Chapter VIII: Section V: Sole Leather

Leather for the soles of boots and shoes is a matter of essential interest to all, and forms one of the best appreciated applications of animal proteids to useful purposes. Methods for its manufacture are as numerous as the factories producing it, hence all that can be done is to describe broadly the general method which is typical of our time, to classify the many varieties into types, and to indicate the recent changes and present tendencies.

Sole leather is mainly manufactured from butt pelt, and the great aim is to produce a firm, thick, waterproof and smooth grained leather which will bend without cracking. It must have a light tan colour to be saleable, and contain as much weight as possible to be profitable.

The modern mixed tannage of "sole butts" or "scoured bends" generally utilizes ox-hides of the Scotch and English markets, though salted Continentals and South Americans are also employed. After the usual soaking a short and sharp liming is given. The special aim in liming sole hides is to obtain the maximum plumping effect with the minimum loss of hide substance. Both these achievements are necessary to obtain good weight. The limes should be kept as clean as possible, which is best obtained by putting clean hides into work. This reduces bacterial activity and loss of hide substance. The "shortness" of the process is attained by the use of sodium sulphide (from 2 to 16 ozs. per hide of sulphide crystals), by which depilation may be accomplished easily in about nine days. The amount of sulphide should be increased somewhat in the short-hair season and in cold weather. Some factories take up to about 12 days using less sulphide, whilst others will lime in about a week by using the larger quantities. The amount of lime used varies enormously, and is invariably in great excess of the actual requirements. "Probably 2-3 per cent. on the green weight of the hides is all that can be really utilized, the remainder being wasted."[5] This amounts to about 2-1/2 lbs. lime per hide, but in practice it is more frequent to find 7, 8, 9 or even 10 lbs. per hide being used. The excess is innocuous, owing to the limited solubility of lime. Some excess is desirable, to replace in the liquor the lime adsorbed by the goods in plumping, to assist bacterial activity, and also because in sharp lime liquors the undissolved portions do not remain so long in suspension. The use of sulphide and other alkalies does not "make it possible" to reduce the amount of lime used, it merely renders the excess more superfluous. The use of sulphide not only shortens the process, but also sharpens it, on account of the caustic soda produced by hydrolysis. Usually for sole leather, however, it hardly sharpens it sufficiently, and it is very common to add also caustic soda (or carbonate of soda) to the limes. About 2 ozs. caustic soda (or its equivalent in carbonate) is used per hide. The hides are limed generally by the three-pit system, giving about three days in each pit. They should be handled each day in the first pit (old lime) and once in the other pits.

[Footnote 5: Procter, "Principles of Leather Manufacture," p. 129.]

Unhairing and fleshing by hand labour is still common, in order to avoid great pressure on the plumped hide. Scudding should be very light, and in some yards is entirely omitted. Only the lime on the surface of the hide should be removed by deliming, and this immediately prior to the insertion of the butts into the tan liquor. This is to ensure good colour and yet keep the butts plump. Boric acid is the best for this purpose, using 10-15 lbs. per 100 butts. The goods are inserted (and preferably rocked) in a dilute solution for a few hours only. About the same quantity of commercial lactic acid may be substituted for the boracic. This deliming can also be accomplished by adding the acid to the worst suspender in the tanyard.

To obtain firmness and plumping it is necessary that the early liquors in tanning should be more acid than for other leathers. With old methods of tanning one could trust to the natural sourness of the liquors to complete the deliming and replump the goods with acid. In such cases any deliming was also unnecessary. In the modern yard, however, we get "sweet" liquors coming down the yard, partly on account of the greater proportion of extract used and partly because the liquors themselves are not so old. Hence it is now practically always necessary to acidify artificially the tan liquors. This may be done by adding a few gallons of lactic, acetic, formic, or butyric acid to the handlers and suspenders, especially in the winter and spring. It is now increasingly common to place sole butts in a special acid bath after they have been in tan liquor for about a week. This bath is often made from sulphuric acid, and may be 1 or 2 or even 4 per cent. in strength.

