Chapter XIII: Deliming, Bating, Puering and Drenching (2)

Wood attributes the differences in action between dog-dung and bird-dung not only to different bacteria, but to the fact that in the latter case the urinary products, and especially uric acid are contained in the dung.

From the results of these and similar researches, Wood in England, and Popp and Becker in Germany succeeded in producing a practical artificial puer, which they now manufacture in conjunction under the name of “Erodin.”

“Erodin” consists of a solid nutrient medium and a liquid “pure culture” of the bacteria necessary to effect the required bating or puering.

The following are the directions for working with erodin bate, as supplied by the manufacturers:--

“For 100 lb. of wet skin washed ready for bating, about 1 lb. of erodin is required. Or in the metric system, 1 kilo. wet skin requires about 10 grm. erodin. The strength or concentration of the bate must not fall below 3 grm. per litre of bate liquor, i.e. ¹⁄₂ oz. per gallon.

For preparing the bate a sufficiently large cask or tub carefully _cleaned_ and steamed out is placed near the bating paddle. The cask should be fitted with a steam pipe easily screwed on and off, and also furnished with a _clean_ cover.

The requisite quantity of erodin is weighed out and put into the tub with fifty times its weight of water, and the whole brought up to a temperature reaching but not exceeding 40° C. (104° F.) by direct admission of steam, thoroughly stirred, and the pure culture of _Bacillus erodiens_ added to the mixture. The temperature must not be allowed to fall below 25°C. (87° F.), and a little steam should be admitted first thing in the morning, again at noon, and in the evening, to bring the temperature up to 40° C. (104° F.).

A practical mode of procedure is as follows:--On Friday make up and start fermenting twice as much erodin as will be required for a day’s work. This is allowed to remain under the above-mentioned conditions until Monday. On Monday half the amount will be used for bating; this is replaced by an equivalent amount of fresh erodin powder, dissolved in fifty times its weight of water, which is added to the already fermented erodin in the tub. Proceed in this way each day until the following Friday, when there will be left in the tub sufficient erodin for one day. This is put into a smaller tub for use on Saturday, and the cycle of operation begun again.

One pure culture of _Bacillus erodiens_ should be used for every 11 lb. (5 kilos.) erodin powder or less quantity.

Suppose the amount of erodin required for a day’s work to be 11 lb. (5 kilos.), then on Friday 22 lb. (10 kilos.) erodin must be mashed as above described in 110 galls. (500 litres) water, 2 pure cultures added, and allowed to ferment until Monday.

On Monday half of this is used, and to the remainder 11 lb. (5 kilos.) erodin and 55 galls. (250 litres) water is added. This is repeated on Tuesday, Wednesday and Thursday; and on Friday half is used and the remainder put into a separate cask for use on Saturday, and in the mashing cask a fresh quantity of 22 lb. (10 kilos.) erodin with 110 galls. (500 litres) water is made up for use next week.[104]

[104] Mr. Wood has found that in many cases it is unnecessary to start
afresh at the end of each week, but that additional quantities of
erodin solution with the accompanying bacterial culture may be added
continuously to the stock-tub as required. In puering, the
concentrated solution from the tub may be diluted with 4 to 6 times
its volume of warm water. The diluted liquor should usually only be
used for one pack of skins.

On Saturday the remainder of the old mash is used up.

In case this mode of procedure is for any reason not suited to the conditions of work, erodin may be used by making up every day a fresh quantity with fifty times its weight of water, adding the pure culture, and allowing it to ferment three days before use.” In some cases the solution may be used for several consecutive packs, merely adding water and a small quantity of erodin without a new culture.

Erodin is being used most successfully in several large works both in England and abroad, and on calf-skins and sheep-skins has proved quite as effective and much safer than dog-dung; the skins coming out clean and free from stains. It has been a good deal used in the experimental tannery of the Yorkshire College, and has proved a satisfactory substitute for puer, but with the present bacterial cultures can only be employed warm, and does not answer used cold like the ordinary pigeon-dung bate. No doubt a suitable bacterial medium and culture can be found for cold bating, which for thicker leathers is often preferable to puering, and experiments in this direction are being undertaken.

