Leather: From the Raw Material to the Finished Product

by K. J. Adcock

Sole Leather

Formerly, the finishing of sole leather was a fairly easy matter, as the slow process of tanning with oak bark which was in general use gave the latter a nice fawn colour. The modern tendency, however, is to demand even lighter-coloured leather than that produced by an oak bark tannage, and, as most of the mixed tannages impart a darker colour, the practice of bleaching has been universally adopted. This process is to be greatly deprecated, since it reduces the strength of the leather. Further, it has the serious drawback of removing matters which would add to the waterproof quality of the leather, and yet the boot manufacturers demand light-coloured sole leather. As in many other articles of modern manufacture, quality is sacrificed for appearance. The colour of sole leather is of no importance whatever to the wearer of the boot, and the public should make a strong protest against bleached sole leather, and also against leather weighted with adulterants, if they wish to get boots of good wearing quality. It is difficult to know where the finishing of some of the modern sole leather begins. Formerly, the line of demarcation between tanning and finishing was distinct, for the finishing processes were begun as soon as the hides were sufficiently tanned. This is by no means the case to-day, as, in many cases, the finishing may be said to begin with a supplementary but unnecessary tannage in very strong tanning extracts, with the object of making the leather firm and plump, and, incidentally, imparting additional weight. This practice would be discontinued if sole leather were sold by superficial measurement. On the other hand, those who favour the continuance of the present system of selling by weight assert that the adoption of the measurement system would indubitably result in the production of a lot of under-tanned leather, that mechanical means would be devised to stretch the leather and thus give it an artificial area, and that a fair quantity of the leather produced does not lie flat, and would, therefore, be difficult to measure correctly. A great point in favour of selling by measurement is that there would be no necessity to weight the leather, and a mere visual examination would suffice to determine the quality, whereas it is often difficult to state whether leather has been treated with injurious artificial matters or not, and certainly impossible to determine the extent of this fraud, without a careful and somewhat elaborate analysis. Of course, there are some rough practical tests for ascertaining the quality of sole leather, especially in regard to the extent of its resistance to water. A weighed quantity of the leather is soaked in water for twenty-four hours, taken out, squeezed, dried, and reweighed. If the loss of weight does not exceed 5 per cent., the leather is a good sample and well tanned; on the other hand, it does not follow that a loss, say, of 40 per cent., would indicate adulteration with injurious materials, as the leather may have been tanned with an excess of strong extracts which have not properly combined with the fibres, with the result that the excess is easily removed in contact with water. However, methods of fixing tanning extracts on the fibres of the leather have been devised (see p. [117]). A careful note should be made of the time required to soak the leather through; if water is absorbed rapidly, the leather is, naturally, unfit for soles.

After the tanner has taken great pains to produce the paleness of tint required, the shoemaker buffs away the grain, thereby reducing the resistance of the leather to wear, and then generally covers it with a black, tan, or white dressing. In its turn, this finish is likewise spoiled the first time the leather is worn. There could be nothing more futile than the elaborate finishing of sole leather to make it pale in colour, as it means that fully 80 per cent. of the leather produced has to be bleached in some way or other; and the manner in which this process is effected has a direct influence on reducing the wearing quality of the leather. If, for example, the colour of the leather is brightened with myrobalans, or sumach, or with extracts of these materials, the wearing quality is not affected; unfortunately, this method is not largely used, as it does not increase the weight of the leather and would tend to make it soft. A large proportion of sole leather is treated with strong sulphited extracts, which both bleach and increase the weight of the leather. In some cases, the leather is placed in vats containing hot tanning extracts to increase the bleaching effect and add to the weight. The gain of weight by this surplus tannage may be, and often is, from 8 per cent. to 10 per cent., and, whereas the tanner a century ago used to get only slightly above 50 per cent. of leather on the weight of the raw hide, it is no uncommon thing nowadays to get a yield of nearly 70 per cent.

The tanners are hardly to be blamed for this practice, which really ought to be stopped; it is almost entirely due to the demand of buyers for leather at a low price per lb., irrespective of its superficial area compared with a higher-priced and often cheaper article. It is only fair to state that those boot manufacturers who have taken careful costings of their leather have not suffered great loss in this way, but most repairers buy common leather in order to be able to do their work at fixed prices usual in this trade. A large proportion of this leather is under-tanned or weighted, and, in either case, absorbs water like a sponge. This kind of leather is quite unfit for soles, and ought to be condemned, as it not only absorbs moisture but also retains it for a long time, so that it is frequently a cause of colds and other complaints arising from chills.

If the repairer does not cost his leather as accurately as the wholesale boot manufacturer, the latter often neglects to take into account the quality of the leather, so long as cheap soles can be cut from it.

