Section VIII.—UPPER LEATHERS
The manufacture of leather for the uppers of boots and shoes embraces a bewildering variety of goods, suitable for anything between a baby's shoe and a man's shooting boot. Almost all degrees of lightness, softness, and waterproofness are in demand. A great variety of finish is also involved, determined by the ingenuity of the currier and the ever-changing fancy of the public. Even greater is the variety of methods by which all these results are obtained by methods which superficially seem quite different; the desired qualities being imparted in one case largely by the tannage and in another case almost entirely by the currying. Under such circumstances the selection of types becomes a problem.
The variety, moreover, commences from the earliest stages, the selection of the raw material. Upper leather may be made from light calfskins, heavy calfskins, kips (home and foreign), light dressing hides and heavy dressing hides, which last may replace any of the former after splitting to the required substance. In this section it will be necessary to take kips as typical of the rest, and to use it in a rather broad sense, including heavy calf and light dressing hides.
Speaking quite generally, kips for upper leather receive usually a long and mellow liming, a thorough bating and a sweet and very mellow tannage in weak liquors. In currying they are well scoured and set out, heavily stuffed and stained black, being sometimes finished on the grain and sometimes on the flesh. These outstanding features of upper-leather methods will be further illustrated by a brief account of the tanning of kips (light hide and heavy calf), and outlining the best known types of finish for butt, shoulder and belly.
The goods receive usually a long and mellow liming of 14-16 days, using only lime as a rule. In some factories lime liquors are used repeatedly for successive packs to an almost indefinite extent. Dissolved hide substance, ammonia, mud and dust, and bacteria accumulate for months and sometimes for years. It is obvious that in such liquors "putrefaction" is a more correct term than "liming" for the depilation. Such methods have been used even in recent years, but there has now been a tendency for some time to make the liming more methodical. Such old limes make a leather which is empty, loose, and dull grained, but the defects are minimized by the system of stuffing heavily and finishing the flesh, and hence the ancient lime remained with surprising tenacity. Even so late as 1903 we find that Procter with characteristic caution could write, "Probably no lime ought to be allowed to go for more than three months at the outside limit without at least a partial change of liquor." It is within the writer's experience to find an upper leather factory with limes which had never been emptied for over three years. In other factories, however, there has been a revulsion of feeling with regard to such processes, and it has been found advantageous to adopt a more scientific routine, in which the lime pits are cleaned out at regular intervals. There is little doubt that a mellow liming is desirable, but this can be secured by blending some old lime liquors with fresh lime liquor in a systematic manner. Similar considerations apply to the question of working the various packs through the limes. It is clear that with a mellow liming a one-pit system is quite possibly satisfactory, but the revulsion of feeling against a lack of method produced a method of liming more elaborate than usual, and it is now not uncommon to find kips limed in a "round" of 6-8 pits, the goods passing through each pit. They remain in one pit about two days, and are shifted forward. In the green or old limes the goods are handled up and down. The old limes are, of course, mellower than the new and exert the desired softening effect. The working is quite analogous to that of a round of handlers. Unhairing is sometimes assisted by the use of arsenic sulphide. E.I. kips need a thorough soaking before any liming; several days are usually needed. The old methods involving putrid soaks and stocks may be considered out of date, and it is usual to soften back in caustic soda or sulphide soaks with some assistance by drumming. A little sulphide is sometimes added to the older limes to continue the treatment.
The goods are next thoroughly bated and delimed. The hen or pigeon dung bate is still usual, and probably gives the best results, though closer approximations have been made of recent years on artificial lines. Some bating with solution of hide substance seems necessary for these goods. The lighter goods are often drenched also to complete the deliming, using 6 per cent. bran on the weight of pelt. The heavier goods are more often treated with boric acid after bating, which not only delimes completely and gives a soft relaxed felt, but also acts as antiseptic and stops the action of the bate, a matter of some importance (see Section II.). Lactic acid may substitute boric, in which case about 2 per cent. on the pelt weight of 50 per cent. acid may be required. It is important to avoid a strong solution and local excess, hence lactic acid must be added gradually so that the liquor is never stronger than 0.2 per cent. Drumming and paddling is an advantage in deliming.
