There are several general theories with regard to the fixation of colours in dyeing organic fibres, and it is probable that no one of them affords a complete explanation in all cases. One holds that the action of dyeing is mechanical rather than chemical, the colour adhering to the fibre by surface-attraction; another, that an actual chemical compound is formed between the dye and the dyed material or one of its constituents; and a third, the “solid solution” theory of Witt, is in a sense intermediate, holding that the colouring matter is actually dissolved in the dyed fibre. The idea of a solid solution, strange at first, offers little difficulty on consideration. The colouring metallic salts in tinted glasses exist obviously in solution in the melted glass, and can hardly be said to change their condition in this respect when the glass becomes solid. Gelatine, indiarubber, and perhaps all other colloid bodies, absorb water or other liquids without losing their solid form, and these liquids may fairly be said to be dissolved in the solid. All animal and vegetable fibres are in this respect like gelatine, and during the process of dyeing are swollen with water. It is quite easy to dye a mass of gelatine throughout with most water-soluble dyestuffs. (Compare on these points what is said in [Chapter IX.] on the physical chemistry of hide-fibre.) The distinctions between solution and molecular surface-attraction on the one hand, and certain forms of chemical combination on the other, are not wide ones, and probably all three theories are true in different cases, and shade off into each other by imperceptible gradations. The subject of leather-dyeing is, in fact, a very complicated one, since we are not dealing with a fibre of uniform composition, but with one which has had its structure (both chemical and physical) altered by the processes to which it has been subjected during its conversion into leather.
Although, strictly speaking, the constitution of the gelatinous fibre of the skin is unknown, we are quite justified in stating[171] that, like the amido-acids which are important proximate products of its decomposition, it contains both acid and basic groups, and is therefore capable of attracting both bases and acids. It is well known, for instance, that the neutral fibre is capable of withdrawing sulphuric acid from a decinormal solution with such vigour that the residual liquid is neutral to litmus paper; and it will also absorb caustic alkalies with perhaps equal avidity.[172]
[171] Procter, Jour. Soc. Chem. Industry, 1900, p. 23.
[172] Cp. [Chap. IX.]
It is thus readily dyed by colouring matter of either basic or acid character, and in many cases will even dissociate their salts, dyeing the characteristic colour of the free dyestuff, but possibly at the same time fixing the liberated base or acid with which the colouring matter has been combined. Many tanning processes consist in a somewhat analogous fixation of weak bases and acids, and it is, therefore, to be anticipated that they will profoundly modify the colour-fixing properties of the original fibre, as indeed proves to be the case. Exactly what the result of a particular tanning process in this respect will be is less easy to foresee.
In the ordinary vegetable tanning process, the tannins, which are of acid nature, are freely fixed by the fibre. It is, therefore, not surprising that vegetable-tanned leather most readily fixes the basic colours, especially as these form insoluble compounds with the tannic acids, so that it is quite probable that the dyeing is mainly effected by the formation of tannin-colour-lakes on the fibre, rather than by actual fixation of the colour-base in combination with the original matter of the skin. It is noteworthy, however, that even fully tanned skin has by no means lost its attractions for acid colouring matters, many of which will dye it even without the presence of free acid, though it is possible that the tannic acid performs the function of saturating the alkaline base with which the colour acid has been combined.
It should be pointed out that while the substance of animal skin consists practically of gelatinous fibres, it is covered on the outer surface with a thin membrane of extreme tenuity, called the hyaline or glassy layer ([p. 50]) which, in the living animal, separates the true skin from the epidermis. This layer, the chemistry of which is quite unknown, reacts to colouring matters differently from the gelatinous fibres, and probably is less absorbent for basic colours, and more so for the coloured anhydrides of the tannins, and perhaps for acid colours generally, than is the true skin. As a result, it colours more darkly in tanning, and less so in dyeing with basic colours, and as it is extremely liable to damage in the preliminary operations of removing hair and lime by the tanner, this irregularity of colouring is a serious disadvantage which is most marked with the basic colours. Small quantities of lime left in the skin are also probably important causes of irregular dyeing.
