THE PUERING AND BATING PROCESS.
“Beizen sind Stoffe die mit dem Kalk nicht nur ein chemische Verbindung einzugehen im Stande sind, wodurch derselbe löslich und somit vollständig unschädlich wird, sondern auch den gegenseitigen festen Schluss der einzelnen Wandungen der Zellenelemente mechanisch lockern, den ganzen Bau der Haut in sich nachgiebiger machen und so die Verschiebbarkeit der einzelnen Hautgebilde erhöhen.”—J. C. H. Lietzmann, 1862.
“Bates are materials which are not only able to enter into chemical combinations with the lime, whereby it becomes soluble, and is thereby rendered completely harmless, but they also mechanically loosen the cementing substance of the separate cell elements, and render the whole structure of the hide more pliable, and thus increase the mobility of the various parts.”
The object of bating or puering is to render the skins, and the resulting leather, soft and supple. Skins which have undergone the liming process, must be thoroughly freed from lime before going into the tan liquors, and, for light and soft leathers, they must be reduced or “brought down,” so that the elasticity or resilience of the skin fibres is got rid of, and the skin, when tanned, can be stretched without springing back. This is usually done in the case of light leathers, by passing the skins through a bate or puer, composed of an infusion of dogs’ dung in water at a temperature of 35° to 40° C., until the required result is obtained. This condition is known to the workman by the feel of the skin. A good indication is, that the skin when “down” retains the impression of the thumb and finger if squeezed. A properly puered skin, when dropped on the floor, will also be perfectly flaccid, the folds lying closely together. It may, however, be said that it is only by experience and a kind of instinct that the exactly correct condition of the skin can be judged.
I know of no very early works on leather manufacture giving an account of bating. It was a “secret process,” and the results obtained depended almost entirely upon the judgment of the operator; and this judgment was frequently in error, owing to the fact that he did not understand what took place in the bate.
The earliest account I have been able to find is in a book[5] in the possession of Mr. Seymour-Jones, of Wrexham, entitled “The Art of Tanning and Currying Leather, with an Account of all the Different Processes made use of in Europe and Asia for Dying Leather Red and Yellow, Collected and Published at the Expense of the Dublin Society, to which are added Mr. Philippo’s Method of Dying the Turkey Leather as approved of by the Society for the Encouragement of Arts, etc., and for which he had a Reward of £100, and their Gold Medal, for the Secret. Also the New Method of Tanning invented by the late David Macbride, M.D., London. Reprinted for J. Nourse, on the Strand, Bookseller to His Majesty, 1780.”
In the chapter entitled “Alumed Calf Skins for Bookbinding” (p. 138), after the limed skins have been fleshed, the writer continues—
“To alum them, put into a large vat, three or four pails of dogs’ turd (this dogs’ turd is called alum); on this they fling a large pail of water to dilute it; this done, the workman goes into the vat, and, with his wooden shoes, tramples it, filling the vat half full of water. The alumer, on his part, pours water out of his boiler into this vat, mixing it with the cold water, after which he flings in the skins, stirring them and turning them for some moments with great sticks.”
The work is described pretty much as now practised, the puer tub being kept at a uniform heat by constantly taking out liquor, heating it, and returning it. But neither these goods, nor morocco, are put through a “drench” after the puer, as we do now; they are scudded on the flesh, and well washed several times in clear water before being tanned.
Morocco Leather (p. 204).—The dry skins are soaked three or four days, “pared” on the beam, and unhaired in weak lime pits one month. At Nicosia they put the skins into lime, reduced to powder, for twenty days in summer, or twenty-five to thirty in winter; out of the lime, the skins are well washed and drained. The author continues—
“The river work finished, the skins are put into the dogs’ confit, or mastering; for every four dozens of skins they add one bucket of dogs’ excrement, containing fourteen or fifteen quarts, which is worked up with their hands into a kind of pap and well diluted. The skins are flung in, stirred and worked in the mastering for some minutes, then turned and left to rest.
“They remain about twelve hours in the mastering, which opens them, and takes off the rawness, disposes them to relax, fill and ferment. This excrement, by its alkaline parts, also cleanses them and takes out the grease, which would hinder them from taking the colour. I shall hereafter speak of bran mastering.
“At Diarbekir, they make use of these masterings in a different way. Whilst the skins are drying, they fill great hollows made in the earth, like our lime pits, with dogs’ dung, which is diluted to the consistence of honey, or of thin pap, in which they soak the skins for eight days in winter, and three in summer, treading them each day with the feet. They are taken out of this fecal matter, and well washed with fresh water, after which another mastering is made with bran diluted with water, in which the skins are soaked six days in winter, and three in summer, observing to tread them each day with the feet, the same as in the dogs’ mastering; they are then taken out, washed in fresh water, and prepared for dyeing.”