The continual demand for leather which will yield low-priced soles has brought about another practice which is much worse than the use of hot extracts. In this case, the hides are not tanned thoroughly, but, instead of completing the process with tanning extracts, the leather is treated with cheap chemicals, such as epsom salts (magnesium sulphate), glucose, and barium salts. These add considerable weight and save the cost of expensive tanning materials; further, it is claimed that they give increased firmness and substance to the thinner parts of the leather, which otherwise could not be used for soles. It is conceivable, therefore, that leather of this kind could be cut to greater advantage than that properly tanned, but, since this artificial filling is washed out on the first contact with water and the leather rapidly deteriorates through attracting and retaining moisture, the fraud of the system is at once apparent.

Although the use of leather artificially weighted with hygroscopic chemicals is extremely detrimental to health, it was only during last year that laws were passed forbidding the adulteration of leather, and even these are inadequate. Strange to relate, the two countries where these legal measures have been taken, Australia and South Africa, enjoy for the most part a hot and dry climate, while it is in wet weather that adulterated leather is most harmful. However, to Australia belongs the honour of initiating legal measures against frauds in leather manufacture, and their praiseworthy action must soon be copied by other civilised countries.

On the other hand, it would be inadvisable to prohibit the addition of every other material except those with tanning properties. There are undoubtedly a few substances which, used in combination with the tanning materials, add to the strength and value of the leather, and it is the opinion of some tanners that great developments will take place in this direction. Certain oils, sulphonated in order to render them easily miscible with water, and known commercially as "soluble" oils, exercise a beneficial effect when mixed with the tan liquors, for they lubricate the fibres of the leather, assist penetration of and fix the tannin. The time required for the process is, therefore, much reduced, as strong liquors may be used without harmful effect. The oil also serves as a lubricant for the fibres, obviates the harshness and brittleness usually associated with rapidly-tanned leather, and fixed to the extent of about 3 per cent., must increase its durability. Best of all, from the tanner's point of view, there is a moderate increase of weight. There are two or three special tanning oils on the market.

A vegetable gummy product has lately been introduced to the trade which has been found to give remarkable results when used in conjunction with tanning materials. This gummy matter is extracted from vegetable seeds and is placed on the market under the name of Tragasol. Its composition is somewhat similar to that of tannins, the Tragasol Co.'s analysis showing it to contain 43.51 per cent, of carbon, 6.23 per cent. of hydrogen, 48.38 per cent. of oxygen, 0.39 per cent. of nitrogen, and 1.49 per cent. of ash. It is very largely used for strengthening and sizing textile fabrics, and seems destined to play an important part in the leather trade, for it has tanning properties, increases the strength of the leather, and considerably hastens the process. Its most important advantage is that it permits successful tannage in the drum, thus effecting a great saving of time and labour.

Hitherto, rapid tannages have not been very successful, as they generally made the leather harsh and brittle, and lessened its durability. Leather tanned by previous rapid methods was also less waterproof than that tanned in pits by slow methods. All these defects are avoided by using Tragasol, which, when combined with tanning extracts, forms a curdy precipitate, described by the makers of Tragasol as cutiloid (contraction of cutis and colloid, cutis meaning "skin," and the Tragasol being, chemically, a colloid). The cutiloid tannage increases the water-resistant properties of leather, and prevents the oxidation and consequent darkening of the colour of the tanning material used with it. Tests have been made which show that cutiloid-tanned leather will withstand a fall of water 12 in. high for six weeks before complete penetration takes place; ordinary tannages do not often resist water for more than a few hours.

For the tannage of sole leather in the drum by the cutiloid process, about 120 per cent. of Tragasol and 40 per cent. of chestnut or oakwood extract are required; this quantity is divided into four equal portions, which are added to the drum successively at intervals of one hour. The hides should then be nearly tanned through, and the process is completed with strong extract (chestnut and quebracho, or myrobalans, according to the kind of leather required). After drumming the hides for ten hours, they should remain at rest for two hours, then drummed another hour, and so on, alternately, until the completion of the process, which may require from twenty-four to thirty-six hours, according to the substance of the hides. By this process, the yield of leather may exceed 60 per cent. of the weight of the raw pelts. This percentage of yield may be raised to 70 per cent. or more in the case of sole leather by steeping the hides in hot extract.

The action of acids in swelling gelatine is sometimes abused, particularly on the Continent and in America. As pointed out before, tan liquors must be mildly acid in character before the process can proceed, and, if the organic acids of the tanning materials are insufficient, the addition of a small quantity of formic, or lactic, or acetic acid is necessary; but, in many of the American acid tannages, an excessive quantity of mineral acid is used, with the object of swelling the hides. When in this condition, they rapidly absorb an abnormal quantity of tannin, and the finished leather is unnaturally thick. To make matters worse, the tannage is sometimes not completed, but, instead, the hides are treated ("doped") with hygroscopic chemicals, which not only increase the weight of the leather but also endanger the health of those who have to wear it. Most of the American tanneries make no secret of their use of these unnecessary chemicals, but assert that they are obliged to use them in order to meet competition, and because a large number of bootmakers insist on buying sole leather at a low price per lb. The Leather and Paper Department of the U.S.A. Bureau of Industry analysed several American leathers in 1913 and found that a large majority were artificially weighted with glucose and Epsom salts (magnesium sulphate). Barium compounds are also used for the same purpose, but the American tannages are apparently free of this adulterant. The results were published in a pamphlet by the United States Government, together with a list of the firms whose leathers were tested.

