The Principles of Leather Manufacture

by H. R. Procter

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]

[176] See [App. D.], [p. 488], [498], and Journ. Soc. Chem. Ind., 1902, pp. 156-158.

[177] Cp. Report of Society of Arts Committee on Bookbinding Leathers, 1901.

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.

Fig. 91.—Dyeing in the Tray.

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]).