Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/404

336 LEATHER 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 sub ject 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.

Upper Leather. – Under this head are included the thin, soft, and pliable leathers which find their principal, but by no means exclusive, application in making the uppers of boots and shoes, which may be taken as the type of a class of leathers. Upper leathers are made from such hides and skins as East Indian kips, light cow hide, calf skins, horse hide, and also from split heavy hides. The preparatory dressing of such skins, and the tanning operations, do not differ essentially from those already described. In proportion to the thinness of the skin treated, the processes are more rapidly finished and less complex, while at the same time the skins absorb a large percentage of tanning extract. The lime used for unhairing must be removed in the preliminary stage, with greater thoroughness than is essential in the case of hides for sole leather; and for this purpose the skins are washed in the dash wheel, and undergo a process of bating or grainering. A quantity of pigeon's dung is dissolved in water, and in this the hides are steeped for a week or ten days, with occasional removals and strikings. The theory of this process is obscure, but it has been explained on the supposition that the uric acid of the dung removes the excess of lime, and that the ammonia generated by the putrefaction of the mixture tends to form an ammoniacal soap with any remaining fat of the hide; but as the gelatin of the hide exists in two states, – one the principal, hard, or fibrous portion, and the other (which is more soluble) contained between the fibres, and more affected by agents and putrefaction – this softer portion is removed by grainering, and the leather, when tanned, is light and porous, and more readily permeable by water. Small skins are not fished one by one out of the colouring and handling pits, but the whole contents of the pits are tied together, so that when the upper skin is seized it is thrown over a sparred cylinder erected between each pair of pits, and, the wheel being set in motion, the entire string of skins comes up over its surface and is passed into the neighbouring pit with the utmost rapidity. Such an apparatus is used for handling all small pieces and fragments, as for example the cheeks and bellies of heavy hides.

The time occupied in tanning an upper leather, say an East Indian kip, with a mixed mimosa, sumach, valonia, and terra tannage, may be about three months. In the fine tanning of calf and kip skins on the Continent, for which French and German tanners are famous, the duration of the operation may be from four to eight months.

Splitting. – In the preparation of most kinds of upper leather, the hides are split into two, or three, and sometimes more portions. In the case of a single split the portions form a grain and flesh side; when three sections, or slices, are made they result in grain, middle, and flesh splits. Some tanners split their hides in the green condition, others after colouring, and in many instances the splitting is done, after the leather is fully tanned, by the currier, as a regular part of his operations, this being particularly the case with imported tanned East India kips, and other fully tanned leather of foreign origin. Splitting machines will be alluded to in connexion with the operations of currying.

Currying. – Leather as it leaves the tannery is a com paratively rough, harsh, and intractable substance, and the duty of the currier is to dress and otherwise fit it for the use of the shoemaker, coachbuilder, saddler, and the numerous other tradesmen who work in it. The currier has to smooth the leather, so to pare it down as to reduce inequalities of thickness, to impregnate it with fatty matter in order to render it soft and pliable, and to give it such a surface-dressing, colour, and finish as will please the eye and suit the purposes of its consumers. The operations of currying are complex and varied, each particular class of goods receiving a treatment in many respects peculiar to itself. The fact also that machinery is used by some curriers for nearly every mechanical operation, while others adhere to the old manual system, renders it almost impossible to give in brief an outline of operations which will be consistent with the practice of any considerable number of curriers.

Fig 9. – Currying Knife. Fig 10. – Currying Apparatus. C, pommel; R, raising board; S, slicker.

Regarding currying as principally a handicraft, the following may be taken as an outline of the range of operations for the preparation of a waxed calf leather, the commonest form of upper leather in use. The leather is first made pliable by soaking in water, after which it is shaved on the flesh side, and a tolerably smooth surface is produced. This operation is carried on at a beam, or strong frame of wood, supporting a stout plank faced with lignum vitæ, and set vertically, or nearly so. The knife (fig. 9) is a double-edged rectangular blade, about 12 inches by 5 inches, with a straight handle at one end, and a cross handle at the other in the plane of the blade. The edges of this knife are first made very keen, and are then turned over so as to form a wire edge by means of the thicker of the two straight steel tools shown in fig. 10. The wire edge is preserved by drawing the thinner steel tool along the interior angle of the wire edge from time to time as required, for which purpose the man holds this smaller tool between his fingers, together with the beam-knife. The skin being thrown over the plank, the man presses his body against it, and leaning over the top holds the knife by its two handles, almost perpendicularly to the leather, and proceeds to shave it, shifting it from time to time so as to bring all tho parts under the action of the knife, and frequently passing a fold between his fingers to test the progress of his work. The skin is then placed in hot water, and removed to a mahogany or stone table, to which the wet flesh side adheres, and is worked with a tool called a stretching-iron, or slicker S (fig. 10), consisting of a flat, rectangular piece of iron, copper, or smooth hard stone, fixed in a handle. With this tool a man scrapes the surface of the skin, exerting a strong pressure with both hands, and dashing water upon it from time to time, by which means lumps and inequalities are made to disappear, the leather is equalized and extended, and the bloom is brought to the surface. The superfluous moisture and the superficial bloom are now slicked out, and a stuffing, or dubbing, of cod oil and tallow is rubbed into both sides of the skin, but chiefly the flesh side, by means of a brush, or with the woolly side of a piece of sheep skin. The skin is now dried in a loft, and, as the water only evaporates, the dubbing sinks into the pores. When dry enough for the