Page:The American Cyclopædia (1879) Volume X.djvu/280

 LEATHER mental to the process), and the air is heavily charged with moisture and the ammonia from the hides. The time required to loosen the hair is from one to six days, according to the condition of the hide; but it is of great im- portance that the hide be removed as speedily as possible after the hair will slip, as it is then in the incipient stages of a very rapid decay. From the sweat pit the hides are first milled, with the water running on them, to soften them further and remove the hair, and then worked over a beam, to take off any remaining hair ; they are also, by the more careful tanners, worked on the flesh side to take off any extra flesh or fatty matter which butchers almost always leave on, and then thoroughly rinsed. By the warm sweating process the same ends are obtained, and in about the same time, though it is with greater danger from more rapid decay in unsound hides. Another object to be attained in this stage of preparing the hide is the swelling or distending of its fibres, to allow of the free admission of the tannin to combine with the gelatine in the cells. When lime is used to unhair, it opens the Eores of the hide very effectually* but the .me itself must also be thoroughly removed afterward; this is most commonly done by soaking the hide in what is called a bate, made of hen dung or similar excrement, aided by subsequent thorough washings in water. The swelling of the hide to allow the tanning liquors to enter the pores freely has no little influence in determining the quality of the leather. Some means to effect this are always necessary in tanning hides which have been un- haired by sweating. There are two methods generally followed in America in preparing sole leather, which illustrate the principles adopted by all tanners. These are commonly called the "acid" and "non-acid" processes. In both methods it is acid that plumps the hide and distends its fibres for the reception of the tan liquor ; in the former process, however, a mineral acid is used, either alone, with a large proportion of water, or in connection with more or less of the vegetable acid furnished by the old, sour liquors, to hasten the process of preparing the hides for the tanning proper ; while by the latter process the hides, after being unhaired, are handled for several days in the old, sour liquors, but without the aid of the mineral acid (which almost invariably con- sists of vitriol or sulphuric acid), to effect the same object, that is, distend the fibres and open the pores. In using the vitriol, great care must be observed in the first stages, or the leather will be very dark, and have what is called a very poor grain, i. e., will not make a light-colored bottom for boots and shoes, an object which is very much sought after in all sole leather. The leather is also more likely to be hard and brittle than that prepared by the non-acid method. The old liquors used in the latter method are simply the tanning liquors which have become weak by use, and in which the remaining tannic acid has largely been changed to gallic acid by exposure to the air. The grinding of the bark and the making of the liquors therefrom is a most important part of the work. The bark is stripped from the tree and cut in pieces 4 ft. long ; a month or two is generally sufficient to make it dry enough to grind, though if kept two years in a large, close pile, and not much exposed to wet, it will not be materially injured. It is ground in a large mill, not unlike a coffee mill in principle ; and about the size of a grain of wheat is considered the proper degree of fine- ness. To get the tanning principle from the ground bark, leaching with hot water is preju- dicial, for in that way resin and coloring mat- ter are also extracted ; and leaching with cold water does not extract all the tannin. Both modes are used. Leaching with hot water, or swelling the ground bark with steam before leaching, causes the leather to be darker than that made with liquors which are leached cold. The vats in which the hides are handled and laid away in tanning are of various sizes, either large enough for whole hides or for sides, and are about 6 ft. deep. In handling, the hides are sometimes sewed together at the ends, so that they can be reeled from one vat to an- other ; but this is only to facilitate the work in the frequent changes which have to be made. For the first few days the hides are changed every two or three hours from one liquor to another, the first liquors being very weak, and the last liquors in which the hides are thus handled, at the end of 10 or 15 days, having 10 or 12 degrees of strength. From the handlers the hides are placed in what are called layaway vats, a shovelful of ground bark on the top of each hide, and the whole covered with the bark liquor. It is the custom in America thus to lay away the leather in vats six or seven times, the liquors of the first lay- away being the weakest, and the time the lea- ther is allowed to remain, as well as the strength of the liquors, being increased with each suc- cessive laying away. For the first layaway a liquor of 10 or 12 degrees would be used, and the time would be from one to two weeks; for the second, 15 to 20 degrees, and two to three weeks ; for the third, 20 to 22 degrees, and three to four weeks ; for the fourth, 24 to 26 degrees, and four to six weeks; for the fifth, 28 to 30 degrees, or stronger, and the time in this layaway, or for the sixth and sev- enth, depends on the judgment of the tanner, the quality of the hide, and the kind of leather to be made. This process, taking about four months, is a fair average of time consumed in America for the actual tanning part of ordi- narily heavy hemlock sole leather. Oak tan- ners generally lay away the leather in bark and strong liquors for a little longer period, and this is also done with all extra-heavy hides. The leather is also much more advanced in the handlers in some cases, and on the proper and thorough opening of the pores here, and the