From these primitive methods the majority of tanners, even of the present day, have departed but little, except that, instead of stirring the goods and liquor with a stick, a paddle is used, and the process thus shortened considerably. Before describing the operation of puering, it will be best to describe the preliminary washing of the skins, because, although limed skins may be entered direct into the puer and brought down quite satisfactorily, in this case more puer and more time are required, hence it is usual to get rid of the bulk of the lime by washing them in water before putting them into the puer. In some cases very dilute hydrochloric or other acid is used, in order to shorten the time of watering.
It is now well known that lime cannot be entirely removed from skins by washing in water, no matter how long the washing be continued. A limed skin containing 4·6 per cent. CaO, calculated on the dry weight, was found to contain after washing—
Time of washing | Per cent. CaO | Per cent. lime | |
1 | hour | 3·05 | 34 |
2 | hours | 2·20 | 52 |
3 | " | 1·75 | 62 |
6 | " | 1·55 | 66 |
7 | " | 1·55 | 66 |
24 | " | 1·50 | 67 |
Fig. 1.
If the washing be continued, a less amount of lime is removed in each successive period of time, so that it is evident a point is soon reached at which it becomes a waste of time to continue the washing. In practice this point is reached in about two hours. The progress of the washing is best shown by the curve in the diagram (Fig. 1), in which the ordinates represent the percentage of lime (CaO) in the dry skin, and the abscissæ time in hours. It will be seen that the character of the curve is a hyperbola; such a curve only approaches a line (representing in the example chosen about 1 1/2 per cent. of lime) asymptotically—in other words, it is impossible to wash out all the lime except by an infinite number of changes of water, since each washing removes a less amount than the previous one. The above is a typical case of the washing of limed grains from the splitting machine in the manufacture of skivers. These contain from 4 to 5 per cent. of lime on the dry skin, and, after washing in water in a paddle for six hours, analysis shows them to contain still about from 1·5 per cent. to 1·9 per cent. of lime (CaO).[6]
The effect of washing depends on the character of the water (hard or soft), and also on its temperature. Hard waters should have a small quantity of clean lime added to them before entering the goods, in order to remove dissolved CO2, which, by carbonating the lime on the surface of the skins, renders the grain harsh and the subsequent puering difficult.
With regard to temperature, it is preferable to employ cold water until the bulk of the lime is out, since this dissolves more lime than warm water. 100 c.c. of saturated lime water, at 10° C., contains 0·134 grm. CaO. 100 c.c. at 40° C. contains 0·1119 grm. CaO. Moreover, a comparatively small rise of temperature causes a considerable decomposition in a fully limed skin, by which the skin substance is rendered more soluble, and consequently lost for the purposes of the tanner. For a fully limed skin the limit of temperature is about 82° F. (28° C.), whereas a skin free from lime or alkali may be submitted to a temperature of 120° F. (49° C.) without damage to the fibre. Lamb prefers a short washing of half an hour in water at 35° to 38° C., for the reason that the increased temperature causes the goods to become more flaccid, but this condition is attained at the expense of the loss of skin substance we have mentioned.
Fig. 2.—Cubical Truck.
In order to save time and water, the following method is adopted. The goods are measured by means of a cubical truck on wheels (Fig. 2); it holds 250 kilos of wet skins (550 lb.). Four of such trucks are placed in a wash wheel, and a stream of water from a 1-in. pipe turned on. The goods are run from three-quarters of an hour to one hour; the water is then stopped, and 4000 c.c. of commercial hydrochloric acid (18° Bé.) is run in slowly, in a very diluted state, through a perforated lead pipe. After all the acid has run in, the wheel is run for half an hour, then water turned on again for half an hour in order to wash away the calcium chloride produced. During this last washing, hot water is admitted at the back of the wheel through a perforated pipe, in order to raise the temperature of the goods, so that they do not enter the puer wheel in a cold condition, and thus lower the temperature of the puer liquor. The goods are now ready for puering.
The chemical action of the acid is a very simple one, and is expressed by the formula—
Ca(OH)2 + 2HCl = CaCl2 + 2H2O
74 + 73·1 = 110·9 + 36[7]
It will be noted that the quantity of acid used is only about one-tenth of that required to neutralize completely the lime in the skin. If a greater amount of acid be used the skins begin to absorb acid before the lime in the interior is neutralized; but, provided the lime on the surface of the skins is removed, that remaining in the interior is forced out during the course of the bating process (see Chapter [III].). If more acid were used, then it would require to be added so slowly that time would be wasted unnecessarily in doing the work.
