The Puering, Bating & Drenching of Skins

by Joseph Turney Wood

The acids neutralize any lime which has not been neutralized in the bate, and they do this in a very gradual and mild way, being produced by bacterial fermentation exactly as required, in a similar manner to fermentations taking place in presence of calcium carbonate. The gases evolved bring the skins to the surface of the drench, and distend the fibres; the workman attending to the drenches puts the skins down under the liquor with a pole, but a better way is to have the drench vat fitted with a wooden paddle, which may be pulled round by hand when necessary; in this way the skins are put under the liquor without danger of tearing or trapping them; in the case of grain splits the paddle is a great advantage over the pole.

The skins are allowed to rise two or three times, according to the condition required. For many kinds of alum leather the bate is omitted and the drench only is used.

In English practice the bran is mashed in the vat, in which the drenching is to take place, at a temperature of about 95° F. (35° C.); the skins are then put in and the drench well stirred or paddled until the whole is uniform, the temperature is then usually down to 85° to 83° F. (29·5° to 28·2° C.), at which temperature the fermentation begins. The temperature allowed varies with the state of the goods and the prevailing exterior temperature; in winter it may be higher than in summer, but never usually higher than 90° F. (32·2° C.). In some continental works the bran is mashed in boiling water, allowed to stand for some hours to cool, and only the bran-water is used; in this case the fermentation is different to that in which the bran is mashed at a lower temperature. In the first process the starch of the bran undergoes changes to dextrin and glucoses, which are then fermented by bacteria with the production of acids and gases (see Chapter X.), in the latter case the starch, which has been gelatinized is fermented by other organisms, which probably secrete diastatic enzymes. Less gas is given off in this fermentation than where the bran is mashed at 95° to 100° F. The temperature of fermentation is kept low, usually 25° C. (77° F.). Bran contains a sufficient amount of nitrogenous matter,[151] to furnish the bacteria with nutriment, and until this is exhausted there is no fear of the skins being attacked.

There is also a “sweet” bran drench used on the continent, in which fermentation is not allowed to take place, the skins being paddled in an infusion of bran in water for a short time only, two to three hours at the most. In addition to the mechanical cleaning action, which the fresh bran has upon the skins, there is also a softening effect due to some constituent of the bran, which is at present not understood. One reason for this, is that the usual drench employed being an acid one, research has practically been confined to the latter.

Another method of preparing the bran drench, consists in allowing the required quantity of bran to soak in cold water for several hours; a sufficient quantity of hot water is then added to bring the temperature up to 50° C.; at this temperature the diastatic enzymes in the bran act rapidly on the starch and transform it into easily fermentable sugars. The temperature is allowed to fall to 34° to 35° C., and then a quantity of an old drench is added to start the fermentation. The liquor becomes acid, and in two days usually reaches its maximum acidity, when it is ready for use. The bran is strained off and only the milky liquor used. Such a drench is largely employed in the manufacture of chamois leather, the goods (sheep fleshes) being paddled in it for some hours previous to pressing. The French also use a similar process in the preparation of suede leather. The bran is strained off and only the sour partly fermented and fermenting liquor used. The straining off of the bran also saves working over the beam which would otherwise be required to get rid of the bran adhering to the skins.

It has recently been proposed to employ the bacillus bulgaricus, the organism which causes the fermentation of milk in the preparation of Bulgarian and Turkish “Yoghurt” (Bibliography 124) for drenching. Bacillus bulgaricus is capable of producing up to 2·5 per cent. of lactic acid, or about three times the quantity of that produced by ordinary lactic bacteria. Dr. Hugo Kühl (loc. cit.) proposes to cultivate the bacillus in the mother liquors, which are a by-product in the manufacture of milk sugar, and to use such liquor as a drench. Where skim milk or butter milk is to be had, this may also be used. A similar process is already in use in America, where the evaporated product is put upon the market under the name of Dermiforma (see pp. [189] and [194]).

As a substitute for the drench, and in order to further remove lime after erodin, the late Franz Kathreiner employed a solution of anticalcium, 1 in 500 water, in which the skins remained overnight. Anticalcium is a mixture of sulphonic acids derived from cresols, and has antiseptic properties, so that its action is quite distinct from that of the bran drench. It is put on the market by J. Hauff, Feuerbach, near Stuttgart.

Drench Damage.—Drenching is a very important and useful process in leather manufacture, but, like other fermentive processes, requires great care and experience in its application. Although not so risky as puering, a good deal of serious damage may occur in the drench. The writer knows of no fuller discussion of all the various forms of drench damage than the articles of Eitner and others which appeared in “Der Gerber” during a long series of years (1882–1900). In these articles perhaps every known form of drench damage is described. Procter[152] also gives a concise account of drenching. It will suffice to mention briefly the chief accidents to which the drench is liable; these are generally known as the “turning” of the drench.