MANUFACTURING PROCESSES. The hides of commerce are brought to the tanneries in four different forms: either they are simply or 'fresh' hides, direct from the slaughter-houses. or. in case they have been shipped from a long distance, they are wet-salted. dry-salted, or simply dried. The preliminary process of preparing the hide, for tanning differs some what with the condition in which they are re ceived, salted and dried hides requiring much more thorough cleansing and softening than green hides. The process also differs somewhat in preparing sole-leather. harness-leather, and dressed leather. The first step is to soak the skins or hides in water, to soften them, after which every vestige of adherent flesh is scraped from the inside. They are then laid in heaps for a short time, and afterwards hung in a heated room, by which means a slight putrefactive de composition is started and the hair becomes so loose as to be easily detached. This process of unhairing. called 'sweating, is mostly followed in America for making sole-leathers, while the process of liming or loosening the roots by the milk of lime is used for dressed leather; hut in Great Britain milk of lime is used for depilation of all leathers. The process may be hastened by use of sulphuric or other acid. Hides or skins intended for dressing purposes, as for shoes. coaches. harness. or bookbinding, after the hair is taken off by the lime. have to be submitted to a process (-ailed bating for the purpose of reduc ing the swelling or thickening oeeasioned by the introduction of the lime. and for cleansing the skin from grease and other impurities. This is effected by working the skin in a &elle tion of pigeon's or dog's dung and Warr water. process does something nu re than cleanse the leather; it a marked change in its texture. reducing it to an ex tremely flaccid condition. If the old method of tanning is followed. the hides after India iring are placed in the tan-pits, with layers of oak bark or other tanning materials between them. and when as many layers of hide: and hark are arranged as the pit will hold. water i- let in, and the hides are allowed to remain for an indefinite period to be acted on by the tanning material. Various means for shortening this process have been devised, such as forcing the tanning-liquor through the skin by pressure, sewing the skins together into a bag in which the liquor is suspended. and simply substitut ing for the dry bark which was formerly used liquid infusions of tanning materials, which are gradually increased in concentration as the proc ess advances. The last-named method, though the slowest, is found to produce the best leather, and the process of tanning is still a tedious one, consuming weeks or even months. The general method employed in American tanneries is de scribed by Sadtler as follows; •"fhe to into which the cleansed and prepared hides or 'butts' now cmne is provided with rows of pits running in parallel lines. which are to contain the butts during their treatment with tan-liquor. The butts in most cases are first suspended in weak tanning in fusions before they go into the first. or 'handler' pits. The object of this is to insure the uni form absorption of tannin by the skins, before subjecting them to the rough usage of 'handling; which in the early stages of the process is liable to cause injury to the delicate structure of the skin. During this suspension the skins should be in continuous agitation to cause the tannin to be taken up evenly. Both the sus
pension and the agitation are accomplished gen erally by mechanical means. From the suspend ers the butts are transferred to the 'handlers,' where they are laid flat in the liquor. They are here treated with weak infusion of bark, com mencing at about 15° to 2.0° by the barkometer and are handled twice a day during the first two or three days. This may be done by taking then] out, turning them over, and returning them to the same pit, or more generally by running them, fastened together, from one handler pit to an other. The treatment of the butts in the hand lers generally occupies about six to eight weeks, by which time the coloring matter of the bark and the tannin should have `struck' through about one-third of the substance of the skin. Many of the butts will have become covered, moreover, with a peculiar 'bloom' (ellagic acid) insoluble in water. They are now removed to the 'layers.' in which they receive the treat ment of bark and 'ooze' or tan-liquor in pro gressive stages until the tanning is complete. Here the butts are stratified with ground oak bark or valonia, which is spread between each butt to the depth of about one inch. and a thicker layer finally on top. The pit is then filled up with ooze, which varies in strength from about barkometer at the beginning to 70° at the end of the treatment. For heavy tan nages six to eight layers are required. the dura tion of each ranging from ten days in the be ginning to a month in the later stages. Each time the butts are raised they should be mopped on the grain to remove dirt and loose bloom." Many materials besides oak-bark are now used to make tanning infusion, and some of these, being stronger, have hastened the tanning proc ess. Among the most important of these are valonia, the acorn of an evergreen oak found in Asia Minor and Greece. which contains three times as much tannin as the strongest oak-bark: the sumach; the divi-divi and algarovilla. pods of Routh American trees closely allied to log wood; and the larch, spruce, pine, and hem lock barks. Besides this group there are the tanning materials derived from abnormal growths, caused by the sting of insects or other injuries, as galls (q.v.) and knoppern. The so-called tannage is produced by a com bination of oak and hemlock barks.
Undressed leather, after it is tanned, needs simply to be rendered smooth and compact, which is accomplished by scouring and compressing the surface with stones,brushes, the 'striking-pin'and rollers, all of which processes arc effected by ma chinery. Dressed leathers must, in addition, be 'stuffed' with oils to increase their resistance to water and their flexibility; they must frequently be dyed or stained in black or colors and 'grained.' These processes are also performed by machinery. In 1860 a machine was invented for splitting leather to any desired degree of thinness. The practice previously was to shave the leather down, the shavings being wasted. The process of dressing tanned leather known as currying was formerly a separate industry, but is now ca• ried on as a part of the general business of leather manufacture. A favorite oil used by cur Hers for stuffing leather is the degras, or super fluous oil pressed from shamoyed leather. The demand for this oil is so great that its manu facture has recently become a separate industry.