Sole Leather or Heavy Leather Tanning from Leather Entry in the Encyclopaedia Britannica MDCCCLXXXII
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Sole Leather or Heavy Leather Tanning. – The hides of oxen are received in the tan-yard in four different conditions. These are –(1) market or slaughter hides, which, coming direct from local abattoirs, are soft, moist, and covered with dirt and blood ; (2) wet, salted hides ; (3) dry salted hides ; and (4) sun-dried or “flint” hides,-the three last forms being the condition in which the imports of foreign hides are made. The first operation in the tannery is to clean the hides, to free them from salt, and to bring the hard dry hides to the uniformly soft flaccid condition in which all market hides are obtained. The treatment at this stage requires skill and attention to prevent the more soluble constituents of the hide from dissolving out in the washing and soaking processes, and also to secure the completely softening of the entire substance, also to secure the complete softening of the entire substance, upon which the successful tanning greatly depends. In the case of market hides cleaning and softening are principally effected by washing and soaking in spent limewater, while for dry hides and dry salted hides brine is essential. The softening of these materials is helped and rendered thorough by working them for some time in the stocks (fig.1) after they have been well soaked. After being thus brought as nearly as possible into a uniform condition, all hides are treated alike. The first operation to which they are subjected is depilation, which removes, not only the hair, but also the scarf-skin. This is effected variously in different countries. In England the most common plan is to throw the hide or skin into a strong watery ley of slaked lime, with lime in excess. By this, in a few, days, more or less according to the proportion of lime present, the hair is easily detached, the hair-sheath having been dissolved. The hair was formerly taken off by making a sour liquor from fermented vegetable matter, in which the hides lay for several days; they were also smoked in a damp state for the same purpose ; but both those methods are now abandoned. They are still sometimes, especially on the Continent, sweated, that is, they are laid in heaps and kept wet and warm, a plan which is still adopted in England for skins. In America the sweating is performed cold ; the hides are hung up wet in a damp underground cellar, and are kept moist for ten days or a fortnight. In either of these sweating processes incipient putrefaction takes place sooner or later, when the hair and scarf-skin are easily removed ; but the fatty matter remains, and in some cases prevents the hide from taking the tan.
There have been numerous other methods proposed and patented for unhairing skins, few of which have been received with much favour. Among the agents proposed may be mentioned caustic soda, sulphide of sodium and sulphide of calcium, borax, sugar, and charcoal – substances which it is obvious must act in very different manners. Lime and alkaline solutions not only loosen the hair and scarf-skin, but also “plump” the corium or true skin, that is, they swell it and render it consequently porous and more permeable to the tanning solution. Lime further forms with the fatty matter of the flesh side calcareous soap, thus neutralizing the fat which would otherwise interfere with the tannin. Some tanners, especially Americans, who work the so-called acid process, plump their hides by the use of sulphuric acid, hanging them six or eight hours in a solution containing 1/500th of acid. The plumping is sometimes done as a preliminary operation, and again others add the acid to the colour pits, or the first pit into which the hides go for the tanning process. Among non-acid tanners the plumping of sweat stock in which there is no lime is secured in the weak acid liquors of the colouring and handling pits. In the case of limed stock the hides, at the proper stage, are withdrawn from the pits and stretched over an unhairing beam (fig. 2), when with a working knife (fig. 3, a) a workman partly scrapes partly shaves off the hair and scarf-skin. Another workman in a similar way with a fleshing knife (fig. 3, b) removes the fatty compounds and flesh from the flesh side. For these operations several machines have been adapted, working mostly with revolving knives or cutters, under which the hides or skins pass in a fully extended state. Such machines are, however, only applied to the smaller skins. The next step in the preparation of the hide is to remove from it as thoroughly as possible all traces of lime. This is partly accomplished by going over the hide on the beam with a scudding knife, pressing the combined lime and interfibrous matter out of the tissue. For more complete neutralization of lime in the larger hides the influence of the weak acid of the colouring pits is trusted to. Harness hides are washed by some means in pure water, the most convenient and generally adopted method being to place them in the dash wheel (fig. 4), in which they revolve and tumble about whilst fresh water is continually being poured on them within the revolving wheel.
The hides now come to be trimmed and prepared for tanning in the shape in which they are intended ultimately to be sent into the market. An entire untrimmed hide (fig. 5) is termed a crop ; a side is half a crop, the dividing line of the two sides being shown at EF ; abut is the back portion ABCD, and a bend is half a butt ABFE. G, G are belly pieces, and H, H the cheeks, both together being the offal. When the shoulder (the upper part of the butt) is removed, what remains is a short butt.
