H. R. Procter

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.

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]

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.

In dyeing blacks on other than vegetable tannages, however, chrome becomes of importance, as logwood is principally employed, though sometimes in conjunction with tannin, and often with addition of quercitron or fustic, to correct the bluish shade of the logwood-chrome or logwood-iron lake. It must not be overlooked in practice, that if ferrous salts are mixed with bichromate solutions, the latter are reduced, and the iron is oxidised to the ferric state.

In alumed leathers the fixing power of the original hide-fibre is much less affected than in vegetable tannages. Whatever may be the truth with regard to the latter, there is little doubt that physical influences are at least as important as chemical ones in the production of mineral tannages. The amount of the tanning agent absorbed is greatly influenced by the concentration of the solutions, and in ordinary alum tawing much of the alumina may again be removed by free washing. In this case, the sulphate of potash present takes no part in the operation, but the alumina salt is absorbed apparently as a normal salt. Alum or alumina sulphate alone is incapable of producing any satisfactory tannage without the assistance of common salt, the quantity absorbed being small, and the fibre becoming swollen by the action of the acid. In presence of salt the absorption is greater, and the swelling is prevented. The explanation of this is not to be found in the formation of aluminium chloride, for though this undoubtedly takes place, it has been shown that the action of aluminium chloride without salt is not more satisfactory than that of alum. It has long been known that salt prevents the swelling action of acids on skin, although it does not lessen the absorption of acid; and the fact is capable of explanation on modern osmotic theories (cp. p. 89). The skin so treated is found to be converted into leather, but if the salt be washed out, the acid is retained by the skin, which returns to the state of acid-swollen pelt. It is probable, therefore, that although the acid and alumina are absorbed in equivalent proportions to each other, they are really dissociated, and attached to different groups in the gelatine molecule, and that the effect of the salt is to allow the absorption of the acid without swelling, and, osmotically, to increase the dissociating power of the pelt. If, in place of a normal alumina salt, a basic salt is employed, such as may be obtained by partial neutralisation of the sulphuric acid with soda, satisfactory tannage may be accomplished without salt, a basic compound is absorbed, and the leather is much less affected by washing. In the analogous case of chrome tannage, this basic compound may be still further deprived of its residual acid, by washing the tanned skin with alkaline solutions, leaving a leather which is extremely resistant even to hot water; and a somewhat similar result may be obtained with alumina, though with more difficulty, as apparently a very small excess of alkali destroys the qualities of the leather. (Cp. p. 187.)

The results on dyeing are almost what might have been foreseen. While ordinary alumed leather absorbs both acid and basic dyes readily, the basic chrome leather has practically lost its affinity for the latter. Both chrome and alumina leathers readily absorb vegetable tannins, thus supporting the view that the acid-fixing groups of the gelatine molecule are still unsaturated (tannins are capable of tanning pelt swollen with sulphuric acid and apparently of expelling the acid). In the case of chrome leather the effect of re-tanning with tannins is greatly to lessen its stretch, and if carried too far, to destroy its toughness, but it at once becomes capable of fixing basic dyestuffs. This property is frequently made use of in dyeing, but the effect on the leather must not be disregarded where softness and stretch are important, as in the case of glove-leathers. Polygenetic dyes are, of course, fixed on alum- or chrome-leathers by the alumina- or chrome-mordant, though apparently the bases are not present in the most favourable condition for fixing colours. Thus logwood extracted without alkali dyes tanned leather yellow, alumed leather violet-blue, and chrome leather blackish-violet, and some of the alizarine group dye very well on chrome as its resistance to hot water allows much higher temperatures to be used than with most other leathers. The tannin contained in dyewoods has the effect of lessening the stretch of chrome leathers.

Something should perhaps be said on the dyeing of oil and aldehyde leathers, but the subject has as yet been scarcely treated scientifically, and our practical knowledge of the subject is insufficient to justify theorising. (See, however, p. 496.)

