The various methods of liming, together with analogous variations in tannage, have resulted in great variety in bating. Sometimes up to three days' bating has been given at 70° F., but more often the goods are merely immersed overnight, and then delimed with boric acid, but with sulphide processes it is an advantage to use some of the commercial bates of the ammonium chloride type, and finish off with boric acid. Scudding is always more thorough than for sole or belting, the hides are rounded into long butts which include most of the shoulder "harness backs." The goods are sometimes bate shaved.
A few tannages will now be outlined, in order of historic type.
Type 1 may be taken to represent the so-called "high-class" process in which oak bark myrabs and valonia are the staple materials. A good deal of gambier is also used, and a little myrabs and chestnut extract are helpful in attaining the desired strength of liquor. The "backs" go first through suspenders (8°-30°), which takes up to three weeks, and then in to handlers (30°-40°) for four weeks, consisting of rounds of clear liquor. They next go through a duster round, in which they are put for a week with oak bark and myrabs into a liquor of 45°. Four layers are given (50°, 55°, 60° and 65°), in which the goods remain for two, three, four and five weeks respectively, oak bark being the chief dusting material. The tannage is thus for twenty weeks. Light backs receive less time in the layers (only 11 weeks). If the tanner is also the currier, the fourth layers are omitted. He then saves five weeks and gets the weight in the stuffing.
Type 2 is a tannage in which oak bark and valonia are replaced by myrabs, mimosa bark and chestnut extract. It is therefore considerably cheaper and probably no less durable. Expense is also curtailed in handling. The harness backs go through suspenders (16°-30°) in two weeks, handlers (30°-45°) in four weeks, and then receive four layers of the same strength as in Type 1, but only one, two, three and four weeks respectively. The last layer is omitted for light harness, and an extra layer of 75° is given if the tanner is not the currier also. Thus the usual tannage is 16-20 weeks, of which 10-14 weeks (63-73 per cent.) are in layers.
Type 3 is a tannage which may consist of myrabs (55 per cent.), valonia or mimosa bark 25 per cent., and extract (26 per cent.). The extract is chiefly quebracho, though some chestnut may be used. More valonia and less myrabs may be used if desired (and when possible), and myrabs extract will then replace quebracho and chestnut. The goods are coloured off in drums or paddles, and then pass through two sets of suspenders handlers (20°-55° and 55°-75°). They are handled up and down very frequently in the first set and rapidly pass into stronger liquors. The backs then receive three floaters at 80°, in each of which they remain one week. The tannage is completed by three layers: first, 85° for one week; second, 90° for one week; third, 95° for two weeks. The tannage is thus 11 weeks, of which 7 weeks involve little labour. If the tanner is not the currier, still stronger liquors may be used.
In all these tannages little or no acid is used for plumping, as the natural acids of the liquors are sufficient to ensure what is necessary in this direction for this class of leather. A little organic acid or even boric acid may be used in the earliest liquors for deliming purposes, when necessary. After tanning the goods are dried out and sorted in the rough state. Harness is a somewhat broad term, and there is scope for considerable variety in classification. The hides are sometimes not rounded until after tanning. The finished article may be any grade between heavy harness for artillery and leather for ordinary bridles.
In currying heavy black harness, the backs are soaked and sammed for shaving. Lighter goods may be machine shaved, but the heaviest are shaved lightly by hand over the beam or merely "skived" with the shaving slickers. The neck needs most attention, and it is often advisable to stone by machine and split. The scouring should be thorough, on flesh and grain. This is done by machine, and not only cleans the goods from bloom, dirt and superfluous tan, but also assists in setting out. Sumaching may be for several days, merely overnight or even only for a few hours, being stoned after wetting back to temper. Hand-stuffed goods get a coat of cod oil first, and during the drying are often well set out. Drum-stuffed goods are well set out by machine, and after some drying, stoned and reset by hand. It is now usual to buff the grain, i.e. remove the coarser parts by light shaving. This prevents cracking in the finished article. The goods are blacked with logwood, iron and ammonia, thinly dubbined again, again well set out and tallowed. Setting out, indeed, may be done at any convenient opportunity. The superfluous grease is removed by slicking, scraping, brushing with a stiff brush, and finally with a soft brush.
For brown harness the goods are more carefully selected, more thoroughly scoured and sumached, and bleached frequently with oxalic acid. They are hand stuffed, stained twice, and after the usual setting out, glassing and brushing, are finally rubbed with flannel.
For bridle leather the goods are carefully shaved but are not stuffed, being merely oiled with cod oil on flesh and grain. They are dried out before scouring, and then sized, set out, stained and resized. The goods are heavily glassed during the finishing.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 195, 297.