The actual tanning now commences, and the operations involved may be divided into a series of three – (1) colouring, (2) handling, and (3) the laying away. The colouring consists in exposing the hides in a series of pits containing oozes which are almost entirely deprived of tannin, but in which some amount of gallic and acetic acids have been developed, and which, moreover, contain a large proportion of the colouring matter extracted from the tanning substances. In these pits (also called suspenders) the hides are suspended over poles laid across the pit, and they are moved daily from one to another of a series of four or six, this stage usually occupying about a week. As the hides are moved forward in the series they are exposed to a liquor containing a small and steadily increasing proportion of tannin, and this, it maybe said, holds good till the hide reaches the last lay-away pit, in which the tanning is completed. The objects attained in the colouring pits are the superficial colouring or dyeing of the hide, some amount of plumping from the acids of the ooze, and a dissolving out of remaining traces of lime, principally by the acetic acid to which the hide is exposed. After colouring, the hides pass on to the handlers or handling pits, a round or series of which may consists of from four to twelve, according to the mode of working. In the handlers the hides are spread out horizontally; and in the first series they are “handled” once a day or more frequently if convenient. The handling consists of lifting the hides out of the pit by means of a tanner’s hook (fig.6), piling them on the side till they drain, and returning them into the pit, the hide on the top in one handling going to the bottom in the next. This operation is continued throughout the series ; only as the hides advance the necessity and advantage of frequent handling decreases, while the strength of the tan liquor in which they are handled increases. The whole handling stage consumes on an average about six weeks. Finally, the hides are carried over into the layers or lay-aways. In these the stock is exposed to the strongest tanning liquors, and between the hid3es thin layers of the tanning bark or mixture are strewn. The object of this interstratification is to separate the mass of hides so as to secure the more ready permeation of the entire mass by the liquor, and also to feed and strengthen the ooze itself as its tannin is absorbed by the hides. In these layers the hides are allowed to rest for about six weeks, after which the pits are cleared out, charged with fresh ooze, and filled with the hides and tan as before. These processes may be repeated three or four times before the tanning is completed. When the process is deemed complete, each hide, on being taken out, will be found to be converted into leather, and a portion of its gelatin which has been dissolved from its interior is, by combination with a portion of tannin from the strong solution, deposited upon its surfaces, where it is found in the form of a yellow deposit, technically known as bloom, or pitching, which disguises the under colour of the leather just as if it were covered with yellow paint. This, prejudice says, must be on its surface, or it is not saleable, but it is so much quality and weight lost to the consumer, as he pays for it on the outside of his leather to be worked off in the dressing and currying operations. By some tanning agents – mimosa, for example – there is little or no bloom deposited.
The theory of the formation of the bloom is this. As soon as ooze has penetrated into a hide it loses its tanning material, but by capillary attraction is detained ; this exhausted ooze acts by maceration on the finer and more soluble interstitial gelatine, and dissolves it. In handling, about one-twelfth of this flows out ; the remaining eleven-twelfths accompany the hide into the next stronger solution, of which only one-twelfth is absorbed directly, and a small portion is slowly exchanged by endosmosis and exosmosis. The small portion of strong solution which passes into the pores of the hide contributes to tan the hard fibrous portions not dissolved, and the small portion of weak solution passing out of the hide by exosmosis gives up its dissolved gelatine to the tan of the stronger solution outside to form tannate of gelatine, which partly adheres to the surface as bloom, and partly falls to the bottom of the pit as pitching.
From the time when the raw ox hide is taken in hand till the leather is fully dried, not less than a year is consumed in the case of the best qualities of sole leather. It was formerly the practice in England, as it still is on the Continent, to tan by the process of stratification, for which purpose a bed of bark is made upon the bottom of the pit ; upon this is laid the hide, then bark, then a hide, and so on until the pit is full ; water is sometimes pumped in, and the pit left for some months ; it is then emptied, and the same hides returned with fresh bark and water for a few months longer ; this is repeated again and again, until the tanning is completed, the time varying from one to four years for heavy leather.
