Leather - how its made
In writing of a manufacture so ancient as that of leather, it is difficult to begin without some sort of historical notice; and yet harder to do so with success, since in the earliest historical times the art had attained such a development that its details were no longer a matter of curiosity, and hence little information of its methods has been preserved. We know, from actual specimens, that in Rome, presumably in Greece, and certainly in the still earlier civilization of Egypt, leather was used for most of the purposes for which we use it to-day.
Catching a glimpse of mankind history, we may picture the ways the primitive hunter used animal hides and skins as crude tents, clothing and footwear, as the first uses of leather dates back to the Palaeolithic period. Some savage races still use skins for clothing, and prepare them in the old traditional ways developed within their tribe. We may imagine that the earliest of these depended on greasing, smoking, stretching and softening the skin as it dried; that only later was the use of barks and berries discovered, and even later that of alum. All these assumptions are meant to show that while methods and machines develop, principles remain unchanged; and all the previously mentioned primitive types survive in altered forms in the manufacture of today, alongside others of which our ancestors knew nothing.
Whatever importance the skin may have to the tanner, it was still greater to the animal of which it originally formed part, serving not merely as clothing, but as an organ of feeling and of secretion, and having a complex structure, which to fully understand we should acquire basic knowledge of biology.
Undoubtedly the most important materials for leather manufacturing are the skins provided by domestic animals, and especially by the ox and sheep. An important feature of the industry is that its main raw material is a by-product of our food supply, and no animals are grown especially for their skins. Thus, in this case, demand does not have any noticeable effect on supply, which, with the increased growth of corn, and the diminution of prairie lands, constantly tends to become short of requirements, and consequently dearer.
Among the important factors to the tanner are the sebaceous glands, which secrete fatty matter to lubricate the hair, and which are composed of groups of cells like miniature bunches of grapes situated round the upper part of the hair-sheath, into which they discharge.
Organic Structure of the Calf Skin
Before the actual tannage can take place, much has to be done which is essential to the success of the final result. If the hide or skin just arrived from the butcher and is very fresh, or only lightly salted, the first thing to do is to cleanse it from dirt, blood and salt, by a few hours washing in clean water, ideally assisted by a short treatment in the drum or wash-wheel, which is a rotating cylinder, similar to a water-wheel turned inside out, and provided internally with floats or pegs which carry the hides up above the centre of the wheel and allow them to fall back, while a stream of cold water is run on them from a tube in the axle and escapes through holes in the rim.
To all appearances our savage predecessors preferred their skin-clothing with the hair on, but for most modern purposes its removal is necessary. Unless it is pulped by sulphydrates so that it can be washed off, the hair is removed by scraping with a knife on the "beam", or a skiving machine. In unhairing it is important not only to remove the hair, but, for sole-leather, to clear away as much as possible of the hair root-sheaths and fat-glands, which are often deeply coloured with the pigment of the hair. In the case of lighter and finer leathers, the unhairing is usually accomplished by performing a later operation called "scudding". In carrying out this process, machines similar to those for unhairing are also used, but with sharper edges to the spiral knives.
The principal agents used in the fermentation processes necessary to the leather manufacturer are bacteria, even though, as regards sugars, yeasts play an important part. Despite that bacteria are matters of everyday talk, it is still necessary to describe their character and life-history because the popular conception related to it is frequently so vague. Bacteria are not the only organisms which cause fermentations – an important step in leather processing.
Leather Processing Techniques
The purpose of all the processes so far explained is the removal of the epidermis and its appendages, and the cleansing and purification of the corium, in preparation for its actual conversion into leather. The skin is still merely raw animal matter. While it is still moist, it is soft and pliable but rapidly decomposes; however when it is dried it becomes like horn, stiff and translucent and useless for most of the purposes of leather. In this case it is necessary to render it soft, porous and opaque when dry; imputrescible and enough resistant to water and all these characteristics can be obtained by means of various methods described as follows.
“Pickling” is considered to be one of these methods and although the pickling process is strictly one for preserving the prepared pelt for tanning, rather than of transforming it into leather, it is in itself capable of manufacturing leather and its investigation has thrown so much light on the actual tanning processes. The pickling process is carried out through a preliminary swelling with acid, which is later reduced by a concentrated solution of common salt. The skins treated in this manner can be kept for many months in a wet condition without putrefaction or injury.
Although, at least in Europe, manufacturing alumed leathers is not such an old process as compared of that of producing vegetable tannages, the alumed leather production has been introduced by the Moors into Spain, yet the simplicity of its chemistry makes it worthy of an introductory position. The tanning properties of alum may have been accidentally discovered by its substitution for common salt, since in some countries with warm climate the alum exists as a natural product of the weathering of aluminous shales.
Vegetable tanned leather
This type of leather is manufactured through a process called vegetable tannage, which is performed with use of tannins. The tannins can be described as a class of substances found in many plants, which have the common properties of precipitating gelatine from solution and of converting skin into leather. Although it raises many problems and difficulties in commercial practice, the vegetable tannage process may in theory be regarded as one of the simplest.
Using fats to convert skin into leather is one of the earliest primitive methods of leather production, and it highlights the fact that the main purpose of fats is to coat the fibres and prevent their subsequent adhesion. Although this is an incontestable fact, it does not appear to be the whole truth, since ordinary wash-leather, which is a kind of typical oil leather, can be washed with hot soda solution, which would remove both oils and their oxidation products, and still preserves all the qualities of a very perfect leather.
It seems fitting to end this little article with a small clip that illustrates the steps in leather processing, a domain of great complexity that binds chemistry with technology and tradition to produce one of the most important materials in modern days – leather - engineered to fit a great variety of purposes from boots and shoes, book-binding and upholstery to harness, saddles and coach-work.