Defects in the colour of the finished leather are due to a variety of causes, but many are produced by want of cleanliness and system during the dyeing itself. The greatest care is needed in this respect, and in brush-dyeing a different brush should be used for each different colour, as it is impossible to thoroughly remove all traces of dye by the ordinary methods of cleansing.

Irregular and surface dyeing sometimes occurs owing to too rapid fixation of the colours; while in other cases the affinity of the dye is too small to allow of reasonable exhaustion of the bath. Addition of salts of weak acids, such as potassium hydrogen tartrate (tartar), or of those like sodium sulphate, which form hydric salts, lessen rapidity of dyeing with acid colours; while acids generally increase it, and it is also often increased by addition of common salt, which lessens the solubility of the dye. Weak acids, such as acetic or formic, or acid salts, such as sodium bisulphate, are generally to be preferred to sulphuric acid as an addition to the dye-bath; and if the latter is used, great care is desirable in its complete removal. There is no doubt that the rapid decay of leather bookbindings and upholstery is largely due to the careless use of sulphuric acid in “clearing” and dyeing the leather;[175] and even if it is fully removed, it has saturated all bases such as lime, which are naturally present in leathers in combination with weak acids, and which would otherwise act as some protection from the sulphuric acid evolved in burning coal gas.

“Bronzing,” the dichroic effect produced by light reflected from the surface of many colouring matters, complementary to that transmitted by them and reflected by the surface of the dyed material, is not peculiar to basic colours, but is generally more marked in them than in acid ones. Basic colours, from their great affinity for tannins, and consequent rapid dyeing, are apt to dye irregularly, and without sufficiently penetrating the leather, and if the soluble tannin is not wholly washed out of the skins previously to dyeing, it bleeds in the dye-bath, and precipitates insoluble tannin-lakes, which waste colour and adhere to the surface of the leather. The inconvenience of basic colours due to their too rapid fixation may sometimes be lessened by slight acidification of the dye-bath with a weak acid, such as acetic or lactic. The acid may be still further “weakened” if desired, by the addition of its neutral (sodium) salt. The precipitation of tannin-lakes in the bath may be prevented by previous fixation of the tannin with tartar emetic, titanium potassium oxalate or lactate, or some other suitable metallic salt.

The fading of the colours of dyed goods by exposure to light is a defect which has been much more investigated in the textile industries than in leather manufacture, though in the latter case, and especially with regard to bookbinding and furniture leathers, it is of even greater importance. It is probable that no colours are actually unaffected by strong sunlight, but in many cases the action is so slight that it may practically be disregarded; some of the coal-tar colours, and especially some of the alizarines, being practically permanent, while others, and particularly the aniline colours belonging to the triphenylmethane group, such as magenta, are so fugitive as to be practically bleached by a week of strong sunlight. Chrysoidine and the eosins are also very bad in this respect. The fastness of colours to light is a good deal influenced by the material on which they are dyed, and but little has yet been published of the results of direct experiments on leathers, but Mr. M. C. Lamb has been for some time engaged in a research of this nature,[176] and the subject is now receiving a good deal of attention in other quarters. Experiments are easily made by exposing samples to sunlight under glass or in a south window, a part of the leather being covered with wood or thick brown paper for comparison. The results are often complicated by the tendency of all leathers tanned with tannins of the catechol group, and especially with turwar bark (p. 298), mimosa and quebracho, to darken and redden in sunshine, or even by exposure to diffused light. Pure sumach tannages are nearly free from this defect, and are also much less easily destroyed by the action of gas fumes (sulphuric acid), and the other injurious influences to which books and furniture are often subjected.[177]

Want of fastness to friction or rubbing is a defect generally more important in textiles than in leather, where it is often prevented by glazings or other finishes applied to the surface; but in some cases, and, especially in black leather, it is apt to be annoying. If suitable colours are used, the defect is generally due to the precipitation of loose colour on the surface, either by the too free use of mordants, or the dyeing of basic colours on leathers which have not been sufficiently freed from loose tannin. It is also often caused by “flaming” or the application of colour mixed with the “seasoning” used in glazing, to hide imperfections in the dyeing, or vary its colour. Colour applied in this way is only mechanically fixed on the leather, and is easily removed by moisture, staining articles with which it comes in contact.

