Several acids of the aromatic series have been from time to time recommended as deliming agents, and generally possess the merit of acting at the same time as powerful antiseptics. In this connection it may be well to mention the solution of 1 per cent. of phenol and 2 per cent. of boric acid used by Dr. Parker and the writer for preparing and preserving skins for colour tests (L.I.L.B., p. 133). This answers very well as a bate even when much diluted, and may be rendered cheap enough for use in practice by the employment of a good commercial carbolic acid instead of pure phenol, and the use of sulphuric acid to remove lime from the solution and render it capable of repeated employment. The carbolic acid should not be too dark in colour, and should be carefully dissolved, or “carbolic” stains will result.

“Cresotinic acid,” a mixture of impure acids obtained from cresols in the same way as salicylic acid is manufactured from pure phenol, was introduced as a bate and unhairing and deliming agent by J. Hauff, of Feuerbach.[91] He also claims the use of hydrochloric acid to liberate the acid after it has been combined with lime in the deliming process. It is only soluble to the extent of about 1 in 800 of water, so that, even if used in excess, no dangerously strong solution is formed, but it has a tendency to slightly swell, and somewhat harden, the hides or skins, so that it is perhaps more suitable for sole than dressing leather. It has also powerful disinfectant properties (see p. 29).[92]

Hauff states that a solution of 18 lb. of cresotinic acid in 500 gallons of water at 30° C. will bate one lot of 50 heavy hides, and that the same liquor may be used continuously, by adding 4-5 lb. more cresotinic acid for each successive 50 hides. For bating glove-leather, Hauff recommends the use of 5 kilos. cresotinic acid dissolved in 1000 liters of warm water for every 500 kilos. of wet skins, to which is added ammonia nearly sufficient to neutralise the cresotinic acid, leaving the solution still slightly acid to litmus paper; and he also advises the addition of 5 kilos. of ammonium chloride or sulphate. The goods are paddled in this solution for about half an hour.

“Oxynaphthoic acid,” the corresponding mixed acids of the naphthols (p. 30), has also been patented by Hauff as a bate, since cresotinic acid sometimes acts too powerfully on light skins.[93] He mentions that mixtures of this and cresotinic acid, or salicylic acid, may also be used. Oxynaphthoic acid requires for its solution 20,000 to 30,000 parts of water.

A mixture of the α and β mono- and di-sulphonic acids of naphthalene has also been patented for bating,[94] under the name of “Acrilene bating and puering acid.” 150 calf-skins, weighing 880 lb., were pured in a 3 per cent. solution of the α acid, and gave 266 lb. of leather as against 255 lb. from a lot of similar weight treated with hen-dung, and this gain was more than maintained on stuffing, while the shoulders were plumper and fuller. This patent appears to anticipate a part of Hauff’s claim mentioned in the next paragraph.

More recently Hauff has patented, under the name of “anticalcium,” a mixture of impure sulphonic acids of various cresols and hydrocarbons. This is cheaper than cresotinic acid, and like it, possesses considerable antiseptic powers. One-half to one-quarter per cent. solution will keep hides uninjured for a considerable time, but at this strength it plumps considerably, and seems more suitable as a deliming agent for sole-leather than as a bate for dressing-leather, though it may replace drenching. No doubt, by the use of warm water, and avoidance of excess of acid, skins could be pulled down satisfactorily, or the plumping could be controlled by addition of salt, but the disinfectant powers of the acid would render further treatment with an ordinary bate or puer very difficult.[95]

The “C. T. Bate,” manufactured by the Martin Dennis Chrome Company, is of a very similar character; and is in the form of a greyish crystalline paste, consisting mainly of sulphonic acids of naphthalene and probably other hydrocarbons. It is very possibly made by sulphonating coal-creasote oils, which contain much naphthalene and phenanthrene. The following directions are given by the company for its use.

“1. After unhairing and fleshing from the lime, the skins should be thoroughly washed with water (preferably warm) so as to remove as much lime as possible.

