Leather: From the Raw Material to the Finished Product

by K. J. Adcock

2. Dyeing and finishing of machine belting, strap, harness, and other heavy greasy leathers.

3. Dyeing and finishing of boot, portmanteau, case, bookbinding, hat, and upholstery leathers, and of the numerous fancy leathers.

4. Dressing, dyeing, and finishing of glove and chamois leathers.

There are also a few special kinds of leather which do not come within the scope of this list. The limited size of this book makes it impossible to outline the finishing of all kinds of leather, and only a few of the important varieties can be referred to in the following pages.

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.