A very similar defect may be caused by incomplete washing of the dyed leather, which leaves loose dye from the dye-bath in the goods. To avoid it in glove-leathers, where its occurrence would be particularly annoying, the natural mordant colours are still largely in use, which being precipitated on the fibre in an insoluble form by the mordant or “striker” (generally a metallic salt) are little liable to come off. Basic colours may be fixed by a subsequent treatment with tannin, or by topping with certain acid colours such as picric acid. Some few colours, and especially Martius or “Manchester” yellow (dinitronaphthol) are volatile at a low temperature, and therefore liable to “mark off” or stain any materials with which the dyed fabric, even in a dry state, is placed in contact.

The practical dyeing of leathers varies considerably according to whether they are tanned with vegetable materials, chrome, alumina salts, or chamoising. Vegetable-tanned leathers are dyed either by hand in the “dye-tray,” or in the drum or paddle, the two latter methods being now largely employed. The dye-tray is a shallow vat, about 10 inches deep, and large enough for the goods to be laid flat in it. In the English method, one or two dozen skins, or even more, are dyed at a time, being turned over in the tray by hand, the undermost pair being drawn out and placed on the top (Fig. 91). The method is convenient where only a small number of skins are to be dyed to one particular shade, which is more easily matched as the goods are always under observation, and it has the further advantage that, if desired, the grain sides only of the skins can be coloured, by “pairing” or “pleating” them before dyeing. For this purpose two skins of equal size are laid together flesh to flesh (pairing), or each skin is doubled down the back, flesh side in (pleating), and pressed firmly together with a sleeker on the table, when the skins adhere so closely that if carefully handled, no colour penetrates between them during the dyeing, except a little round the edges. This effects considerable economy of dye-stuff, as the fleshes would absorb a good deal, and for some purposes, an undyed flesh is preferred. In dyeing in the paddle or drum, the skins are merely placed loose in the dye-liquor, so that the fleshes are dyed equally with the grain sides. Paddle-dyeing has the advantage of effecting a considerable saving of labour, as compared with the dye-tray, in which constant handling, which often lasts an hour or more, is required. It also allows of almost equal facility in examining the colour of the skins, which is very important when dyeing to shade; but it is less economical in dye-stuff, as not only the flesh sides are dyed but a much larger volume of liquor is used, and as the dye-bath can never be entirely exhausted, more dye is run away in the used liquor. Drum-dyeing is much less expensive in this respect, as the volume of liquor may be very small, and from the efficiency of the motion, the dyeing is very thorough, and penetrates deeply into or through the skin, which in many cases is advantageous, but it is difficult to dye to exact shade, since the skins can only be examined by stopping and opening the drum. Most dyes are more readily fixed at high temperatures, and in this respect the drum has an advantage over all other methods, as once heated it retains its heat with very little loss to the end of the operation, while both in the paddle and the dye-tray the liquor is rapidly cooled, and special methods of maintaining the temperature complicate the apparatus, and require great care to avoid overheating. It is usually best to work at the highest temperature which the goods will safely bear, and this varies to some extent with the class of goods, chrome tannages and chamois leather being peculiar in standing almost any temperature short of boiling. With vegetable tanned leather 50° C. may be taken as a maximum; but cold wet skins may safely be introduced rapidly into a liquor heated to 60°, as they will cool it sufficiently.