From the multiplicity of germs present, and the adaptability of the dung infusion as a nutrient medium for any putrefactive organisms which may gain access to it, the bating and puering process is necessarily a dangerous one for the goods, always leading to loss of weight, and, if the process is carried on too long, to the more or less complete destruction of the skins. Loss of weight, however, in greater or lesser degree is inevitable, and indeed necessary where a soft leather is to be produced. If the skins are allowed to lie in the bate or puer liquor, mud, containing organisms, and zooglœa-forms of bacteria settle in the folds, and produce marbled markings, streaks and lines by the destruction of the grain surface (hyaline layer). Black or bluish stains are also often produced, known as bate-stains, and either due to bacterial pigments, or in some cases, to the action of evolved hydrogen sulphide on iron present from salting or other sources. Frequent change of position is therefore necessary, especially when the liquor is active from being used at a high temperature, but it does not seem to be desirable to keep the skins in constant motion, and if puering is done in a paddle, it should only be run at intervals.

T. Palmer[105] determined in experiments on pigeon-dung bates that there is considerable loss of nitrogen during the process, and recommended bating in pits from which the air was excluded as much as possible, both as effecting a considerable economy in the dung, and in excluding false ferments, which, he concludes, are mostly aerobic. It is not improbable that the method is advantageous, since it has been shown by Roscoe and Scudder that liquefaction of gelatin only takes place in presence of oxygen, and its partial exclusion would therefore lessen the risk of overbating, and consequent damage and loss of weight.

[105] Leather Trade Circular, 22nd Sept., 1891; 1887, p. 667; and
Sanford, Journ. Soc. Chem. Ind., 1893, p. 530.

Starting from the presumption that bating and puering are, in the main, bacterial processes, more or less successful attempts had been made previous to those of Wood, Popp and Becker, to substitute other fermenting substances for dung; and probably these efforts failed in many cases, not so much because they were wrong in principle, as from want of knowledge of the necessary details, such as the use of proper ferments, and the provision of suitable culture-media. Guano, prepared horse-flesh, urine, yeast, and fermenting vegetables have all been tried. A solution of glucose or treacle of about 10 per cent., to which 3 per cent. of pasty dog-puer is added about a week before use, was tried many years since in a morocco-factory, at the suggestion of the writer, as at least a partial substitute for puer, and is still in use there. The mixture keeps for some time in an active state, and is added to the puer liquors in the same way and in approximately the same proportions as the dung paste. Similar in principle is the solid bate supplied by an American firm, in which glucose is mixed with a small amount of nitrogenous matter and phosphates, together with a lactic ferment, and which only requires dissolving in warm water some little time before use. Its results are good for some purposes, but rather resemble those of a drench than a bate. In a similar way, puer may be added to bran-drench liquors, and induces in them a fermentation which brings the skins down much lower than the ordinary drench. It is probable that a weak glucose solution, with traces of mineral constituents similar to Cohn’s solution (see L.I.L.B., p. 269) and “set” with sour milk, or fermenting drench-liquor, might in some cases be used with advantage for drenching, with a saving of cost. A writer in ‘Hide and Leather’ describes a bate in which two parts by weight of glucose are dissolved in about 25 parts of water, and fermented, for about three days, till a foam gathers on the top, with about one part of old bran drench-liquor, or 0·1 part of pressed yeast, and then made up with water to 1000 parts. The goods are bated 24-36 hours at a temperature of about 35° C, and the bate is strengthened for a second pack with about one-fifth of the original glucose, a new bate being made at the end of a week, and set with one part per thousand of the old one. A short bating of say 10 hours produced very nice harness-leather, but the general tendency was to make the goods looser and more spongy than a dung-bate. It is obviously not a matter of indifference whether old drench, or yeast, is used to start the fermentation, since in the latter case only alcohol could be produced directly by the ferment introduced, though this might be fermented later, by other accidental organisms, into acetic acid. These mixed bates, containing glucose, are however probably wrong in principle, since the true puering and bating bacteria will not thrive in presence of acids, and require nitrogenous nutriment.

