CHAPTER XXV.
DYES AND DYEING.
Before the discovery of artificial organic dyestuffs, the only colouring materials known to industry were those of mineral and direct organic origin; and on this account the dyeing of leather was formerly subject to great difficulties and limitations.
The discovery of the means of artificially preparing an organic dyestuff (mauve) by Perkin some forty-five years since, opened up a new field for research, and since that time, the list of commercial dyes has so increased that there is now scarcely a tint or shade which cannot be accurately matched and reproduced by the coal-tar colours. These colours are often spoken of as “aniline dyes” owing to the fact that many of them, and especially the earlier ones, have been derived from aniline, one of the products of coal-tar; but more recently, a considerable number of important colours have been prepared from other constituents of the tar, and it is therefore more correct to term the whole of the dyes obtained, either directly or indirectly, from coal-tar, the “coal-tar colours.”
The coal-tar colours are generally soluble in water, or mixtures of water and alcohol, and the majority of them combine with the fibre of the leather without the use of any mordant, so that in most cases it is only necessary to apply a solution of the dye direct to the leather, though their suitability for the purpose varies considerably. A few which are only soluble in oils or hydrocarbons, are not suitable for leather-dyeing, though they may sometimes be utilised in conjunction with fats in currying; and there are also certain colours which are not applied to the fibre ready formed, but are developed on it by subsequent chemical treatment, and which have only been applied to a limited extent to leather.
A number of the coal-tar dyes, which are produced in the crystalline form, have a totally different colour when solid to that of their solutions, and to the colour they produce when dyed. A well-known instance of this is magenta or fuchsine, which forms glistening green crystals, while in solution it is a brilliant red dye. The colours of the crystals are usually complementary to those of the solution, thus several blues have the appearance of metallic copper, and violets, such as methyl-violet, are greenish-yellow, generally with a pronounced metallic lustre. This peculiarity is the cause of the defect in dyeing known as “bronzing,” in which the dye, when applied in too concentrated a form, takes a surface-shimmer of its complementary colour.
The coal-tar colours are mostly either “acid” or “basic.” The former are the salts of organic colour-acids with inorganic bases (generally sodium) and are usually readily soluble in water, but frequently do not fix themselves on the fibre till the colour-acid is set free by the addition of some stronger acid to the bath, and in many cases the free colour-acid is of different colour to its salts. The “basic” colours are salts of colour-bases (organic bases of the nature of very complicated ammonia-derivatives) with acids (mostly hydrochloric, sulphuric or acetic). Most of those in commercial use are soluble in water, though a few require the addition of alcohol. The colour-bases themselves are usually insoluble in water, and therefore precipitated by alkalies, and in some cases they are also colourless. The basic dyes have generally greater intensity of colour than the acid dyes, but large classes of them are very fugitive when exposed to light, and in strong solution many others are very liable to “bronze,” a defect which is generally less marked with the acid colours.[170]
[170] It has recently been shown by Lamb (see [App. D], [p. 498]) that many basic colours are much faster to light on leather than on textiles.
As it is not obvious at first sight whether a given dye is acid or basic, a reagent to distinguish them is useful. For this purpose a solution of 1 part of tannic acid and 1 part of sodium acetate in 10 parts (by weight) of water is conveniently employed, which gives coloured precipitates with basic dyes, but is not affected by acid ones. The fact that basic dyes are precipitated by tannins influences their use in leather dyeing, not only as regards their fixation on the leather-fibre by the tannin which it contains, but as the cause of their precipitation in the dye-bath if great care is not taken to avoid the presence of tannins in a soluble form. The use of the sodium acetate is to combine with the mineral acid of the colour-salt, which if left free would prevent complete precipitation, substituting for it acetic acid, which is much weaker, especially in presence of excess of sodium acetate (cp. [p. 81]).
In using the terms “acid” and “basic” with regard to dyes, it is not to be understood that the dyestuffs as employed are acid or alkaline in the sense that vinegar is acid, and soda basic, but merely that the actual colour-constituent of the salt is in the one case of an acid nature, and set free by stronger acids, and in the other case is basic, and liberated (and often precipitated) by stronger alkalies.