The Puering, Bating & Drenching of Skins

by Joseph Turney Wood

In the system water-hide substance the researches of Wiedemann and Lüdkeing have shown that swelling is accompanied by evolution of heat. Since a rise of temperature is favourable to a system formed by absorption of heat, it therefore hinders swelling and vice versa. This is confirmed by practical experience, and most bating operations are conducted at temperatures between 35° and 40° C.

With regard to the influence of pressure on swelling, a similar law holds good as for temperature, which may be expressed as follows. When a chemical system is compressed at constant temperature, its equilibrium is shifted in that direction by which the reaction is accompanied by diminution of volume. Quincke, however, has shown what appears to be a paradox, namely, that swelling is accompanied by diminution of volume, i.e. the swelling substance, plus the water taken up, occupies a smaller space than the sum of the two constituents taken together; therefore, an increase of pressure must be favourable to swelling, and conversely a diminution of pressure is favourable to “falling.”

We must clearly bear in mind that in all cases of swelling, it is the entry of water into the system which is the cause of the swelling, and since puering is a process which acts in a contrary direction, i.e. the skin falls, this means that water is expelled from the system, skin plus water. By direct determination, from 3 to 8 per cent. of water is expelled during puering. Calculations made from density determinations are masked by the fact, above referred to, that the density of the skin changes during puering.

A fallen skin contains less water than when in the swollen condition, but the difference between the percentages of water in the skin in the two states is only relatively small, and obviously insufficient in itself to account for the great difference in the physical properties of the skin in the two conditions. It is evident, therefore, that the state in which the water is held in the two stages must be different. It is reasonable to suppose that, in the case of swollen skin substance, the whole of the capillary cavities of the organized structure of the skin are completely filled with water, which, owing to its incompressible nature, confers upon the skin elastic and unyielding properties which it always possesses in this condition. The water, entering the organized cell structure in the first place by osmotic pressure, is fixed there and confers upon the skin, in part, its own property of incompressibility. It is a well-recognized fact that, during the puering process, large quantities of the skin substance are dissolved by the action of the enzymes in the bate, and it is probable that the finer organized structure is first attacked. The walls of the cells which hold the water are partly destroyed, leaving the skin with, quite possibly, the same amount of water contained in it as a whole, but with the water dispersed at large throughout its interior structure instead of being held in the fibres. The result of this is, that when a small area of the puered skin is subjected to local pressure the water in the portion undergoing compression is free to move into the adjacent portions of the skin, and there is no force acting upon it, except that of capillarity, to cause it to return to that area when the pressure is removed from it. When a portion of a puered skin is compressed, the surrounding part is always swollen to an equivalent amount. That this view is, in the main, correct is supported by an observation of Abt (Paris), that, while cell nuclei may be demonstrated in the layers near the epidermis of an unpuered skin, they are entirely absent from a puered skin, showing that the puer has completely dissolved the nuclear structure of the cells.

The swelling caused by alkalis is of a different nature to the swelling caused by acids. Procter has pointed out[59] that alkaline swelling is not repressed by sodium chloride, even when caused by sodium hydrate, but is repressed by sufficient concentration of the hydroxyl ion in the outer solution.

Müller[60] points out that jellies containing certain non-electrolytes (glucose, glycerine, alcohol) render the diffusion of soluble matter much slower than pure jellies of the same concentration, but the presence of urea favours the permeability of gelatin and agar jellies. The differences in the velocity of diffusion of various salt solutions through jellies of the same strength, are thus not due altogether to the greater diffusibility of the particular salt solution, but are to be attributed to the influence of the salt on the permeability of the colloidal medium. Limed skin treated with a 1 per cent. solution of urea at 43° C. is depleted or “falls,” but both the solution and the skin remain alkaline.

Přibram[61] has recently shown that the swelling and contraction of muscle are to be ascribed to absorption and expulsion of water from the cells of the muscle; the rapid changes taking place, are brought about by changes in the concentration and composition of the liquid components of the muscle. Lactic and phosphoric acids are produced with enormous rapidity, and of relatively high concentration; during a short period these cause instantaneous changes in the swelling of the muscle cells, followed by a return to their original state. For the evidence of this the original paper must be consulted, but it goes to show that the equilibrium of the system—colloids, electrolytes, water—depends on the proportions and quantities of each of these constituents. The phenomena we are considering thus conform to the general law of mass action (Guldberg and Waage).

Influence of Solid Matter in the Bate.—It was first pointed out by Wood (J.S.C.I. 1899, p. 991) that a filtered puer liquor had less action on the skin than an unfiltered liquor containing much finely suspended matter.

On adding an inert solid (kaolin) to the filtered bate, the action was hastened. No doubt much of the colloidal matter, and with it some of the enzymes, are removed by filtration, but nevertheless, the suspended particles in the bate have some effect on the process.

In a most interesting paper, Perrin[62] has recently shown, that the granules in suspension in a colloidal liquid function like the invisible molecules of a perfect gas with a molecular weight of 3·3 x 10^-9. Such granules are endowed with the molecular (Brownian) movement, and therefore may exert a mechanical effect on the fibres of the skin. Although the mass of the particles is large[63] compared with molecular dimensions, they are small enough to penetrate the pores of the skin, and where puering is carried too far, may become deposited beneath the fine hyaline layer, and thus render it cloudy and unsuitable for fine colours.