As regards the relative effect of dog- and hen- or pigeon-dung bates, the chief of the published experiments are those made by W. J. Salomon at the Vienna Versuchsanstalt für Lederindustrie,[106] in which he determined the relative solvent power of equal quantities as being, for dog-dung 2¹⁄₂, for pigeon-dung 2, and for hen-dung 1. It is obvious that these figures, though interesting, must be taken with some reserve, as the composition even of pure dungs is by no means constant, depending on the feeding of the animals, and adulteration is common. The writer has heard stories of a certain dealer who used to fabricate his product from clay by the aid of a popgun, though he does not vouch for the statement! It is generally held that the action of bird-dung is more penetrating, but less softening and loosening than that of dog-dung, which is thus generally used for descriptions of leather where great softness and stretch are required. It is to be remembered in this connection that bird-dung bates are generally used cold, and hence are much slower in their action, which allows them time to penetrate thicker hides more uniformly. Few analyses of the dungs used in leather manufacture have been published, and these mostly with a view to manurial value. Schulze[107] gives the result of forty analyses of pigeon-dung as follows:--

Min. Max. Mean.
per cent. per cent. per cent.
Water 3·80 40·00 21·00
Nitrogen 1·47 5·04 2·53
Phosphoric acid 1·00 2·77 1·79
Potash 0·71 2·57 1·46

One sample contained 43·3 per cent. of sand!

[106] Tech. Quart., 1892, v. p. 81.

[107] Der Landwirt, 1895, li. p. 301.

Wood[108] quotes the following:--

_Hen-Dung._
Per cent.
Water 60·88
Organic matter[109] 19·22
Phosphates 4·47
Calcium carbonate and sulphate 7·85
Alkaline salts 1·09
Silica and sand 6·69

_Dog-Dung._
Water 31·0
Ca 43·0
Na, K, Mg 0·8
PO₄ 3·4
CO₂ 7·5
Organic matter 14·2
Traces Fe, Cl, Si, loss 0·1

[108] Journ. Soc. Chem. Ind., 1894, p 220.

[109] Containing nitrogen equal to 0·74 per cent. of ammonia.

This was apparently a sample from a dog fed on bones; that from the kennels, which is more commonly used in leather manufacture, contains much less lime; a sample analysed by Wood gave 4·7 per cent. mineral matter, 9·7 per cent. organic, and 85·6 per cent. of water, part of which was no doubt added.

_Analysis._--Little or no attention has been paid to the analysis either of dungs for bating purposes, or of the bating liquors, and although the total cost of manure bates is a high one, it is evident that such low- priced and irregular articles will not pay for elaborate analysis. Probably in some cases it would be worth while to make a determination of moisture and organic and mineral constituents by drying and ignition. Where a further investigation is desired, the determination of the soluble matter by filtering and evaporating a portion of the solution to dryness, and that of the nitrogen by Kjeldahl’s method (see p. 70), would be advisable, and of course in the future, when the subject is better understood, a bacteriological examination may be useful. If it is desired to estimate the solution of hide-substance in the use of bate or drench liquors, the determination of the nitrogen in a measured quantity by Kjeldahl’s method will afford the best basis of calculation, allowance being made for the nitrogen present in the original bate liquor. Hide-substance contains about 17·8 per cent. of nitrogen. In many cases, simple weighing of the solid residue, left on evaporating the liquor to dryness and drying for several hours at 100° C., with subsequent ignition to determine lime and other mineral matters, will suffice.

The quantity of hen- or pigeon-dung used in bating hides is very variable, but may be stated at from 12 to 60 litres per 1000 kilos of raw hide, in at least 2000 litres of water. The bate is generally used cold, the hides remaining in it 4-8 days, with frequent handling; but some tanners, especially in the United States, prefer bating in a paddle or drum at a temperature of about 35° C., in which case the time must be diminished to a few hours. The dung is best infused with warm water in a separate vessel,[110] and allowed to ferment for at least a week without use, when it will be found to swarm with micrococcus-chains. Only the clear liquor should be run into the bate-pit, the sediment and dirt being thrown away, or used as manure. In this way the danger of stains and flaking is much reduced. Bates may be mended with fresh portions of dung-infusion for several successive packs of hides, but should not be used too long, as they gain in solvent power by the dissolved hide-substance and the increased fermentation, and the method is not without risk.

[110] This seems to have been first suggested by T. Palmer, Eng. Pat.
13,636, 1886.

After bating, the hides are usually “worked” (“scudded,” “fine-haired”) on the beam, to remove dirt and grease, but in America a wash in the wash-wheel is often considered sufficient. Goods are occasionally “stocked” (p. 116) from the bates, but this is not to be recommended, as it is likely to drive out much of the partially dissolved hide-substance and produce undue looseness and loss of weight.