The Continental method of dyeing in two trays may be mentioned here, as it produces very rapid and even dyeing, with considerable economy of dye-stuff, and the principle is capable of application to other methods where a large number of skins have to be dyed to the same colour. As generally carried out, two trays are employed, each about 4 feet long, 18 inches wide, and 10 inches or a foot deep, and these are usually made with a sloping bottom, or propped up in such a way that the dye-liquor all runs to the further side of the tray. A single pair of skins is usually dyed at once (in about 6 liters (5 qt.) of liquor for sheep and goat). To begin with, the first tray is filled with a very weak liquor, and the second with one of about half strength. The goods are entered in the first tray, turned a few times, and passed into the second; the liquor in the first is run away, and it is re-filled with one of the full strength, to which the goods are then transferred, and dyed to shade. The second tray is much reduced in strength by the skins, and now serves as the weak liquor for a fresh pair, which in its turn passes into that from which the goods have been dyed out, and then into a new liquor; each pair of goods thus passing through three baths, of which the last is of full strength, and which quickly brings up a full and even colour. In the ordinary English method, the goods must, for the sake of economy of dye-stuff, be dyed out in a nearly exhausted bath, which is a tedious operation, the last stage of dyeing often taking a time far longer than that required to bring the goods nearly up to shade, and even then failing to produce a good and full colour. This evil may be lessened by adding the dye-stuff in several successive portions, as the bath becomes exhausted, but cannot be altogether avoided with a single tray, if any reasonable exhaustion of the bath is to be attained. At first sight it seems a very slow process to dye the goods in single pairs, but this is to a great extent compensated by the rapidity with which they take on colour. In the Continental system, the dyes, mostly of the coal-tar series, are used as strong solutions, and each new dye-bath is made up by filling the tray with a definite volume of hot water and adding a measured quantity of the dye-solution.

The re-use of partially exhausted dye-baths is generally limited to cases where either single dyes, or mixtures of very equal affinity for the leather are employed, since where dyes of unequal affinity are employed, one is more rapidly removed than the other, and the shade of the dye-bath is altered. Many dyes sold as single colours are really mixtures,[178] and alter in shade if successive quantities of leather are dyed in their solutions. Basic dyes are also apt to be precipitated by traces of tannin washed out of the goods, and thus rendered unfit for use a second time. This may be avoided by suitable preparation of the goods (see p. 411).

Much of the success of practical leather-dyeing depends on proper selection and preparation of the goods. Sound uninjured grain is a matter of first importance; no satisfactory dyeing can be expected on skins which through carelessness in soaks, limes, or bates, are tainted by what is known as “weak grain,” caused by destruction or injury of the delicate hyaline layer, which forms the natural glaze and outer surface of the skin (p. 50). For such goods, “acid” are to be preferred to “basic” dyes, the latter having an especial tendency to dye darker and deeper where the grain is imperfect. Goods of different tannages and colours should never be dyed together, as they are certain to produce different shades in the same dye-bath. Tanned skins which have been dried, especially if they have been in stock for some time, should be thoroughly softened by soaking in tepid water and drumming, a temperature of between 40° and 45° C. being most advantageous. Skins, such as calf of mixed or bark tannage, must now be freed from all bloom by scouring with brush and if necessary with slate or stone, but great care is requisite to avoid injury to the grain. A little borax or other weak alkaline solution assists in removing bloom. Fresh sumach-tanned skins merely require setting out with a brass sleeker, but those which have been long dried often dye more evenly and readily if they are re-sumached.

Dark coloured tannages, such as Australian bazils, and East India sheep and goat tanned with cassia bark, are always improved by sumaching, and if for light colours, by first stripping a portion of the original tan by drumming for a quarter of an hour with a weak (¹/₄ per cent.) solution of soap powder or borax at a temperature of 30° to 35° C. and then passing (after well washing in warm water, but with as little exposure as possible to the air) through a weak sour of sulphuric acid of 1-2 per cent. The acid should now be as thoroughly removed as possible by washing in water, and the goods should be sumached. The process, and especially the use of sulphuric acid, is always deleterious to the skins, and is one of the causes of the early decay of coloured bookbindings and furniture leathers. Lactic, formic, or acetic acid may be substituted for sulphuric with safety, and the risk of injury from sulphuric, which generally is only apparent after the lapse of a considerable time, is a good deal lessened by adding to the sumach liquor a small quantity of potassium tartrate, sodium acetate or lactate, or some other salt of a weak organic acid, which is thus substituted for the much more dangerous sulphuric. Except in cases of absolute necessity for the production of light shades, the use of sulphuric acid should not be resorted to, and then only for goods which are not expected to possess great permanence. For light shades for bookbinding and upholstery, good sumach-tanned leathers and organic acids only should be employed. Alkaline treatment also demands great caution, as excess of strong alkalies is very injurious to the leather. Another objectionable method for the preparation of leather for very light shades, is the use of the lead-bleach described on p. 399.