2. If, in the liming process, the sulphide of sodium is used in combination with the lime, it will render the lime more soluble and therefore more easily removed with water.

3. The more completely the skins are cleansed with warm water the less will be the quantity of bate required.

4. After washing, the skins should be thoroughly worked on the beam, especially on the grain.

5. A solution of C. T. Bate is now prepared in the proportion of from one-half pound to one pound of bate in 100 gallons of warm water (90° F.). In making the solution do not have the water over 140° F. Under no circumstances boil it.

6. If the hides or skins have been treated as above indicated, one pound of bate should be sufficient for 400 pounds wet hide, washed from the limes. The hides or skins are placed in the bating solution and worked for an hour. They are then allowed to rest in the solution with occasional stirring for some hours or over night.

7. The length of time that the bating should continue will depend upon the degree of softness and pliability required in the leather. For instance, for sole-leather fifteen minutes is sufficient; for satin leather thirty minutes; for glove-leathers four to six hours or even longer.

8. On removing the skins from the bating solution it is sometimes desirable, especially for the finer grades of leather, to wash them in warm water and again work them over the beam. They are then ready to be placed in the tanning liquors.

9. In preparing the bating solution for the second pack, draw down the old solution one-third and replace with fresh water; then add in solution just one-half the quantity of bate used at first, and so on with each succeeding pack.

10. When fresh white limes are used toward the end of the liming process, and a manure bate is deemed necessary to reduce the harshness of grain caused by the fresh lime, it is very beneficial to give the skins from the manure bate a drench of C. T. Bate, thereby arresting the bacterial action of the manure bate, preserving the grain, besides cleansing, bleaching and neutralising the skins preparatory to placing them in the tanning liquors.

11. Again, when it is considered desirable to use a manure bate, it is good practice to treat the skins as above indicated (down to item No. 7), and then place them in the manure bate. By this previous treatment the antiseptic action of the C. T. Bate tends to arrest the destructive bacterial action of the manure bate, thereby lessening the risk of damage to the grain. In all cases where the value of the leather is dependent on the quality and perfection of the grain, this is an important advantage to gain.”

All these coal tar “bates” are rather suitable to replace drenching than bating or puering, as their effect is mainly that of removing lime. From their antiseptic character they are very useful in stopping the effects of putrefaction, and preventing ferments being carried into the tanning liquors, and skins may safely be kept at least for some days in weak solutions, but any necessary fermentive puering or bating should usually be done before and not after their use.

A writer in the ‘Gerber,’ 1875, p. 279, recommends the use of dilute solution of sulphide of sodium as a bating agent. Possibly it removes lime as sulphydrate, and the writer named seems to have obtained good results with glove lamb-skins. In experiments made at the Yorkshire College, a solution of 4 grm. per litre used on 40 grm. of pelt was found to plump it considerably, but probably a much weaker solution might be sufficient and more satisfactory. Polysulphides, such as “liver of sulphur,” or the yellow solution obtained by boiling dilute sodium sulphide or sodium hydrate solution with excess of sulphur, have great power of “bringing down” the pelt, and seem well worthy of experiment as bating agents.

In India, the pods of the babool (Acacia arabica) are much used as a bate, the infusion being allowed to ferment. In their dry state they contain about 12 per cent. of an easily changeable tannin, which does not precipitate lime-water, and which by fermentation is very probably converted into gallic acid. The use of gallic acid itself as a bate has been patented by Albert Hull,[96] and would undoubtedly accomplish the removal of the lime if used in sufficient quantity; but as he only uses a solution of 25 mgr. per litre (one part in 40,000) any effect must be mainly due to the washing with water. Gallic acid forms dark oxidation products with lime.

Of the fermentive methods of removing lime, “drenching” with fermenting bran-infusions is the simplest in theory, and has been very carefully investigated by Mr. J. T. Wood.[97] It will, therefore, be convenient to consider this process first, although it is frequently employed as a means of cleansing and slightly plumping the skin after the lime has been removed by puering or bating. In calf-kid manufacture, however, it is now used without previous puering, and in some other cases it is substituted for the use of dung bates. The most important of the active ferments are two species of bacteria, named by Wood Bacterium furfuris α and β, which are very similar in their form and action (see L.I.L.B., p. 264), but produce a somewhat better fermentation together than separately. They are shown in Figs. 34 and 35.