Mordants are chemicals used to enable the fibre to fix dyes for which it would not otherwise have sufficient attraction, and hence are generally substances which have affinity both for the fibre and the dye. Thus cotton, which does not itself attract the basic colours, is mordanted for them by a solution of tannin, which it attracts, and which, in its turn, attracts and fixes the colours. In many cases, however, the function of mordants is more complex, not merely fixing the dyestuff, but often modifying, or even producing its colour. Thus tannin dyes black on an iron mordant, though it is itself colourless. Such mordants may be applied after the colouring matter, where it has sufficient attraction for the fibre to be taken up alone, but does not produce the required colour. This process is often called “saddening,” as the colour is generally darkened. A familiar instance is the use of iron solutions to darken or blacken tannin or logwood. There is scarcely any distinction in theory between mordants of this class and the constituents of dyes which are successively applied to the leather in order to produce the colouring matter on the fibre. Among these may be mentioned several mineral salts which were formerly employed in leather dyeing, though their use is now nearly obsolete. Iron salts are easily fixed by leather, whether tanned or tawed, and in the former case produce a dark colour by action of the tannin. On subsequent treatment with a solution of potassium ferrocyanide, a deep blue is formed (Prussian blue). If copper acetate or ammoniacal solution of copper sulphate be substituted for the iron salt, a deep red-brown ferrocyanide is produced. Yellows are sometimes dyed by first treating tanned leathers with lead acetate, which is fixed by the tannin, and then with potassium bichromate, by which yellow lead chromate is produced. A more important use of lead is in the so-called “lead-bleach,” which is really a white pigment-dyeing with lead sulphate. The tanned leather, after washing, is first treated with a solution of lead acetate (usually “brown sugar of lead” of about 4 grm. per liter), and subsequently with a dilute sulphuric acid of about 30 grm. of concentrated acid per litre, and then thoroughly washed to free it from acid. The process is often used as a preparation for dyeing pale shades, as many of the aniline dyes are easily fixed on the bleached leather, but is subject to the disadvantage attendant on all pigments containing lead, of becoming rapidly darkened by traces of sulphur or sulphuretted hydrogen, such as are constantly contained in lighting gas, or arise from the putrefaction of organic matters. The use of acid is also liable to cause early decay of the leather.
A large proportion of the coal-tar colours contain amido-groups (NH2 groups) which, when treated on the fibre with nitrous acid (or an acidified solution of sodium nitrite), become “diazotised” (converted into —N : N— groups with elimination of OH2). On further treating the diazo-compound with solutions of amines or phenols, combination takes place, and new azo-colours are formed in or on the fibre, often remarkably fast to washing or rubbing. Since these qualities are less important in leather than in textiles, and the process is moreover somewhat delicate, and the nitrous acid is apt to injuriously affect the leather, these processes have been little used in leather-dyeing, and are only mentioned here for the sake of completeness.