The devices and processes which have been proposed and to some degree put in operation with the view of shortening the time occupied in tanning are beyond all enumeration. In scarcely any case have time-abridging processes proved successful in practical working, so far as the production of good leather is involved ; and now the opinion appears to be completely established that, for the thorough tanning of heavy leather, a slowly operation influence and consequently long time are essential. The devices for the hastening of tanning have for the most part turned upon some plan for forcing the tan liquor into and through the pelt, or for alternate soaking and squeezing of the hides. Among the plans which have been tried on a commercial scale may be enumerated tanning by the application of hydrostatic pressure to force the liquor through the hides, a method which failed simply because the pressure was equal on both sides. The vacuum tanning principle is another which has been extensively tried, only to issue in disappointment. It consists in hanging the hides in a pit or a cylinder so constructed that the air can be exhausted by an air pump, after which tan liquors are forced into the vessel, air readmitted, and again withdrawn. Hides, however, loaded with water swell little under diminished atmospheric pressure, and the practical difficulty of procuring and maintaining a vacuum in tan pits is very great. More promising results have been obtained by setting up in tan pits the physical process of endosmosis and exosmosis. This is done by sewing up hides two and two as bags which, being filled with solution differing in specific gravity from the tan liquor in which they are immersed, thereby set up transfusion through the hide. This process failed chiefly through the hardness of the leather it yielded. A plan of sewing hides into bags and suspending them filled with strong tan liquor, which as the fluid exuded was renewed, was also tried for some time. Again, it has been attempted to keep the hide suspended stationary in the pits and move the liquors instead of carrying over hides from one pit into another. A more recent device, which may not yet be fully tested, consists in keeping up the strength of the liquor by a continuous circulation through pipes from the stronger into the weaker infusions. By this system of circulation, instead of the oozes in which hides are immersed becoming weaker and weaker the longer they rest in the liquor, the ooze is kept up at least to its original strength, and it may indeed, if desirable, be increased in proportion as the tannin combines with the hide.
Heavy hides for sole leather, belting, and similar purposes do not require to undergo any elaborate dressing or currying. When finally removed from the tan pits they are piled grain to grain and flesh to flesh to drain, care being taken that no tan liquor is allowed to lurk in the pile, which is covered over from the light. When sufficiently drained, they are brushed or sourced to free them from adhering impurities, and removed to the drying loft, where, after lightly rubbing over with oil, they are hung on poles to dry. In the loft steam-heated pipes keep a dry atmosphere during winter, and enable the attendants to regulate and control the drying of the leather. The leather when dried in this condition is rough tanned, and fro finishing as sole leather it has to be struck out or “pinned” and compressed by rolling. For striking or pinning by hand the hide is dampened with water, thrown over a beam, and worked all over the grain side with a striking pin (fig. 3 c). This operation smoothes and levels the grain, removes smaller wrinkles, and to some extent compresses and solidifies the leather. Striking machines (fig. 7) are now very generally used for the operation. These consist of a drum or cylinder having a parallel series of projecting knives, or plates of gun-metal, set angularly across its surface. Underneath the drum is a brass bed, fixed on a yielding cushion, which can be pressed up or eased by means of a foot lever, according as the leather operated on is thick or thin. The drum is made to resolve at a very rapid rate, the blunt edges and external angles of the nives thereby striking the surface of the leather with great violence, and thus the grain is struck out, smoothed, and compressed in a very rapid and compressed manner. Finally, the leather is rolled and compressed on a level zinc-lined wooden bed by a heavy hand roller, such as is shown in fig. 8, or on the platform of one of the numerous forms of machines designed for that purpose.
The yield of leather from a given weight of dry hide varies very much according to the different styles of tannage and materials used. As a mean outcome, it may be said that 100 lb of green hide, tanned with from 300 to 400 of oak bark, will yield 40 to 50 lb of leather ; 100 lb of green hide, however, when deprived of hair, flesh, and moisture, will weigh only 18 lb, and, taking 100 lb of dry hide, which, fleshed and unhaired, weighs 85 lb, the yield of leather will be from 180 to 200 lb according to tannage. The percentage of tannin alone absorbed from different tanning agents has been found to be for hemlock, 64.2 ; pine, 90.8 ; chestnut, 85.2 ; oak, 76.9 ; oak, three years in pit, 70.2. Heavy leathers, being sold by weight, are subject to adulteration, and have fictitious weight given them without any benefit to the material, but rather the opposite, by impregnation with such salts as sulphate of magnesium or chloride of barium, or with glucose, the last being the most frequently used adulterant.