Animal Proteins Part 5

b.u.t.t layers b.u.t.t suspender b.u.t.t suspenders (and + extract) Handlers (+ extract) | | | | V V | Belly layers Shoulder layers | (80-60) (80-60) | / | V V | Offal handlers (55-40) | | V V Offal drums Offal suspenders (40-18) (18) / / / / / / V V Drain

The tanned b.u.t.ts are piled for 2-3 days, sometimes rinsed to remove dusting material, and then scoured either by machine or by drumming with sumac and extract. This removes bloom, but causes some loss of weight.

"Vatting" or "bleaching" now follows, in which it is attempted not only to bleach the colour of the leather, but also to impart as much weight as possible. The vat liquor is made several degrees stronger than the last layer by means of quebracho bleaching extract and good coloured chestnut or myrabs extract. The liquor is kept warm by a steam coil, at about 100 F., but not much more without risk. The goods remain in the bleach liquor 2-3 days and are then horsed or suspended to drain. Sumach is sometimes used in the vats. A new vat liquor must be made up after some weeks' use. The goods are sometimes rinsed in weak sumac liquor before vatting to get good penetration, and sometimes after to ensure good colour.

The b.u.t.ts are next oiled and hung up in a dark shed and allowed to dry slowly and evenly to an "india-rubbery" consistency and rather slimy feel. They are then "struck out" by machine, wiped, re-oiled and again hung up to dry, preferably with sulphonated oil. After a short drying to a suitable and even condition they are "rolled on," and, possibly after further drying, "rolled off" with greater pressure, and then dried for a day or two with the help of a little steam. Finally they are machine-brushed and sent to the warehouse, where they are weighed and cla.s.sified.

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 b.u.t.ts, 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 b.u.t.ts" 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 b.u.t.ts 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 b.u.t.ts 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.).

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 ill.u.s.trated by the tanning and finishing of "strap b.u.t.ts." 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 b.u.t.ts. Crackiness is a fatal defect in strap b.u.t.ts, so that a sound grain must always be obtained. Generally speaking, therefore, strap b.u.t.ts 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 b.u.t.t, 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.

Distinctly more myrabs are used than in sole leather tannages, in the dry material, and amongst the extracts chestnut is preferred to quebracho, and myrabs to mimosa bark, though all these may be used in some degree. In the past the most favoured extract has been undoubtedly gambier, which gives a tannage which is easily curried and imparts the required mellowness to the uncurried leather. The great expense of this material, however, together with the advent of drum stuffing and shorter tannages in stronger liquors, have tended to cause a considerable reduction in the proportion used for strap b.u.t.ts, and to limit its employment to the earlier stages of tanning.

The same tendencies for reducing the time taken to tan, employing stronger liquors, and securing economy of labour in handling, have been evidenced in the tannage of strap b.u.t.ts as in sole b.u.t.ts. It is nevertheless true that, broadly speaking, strap b.u.t.ts receive rather more handling and rather weaker liquors than sole b.u.t.ts. A greater amount of mechanical a.s.sistance is also employed with early stages (paddling, drumming, rocking). This is less objectionable for curried leather than for sole b.u.t.ts. The handling is more usually in suspension.

The liquors are usually worked straight down the yard as a greater mellowness is needed in the early liquors than for sole b.u.t.ts. The offal is given a separate tannage and often used for different purposes, _e.g._ the shoulders for welting and the bellies for fancy goods.

Plumping with sulphuric acid is generally considered inadmissible for strap b.u.t.ts. It has been shown that leather containing sulphuric acid tends to perish after the lapse of a number of years. Sole leather will be worn up before this effect is observed, but belting is an article which is intended to last much longer, and the use of sulphuric acid is consequently inadvisable. Plumping must be obtained, to a considerable extent, but must be achieved by the organic acids (lactic, acetic, formic and butyric acids). A few gallons of such acids are consequently added to the handlers, especially in the winter and spring. Less may be used in the autumn, when the layer liquors which fermented in the summer months have worked down to the suspenders. A mixture of these acids is usually better than any one alone, for they not only differ very considerably in price, but also have different powers of neutralizing lime and plumping the goods. Lactic acid (M.W. 90), Acetic acid (M.W.

60), and formic acid (M.W. 46) are each mon.o.basic acids; consequently 3 lbs. formic will neutralize as much lime as 4 lbs. acetic or 6 lbs.

lactic. Their plumping powers are somewhat influenced by the anion. In determining what quant.i.ties to take, the commercial strength of the acids must also be considered. Formic is often 80-90 per cent. pure, acetic 60-80 per cent., and lactic 40-60, but may be as low as 25 per cent. The blend must be adjusted accordingly. As strap b.u.t.ts do not need the firmness of sole leather, less of these acids may be used than for sole b.u.t.ts.

The exact nature of the tannage and the strength of the liquors is largely influenced by commercial considerations. If the manufacturer is both tanner and currier, he need not go to such great expense in strong liquors and in time in layers, for he can obtain some of this weight in currying. If, however, the tanner sells the b.u.t.ts rough dried, he must naturally aim at obtaining greater weight in tanning.

The actual details of the tanning processes are as usual very varied, but may be cla.s.sified according to type, just as in the case of sole b.u.t.ts.

Ill.u.s.trations will now be given.

=Type 1=, which may be compared with Type 1 for sole b.u.t.ts, is a tannage of about 5 months. The goods pa.s.s through suspenders (8-30) in 2-1/2 weeks, and then pa.s.s to the handlers (30-50), in which they remain a month; they are then put into suspension again and pa.s.s through the suspender handlers (40-55), which takes 2-1/2 weeks. In this round much gambier is added, and the goods are frequently handled. Four layers are usually given, viz. first layer 55, one week; second layer 60, two weeks; third layer 65, four weeks; and fourth layer 75, four weeks.

