Many coal-tar colours have, indeed, been recommended as antiseptics or as medicinal remedies, e.g. Methyl Violet, Auramine and Methylene Blue, because of their special physiological action. In histology and bacteriology many coal-tar colours have rendered excellent service in staining microscopic preparations, and have enabled the investigator to detect differences of structure, &c., previously unsuspected. In photography many of the more fugitive colouring matters, e.g. Cyanine, Eosine, Quinoline Red, &c., are employed in the manufacture of ortho-chromatic plates, by means of which the colours of natural objects can be photographed in the same degrees of light and shade as they appear to the eye—blue, for example, appearing a darker grey, yellow, a lighter grey, in the printed photograph.
Since the year 1856, in which the first coal-tar colour, mauve, was discovered, the 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 idea that the vegetable dyestuffs are superior in fastness to light is gradually being given up, and, if one may 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.
Authorities.—Macquer, Hellot and le Pileur d'Apligny, The Art of Dyeing Wool, Silk and Cotton (London, 1789); Bancroft, Philosophy of Permanent Colours (2 vols., London, 1813); Berthollet-Ure, Elements of the Art of Dyeing (2 vols., London, 1824); Chevreul, Recherches chimiques sur la teinture (Paris, 1835-1861); O'Neill, The Chemistry of Calico Printing, Dyeing and Bleaching (Manchester, 1860); Dictionary of Calico Printing and Dyeing (London, 1862); Schützenberger, Traité des matières colorantes (2 vols., Paris, 1867); Bolley, Die Spinnfasern und die im Pflanzen- und Thierkörper vorkommenden Farbstoffe (1867); Crookes, A Practical Handbook of Dyeing and Calico-Printing (London, 1874); Jarmain, Wool-Dyeing (1876); O'Neill, Textile Colourist (4 vols., Manchester, 1876); Calvert, Dyeing and Calico Printing (Manchester, 1876); Moyret, Traité de la teinture des soies (Lyon, 1877); O'Neill, The Practice and Principles of Calico Printing, Bleaching and Dyeing (Manchester, 1878); Girardin, Matières textiles et matières tinctoriales (Paris, 1880); Hummel, The Dyeing of Textile Fabrics (London, 1885); Sansone, Dyeing (Manchester, 1888); Witt, Chemische Technologie der Gespinnstfasern (Brunswick, 1888); Benedikt and Knecht, The Chemistry of the Coal-Tar Colours (London, 1889); Hurst, Silk Dyeing, Printing and Finishing (London, 1892); Noelting and Lehne, Anilinschwarz (Berlin, 1892); Knecht, Rawson and Loewenthal, Manual of Dyeing (London, 1908); Steinbeck, Bleichen und Färben der Seide und Halbseide (Berlin, 1895); Gardner, Wool-Dyeing (Manchester, 1896); Rawson, Gardner and Laycock, A Dictionary of Dyes, Mordants, &c. (London, 1901); Gros-Renaud, Les Mordants en teinture et en impression (Paris, 1898); Georgievics, The Chemical Technology of Textile Fabrics (London, 1902); Paterson, The Science of Colour Mixing (London, 1900); Paterson, Colour Matching on Textiles (London, 1901); Beech, The Dyeing of Cotton Fabrics (London, 1901); Beech, The Dyeing of Woollen Fabrics (London, 1902); The Journal of the Society of Dyers and Colourists (Bradford, 1885-1908) and the publications of the colour manufacturers.
(J. J. H.; E. K.)
[1] The term "dry dyeing," which is carried out only to a very limited extent, relates to the dyeing of fabrics with the dyestuff dissolved in liquids other than water, e.g. benzene, alcohol, &c.