Hence it is evident that, for hides dried at low temperatures, short soaking in fresh and cold water is sufficient, and, except in warm weather, there would be little danger of putrefaction. With harder drying, longer time is required, and it may be necessary to use brine instead of water. A well-known tanner recommends a solution of 30°-35° barkometer (sp. gr. 1·035, or about 5 per cent. of NaCl). This will have a double action, not only preserving from putrefaction, but dissolving a portion of the hide-substance in the form of coriin. Although this is undoubtedly a loss to the tanner, it is questionable if there is any process which will soften overdried hides without loss of weight: since even prolonged soaking in cold water at too low a temperature to allow of putrefaction will dissolve a serious amount of hide-substance. Water containing a small quantity of carbolic acid has been recommended for the purpose, and will prevent putrefaction, while it has no solvent power on the hide, but, on the contrary, will coagulate and render insoluble albuminous matters. Concentrated carbolic acid, however, tans the grain and renders it incapable of colouring in the liquors. Borax has been proposed for the same purpose, and, in strong solution, certainly prevents putrefaction, but is probably too costly. Sodium sulphide and other sulphides seem to have considerable effect in softening dried hides, from their property of attacking hard albuminous matters, without injuring the true hide-fibre.
For some descriptions of hides, however, and notably for India kips, putrid soaks seem actually to be an advantage, the putrefactive action softening and rendering soluble the hardened tissue. In India the native tanners soften their hides in very few hours by plunging them in putrid pools, into which every description of tannery refuse is allowed to run. Putrefactive processes are always dangerous, as the action, through changes of temperature, or variation in the previous state of the liquor, is apt to be irregular, and either to attack one portion of the hide before another, or to proceed faster than was expected. Hence hides in the soaks require constant and careful watching, and the goods must be withdrawn as soon as they are thoroughly softened, for the putrefaction is constantly destroying as well as softening the hides. It is possible that putrefactive softening is less injurious to kips, and such goods as are intended for upper-leather, than to those for sole purposes, as it is generally considered necessary in the former case that the albumen and interfibrillary matter be removed, and that the fibre be well divided into its constituent fibrils for the sake of softness and pliability; so that the putrid soak, if acting rightly, only accomplishes a part of the work which would afterwards have to be done by the lime and the bate. The actual fibre of the hide seems less readily putrescible than the albuminoid parts; hence the putrefaction may soften the latter better, and even at less expense of valuable hide-substance, because more rapidly, than fresh water. On this point, there is room for investigation. Putrefaction is a general name for a class of decompositions which are caused by a great variety of living organisms, each of which has its own special products and modes of action. It is quite possible that, if we knew what precise form of putrefaction was most advantageous, we might by appropriate conditions be able to encourage it to the exclusion of others, and obtain better results than at present. It will be necessary to revert to this subject when speaking of the bates used in preparing dressing-leather, which also owe their activity to putrid fermentation.
Beside merely soaking the hides, it is necessary to work them mechanically, to promote their softening, which was formerly accomplished by "breaking over" the hides on the beam with a blunt knife. This process is now usually superseded or supplemented by the use of the "stocks"; these consist of a wooden or metallic box, of peculiar shape, wherein work 2 very heavy hammers, raised alternately by pins in a wheel, and let fall upon the hides, which they force up against the side of the box with a sort of kneading action. The ordinary form of this machine is shown in [Fig. 22]. A more modern form, which seems to possess some advantages, is the American double-shover, seen in [Fig. 23].
Fig. 22.
Fig. 23.