The tannage is thus 20 weeks, of which 11 weeks (55 per cent.) are in layers. Extra layers may be given to heavier goods, using stronger liquors made up with extract. All liquors work straight down the yard.

The tannage consists of 35 per cent. myrabs, 35 per cent. valonia, 10 per cent. Natal bark, and 20 per cent. extract, chiefly gambier, though some chestnut and quebracho are used.

=Type 2= represents the modern tendency to use stronger liquors and a shorter time. The strap b.u.t.ts pa.s.s through the suspenders (22-50) in 1-1/2 weeks, during about a third of which time they are rocked. They next pa.s.s through two sets of suspender-handlers (50-67 and 67-80), which takes a month, and thence to the layers. Three layers are given (85, 90 and 100), in which the goods remain one, three and four weeks respectively. The tannage is thus 13-1/2 weeks, of which 8 weeks (nearly 60 per cent.) are in layers. The liquors work down the yard. Longer time may be given to heavier goods. The tannage consists of 40 per cent.

myrabs, 35 per cent. valonia or Natal bark, and 25 per cent. extract, chiefly chestnut, though some gambier may be added to the suspenders.

However tanned, strap b.u.t.ts are first dried out rough over poles. This a.s.sists in making the tannage permanent, on account of secondary changes discussed in Section III. They are next wet back for currying by soaking in water or sumach liquor for a few hours and piling to become soft and even. The first operation is "skiving," which is a light shaving on the flesh side, carried out by a sharp slicker with a turned edge. The b.u.t.ts are next scoured thoroughly by machine on both flesh and grain, and sumached in a vat for several hours at 100 F., after which they are slicked out and hung up in a cool shed to samm for stuffing.

Hand stuffing is often still preferred, with tallow and cod oil. The b.u.t.ts are next set out, and it is important that this should be thoroughly done. Machines are now generally used, and the goods are often reset after further drying. After drying out completely they are given a light coating of tallow and laid away till wanted for cutting up into straps, which is now done by machinery.

A Continental method for making belting leather is to give 6 weeks in a suspender set (70-24) of twelve pits arranged on the press system, running two fresh liquors a week, and to give them two layers (24 and 28) of 6 and 8 weeks. The material is chiefly pine bark, but some oak bark, valonia, myrabs and quebracho are also used. The goods are stuffed by "burning in," molten fat being poured on the flesh side.

REFERENCE.

Bennett, "Manufacture of Leather," pp 194, 295.

SECTION VII.--HARNESS LEATHER

When discussing the question of oak bark (Section III.), reasons were advanced for its decreased use and popularity. These were quickly appreciated in the sole leather trade, but the obsolescence of oak bark in the dressing-leather section was much more prolonged, partly because there was less pressing need to obtain good weight in the actual tanning, and partly because in some branches of dressing leather, such as belting and harness, a leather was required of great durability and toughness, for which qualities oak bark tannage had a deservedly high reputation. Hence harness leather manufacture affords a good ill.u.s.tration of the transition between the methods of the late nineteenth and those of the twentieth century. With the use of oak bark lingered the old methods of liming, bating and tanning in weak liquors for a long time with plenty of gambier. Hence in this section it will be necessary to observe a gradual transition of method, both in wet work and tanning. It should be pointed out that this transition has not been and is not going on in all factories at the same rate. Many factories remain in which the old methods are still preferred at some stages of the manufacture, and some remain in which many of the changes indicated below have not taken place at all. The leather trade has always been considered conservative in its methods, but it should be realized that much of the prejudice in favour of old methods is due to the public, and that after all tanners and curriers, like other business men, have to suit their customers. The march of industry is not like a regiment in line; it is rather more like nature, a survival of the most adaptable.

Hides for harness leather are limed in various ways, of which the following are types.

1. A rather mellow liming of 10-15 days (longer than for sole leather), in which nothing but lime is used, and a certain amount of old liquor used in making up the new limes. The liming was carried out by the one-pit system, but the goods and liquors were kept clean by a good soaking process. Hence the loss of hide substances was not very great; goods so treated were bated before tanning.

2. A shorter liming than the above by the three-pit system. This saved time (taking 9-10 days), saved hide substance, and ensured greater regularity of treatment. The limes were about as mellow, but a little sulphide (2-4 ozs. per hide) was used to a.s.sist the depilation, especially during the short-hair season. These goods were also bated.

3. A distinctly longer liming, 15-16 days, in mellower limes. This differed from Type 1 also in the respect that greater regularity was ensured by the three-pit system; a foot or two of old liquor was used in making up the new lime. More hide substance was lost than in either of the above processes, but this was deliberate, the object being to dispense with bating, which is always light for harness hides. Thus a longer and mellower but systematic liming was used as a subst.i.tute for shorter liming and bating. No sulphide was used in this process.

4. A short liming of 6-7 days, using up to 12 ozs. of sulphide per hide.

The object here is to save time and hide substance. The three-pit system is preferred. Bating again becomes necessary, but the pigeon-dung bate is replaced by artificial bates, less objectionable, quicker, and more scientific in management.

5. A still shorter process of about five days, using still more sulphide (about 16-20 ozs. per hide), together with some calcium chloride to reduce harshness. In such a method there is a tendency to revert to the one-pit system, which involves rather less labour. The three-pit system shows to a great advantage in the longer processes of liming when the process is reduced to five days; there is little difference between the two, for a one-pit system is a two-liquor method. Hence again an artificial bate is used.

The various methods of liming, together with a.n.a.logous 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 b.u.t.ts which include most of the shoulder "harness backs." The goods are sometimes bate shaved.