As the tannage proceeds and penetrates further into the hide, the liquors are used stronger, as the outside, once tanned, is to a large extent protected from their action, and it is only by continuously increasing the strength of the liquors that more tannin can diffuse into the interior, since diffusion only takes place from a stronger into a weaker liquor. The liquor in the interior of the butts is always exhausted of tannin so long as any part of the hide-fibre remains untanned, but as the layer of tanned fibre between this and the outside gets thicker, a greater difference is required to maintain a reasonable rate of exchange, just as a greater head of liquor is required to maintain a flow of liquor through an increased number of percolation-leaches. If the strength of the liquor outside be allowed to fall off, this graduation of strength from the outside to the inside of the butt is disturbed, and takes some time to re-establish. As the liquors become stronger in tannin, they may also become somewhat stronger in acid, since, as has been stated, the two act to some extent in opposition to each other. The acid-swollen fibre absorbs the tannin more slowly than if it were in more neutral condition, but it absorbs it apparently in larger quantity, and at any rate, makes a firmer, solider, and less flexible leather.
It has been mentioned that in the latter stages of the process, solid tanning materials are generally strewed between the butts in the tanning liquor. It may be pointed out that many materials vary in their tanning effect, according to whether they are used in solid form or merely in liquors. It has been shown by Youl and Griffith[126] that such materials as valonia, oakwood and chestnut extracts, and myrobalans, which contain both gallotannic and ellagitannic acids, lose strength rapidly when kept in the form of liquor, the ellagitannic acid becoming decomposed with separation of insoluble ellagic acid. Now it is just this ellagic acid, which deposited in or on the leather, gives weight, solidity and bloom, and the investigation points out not only an important source of loss in the tanning industry, but also, why valonia, which in sole-leather tannage is known to give hard and heavy leather, can be used in large quantities on dressing leathers in Yorkshire, with gambier, in the form of liquor, giving a soft and mellow leather almost destitute of bloom. If weight and solidity are required from the use of such materials, it is obvious that they must be brought into immediate contact with the leather to be tanned, so that as large a part of the bloom as possible is deposited in, and not outside the leather. With many other materials, such as hemlock, quebracho, and mimosa, which yield no bloom, but “difficultly soluble” tannins (reds or phlobaphenes), the same rule holds, since in contact with the hides, the small proportion of these materials which is soluble in the liquors, is replaced from the materials as rapidly as it is absorbed by the leather, while, when liquors or extracts only are used, the greater part of these solidifying and weight-giving constituents remain unutilised in the spent tanning materials. At the same time, the long “layers” afford an opportunity for the acetic and lactic fermentations to go on which are the principal source of the natural acidity of liquors. It must be understood that what are called layers in England, are not to be identified with the Sätze, but rather with the Versenke of the German tanner, the former being layers given in much the same manner as was current in England 150 years ago; in which the leather, with thick layers of tanning material between it, is laid into the empty pit, which is afterwards filled up with liquor, often of a comparatively weak character. In such layers, the acidification, and the solidification of the leather both go on to a still greater degree; the acid formed, apparently gradually penetrating to the heart of the leather-fibres, and producing a solidity, and cheesy texture which can hardly be obtained by layers of the English kind; which nevertheless have the advantage in rapidity and cheapness.
[126] Journ. Soc. Chem. Ind., 1901, p. 428.
In drying sole-leather, one of the great objects which must be aimed at is to remove the dark coloured liquor, with which the goods are saturated, from the surface, and to prevent further portions of it from finding their way there from the interior. If a strip of filter-paper be allowed to rest with one end in a basin containing a little liquor, and be placed in a draught of air, the exposed end of the paper will rapidly become dark brown or black, the liquor which evaporates there being constantly replaced by fresh portions sucked up by capillary attraction from the basin. A similar action is constantly seen, when filtering liquors through paper, if the latter be allowed to project above the edge of the funnel. Precisely the same effect occurs, perhaps increased by the oxidation of the tannins, on the edges and other parts of a butt which are most exposed to draughts of air. The use of oiling the grain is not only, to a certain extent to protect it from oxidation, but also to check evaporation, and the consequent accumulation there of the dark-coloured solids contained in the liquor. A very similar result is attained by wetting the grain-side, and allowing as much of the evaporation as possible to take place from the flesh.