Neither species has any direct action on the hide substance, but ferments the glucose produced by the action of the cerealin of the bran on the starch which is present. A considerable quantity of hydrogen, with carbon dioxide, nitrogen and small quantities of hydrogen sulphide, are produced during the fermentation, together with lactic and acetic, and traces of formic and butyric acids and amines. Active drenches contain 1-3 grm. of mixed acids per liter, to which they owe their action, a perfectly satisfactory drenching being produced by an artificial drench containing 0·5 grm. of glacial acetic acid and 1 grm. of lactic acid (sp. gr. 1·210) per liter in which the skins were worked for 1¹⁄₂-2 hours, while 12-16 hours would have been required in the ordinary drench. An experimental drench gave the following results on analysis:—

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.

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.

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]

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.

Wood is of the opinion that the ferments found in bran do not originate in the drench itself, but come from the bated skins, as the drench-bacteria soon die out without finishing the fermentation, and constant renewing of the nutrient material is necessary (cp. p. 18).

Bating and puering, though differing practically in many ways, are identical in theory, and most of what follows applies to both of them. The action is much more complex than that of the drench, involving both chemical reactions and those of organised and unorganised ferments, and it is a matter of no little difficulty to say what proportion of the observed effect should be ascribed to each of these agencies.

Formerly, the principal effect was attributed to organic salts of ammonia and its homologues, and to amido-acids which combine with lime. Phosphoric acid is also present, and if any exists in the form of soluble salts, it will combine with lime, and render it insoluble and inactive. It is probable, however, that most if not all the phosphoric acid is already in the form of tricalcium phosphate, and therefore without effect.

It is now, however, recognised that the effects of these chemicals are of no importance as compared with the products of bacterial action, and the researches of J. T. Wood have cleared up much that was until recently quite inexplicable.[101]

Much effect has been ascribed to the digestive ferments, such as pepsin and trypsin, which are present in fresh dung. It is known that the animal organism secretes these in considerable excess of its requirements, but it is doubtful whether any exist undecomposed, even in fresh dung; though they are apparently more resistant to putrefaction and decomposition than would a priori have been expected of such complex organic compounds, and there is therefore a possibility of their existence in the dung, even as it comes to be used in the tannery. Both pepsin and trypsin are enzymes (see p. 16), and belong to the great class of albuminoids. They are soluble in water, but insoluble in alcohol, and hence are precipitated by the addition of the latter to their solution, but are not altered by it, and regain their activity on solution in water. By heat they are coagulated and decomposed, and their activity permanently destroyed.

Pepsin is the active principle of the secretion of the glands of the stomach, and large quantities are prepared for medical use as an aid to digestion from the stomachs of pigs. Pepsin only acts in slightly acid solution, and, though fresh bate liquor is slightly acid to litmus, it speedily becomes alkaline from the lime of the skins and the ammonia present, so that the action of pepsin in a bate can only be a very limited one. Wood[102] compared the action of a 1 per cent. solution of pepsin, acidified with 0·2 per cent. of hydrochloric acid, with that of a dogs’ dung puer liquor, both at the temperature of 40° C. At the end of one hour the skin in the pepsin-solution was considerably fallen, but that in the puer-solution was almost dissolved. Since the solution here employed was much stronger than is likely to occur in practice, and the conditions much more favourable to its action, it may be assumed that the practical effect of traces of pepsin in the bate may be neglected.

