The Vegetable Tanning Process - A Collection of Historical Articles on Leather Production. Various
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the surface, producing perhaps a “semi-permeable membrane,” which it is very difficult for the tannin to penetrate later. Such liquors also cause “drawn grain,” for if the surface of the hide is fixed by too early tanning, and the interior afterwards swells in thickness and contracts in area, the grain becomes puckered in wrinkles which are the larger and coarser, if through ill-managed liming the fibrous texture immediately below the surface is too much loosened.
The “handlers” are a series, conveniently say of 12 pits, in which the butts are laid flat instead of suspended, since in this way a much larger number can be got into each pit; and the hides and not the liquors are changed from pit to pit. The “pack” from the suspenders is, of course, brought first into the weakest and oldest pit; the “forward” or most tanned pack is removed to the “layers,” or in some cases to a second and stronger “shift” of handlers; and the intermediate packs are each moved forward one pit, all getting thus a change into a stronger liquor. As, however, the strength of the liquors must be maintained by fresh liquors, if possible from the “layers,” the new “forward” pack is again raised (conveniently next day), the whole series is again shifted forward, and the liquor vacated by the last pack is run to the suspenders, and replaced by a new liquor to receive the forward pack. If this and the bringing in a new pack takes place on alternate days, the passage of a pack through the entire “shift” or “round” will occupy four weeks, including Sundays. It is most usual to add to the liquor of the forward pack a few pailfuls of finely ground material, bark, myrobalans, or valonia. The object of this is less to increase the strength of the liquor than to prevent the actual contact of the butts. Before bringing the later packs into a pit, this “dust” is well plunged up to distribute it as evenly as possible between the butts. It is not uncommon to have two or even three head packs or “dusters” which are moved alternately so as to give them a longer time in the new liquors, but this introduces complications which cannot be discussed here.
In the handlers and layers the goods were formerly handled with sharp steel hooks on long poles, but as these are apt to produce serious scratches they have been to a large extent abandoned for the method of handling with strings, described on p. 27, which is also much quicker, and avoids the need of partially skilled labour.
If the liquors are well maintained, and the goods are light, they will probably now be ready for the “layers” that is to say, thoroughly tanned through, but still wanting in solidity and weight. They are no longer capable of rapidly absorbing tannin, but can still fix considerable quantities of bloom and reds (p. 101) on and between the fibres. These difficultly soluble matters are supplied by strong and fresh liquors which are supersaturated, and deposit them not merely on and in the hides, but in the liquors and on the sides of the pits. It is hence desirable to bring the actual tanning materials as closely as possible in contact with the leather, and to maintain the strength of the liquors during the comparatively long periods for which the butts must remain undisturbed. To accomplish this, much larger quantities of strong solid materials, such as valonia, mimosa bark, and (in moderation) myrobalans are employed than in the handlers, and these serve also the purpose of separating the butts and maintaining a larger volume of liquor between them. Instead of throwing this material in three or four portions into the liquor, and trusting to its distributing itself between the goods, each butt as it is drawn on to the surface of the liquor, receives a regular sprinkling; a second is drawn on and similarly sprinkled, and so on, and the whole are allowed to sink as evenly as possible into the liquor. This method of making a layer is now universal in England, but an older method is still largely in use on the Continent, in which the goods (generally whole hides) are spread in the vats with thick layers of dry material between them, and the liquor is only run on when the pit is filled, which is then often left undisturbed for three months or more. The method dates from a period when the solid materials rather than the “weak” liquors were relied on to supply the tannin, and has the disadvantage that the hides are often deeply pitted by fragments of tanning material pressed into them, but it must be admitted that the leather produced in this way, though sometimes unsightly, is very solid and durable.
The liquors used for the last layers are the strongest which can be made and, since the almost universal use of oakwood and chestnut extracts, frequently reach. 100° or even 120° of the barkometer (S. G. 1·100—1·120) while in pure oak-bark tannage it is difficult to exceed 30°, and then only by strengthening repeatedly used liquors. The goods, when it is desired to change the liquors, are pulled out, the liquor pumped off and the partially exhausted solid material removed for further extraction, and the goods are returned to a new liquor in the same pit. Layers of a month to six weeks were formerly the rule, but it is better practice to begin with one even as short as a week or ten days, gradually extending the time as the goods become more fully saturated, and exhaust the liquors more slowly. It is only by constantly maintaining and increasing the strength that the goods will continue to “feed,” and it is waste of time to allow them to lie in a liquor which has fallen below its original strength. Of course, if layers are to be short, less solid material is given. Light goods are fully tanned in two or three layers; heavy ones may require four or five, but the careful tanner will investigate how far the gain of weight and solidity repays for the cost of additional time and material, which, as the goods become fully tanned, produce but little effect.
After the last layer, goods were formerly merely washed in a weaker liquor before being sent to the drying shed, but some sort of bleaching is now almost universal where extracts are largely used. A strong warm myrobalans liquor produces a good effect, but recently “vatting” in warm strong solutions of “bleaching extracts” after scouring has become customary. These are mostly quebracho extracts containing much bisulphite of soda, which not only bleaches by its sulphurous acid, but has the property of dissolving and removing the “reds” deposited in the leather. The effect is to render the leather brighter in colour without removing much weight, but at the same time to make it more porous and much more permeable to water. It is hard, however, to see how the practice is to be avoided so long as the public and the shoe-manufacturers continue their absurd demand for soles of light and bright colour!
The finishing of sole-leather is simple in theory, but, especially with heavy extract tannages, not at all easy in practice. Before drying, the goods are very lightly oiled on the grain side, usually with crude cod liver oil, with the object both of lessening oxidation by the air, which darkens colour, and of checking evaporation from this side, which would tend to bring the dark liquors, still present in the interior, to the surface by capillarity. The first drying must be slow and even, and at a low temperature, and if possible is accomplished without artificial heat. When the goods are half dry (or “sammed”) they are laid in a pile, damped in dry places, and often allowed to heat a little by the incipient growth of mould, which, though somewhat dangerous, facilitates the next process, that of “striking.” This, if done by hand, is accomplished on a horizontal wooden beam of rounded section on the upper surface, and about 7 ft. long. The tool used is called a “striking pin,” and is a two-handled blade of triangular section with three edges, with which the workman stretches and smooths the grain side of the leather, leaning over the low beam, and putting his full weight on the tool. If it is desired to strike the bloom “out,” so as to show the original colour of the leather as fixed in the suspenders, the surface is kept thoroughly wet, often with slightly soapy water, the tool used is pretty sharp, and is aided by a stiff brush. If the bloom is to be struck “in,” and afterwards concealed by colour, a blunter tool, a dryer leather, and a little oil are employed, and loose bloom is afterwards washed off. The process, however, is now generally performed by a machine carrying four blades or “slickers” on separate spring-arms which work outwards from the centre of the butt, and lift on the return stroke (Fig. 11). In place of removing the bloom by striking it is now generally removed by scouring tools of smooth sandstone, aided by brushes and plenty of water, before vatting and usually by machine. After the first striking, the goods are dried a little further, piled again overnight, and again struck to smooth them and remove tool marks, often after rubbing the surface with an oily rag. After the second striking, they are rolled, now almost universally by machines of the type shown in Fig. 12, coloured if