The use of the natural polygenetic colours in dyeing leather of vegetable tannage, which was once universal, is gradually disappearing, except for the production of blacks. Leather cannot be very satisfactorily mordanted for these colouring matters; but they have some natural attraction for the leather itself, and are generally dyed first, and their colours afterwards developed by metallic mordants such as iron, chrome, tin salts, and alum, which act not only on the absorbed dyestuff, but frequently on the tannin and colouring matters derived from the tanning materials. For black-dyeing, the use of coal-tar colours, either alone, or to deepen the colours produced by iron, is gradually extending. Claus and Rée’s “Autho-black,” the “Corvolines” of the Badische Co., and Casella’s “Naphthylamine Black,” “Aniline Grey,” and “Naphthol Blue-black” may be mentioned as useful colours. As coal-tar blacks are mostly dark violets rather than dead blacks, their colour may be deepened by the admixture of suitable yellows or browns, and this has already been done in one or two of the colours named. Apart from the coal-tar colours, black dyeing is generally produced by the action of iron (and chrome), either on the tannin of the leather itself or on logwood. As the leather is frequently greasy, and the satisfactory formation of a tannin- or logwood-lake can only take place in presence of a base to absorb the liberated acid of the iron salt, the skins are either brushed with, or plunged in, a logwood infusion, rendered alkaline with soda or ammonia, or the tanned leather receives a preliminary treatment with weak soda or ammonia solution. As such solutions act powerfully on tanned leathers, rendering them harsh and tender, great care must be taken to avoid excess. The effect of this alkaline treatment is not only to assist the wetting of the greasy surface, but to prevent too deep penetration of the dye, by causing rapid precipitation of the colour-lake. In recent times, however, leathers are sometimes demanded in which the colour goes right through, and in this case it might be well to reverse the treatment, beginning with a weak solution of a ferrous salt, perhaps with addition of sodium acetate or potassium tartrate, and finishing with alkaline logwood, as without alkali the full colour is not developed. The use of iron salts is not very satisfactory in regard to the permanence of the leather; and in this respect it is of great importance that they should not be used in excess, and that any strong acids they contain should be saturated with permanent bases, and if possible washed out. Leather-surfaces blacked with iron almost invariably ultimately lose their colour, becoming brown if tannins, and red if logwood has been employed, and at the same time the leather surface usually becomes brittle or friable. This is to a large extent due to the effect of iron oxides as oxygen-carriers. Exposed to light, they become reduced to the ferrous state, oxidising the organic matters with which they are combined, and in the dark they re-oxidise, and the process is repeated. It is therefore of the first importance that excess of the organic colouring matter should be provided, and that the quantity of the iron should be as small as possible, and in stable combination. These points are greatly neglected in practice, especially where blacking is done by the application of iron salts without logwood, when the evils mentioned are intensified by the actual removal of part of the tannin of the leather, and perhaps by the combination of ferric oxide with the skin-fibre itself, forming a brittle iron-leather. Treatment with alkaline sumach-, gambier- or logwood-solutions, both before and after the application of the iron, would lessen the evil. Iron-logwood blacks are much less permanent, and fade more rapidly under the influence of light and air than iron-tannin blacks. The use of iron-blacks on curried leathers seems considerably to increase the tendency to “spueing,” a defect due to oxidation of the oils (see [p. 390]). Copper salts mordant logwood a very dark blue, which is much more stable than the iron compound, and hence are often used advantageously in mixture with iron salts. In practice, iron blacks are generally oiled in finishing, and this renders them more permanent, both by protecting the lake from air and by forming iron soaps which are stable. The use of actual soaps in blacking and finishing is not unknown, and probably deserves more attention. Hard soaps of soda and stearic acid,[173] form an excellent finish where a moderate glaze is required, the soap jelly being applied with a brush very thinly, allowed to dry thoroughly, and polished with a flannel or brush, or glassed. Many acid colours are soluble in such soap jellies, which may thus be employed for staining. Similar but harder finishes, and capable of being glazed to a high polish, are made by dissolving shellac with dilute borax or ammonia solutions.[174] Both of these finishes are useful in lessening the tendency of iron blacks to smut or rub off, a failing which is due to the precipitation of loose iron-lakes on the surface, instead of in combination with the fibre, and is particularly obvious where “inks” or one-solution blacks are employed, or where the mordant and the colouring matter solutions are allowed to mix on the surface of the leather. Such “inks” are generally made with a ferrous salt and logwood or tannin, together with some aniline black, and the colour-lake should only be formed on oxidation. Chrome is not much employed in blacks with vegetable tannages, as it only produces blacks with logwood, the chrome compounds of tannins having no colouring value; and bichromates used at all freely being very injurious to the leather.