Trypsin or pancreatin[103] if present, is more likely to have an effect, since it is active in neutral and in alkaline solutions. It is the product of the pancreas, and is largely concerned in intestinal digestion. Chemically it much resembles pepsin, but is more resistant to heat, retaining its power of digestion after heating to a temperature of 160° C. in a dry condition. Its warmed solution dissolves fibrin almost instantly, and in large quantity, and peptonises gelatin and hide-fibre, so as to render them soluble in water. Wood found that a 1 per cent. solution of pancreatin acted far more rapidly than a solution of pepsin of equal strength. At 40° C. in neutral solution, the skin fell rapidly, and the action continued even in the cold. In 15 hours the liquid was swarming with minute bacteria. At the suggestion of the Author, the experiment was therefore repeated, with the addition of 15 per cent. of chloroform, which prevented the development of bacteria, while it did not stop the action of the pancreatin. The skin fell as before, but in neither case had it the peculiar touch of puered skin, nor were the characteristics of the leather produced from it the same. We may therefore conclude that, though trypsin may contribute to the action of the bate or puer, it can only do so in a minor degree, and that the principal effect of the bate or puer is due to other causes. It is certain, however, that fresh bird-dung, and probably that of all animals, contains ferments capable of liquefying gelatin. An instance of this is found in the observation, common in glue manufacture, that if the dropping of a sparrow falls on a cooler full of solidified gelatine size, it will liquefy a track quite down to the bottom of the cooler. Trypsin, or at least the secretion of the pancreas, as well as the gall from the liver, have great power of wetting and emulsifying fats, and this has possibly something to do with the action of the bate in enabling the skins to be cleansed of fat.

Bacterial fermentation and its products are however the main factor in the action of puers and bates, and on this subject we owe most of our knowledge to the work of J. T. Wood, since, though Popp and Becker have worked over much of the same ground, they have not nearly so freely published their results.

Wood showed that a fresh puer liquor, even when boiled for half an hour and so freed from living organisms and albuminoid ferments, has still considerable action on a limed skin, though much less than the unboiled puer. He found that this action was principally due to amines and their compounds with organic acids, which removed lime, but did not remove the interfibrillary substance or give the proper feel of puered skin. A very similar result was obtained with aniline (phenyl-amine) hydrochloride in 1 per cent. solution.

A considerable variety of bacteria from dung and other sources were cultivated in various media and their puering power tested, but though greater than that of the unorganised chemical compounds such as amine salts and organic acids, it was in no case equal to that of an ordinary puer, or sufficient for practical use. When, however, a small quantity of the amine salts obtained from the puer were added to a mixed bacterial culture the effect on the skin was almost as rapid and considerable as with an actual puer.

In order to determine whether the puering effect was due to the direct action of the bacteria or to their enzyme-products, the latter were separated from a filtered puer solution by adding it to a large volume of 98 per cent. alcohol in which the enzymes are insoluble. When redissolved in water, they had a decided puering effect, and a solution of 0·5 grm. of the mixed enzymes and 0·5 grm. of the mixed amine hydrochlorides in 100 c.c. of water at 350° C. brought down a piece of limed sheep-skin in thirty minutes exactly like a puer. The action is therefore dependent on the mutual action of the enzymes and amine salts, but as the separation of these would be too costly for practical use, and the puering proved more effectual when they were formed in contact with the skin by active bacteria, Wood adopted the method of preparing a suitable sterilised nutritive liquid, which was inoculated before use with a mixed culture of suitable bacteria. For laboratory purposes a suitable culture-medium was obtained by digesting 10 grm. of gelatine with 5 grm. of lactic acid (reckoned water-free) and 100 c.c. of water for three hours in a closed vessel on the water-bath. The resultant solution was neutralised with sodium carbonate and diluted to 1 litre with addition of a small quantity of potassium phosphate.

The bacteria of fresh dog-dung were not found to possess a satisfactory puering effect, but those from dung which had been fermented a month (as in practice) gave a result nearly equal to actual puer. A still better result was obtained by a mixed culture from the roots of wool loosened by sweating. The bacteria were principally of two species, of which neither separately was capable of satisfactory puering; but which together acted more rapidly than an actual puer. These bacteria do not liquefy gelatine.

During the course of his experiments, Wood found that filtered puer solutions were less active than turbid ones and that their activity was increased even by the addition of inert substances, such as kaolin.

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]

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.

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:—

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