The sumaching is best done in a drum, at a temperature of about 40°. Lamb advises that 1 to 2 lb. of sumach per dozen is sufficient for calf, and recommends running in this liquor for two or three hours. The skins are then rinsed in water to free them from adhering sumach, and set out on a table with a brass sleeker, and are now ready for dyeing with “acid” dye-stuffs. If “basic” dyes are used, thorough washing in several tepid waters is necessary to free them from the loose tannin; and if deep colours are to be dyed, it is better, instead of too much washing, to fix the tannin, which then serves as a mordant for the colour. For blues, blue-greens, or violets, this is done with a solution of “tartar emetic” (antimony potassium tartrate, of 5 to 20 grm. per liter according to the amount of tannin to be fixed, often with addition of some common salt), which produces no alteration in the colour. For browns, yellows, deep reds, or yellow-greens, it is advantageous to use titanium-potassium lactate or oxalate (2 grm. per liter), which in combination with the tannin produces a very permanent yellow coloration on which the basic colours dye freely. In many cases the titanium salt is best applied after dying with one of the dyewoods (Dreher).

The basic colours usually require simple solution in hot water before adding to the dye-bath, and are used in quantities of 0·5 to 2·5 grm. per liter of dye-bath, according to their colouring power, which varies a good deal, and to the depth of shade required. The solutions should not be boiled, and some colours are injured by too high a temperature. Some colours dissolve incompletely, and require filtration through a cotton cloth. As basic colours are precipitated by calcium carbonate, it is important that “temporary” hard waters should be neutralised with acetic or lactic acid till they faintly redden litmus; and in the case of colours which, from their attraction for the leather fibre, dye too rapidly, and consequently unevenly, better dyeing is often obtained by the use of a small excess of acetic acid, which also increases the solubility of the colour. Too much acid, however, will prevent the proper exhaustion of the bath. Some few colours, now little used, require to be dissolved in the first instance in a little methylated spirit; and the addition of spirit will often assist dyeing and staining where the leather is slightly greasy, though considerations of cost generally prevent its use. Sodium sulphate is not unfrequently added to dyeing baths to improve equality of dyeing; and with some of the cotton dyes common salt is used to lessen their solubility and facilitate the exhaustion of the dye-bath.

“Acid” colours usually dye better if acid is added to the bath, to liberate their colour-acids, and for this purpose sulphuric acid is generally used in weight about equal to that of the colour used. Its use is, however, objectionable, in this case, for the same reasons as in bleaching, since it is impossible by mere washing to remove it entirely from the leather, which it ultimately rots when concentrated by exposure to a dry atmosphere or high temperature; and it is better to use formic or acetic acid to the extent of two or three times the weight of the dye-stuff. Sodium acid sulphate may also be used, but is probably more objectionable than an organic acid. Many acid colours, however, dye quite satisfactorily from a neutral bath. The acid colours are used in somewhat similar quantities to the basic, but are generally inferior in colouring power, though they dye more evenly, especially on defective grain, and are often more permanent to light.

Mention has already been made of the polygenetic or mordant dye-stuffs, which are still used to some extent for dyeing glove-leathers, and of which logwood is important in dyeing blacks. Fustic and Brazil-wood (peach-wood) are not quite gone out of use among old-fashioned dyers, even for dyeing moroccos and other coloured leathers of vegetable tannage. Peach-wood, with a tin mordant (generally a so-called “tin spirits” made by dissolving tin in mixtures of hydrochloric and nitric acid) was formerly much used in dyeing cheap crimsons, but is now quite displaced by the azo-scarlets. The acid tin-solutions were frequently very injurious to the leather.

