It is probable that other organisms are capable of producing similar fermentations, and it is not certain that in all tanneries the same ferments are present. Mr. A. N. Palmer states that at the Cambrian Leather Works at Wrexham, he has been unable to detect lactic acid in the drenches, all the acids present being of the acetic series.
The drench-ferments investigated by Wood are incapable of attacking or injuring the hide, and, in his opinion, when the skin is attacked, it is generally due to putrefactive and gelatine-liquefying organisms introduced from the bates, or from the air in hot sultry weather. Drenching takes place most safely and satisfactorily at temperatures not exceeding 30°-35° C., when the process is usually complete in 12-24 hours. In hot sultry weather a butyric fermentation of an active character sometimes suddenly takes the place of the normal one (Ger. Umschlagen), the skins swell rapidly, become translucent (glasig) and finally dissolve to a jelly. If tanned in the swollen condition, tender and useless leather results, and the injury, once begun, proceeds with alarming rapidity, skins being sometimes completely ruined in a few hours. Prompt action is therefore necessary, and the first step to take is to add salt, which checks the fermentation, and acts in the same way as in the pickling process, controlling the action of the acid, and producing a sort of tawing. Such skins will yield sound leather, though the grain is apt to be somewhat drawn. If the skins can be immediately got out of the drench, the acid may be neutralised by the cautious addition of ammonia, soda, or whitening to the water in which they are placed, preferably in a paddle, and if they are insufficiently drenched they may then be paddled in tepid water, though this is hardly likely to be needed, as the effect of the acid is to remove the lime very completely. The objection to the use of whitening, which otherwise is the safest and best material to employ for removing acid from pelt, is that it is apt to become mechanically fixed in the grain, and, thus, to produce bad colour with vegetable tans. For white or chrome leather it would do no harm. Precautions to prevent the recurrence of the injury are to keep the temperature of the drench low, and to free the bran from flour by washing in two or three cold waters, before adding to it the hot water with which the actual drench-liquor is made, since the flour, or at least its starch, is the source from which the butyric acid, as well as the lactic, is formed. In cold weather, where drenching is proceeding in a normal way, the flour is useful, since it is the natural nutriment of the drench-ferment; and, in England, flour is frequently added purposely to the bran to increase the activity of the drench. To retain the flour, the bran may be washed first with boiling water, which gelatinises the starch and makes it adhere to the bran, and, according to Eitner, removes a sticky fatlike matter from it, and fits it better to remove the fat of the skin. After soaking in hot water for two hours, it is washed in several cold waters and infused at about 40° C. for use.[98] Many tanners use the bran without previous washing, but if much flour is present it rises to the top with the gas evolved by the fermentation, and forms a pasty mass on the skins, which interferes with even drenching.
[98] Gerber, 1882, p. 246.
The quantity of bran used in ordinary drenching is very variable, but about 4 parts per 1000 of water used and from 5 to 10 per cent. on the weight of pelt may be taken as an average quantity, more being frequently employed. The temperature may vary from 10° up to about 30°-35° C., and the time inversely from days or weeks down to two or three hours, according to the temperature of the drench, the amount of ferment present, and the thickness and character of the skins. The skins are usually thrown into the freshly prepared drench, to which a few pailfuls of old drench-liquor is frequently added as a ferment. Fermentation soon sets in, and the gas evolved causes the skins to float to the surface; this is called the “working” of the drench. Thin skins may be sufficiently drenched after once rising, while thick ones require to be put down two or three times. A certain sign of sufficient drenching is the appearance of small blisters on the grain, caused by the evolution of gas in the substance of the skin. When these are seen the drenching should be at once discontinued, as otherwise the blisters will increase in number and burst through the grain, causing minute holes or “pricks” (one of the many forms of the complaint called in German Pikiren or Piquieren). When a bubble of air is enclosed in a fold of the sufficiently drenched skin and pressed, it raises the grain without actually separating it from the substance of the skin. The properly drenched skin also falls easily in folds when held between the hands either lengthways or crossways, and if thin, the skin tightly stretched over the hand shows grains of bran underneath it as little lumps, round which the skin clings to the hand. The drenched skin should not be transparent, but white and soft; and when pressed should retain the mark of the finger. Some experience is required to determine certainly the point of sufficient drenching, which, of course, varies with the character of the skins, and the kind of leather which is to be produced; and the feel of the skin to a practised hand is one of the most important criteria.
A writer in the ‘Gerber’[99] divides drenching into three classes—“sweet,” “alcoholic” and “sour.” Sweet drenching is done in a bath of tepid bran-water, made by infusing in hot water and drawing the clear liquor off the bran, which settles to the bottom. The skins are only allowed to remain in 2-3 hours, or not long enough for fermentation to set in. The process is only suited for very thin or soft skins, which will not stand any further loosening. The use of bran-water has the advantage of saving the labour of “branning,” or removing adhering bran with the knife on the beam, but it is doubtful if unfermented bran has much actual effect. Bran-water can, however, be used for drenching by fermentation, and for small glove-lamb has largely superseded the older method. The mechanical action of the bran in cleansing the pelt is however often useful. In sour drenching the bran is allowed to steep and soften in cold water for many hours, and boiling water is then added till the temperature is raised to 75° C., and it is allowed to infuse with frequent stirring for some hours, and after cooling to 45° a considerable quantity of old drench-liquor is added as a ferment. If the drench is used warm (30°-35°, or, in cold weather, even 40° C.), the skins only remain in 1-3 hours, but if cold the drenching can be extended over a period of 2-3 days, the skins being frequently handled. This modification is suitable for glacé-kid and the harder sorts of skins, but glove-lamb are always treated by the warm and rapid process. What the writer in the ‘Gerber’ describes as the “alcoholic” bran-drench is probably the method of fermentation investigated by Mr. Wood, in which ordinary inflammable gases, but no alcohol, are produced.
[99] Gerber, 1888, p. 257.
A normal drench plumps the goods slightly, but if it contains much of the putrid ferments carried in from the bate or puer the skins fall in it as they would do in a bate. To increase this effect, putrid soak-liquor is sometimes added to the drench, but with doubtful advantage.
In drench-liquors the total acidity may be determined by titration with lime-water or N⁄10 caustic soda, with phenolphthalein as indicator; and the volatile acids may be distilled off as described under the analysis of tanning liquors (L.I.L.B., p. 126). For more complete methods of analysis the reader is referred to Messrs. Wood and Willcox’s paper on the “Nature of Bran Fermentation.”[100]
[100] Journ. Soc. Chem. Ind., 1893, p. 422.
Drenches are said to “work” somewhat better if made with water containing nitrates, and this is quite probable; but the necessary nitrogen can easily be supplied if required by the addition of a very small quantity of saltpetre.