After liming it is necessary to unhair the goods. This is done by stretching a hide over a tanner’s beam (fig. 3), when with an unhairing knife (a, fig. 4) the beamsman partially scrapes and partially shaves off the hair and epidermis. Another workman, a “flesher,” removes the flesh or “net skin” (panniculus adiposus), a fatty matter from the flesh side of the skin, with the fleshing knife (two-edged), seen in b, fig. 4. For these operations several machines have been adapted, working mostly with revolving spiral blades or vibrating cutters, under which the hides pass in a fully extended state. Among these may be mentioned the Leidgen unhairer, which works on a rubber bed, which “gives” with the irregularities of the hide, and the Wilson flesher, consisting of a series of knives attached to a revolving belt, and which also “give” in contact with irregularities.
At this stage the hide is divided into several parts, the process being known as “rounding.” The object of the division is this: certain parts of the hide termed the “offal” are of less value than the “butt,” which consists of the prime part. The grain of the butt is fine and close in texture, whereas the offal grain is loose, coarse and open, and if the offal is placed in the same superior liquors as the butt, being open and porous, it will absorb the best of the tannin first; consequently the offal goes to a set of inferior liquors, often consisting of those through which the butts have passed. The hides are “rounded” with a sharp curved butcher’s knife; the divisions are seen in fig. 5. The bellies, cheeks and shoulders constitute the offal, and are tanned separately although the shoulder is not often detached from the butt until the end of the “suspenders,” being of slightly better quality than the bellies. The butt is divided into two “bends.” This separation is not made until the tanning of the butt is finished, when it is cut in two, and the components sold as “bends,” although as often as not the butt is not divided. In America the hides are only split down the ridge of the back, from head to tail, and tanned as hides. Dressing hides are more frequently rounded after tanning, the mode depending on the purpose for which the leather is required.
| Fig. 5. |
The next step is to remove as much “scud” and lime as possible, the degree of removal of the latter depending upon the kind of leather to be turned out. “Scudding” consists of working the already unhaired hide over the beam with an unhairing knife with increased pressure, squeezing out the dirt, which is composed of pigment cells, semi-soluble compounds of lime, and hide, hair sacks and soluble hide substance, &c. This exudes as a dirty, milky, viscid liquid, and mechanically brings the lime out with it, but involves a great and undesirable loss of hide substance, heavy leather being sold by weight. This difficulty is now got over by giving the goods an acid bath first, to delime the surface; the acid fixes this soluble hide substance (which is only soluble in alkalies) and hardens it, thus preventing its loss, and the goods may then be scudded clean with safety. The surface of all heavy leathers must be delimed to obtain a good coloured leather, the demand of the present day boot manufacturer; it is also necessary to carry this further with milder leathers than sole, such as harness and belly, &c., as excess of lime causes the leather to crack when finished. Perhaps the best material for this purpose is boracic acid, using about 10 ℔ per 100 butts, and suspending the goods. This acid yields a characteristic fine grain, and because of its limited solubility cannot be used in excess. Other acids are also used, such as acetic, lactic, formic, hydrochloric, with varying success. Where the water used is very soft, it is only necessary to wash in water for a few hours, when the butts are ready for tanning, but if the water is hard, the lime is fixed in the hide by the bicarbonates it contains, in the form of carbonate, and the result is somewhat disastrous.
After deliming, the butts are scudded, rinsed through water or weak acid, and go off to the tan pits for tanning proper. Any lime which remains is sufficiently removed by the acidity of the early tan liquors.
The actual tanning now begins, and the operations involved may be divided into a series of three: (1) colouring, (2) handling, (3) laying away.
The colouring pits or “suspenders,” perhaps a series of eight pits, consist of liquors ranging from 16° to 40° barkometer, which were once the strongest liquors in the yard, but have gradually worked down, having had some hundreds of hides through them; they now contain very little tannin, and consist mainly of developed acids which neutralize the lime, plump the hide, colour it off, and generally prepare it to receive stronger liquors. The goods are suspended in these pits on poles, which are lifted up and down several times a day to ensure the goods taking an even colour; they are moved one pit forward each day into slightly stronger liquors, and take about from 7 to 18 days to get through the suspender stage.
The reason why the goods are suspended at this stage instead of being laid flat is that if the latter course were adopted, the hides would sink and touch one another, and the touch-marks, not being accessible to the tan liquor, would not colour, and uneven colouring would thus result; in addition the weight of the top hides would flatten the lower ones and prevent their plumping, and this condition would be exceedingly difficult to remedy in the after liquors. Another question which might occur to the non-technical reader is, why should not the process be hastened by placing the goods in strong liquors? The reason is simple. Strong tanning solutions have the effect of “drawing the grain” of pelt, i.e. contracting the fibres, and causing the leather to assume a very wrinkled appearance which cannot afterwards be remedied; at the same time “case tanning” results, i.e. the outside only gets tanned, leaving the centre still raw hide, and once the outside is case-hardened it is impossible for the liquor to penetrate and finish the tanning. This condition being almost irremediable, the leather would thus be rendered useless.
| Fig. 6.—Tanner’s Hook (without handle). |
After the “suspenders” the goods are transferred to a series of “handlers” or “floaters,” consisting of, perhaps, a dozen pits containing liquors ranging from 30° to 55° barkometer. These liquors contain an appreciable quantity of both tannin and acid, once formed the “lay-aways,” and are destined to constitute the “suspenders.” In these pits the goods, having been evenly coloured off, are laid flat, handled every day in the “hinder” (weaker) liquors and shifted forward, perhaps every two days, at the tanner’s convenience. The “handling” consists of lifting the butts out of the pit by means of a tanner’s hook (fig. 6), piling them on the side of the pit to drain, and returning them to the pit, the top butt in the one handler being returned as the bottom in the next. This operation is continued throughout the process, only, as the hides advance, the necessity for frequent handling decreases. The top two handler pits are sometimes converted into “dusters,” i.e. when the hides have advanced to these pits, as each butt is lowered, a small quantity of tanning material is sprinkled on it.