The wood-infusion, rendered slightly alkaline with soda, ammonia or, formerly, with stale urine, is usually dyed first on the leather, and followed by the mordant “striker”; ferrous or ferric solutions, and potassium bichromate being used for dark colours, and tin salts, or sometimes alum, for the brighter ones. The mordant is sometimes added to the dye-bath towards the end of the operation, but is better used as a separate bath, as it is apt to produce a precipitate of colour-lake on the surface of the skin, which rubs off on friction. In some cases, and especially in black dyeing, the strong infusion of dye-wood, and the necessary “striker” are successively applied by brushing instead of in the dye-tray.

Logwood and Brazil wood are both CÆsalpinias closely allied to divi-divi. Logwood is CÆsalpinia (see p. 287) Campechianum. Its colouring matter is hÆmatoxylin, a substance nearly allied to tannins, and almost colourless; which on oxidation gives hÆmatin, which dyes a yellow-brown, only developing other colours by the aid of mordants. Logwood chips are extracted by boiling or heating under pressure for some time with water; and as hÆmatin gives dark purplish-red compounds with alkalies, soda or stale urine is frequently added under the mistaken belief that it produces a better extraction, but really leads to waste of colouring matter by oxidation. It is best to extract with water alone, and add any necessary alkali to the infusion before use. 1-2 lb. of wood per gallon is frequently employed in making the infusion, and as this proportion of water is quite insufficient to properly extract the wood, the residue should be boiled with one or more further quantities, which are employed in turn for extracting fresh portions of wood. Logwood dyes best at high temperatures, and especially in the case of chrome leather with which a temperature of 80° C. may be safely used. The presence of a trace of a salt of lime is advantageous, and with very soft waters a little lime water or chalk may be added to the logwood liquor.

In blacking skins, the strong infusion is rendered slightly alkaline with sodium carbonate or ammonia, and brushed undiluted on the leather. If employed as a bath, a somewhat weaker infusion is used, and the leather is frequently treated first in an alkaline bath, to which a small quantity of potassium bichromate is often added. The object of the alkali is not only to assist in the formation of the colour-lake, by saturating the acid set free from the iron-salt used as a striker, and thus to prevent the colour from penetrating the leather too deeply, but, at the same time, to overcome the resistance to wetting caused by grease or oil which the leather may contain. It must thus be used more freely when stuffed leather is to be blacked, but excess should be carefully avoided, as it easily renders the leather tender and brittle. The potassium bichromate oxidises the hÆmatoxylin, or the ferrous salt subsequently applied, and forms a nearly black chrome-logwood lake.

The iron solution is generally either of ferrous sulphate of perhaps 5 per cent. strength, or commercial “iron-liquor,” which is a “pyrolignite” or crude acetate of iron, containing catechol-derivatives and other organic products from the distillation of wood, which act advantageously, both as antiseptics, and in preventing the rapid oxidation which occurs when pure ferrous acetate is used. Iron-liquor is generally to be preferred to ferrous sulphate (“green vitriol”), as the sulphuric acid of the latter, unless completely neutralised by the alkali employed in preparation, acts in the end disastrously on the leather. Commercial iron-liquor is often adulterated with ferrous sulphate, which may be detected by its giving a precipitate with barium chloride. Great care should be taken not to use iron in excess of the logwood or tannin present, as it otherwise takes tannin from the leather itself, making it hard and liable to crack, while any uncombined iron acts as a carrier of oxygen, giving up its oxygen to the colouring matter or tannin with which it is in contact, and again oxidising from the air, and so causing “spueing” or oil-oxidation, and other evils.

Good blacks which are more permanent than those with logwood, may be obtained by merely treating leather containing an excess of oak-bark tannin or sumach, first with an alkaline solution (not at the most stronger than 2¹/₂ per cent. of liquid ammonia, or 5 per cent. of soda crystals), and then with iron-liquor. If it is not certain that the leather contains excess of a suitable tannin, a tannin-solution must be employed like the logwood infusion, or the leather must be sumached. The addition of some sumach to logwood liquor is often advantageous, and a blacker (i.e. less blue) black, especially on alumed leathers, is obtained by using a proportion of fustic. Solutions made by boiling 10 per cent. of cutch with 5 per cent. of sodium carbonate give good blacks with iron-liquor, and do not make the leather tender, and they can be used in mixture with logwood. Many commercial logwood extracts contain chestnut-wood extract as an adulterant.