The process of sole leather tanning has been discussed in considerable detail on account of its simplicity and importance. It is now time to point out in what respects the tannage of the lighter leathers differs from it in principle. Taking the case of ordinary dressing leathers, such as kips and shaved hides, the first point to remember is that these goods come into the liquors not merely almost entirely deprived of lime by bating, but in a very flat and fallen condition from the action of the bacterial ferments of the bate. As a general rule in this country the colouring is done in paddles, but where a very smooth grain is required, the use of suspenders is to be recommended, and in America is largely adopted. Indeed in the States the entire tannage of much of the cheaper leather is done in suspension, and the sides are only removed from the laths to which they have been nailed, when they are required for splitting. It is obvious, from what has been said of sole leather, that as the hides are brought into liquors in a very fallen and extended condition, the grain will be likely to be wrinkled; and indeed this is sure to be the case unless, by suspension, the hide is more or less kept in tension till its fibres are fixed by tanning. The free motion in the paddle favours the formation of a “pebbled” grain, since the hide is bent now this way, now that, and minute wrinkles and creases are formed in all directions. For many purposes, and especially if a grain is afterwards to be raised by “boarding” the curried leather, this graining in the paddle is not disadvantageous, so long as it is not excessive. In some other cases it causes much trouble and labour to the currier before it is removed, and if the English tanner and currier are ever to compete with the American in smooth grain finishes, it will be necessary for them to obviate this source of wasted labour. The graining is the less considerable, and the easier to remove, the weaker and more mellow are the liquors employed in colouring and the more gradually their strength is increased.
The production of a soft leather depends on the fibre being tanned in a fallen and unswelled condition. It is for this reason that bating is in many cases essential, though where somewhat firmer leathers are required, mere reduction of the swelling by removal of the lime is sufficient. For the same reason, no acid-swelling is permissible either before tanning, or in the liquors, and though liquors for soft leathers must be rather acid than alkaline, they are incapable of removing any large quantity of lime, and for the best results, the deliming must be complete before tanning. As mere bating or puering is mainly designed to reduce swelling by the action of bacterial products ([p. 172]), and is not a very efficient means of removing lime, it is desirable where it is employed, to supplement it by some more active deliming process. In the lighter leathers, drenching ([p. 166]) generally fulfils this purpose and many of the more intelligent tanners now give bated hides a bath in boric acid before tanning, which not only removes the last traces of lime without acid-swelling, but checks the bacterial fermentation, and prevents its introduction into the liquors. In gambier tannages, a decidedly better colour is obtained by this treatment ([p. 228]).
In most cases the production of bloom is not desired in dressing leather tannage, and is prevented by relying chiefly on liquors, and avoiding the use of bloom-giving solid materials, which include most pyrogallol tannins. Dressing leather tannages can frequently be advantageously hastened by drumming: which by continuously bending the leather in all directions, constantly widens and contracts the spaces between the different fibres, and, as it were, pumps the liquor through the skin. The softness of dressing leathers is increased, and the hardening action of acids present in the liquors is prevented by the addition of salt, or of some sulphates (sodium, magnesium, ammonium) which exercise a sort of pickling action on the fibre, and prevent its swelling, but at the same time tend to light weight and a somewhat empty tannage. It by no means follows that a hide or skin which is thoroughly coloured through, is really fully tanned; as, though the fibres may be actually tanned or coated on the surface, time is required for the tannins to penetrate them to the centre. This incompleteness of saturation is often found in drum tannages. Such leathers are generally tough, and gain weight and softness in currying. In order to “carry grease” well, that is, to absorb a large quantity without appearing greasy, it is essential that the fibre-bundles should be thoroughly split up or differentiated; and the degree to which this is attained largely depends on the extent of liming. There is also considerable difference in different tannages, as to the amount of grease which they will carry.
It is now not uncommon to combine a degree of alum or chrome tannage with vegetable tannage in the finer dressing leathers. For further information on this the reader must be referred to the [next chapter].
The finest sorts of leather such as goat, calf, sheep and seal for bookbinding, upholstery and the like, are mostly tanned with sumach; paddles and drums being largely used to quicken the operation. Leather tanned with sumach has been proved by the researches of the committee of the Society of Arts on the decay of bookbinding leathers[127] to be the most durable leather for this purpose, some other tanning materials of the pyrogallol class coming near it in this respect, while all catechol tannages are found peculiarly liable to destruction by the action of sunlight, dry heat, gas fumes, and traces of sulphuric acid from other sources, although in many cases more durable than the pyrogallol tannages when exposed to mechanical wear and moisture, as is the case with shoe-leather ([p. 298]). East India sheep- and goat-skins (so-called “Persians”) are tanned with the catechol tannin of turwar or cassia bark.
[127] Soc. of Arts Journ., 1901, p. 14.