Ordinarily in the production of leather only the corium, or true skin is used, and in order to obtain it in a suitable condition for the various tanning processes, the hair or wool, together with the epithelium, must be completely removed without damaging the skin itself; and especial care must be taken that the grain, or portion next to the epidermis, does not suffer any injury during the treatment. All the methods employed depend upon the fact that the epidermis cells, especially the soft growing ones next to the corium, and those of the epidermis layer which surround the hair-roots, are more easily destroyed than the corium itself owing to their different chemical character. The “unhairing” process consists essentially in breaking down these cells by chemical or putrefactive agents, and removing the hair together with the rest of the epidermis by mechanical means. Of the various substances which may be used for this purpose, lime is one of the most convenient, as its solubility in water is so slight, that a solution of such a strength as to injure the hide cannot be easily made. Caustic alkalies, on the other hand, are much more soluble, and unless care be taken to use only the proper quantity, a dangerously strong solution may be made with consequent damage to the skin. The addition of small amounts of sulphides to the lime-solution accelerates the unhairing owing to their special solvent action on the epidermis-structures, and also in the case of alkaline sulphides, by the caustic alkali which is produced by their reaction with the lime. Even if used alone, strong solutions of alkaline sulphides rapidly destroy both hair and epidermis, converting them into a mass which may be swept off the skin like wet pulp, and yet they have practically no injurious action on the true skin.
In the “sweating” process the epidermic cells are broken down by putrefactive organisms and their products, so that the hair becomes loose and may then be either rubbed or scraped off. Ammonia, which is produced during the putrefaction, has also an important solvent action, and its presence doubtless tends to quicken the processes both of unhairing and of destruction.
To obtain useful knowledge of the structure of any particular skin, it is not necessary to have a very elaborate or expensive microscope, and it is quite possible to obtain useful information merely by the use of a good pocket lens, as for instance, in the examination of various forms of “grain,” and the embossing of one skin to imitate another.
For further details of the manipulation and selection of the microscope, the reader must consult L.I.L.B., pp. 234 et seq.
CHAPTER VIII.
THE CHEMICAL CONSTITUENTS OF SKIN.
The chemistry of the various constituents of skin is still very imperfectly understood, but Beilstein, in his great handbook of organic chemistry, places gelatin, albumens and keratins in the “aromatic” series, and implies therefore that they contain the “benzene” ring. It is at least certain that all are very complex.
The epidermis structures belong to the class of keratins, which are closely related to coagulated albumin; while the white fibres of the corium (or true skin) are either identical with gelatin, or only differ from it in their molecular condition or degree of hydration. This gelatinous tissue constitutes the bulk of the corium, but it also contains albumen as a constituent of the lymph and blood which supply its nourishment, keratins in the epithelial structures of the blood and lymph vessels, and “yellow fibres,” which are perhaps allied to the keratins, but which cannot well be isolated for analysis.
The white connective tissue of the corium is converted into gelatin (glutin) by boiling with water. Owing to the impossibility of obtaining unaltered hide-fibre free from the other constituents, and still more to that of deciding to what point it should be dried to remove uncombined water, it is impossible to prove by analysis whether its composition is identical with that of glutin; but as the white fibre constitutes by far the largest part of the corium, and the other constituents do not differ largely from it in their percentage composition, an analysis of carefully purified corium is practically identical with that of the actual fibre. The following analyses of hide and gelatin are therefore of interest.
The analyses of Von Schroeder and Paessler[22] are of special importance as being the average of a large number of separate determinations. Their nitrogen determinations are by Kjeldahl’s method. Small amounts of ash and traces of sulphur are neglected, and probably included in the O, which is obtained by difference.