Instead of dyeing in the bath, it is very common, especially for the cheaper leathers such as linings, and coloured leathers of the commoner sort, to apply the colour by brushing (commonly called “staining”). Many colours, however, which dye well with time and warmth, are inapplicable in this way, and only those should be used which have a strong attraction for the leather, and hence go on well in the cold. If “acid” colours are employed, it is essential to select those which can be used in neutral solution, or at most with addition of some mild organic acid such as formic or acetic, since, as the leather is not washed after staining, the sulphuric acid would remain in it, and would ultimately destroy it. Where leathers have a hard and repellent surface, the addition of a little methylated spirit to the dye is often very useful. The colours are used in solutions of from ¹/₄ to 1 per cent., which should be quite clear and free from sediment. Difficultly soluble colours must be used in weak solution, or the dye kept warm while in use. Dye-solutions will not generally keep for any great length of time without change.

Before staining, the leather must be carefully “set out,” or otherwise made as smooth as possible, and the staining is generally done after most of the other operations of currying or dressing have been completed. Staining is best begun with the leather in a slightly damp or “sammied” condition, and the colour is applied evenly with a softish brush in two or three coats, the leather being slightly dried after each. As a rule the more coats are applied, the more even is the work; but to save cost of labour it is common on cheap goods to be content with two, of which the first is given, preferably with a weaker solution, to the dry leather. Where the leather is “weak-grained” it is sometimes advantageous to size it first with a weak solution of gelatine, gum tragacanth, or linseed mucilage, and similar solutions are often used to fix the colour and give a higher gloss. The stearine-glaze mentioned on p. 401 may also be used for this purpose, and a weak solution of it is sometimes employed as a vehicle for the acid colours. Acid yellows and browns may also be dissolved in the undiluted glaze where only a pale colour is required, or to heighten the colour of leather already stained. A list of suitable colours for staining is given in the Appendix, p. 486.

It rarely happens in leather dyeing that the required colour can be given by the application of a single dye, most of the shades now required being produced by mixtures. It is, therefore, necessary to say a few words on the theory of colour combinations.

White light is of course composed of a mixture of all the spectrum-colours, and can be separated into them by the prism. It is probable, however, that the eye is only capable of three distinct colour-sensations, and that all the colours we perceive are represented by the excitement of these in different proportions, the actual colour-sensations being red, blue-green, and violet.[179] If we interpose a piece of yellow glass between the eye and white light, the violet and blue are absorbed, and the remaining red and green rays combine to produce the sensation of yellow. If pure blue glass is used, the red is absorbed, and we have blue as the result of the remaining mixture of green and violet. Red glass absorbs the whole of the green, and greenish-blue, allowing red and much of the violet to pass. Thus, if we combine blue and yellow glass, only the green is allowed to pass, and similarly with red and blue glass, green and blue is cut out, and only the violet remains. Thus red, yellow, and blue are frequently called the primary colours, and by combining all three in equal proportions all colours are cut out, and black or grey results. The blue and violet which are stopped by yellow glass are those colours which would produce the sensation of violet-blue, and hence the latter is called the “complementary colour” of yellow, and so on with the rest. It will be noted that all the colours of coloured objects are produced by absorption of a part of the light, and therefore coloured bodies are always darker than white ones, and where a colour is mixed with its complementary in suitable proportion, all colours are absorbed and black or grey is produced.

[179] The subject of colour is too complicated to be adequately treated here; and for fuller information, readers are referred to Abney’s ‘Colour Measurement and Mixture,’ S.P.C.K., London, 1891. It may, however, be pointed out that, while the true primary colour-sensations are unquestionably red, blue-green and violet, and by mixture of light of these colours, all other colours, including white, can be produced; the primary pigments or dyes are red, yellow, and blue; the effect being produced in the former case by the addition of colours, and in the latter by their subtraction.