The actual tanning of sole butts lasts three to four months, and just prior to the war the tannage consisted often of about one-third myrabs, one-third valonia, and one-third extract. The myrabs and valonia were leached together, and the extract added to the best leach to make layer liquors of the required strength. Some mimosa bark was generally used also, and now it is extensively employed to replace the valonia. The most widely preferred extract is chestnut, but quebracho, myrabs extract and mixtures have also a prominent place, and mimosa bark extract an increasing importance. It is recognized that this tannage is if anything too mellow, and that if only a smooth grain and plump butt can be ensured in the first weeks of tanning, it is much better for sole leather to employ the most astringent tans possible and the sharpest liquors (_i.e._ liquors with a small relative proportion of soluble non-tannin matters). Hence there is the tendency in sole-leather tanning to employ fresh clear liquors for the butts and use up the more mellow liquors on the "offal" (shoulders and bellies).

Four types of sole butt tannage will now be described, all of which illustrate the methods employed in a modern mixed tannage.

=1.= The first type consists in a four-months tannage, in which the liquors are worked down the yard.

The butts pass first through the suspenders (20°-40° Bkr.) in about a week, and are rocked in the first liquors. They next enter the handlers (40°-55°) rounds of eight pits, six floaters and two dusters. Myrabs, or a mixture with algarobilla is used as dusting material. The goods remain in this set for two weeks, and should then be struck through. The suspender handlers (55°-65°) are next entered, in which they remain up to three weeks in suspension, being shifted forward on alternate days. The goods now enter the layers, of which four are given: first 70° for one week; second 75° for two weeks; third 80° for three weeks; and fourth 90° for a month. The goods thus take sixteen weeks to tan, of which ten weeks (62-1/2 per cent.) are in layers.

The system of working the liquors is expensive, and is only possible if the butt liquors can be spent out by the offal. The best or fourth layer, 90°, is made from the best leach liquor, 65°, and extract (chestnut with some oakwood or mimosa bark). After use it becomes the second layer, 75°. The third layer, 80°, is also made from fresh leach liquor and extract (chestnut with some myrabs or mixed extract). After being used thus it is used for the first layer, 70°. The used first and second layers are mixed together and used partly to form the belly layers, and partly to make a sharp liquor for the handlers (55°-40°) by diluting with 40° leach liquor and adding quebracho extract. The old handler liquor is run to the suspenders (40°-20°), and finally used for colouring off the offal in drum or paddle 18°. The suspender handlers (65°-55°) are made from fresh leach liquor and chestnut extract. They are afterwards used to make shoulder layers. The course of the liquors is shown in the diagram below.

It will be seen that fresh leach liquor and fresh material are used to each set except the suspenders, which must have some mellowness to ensure plumping and smooth grain. Layer liquors are used twice only, and then (when only five weeks old) pass to the handlers. These are further sharpened by fresh leach liquor and fresh extract and dry materials. The forward handlers are fresh liquors with fresh extract. This tannage is fairly typical of high-class sole leather, in which the liquors are worked down the yard, but worked towards the offal, which thus receives liquors with relatively greater proportions of mellow tans and soluble non-tans.

----------Leach liquor-------------------
| / \ |
| V \ |
| 4th layer, 90° \ |
| | V |
| V 3rd layer, 80° |
| 2nd layer, 75° | |
| \ V |
| \ 1st layer, 70° |
\ \ / |
\ \ / |
\ V V |
\ (mixture)[-->bellies] |
\ / V
V V Suspender handlers-->(65°-55°)(shoulders)
Handlers, 55°-40°
|
V
Suspenders [-->offal]

=2.= The second type consists in a tannage of about four months, in which the liquors are not worked down the butt yard. In this method also there is an attempt to save much of the labour in handling, first by shortening the time in the handlers by one week (as compared with the above), and second by fusing the two progressive handler sets into two sets of equal strength, through which the goods pass more slowly and with less disturbance.