The tannage is light in most cases, partly because some of the finished goods are sold by area, but partly also because even if sold by weight, the weight is obtained quicker and more easily by stuffing, which course is also often preferable to obtain the desired mellow feel, waterproofness and durability. Hence it is seldom that strong liquors are employed. The tannage is also mellow, on account of the softness and pliability required; no acids are consequently employed, and no material which is liable to yield sour liquors. Gambier is easily the first favourite amongst the tanning materials, whilst oak bark comes second. It should be observed, however, that a hypothetical tannage of equal weights of cube gambier and oak bark is in reality a tannage by four-fifths gambier and one-fifth oak bark, on account of the relatively greater strength of the former. This observation is so apposite with respect to some tannages that it is nearly correct to say that the tannage is gambier and the oak bark an excuse for having leaches through which the gambier liquors may be run occasionally to clear and to sharpen slightly. No serious theoretical objection to such a method is possible if the liquors are weak and the system of working the liquors is scientific and the process carefully regulated. Upper-leather tannages, however, have scarcely merited scientific praise. It is often a case, not of poor methods, but of no method at all. The same lack of system, principle, and regularity observed with regard to the limeyard has been equally obvious in the tanyard, when perhaps the need was even greater. Even a mellow tannage has varying degrees of mellowness possible to it; there still remains the question of the soluble non-tans. However, method in the upper-leather tanyard has often been conspicuously absent. There has been many a factory where any one tan liquor was as good as any other in the yard. In the writer's experience are two such cases: in one the liquors were all 25° Bkr., in the other they were all 0° Bkr. In such cases, handling the goods from pit to pit is somewhat futile, and handling forward from set to set still more so. Hence it is possible to find dressing leather tanned by putting it slowly through one round of handlers, adding a few buckets of gambier where it apparently is necessary. It is, from one point of view, surprising to see what serviceable and excellent-looking upper leather can be manufactured by such happy-go-lucky processes. It is, however, also possible to see how this may occur. Gambier is a stable tan, and no souring and little decomposition take place in gambier liquors. It is also extremely mild and non-astringent, and is always used in weak liquors. The hides, moreover, are completely delimed, and there is little danger of bad or uneven colour. Tanning under these conditions is at its easiest; it is almost more difficult to spoil the goods than make them right. Under such conditions tanning deteriorated rather than improved in method. When neglecting it made little difference to the finished leather, it was neglected.
This state of affairs, however, was embarrassing whenever a tanner wished to try any other tanning material. The expense of gambier and oak bark made valonia and mimosa bark into obviously desirable alternatives and substitutes. Methods which would tan with gambier, however, would not work with Natal bark or valonia, and many a tanner has had to revise his method of tanning from end to end. The use of myrabs also raised the problem of souring, and it has become evident that "working the liquors down the yard" is as desirable a method for dressing leather as after all other tannages. It will be clear from the above that types of upper-leather tannages are less typical than for other leathers, but nevertheless the more progressive manufacturers have for some years now been working on sounder lines, economically and scientifically. In such cases it is now usual to pass the goods through at least two sets of handlers, and through liquors of gradually increasing strength. Occasionally dusters or layers are given, especially for the heavier goods. The tannage is nearly always commenced now by paddling the goods in the oldest liquor. This paddling may be anything from half an hour up to twenty-four hours. It is sometimes desired to work up a "grain," and the old liquor is then often sharpened by the addition of fresh gambier or leach liquor.
The same tendency to save labour in handling is to be observed in upper leather tannages as in sole and other dressing leather factories. There is also a tendency to obtain rather more weight in tanning by using stronger liquors, and in the heavier goods to shorten somewhat the time taken. The following methods may be taken to illustrate modern processes, in order of evolution. They all last about seven weeks.