The offal is often drum oiled. It needs more striking and is more difficult to obtain in suitable condition for striking, rolling. It is treated similarly to butts, but often also goes for dressing leather, and may be split. It is of some interest to compare the above processes with that once very popular manufacture of "bloomed butts" in the West of England from South American salted hides. These receive a liming from 12-14 days, using 12-16 lbs. of lime per hide. They receive then a tannage of about 9 months, comprising 3 weeks in suspenders (20°-40°)—very sour and mellow liquors—4 weeks in handlers (40°-55°), 4 weeks in dusters (60°), 4 weeks in round made from hemlock extract (60°), and 20 weeks in six layers (60°-90°) in which they were dusted heavily with valonia. Oakwood extract was used for the layers, which took 57 per cent. of the total time. The butts were scoured in a much-dried condition, so that only the loose and surface bloom was removed. No bleaching was given in the modern sense.
In the old oak-bark tannage of sole leather up to 12 months were taken for tanning, two-thirds to four-fifths of which time the goods were in layers. The strongest liquor rarely exceeded 50° even where valonia and gambier were also used, and rather more than 30° if not.
It will be understood from the above that the tendency for many years has been to shorten the time and the labour required for tanning. Drum tanning is obviously the next stage in shortening the time. In one such process the butts are put through suspenders (25°-40°) for 2 weeks, drummed for 12 hours in an 80° extract liquor, and finally in a neat extract 200° for 36 hours. Drum tanned sole leather, however, is not as yet of good quality; the grain is not smooth, and the heavy weight finish (striking and rolling) needed to counteract this tendency is liable to cause poor "substance." The leather, too, readily wets and goes out of shape. Possibly some drumming may be adopted to save time in the early layers, but the most serious rival to the 3 months' tannage is the waterproof chrome sole leather (Part III., Section V., p. [173]).
REFERENCES.
Parker, J.S.C.I., 1902, 839.
Procter, "Principles of Leather Manufacture," p. 220.
Bennett, "Manufacture of Leather," pp. 179, 259.
Bennett, J.S.C.I., 1909, 1193.
SECTION VI.—BELTING LEATHER
The manufacture of belting leather is well illustrated by the tanning and finishing of "strap butts." In general, the tannage presents many points of great similarity with the tannage of sole leather; indeed, the resemblance is so close that in some factories there is little difference observed, and the currying and finishing operations are relied on to produce the desired difference in final results. Nevertheless, there is considerable difference in the type and ideal of the two leathers, which may be expressed in trade parlance as a greater "mellowness" for the belting leather, and in the best methods of manufacture this fact is in evidence throughout the whole process of manufacture.
In liming, there need be little difference between sole and belting hides, and a sharp treatment of 9-10 days, by the three-pit system, with a day or two extra in the coldest weather, would meet ordinary needs. For the conservation of hide substance and for the saving of time a shorter liming is sometimes given, in which more sulphide is employed than is usual for sole leather. Even the very short processes of liming, 1 to 3 days, which involve the use of strong solutions of sodium sulphide, have been successfully employed for belting leather. The tendency to harsh grain with such processes is not so serious a defect with belting as with sole leather, and can be minimized by careful deliming. American and Continental factories tend to favour the use of those quick processes which employ warm water in addition to sulphide. The hides after a short liming in sulphide limes are immersed in warm water, which greatly accelerates both the chemical and bacterial actions. For example, after about 3 days' liming, in which both old and new limes are used as usual, the hides may be thrown into water from 100°-105° F., and will be ready for depilation in 7 or 8 hours.
Even a stronger liming may be given, especially if the soaking is unusually prolonged. Such processes undoubtedly save hide substance, and the pelt is obtained more free from lime, but they have the disadvantage that the natural grease of the hide is only imperfectly "killed" (i.e. saponified or emulsified), and may interfere with the normal course of the tannage. The plumping is also apt to be insufficient. On the other hand, liming processes are also used in which a mellower liming or a longer liming is preferred in order to produce the desired degree of softness and pliability in the finished leather. Belting must not be too soft, of course, and it will be clear that the required difference from sole leather can be produced either in liming or tanning or partly in both. These considerations also decide whether bating is to be omitted or not. A hard astringent tannage in sour liquors after a sharp liming might make bating essential, but in these days it is usual to avoid it and produce the effect in other ways. A light bating of a few hours is sometimes given, but it is more unusual to delime the grain thoroughly with boric acid, using up to 20 lbs. per 100 butts. Crackiness is a fatal defect in strap butts, so that a sound grain must always be obtained. Generally speaking, therefore, strap butts receive more washing in water, and rather more deliming than sole leather, even when they are not bated. It is also usual to scud much more thoroughly, and to round a larger proportion of butt, especially in length.
The tannage is usually carried out with a blend which includes a much greater proportion of the fruit tans, and correspondingly less of extract.