The goods go through the suspenders (10°-25°) in about a week, rocking in the early liquors, and then into large rounds of handlers (30°-45°) for one month. The handlers consist of floaters and several dusters, in which the butts are laid away with 1-3 cwt. myrabs. The goods next enter the layers, of the same strength as in Type 1, and in which they remain the same time. The total tannage is thus 15 weeks, of which 10 weeks (nearly 67 per cent.) are in layers.

The best or fourth layer is made up from leach liquor and extract, and is then used successively as a third, second and first layer, and then passes to the offal layers. The handler liquor is made entirely from fresh leach liquor and quebracho extract, and is a sharp liquor of greater strength than its Bkr. strength would indicate. The old handler liquor is run to the butt suspenders. The course is represented thus:--

Leach liquor
/ |
/ 4th layer, 90°
/ |
/ 3rd layer, 80°
/ |
/ 2nd layer, 75°
/ |
/ 1st layer, 70°-->[offal layers]
V
Handlers (45°-30°)
|
V
Suspenders (25°-10°)

=3.= The third type consists of a short three-month's tannage in which the liquors are worked straight down the yard. To compensate for the short time it is necessary to have stronger layer liquors in which the goods spend a still greater proportion of their total time. The stronger liquors involve a greater proportion of extract, particularly of quebracho, which fact causes the whole of the liquors to be sharper than their Bkr. strength indicates, and justifies them being worked straight down the yard.

The goods go through suspenders (20°-40°) as usual one week, and then pass into suspender-handlers (40°-60°) for two weeks, and thence to the layers. In the first two of these (65° and 70°) they are actually in suspension, a week in each liquor. They are then dusted down for ten to eleven days, first in 85° and then in a 95° liquor, and finally for a month in a liquor of 110°. The total tannage is thus twelve weeks, of which nine weeks (75 per cent.) are in layers. There is considerably less handling than in Type 2, and it is more convenient, the goods being in suspension.

=4.= The fourth type is also a three-month's tannage. In this it is attempted to obtain even greater weight with still less labour. The layer liquors are kept much stronger by the more extensive use of extract, and this makes it impracticable as well as too costly to run these liquors down the yard. They are therefore repeatedly strengthened with extract and used again.

The goods go through suspenders (20°-40°) as usual one week, and then through a round of suspender-handlers (40°-55°) consisting of fresh sharp liquor from the leaches together with quebracho extract. They are in this set two weeks, and then are laid away. They receive three layers: first, 105° for 2 weeks; second, 110° for three weeks; and finally, 120° for a month. Of the twelve weeks, therefore, nine weeks (75 per cent.) are spent in layers. In this method the goods are immersed in 3 per cent. sulphuric acid after passing through the suspenders.

There is possible, of course, a tremendous number of variants of the above types. The number of handler rounds is determined by the number of butts being dealt with. With a large number it is more easily possible to arrange for them to be in progressive strength as in Type 1. There are also many systems of working the layers, of which the most notable is to make the second or third layer from fresh leach liquor and extract, and strengthen it with extract for the succeeding layers. It is then used as a first layer and worked down the yard.

The bellies and shoulders often go through separate sets of liquors, but it is common to put them through suspenders, and even handlers together. They receive, of course, a distinctly shorter tannage, and are often drummed with extract before laying away or after the first layer. By way of illustration, the course of the offal and their liquors may be given in the case of Type 1. The shoulders and bellies are coloured off in a paddle or drum with old butt suspender liquor, which is then quite exhausted. They then pass through suspenders (18°-40°) together in 4-5 days, and go through a handler round (40°-55°) for 3 weeks, including one duster. The bellies are removed after 2 weeks, and given three layers (60°, 70°, 80°) of a week each. They receive, therefore, nearly 6 weeks in all. The shoulders also have three layers (60°, 65° and 80°) of 2, 3 and 4 weeks respectively.

The course of the liquors is shown thus:--

Butt layers Butt suspender Butt 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 butts 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 butts 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 classified.

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.).

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.