It is difficult to give any definite marks of sufficient bating other than the soft and fallen feel of the hides, which is easily recognised by a practised hand. One of the earliest signs of commencing overbating is the occurrence of bluish patches, or a bluish tinge somewhat similar to an iron-stain, which, if slight, generally disappears in a few days after the hides are taken into the liquors. Hen- and pigeon-dung is probably best kept air-dried, though, if very wet, or for convenience for immediate use, it may be kept in paste like dog-dung.

Dog-dung should never be allowed to lie exposed to the air, or it putrefies and turns black, the bating ingredients are destroyed, and it will not puer the goods which turn black and putrid without softening. Dung should, therefore, be mixed to a paste with water and kept in tanks, so as to be but little exposed to the air, when it will retain its puering properties for a long time unaffected. Fresh dung should be allowed to ferment for at least a week before use. No accurate statement can be made as to the quantities required. Eitner states that 1-1¹⁄₂ pails of dung-paste (say 14-20 litres) is sufficient for 200 medium to large lamb-skins for glove-kid. It should be sufficient to make the water quite turbid, but not thick or soupy. For lamb-skins a temperature of 18°-20° C. is suitable, which may be raised in very cold weather to 25° C., to allow for cooling. The time required is from two hours for the thinnest slink skins, to 12-14 hours for strong ones. It is well to use wooden, and not iron, utensils for handling the dung, and it should be strained through a coarse cloth after diluting with water. As has been remarked, it is not desirable to keep the skins in constant motion in the puer; they should be stirred or paddled for the first 20-30 minutes, and then for 10 minutes every hour for five or six hours, after which they can be allowed to lie for a longer period without injury. Puering is sufficient when the skins feel quite soft and flaccid, hanging in folds in any direction and allowing the flesh to be scraped off with the finger-nail.

Wood recommends that, for the puering of sheep-skins, dung should be allowed to ferment one month before use, and states that it deteriorates if kept over three months. The puering products are the result of the successive action of many sorts of bacteria, and Wood is of opinion that those actually concerned in puering originate from the air, or from the vessels in which the dung is stored, and are not present in it when excreted. Borgman[111] advises that the dung should be kept in a dry condition, and only made into a paste between a fortnight and three weeks before use, by covering in a clean cask with cold water, and on the following day mixing to a smooth paste with a clean wooden “poss-stick,” made from wood free from tannin. The cask should then be covered up, and allowed to rest undisturbed till required. Clean extract-casks are very suitable for the purpose, if carefully and repeatedly steamed out, and Borgman advises that a regular series should be arranged, so as to supply the dung required, the date of mixing being carefully marked on each cask. Throughout the process the utmost cleanliness should be observed, and the casks should be carefully steamed out as soon as emptied. Immediately before use the dung-paste should be heated by steaming nearly but not quite to boiling point, care being taken to avoid the introduction of condensed water containing iron, and the dung thoroughly mixed with a large quantity (say 100 gallons) of water at 45°-50° C., allowed to settle, and drawn off through a basket, and strained into the puering paddle through a second basket lined with coarse open canvas (such as is used by plasterers to cover windows while the plaster is drying). A further quantity of warm water should be poured on the residue in the mixing tub, and used for diluting that in the paddle to the proper volume. The temperature of the liquor may reach 42° C. before the skins are introduced. The liquor should be of a light colour, greenish to brownish yellow; if darker, it indicates decomposition of the dung by improper storing, or too long fermentation, and will be liable to cause staining and injury to the skins. About 33 liters of dry dung is required per 100 kilos. of wet skin prepared for puering (33 gallons per 1000 lb.). Dry dung should be of yellow to brown colour, dark brown or black dung is spoiled and unsuitable for use. Wet dung is more difficult to judge, but very dark brown or black should be rejected, as well as that with a very strong smell, indicating that it has already fermented. Borgman’s directions bear the stamp of experience and common sense, and the book as a whole repays study.

[111] ‘Die Feinleder-Fabrikation,’ Berlin, 1901, p. 69.

Borgman recommends that the skins should be warmed by paddling for some time in water of about 40° C. to which a couple of pails of puer-paste have been added, before bringing them into the puer, the temperature of which they should reduce to perhaps 38° C. The puered skins should feel silky on the grain, and even somewhat slippery, and when pressed between the finger and thumb a dark impress should be left, and the flesh should be tender and easily scraped off. The requisite condition will, however, vary somewhat with the kind of skins, and the purpose for which they are intended. After puering, the skins may be paddled for half an hour in water of about the same temperature as the puer.