effect of tightening the hair. The real cause of the loosening
of the hair is that the bacteria in the old lime creep down the
hair, enter the rete Malpighi and hair sheath, and attack and
decompose the soft cellular structure of the sheath and bulb,
also altering the composition of the rete Malpighi by means of
which the scarf skin adheres to the true skin. These products
of the bacterial action are soluble in lime, and immediately
dissolve, leaving the scarf skin and hair unbound and in a condition
to leave the skin upon scraping. In this first “green” lime
the action is mainly this destructive one, but the goods have yet
to be made ready to receive the tan liquor, which they must enter
in a plump, open and porous condition. Consequently, the
“green” lime is followed with two more, the second being less
charged with bacteria, and the third being, if not actually a new
one, a very near approach to it; in these two limes the bundles
of fibre are gradually softened, split up and distended, causing the
hide to swell, the interfibrillar substance is rendered soluble
and the whole generally made suitable for transference to the
tan liquors. The hide itself is only very slightly soluble; if care
is taken, the grease is transformed into an insoluble calcium
soap, and the hair is hardly acted upon at all.
The time the goods are in the limes and the method of making new limes depends upon the quality of the leather to be turned out. The harder and tougher the leather required the shorter and fresher the liming. For instance, for sole leather where a hard result is required, the time in the limes would be from 8 to 10 days, and a perfectly fresh top lime would be used, with the addition of sodium sulphide to hasten the process. Every tanner uses a different quantity of lime and sulphide, but a good average quantity is 7 ℔ lime per hide and 10-15 ℔ sodium sulphide per pit of 100 hides. The lime is slaked with water and the sulphide mixed in during the slaking; if it is added to the pit when the slaking is finished the greater part of its effect is lost, as it does not then enter into the same chemical combinations with the lime, forming polysulphides, as when it is added during the process of slaking.
For softer and more pliable leathers, such as are required for harness and belting, a “lower” or mellower liming is given, and the time in the limes is increased from 9 to 12 days. Some of the old mellow liquor is added to the fresh lime in the making, so as just to take off the sharpness. It would be made up as for sole leather, but with less sulphide or none at all, and then a dozen buckets of an old lime would be added. For lighter leathers from 3 to 6 weeks’ liming is given, and a fresh lime is never used.
“Sweating” as a method of depilation is obsolete in England so far as heavy leathers are concerned. It consists of hanging the goods in a moist warm room until incipient putrefaction sets in. This first attacks the more mucous portions, as the rete Malpighi, hair bulb and sheath, and so allows the hair to be removed as before. The method pulls down the hide, and the putrefaction may go too far, with disastrous results, but there is much to recommend it for sheepskins where the wool is the main consideration, the main point being that while lime entirely destroys wool, this process leaves it intact, only loosening the roots. It is consequently still much used.
Another method of fellmongering (dewooling) sheepskins is to paint the flesh side with a cream of lime made with a 10% solution of sodium sulphide and lay the goods in pile flesh to flesh, taking care that none of the solution comes in contact with the wool, which is ready for pulling in from 4 to 8 hours. Although this process may be used for any kind of skin, it is practically only used for sheep, as if any other skin is depilated in this manner all plumping effect is lost. Since this must be obtained in some way, it is an economy of time and material to place the goods in lime in the first instance.
Sometimes, in the commoner classes of sole leather, the hair is removed by painting the hair side with cream of lime and sulphide, or the same effect is produced by drawing the hides through a strong solution of sulphide; this completely destroys the hair, actually taking it into solution. But the hair roots remain embedded in the skin, and for this reason such leather always shows a dirty buff.
Arsenic sulphide (realgar) is slaked with the lime for the production of the finer light leathers, such as glace kid and glove kid. This method produces a very smooth grain (the tendency of sodium sulphide being to make the grain harsh and bold), and is therefore very suitable for the purpose, but it is very expensive.
Sufficient proof of the fact that it is not the lime which causes skins to unhair is found in the process of chemical liming patented by Payne and Pullman. In this process the goods are first treated with caustic soda and then with calcium chloride; in this manner lime is formed in the skin by the reaction of the two salts, but still the hair remains as tight as ever. If this process is to be used for unhairing and liming effect, the goods must be first subjected to a putrid soak to loosen the hair, and afterwards limed. Experiments made by the present writer also prove this theory. A piece of calf skin was subjected to sterilized lime for several months, at the end of which time the hair was as tight as ever; then bacterial influence was introduced, and the skin unhaired in as many days.
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| Fig. 3.—Tanner’s Beam. |
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| Fig. 4.—Tanner’s Knives and Pin. |
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.
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
