It is impossible in the space of a short chapter to give an adequate explanation of the physical changes taking place during the puering process. An outline only can be given, and perhaps a few signposts to indicate for those interested, in what direction further work can be usefully done. It is obvious that if more be attempted, a treatise on physics and physical chemistry would be required; such a work is beyond the scope of the present volume. It is to be hoped that the whole of the questions dealt with here, including the physical chemistry of skin and of the whole tanning process, will shortly be fully treated by the master hand of our greatest tanning chemist, Professor H. R. Procter.
Since tanning, in the earlier stages, is for the greater part a physical absorption, or colloidal co-precipitating action,[44] the physical state of the skin fibre, or condition of the skin before it enters the tanning liquor, is of the greatest importance. In fact, the whole of the operations to which the skin is subjected previous to tanning are directed towards changing its physical state, and the chemical changes undergone are small and principally hydrolytic.
The two most important physical influences over which we have control are pressure and temperature.
The pressure during puering is practically constant, viz. the atmospheric pressure, but, as we shall see later, a diminution of pressure is favourable to falling or depletion. It would, therefore, be interesting to conduct experiments on puering in vacuo, or under reduced pressure.
Before considering the effect of temperature and the changes in the volume of the skin during puering, it will be well to consider the properties of the skin and the puer liquor.
In the skin prepared for tanning, practically all the keratin and epidermis and soluble matters are got rid of, and we have a mass of fibres which are composed of collagen, a proteoïd, which by prolonged boiling with water is converted into gelatin.
Neither the chemical formula nor the molecular weight of collagen is known with certainty, but, from a series of ultimate analyses, the change of collagen into gelatin is represented by Hofmeister[45] by the following equations:—
| C102H149N31O38 + H2O | = | C102H151N31O39 |
| collagen | gelatin |
from which it will be seen that he assigns a molecular weight of 2416 to collagen, and 2434 to gelatin. Other considerations, however, make it probable that the molecular weight is double the above figures.
Gelatin is a typical colloid, and we may consider that skin is practically a colloid with a structure (a very important point, as we shall see later), but behaving in many ways as gelatin. The puer solution is to a large extent colloidal. Therefore, both the skin and the puer are in the colloidal state. In the typical crystalloid solution of an electrolyte, the dissolved body is separated into its molecules, and to a large extent into individual ions, while, in the colloid state, the units of distribution are either large and often conjugated molecules, or more frequently minute particles composed of many molecules united by cohesive attraction.[46] In the case of the skin the molecules are not free to move, but are held in place by the structure of the skin, and the fibres thus act as semi-permeable membranes, with capillary spaces between them, in which water and other fluids are merely held by capillary attraction.