While the European tanners do not weight their leather so freely as the Americans with these chemicals, the practice of swelling the hides unduly is not unknown, with the difference that, especially on the Continent, the swollen fibres are fixed by treating the hides with a weak solution of formaldehyde, which also acts as a preliminary tanning agent. The tannage can then be rapidly completed with fairly strong extracts, as the formaldehyde prevents the contraction of the grain which would ensue if untreated raw pelts were placed in strong tan liquor. The acid-formaldehyde process is risky, and cannot be recommended. The use of the artificial tannin, neradol, has replaced it in many Continental sole-leather tanneries. A preliminary tannage with neradol forms a good mordant for the use of the strong extracts.

Fig. 26

PINNING SOLE BUTTS

Whatever the method used to improve or depreciate the natural tannage, the surplus material left on the surface is raised or brushed off, and, after the leather has been left to drain for a few hours, it is sent to the finishing department, which is usually the drying shed. There, it is left piled up for a day or two, until it is in the right condition for oiling. The leather must be oiled, otherwise the grain would be harsh and brittle when dry, and would darken in colour, especially at the edges. Cod oil is generally used, and sometimes a little mineral oil is added to it to prevent possible impurities in the fish oil rising to the surface of the leather or causing damage in other ways. A cod oil purified by distillation would, however, be quite safe to use, and there seems to be no reason why tanners should use crude oils so frequently as they do, now that a large number of oils specially prepared for the leather trade are available. Linseed oil is also used in admixture with cod oil, its oxidising properties hastening the process of drying. The leather is liberally oiled with a brush or swab on the grain side and immediately hung up in the drying shed, where it is left until it reaches what is technically known as a "sammed" condition (i.e., just damp enough to exude no water when the leather is doubled over). It is then struck out ("pinned") on the grain side, either with a hand tool (Fig. [26]) or by machine. In the former case, the leather is placed on a long wooden beam and the pinner works from the left side, keeping two of the three edges of his knife flat on the leather while making heavy forward strokes. As the scum is worked out, the operator mops it up with a wet cloth. Machinery has replaced hand labour in this operation in all modern tanneries. The pressure of the tool removes scud and dirt, varying in quantity according to the amount of "bloom" contained in the particular tanning materials used. The leather is then laid in pile for a few hours, when it is in a fit condition for the first rolling, which is known in the trade as "rolling on." This operation is generally done by machine (Fig. [27]) nowadays, but the old method of using a hand roller heavily weighted with an iron box (Fig. [28]) is still practised in a few yards. After rolling, the leather is hung up again until nearly dry, when it is taken down and sized with a weak colouring matter, made up of annatto or tumeric, with chalk or whitening dissolved in white vinegar (acetic acid) and diluted. It is then rolled again ("rolling off") and polished with a piece of flannel. Finally, it is hung up to air off, and is then ready for the warehouse.

Fig. 27

ROLLING MACHINE

In many yards the finishing consists merely in striking out the leather, rolling it when properly tempered, hanging up until nearly dry, and re-rolling. An American machine for giving the final polish to sole leather has lately been introduced.

Instead of the rolling machine, some French tanners use the hammering machine, which gives good results, but is necessarily slow and frightfully noisy in action. The steel hammer moves up and down with remarkable rapidity, but only a small area is covered by each blow.

Fig. 28

OLD METHOD OF ROLLING SOLE LEATHER

The artificial drying of sole leather, or of any other kind, is a matter that needs careful attention, for it is obvious that great delay would occur, and a large amount of business would be lost, if tanners relied solely on the climatic conditions of the British Isles; in fact, the weather is so rarely fit for drying leather that every up-to-date tannery contains some artificial aids, the need for which has been more pressing since a large proportion of leather contains a surplus of tanning matter which is liable to oxidise in contact with strong light and air, thereby darkening the leather and making it brittle.

Among the satisfactory drying plants are those of Howard-Smith & Co., and the Sutcliffe Ventilating and Drying Co., Ltd. Both work on the theory of fanning a continuous current of hot air through the drying-room, so that the moisture evaporating from the leather is constantly drawn off while the apparatus is at work.

Fig. 29

FAN
(Howard-Smith system)

The Howard-Smith plant consists of a series of steel tubes into which hot water or steam is pumped. These pipes are enclosed in a chamber in which the air naturally acquires great heat. A fan (Fig. [29]) is fixed in one side of this chamber (Fig. [30]) and, when in motion, drives the hot air through a wide tube which leads to the drying chamber.

Fig. 30

FAN FIXED IN CHAMBER
(Howard-Smith system)

Figure 31 shows the Sutcliffe Fan and Heater installed in a tannery. At least two hygrometers should be hung up in a drying-room to determine the amount of moisture in the air.