With regard to the use of other acids for deliming, Lamb considers formic acid preferable to hydrochloric, and states that the removal of lime is more effectively accomplished by its use. He attributes a pulling down action to the calcium formate produced, so that less formic acid is required than would be equivalent to hydrochloric, merely regarded as a solvent for lime. We shall refer to this point again in Chapter [II]. It may be stated here that the author has obtained better results by the use of a mixture of formic and acetic acid, in equal proportions, than from either acid alone, but that the cost of working is considerably greater than with hydrochloric acid.
Lactic acid is in fairly common use as a deliming agent. In practice 1 per cent. lactic acid (50 per cent. strength) calculated on the wet weight of pelt, is used, added gradually in small quantities. This does not remove the whole of the lime, but sufficient for the skins to puer quickly.
Some tanners measure their acid per dozen skins. In a case which came under my notice, 1200 c.c. 90 per cent. formic acid were being used for ten dozens medium goat skins at a temperature of 35° C., time 1 1/4 hours.
The following table, due to Professor H. R. Procter,[8] gives the cost of dissolving 1 lb. of lime at present approximate prices of the various acids. The dissociation constant K shows their relative “strength;”[9] the equivalent, Eq., the weight in lb. of 100 per cent. acid required to dissolve 28 lb. of lime. Ordinary wet limed hides, unhaired and fleshed, contain only about 4 lb. lime per 1000 lb. wet weight.
Cost of Acids to remove 1 lb. Lime.
Acid | Eq. | K | Cost | Strength | Cost for | |
s. | d. | d. | ||||
| Hydrochloric | 36·5 | say 200 | 3 | 3 | 31·5 | 1·4 |
| Sulphuric | 49·0 | " | 4 | 0 | 95·0 | 0·8 |
| Oxalic | 63·0 | 0·1 | 30 | 4 | 99·0 | 8·1 |
| Formic | 46·0 | 0·0214 | 35 | 0 | 87·4 | 7·0 |
| Lactic | 90·0 | 0·0138 | 26 | 0 | 49·7 | 18·0 |
| Acetic | 60·0 | 0·0018 | 18 | 0 | 40·0 | 10·0 |
| Butyric | 88·0 | 0·00115 | 21 | 0 | 82·8 | 8·0 |
| Boracic | 62·0 | 0·00000001 | 27 | 0 | 99·0 | 6·5 |
It will be seen from the above table that sulphuric is the cheapest acid to use; but, owing to the insoluble nature of the calcium sulphate, it is best to use hydrochloric acid. Procter has suggested using a mixture of sulphuric acid and common salt in molecular proportions, so as to avoid the iron which is generally present as an impurity in commercial hydrochloric acid. The author has tried this, and found it perfectly satisfactory. For a complete discussion of chemical deliming, see Procter’s “Principles of Leather Manufacture,” chapter xiii.
Quite recently butyric acid, which is now being manufactured on a commercial scale by the process of Dr. Effront (Brussels), has come into use for deliming.[10] and is likely to become a useful acid, as it can be manufactured cheaply. According to Parker, it dissolves less skin substance than formic, acetic or lactic acids, which observation is confirmed by Thuau. The quantity required per 100 lb. of ordinary washed sheep skin is about 1/2 lb. of the 80 per cent. acid. It is used in exactly the same way as other acids, i.e. it should be added in small quantities, suitably diluted, at successive intervals of time.
Acid salts are also used for deliming, and Procter suggested sodium bisulphate NaHSO4 (“Principles,” p. 155). More recently, sodium bisulphite (NaHSO3) has been recommended;[11] the skins are tumbled for half an hour with a dilute solution, which combines with the lime according to the following equation—
(1) Ca(OH)2 + NaHSO3 = CaSO3 + NaOH + H2O
if sulphides are present the reaction is—
(2) Na2S + 2NaHSO3 = H2S + 2 Na2SO3
After this treatment the theoretical quantity of hydrochloric acid, diluted with plenty of water, is run in through the axle of the tumbler, and the skins run another half hour. The reactions are—
(1) CaSO3 + 2HCl = CaCl2 + SO2 + H2O
(2) Na2SO3 + 2HCl = 2NaCl + SO2 + H2O
The sulphurous acid evolved in this way slightly swells the skins, and also removes stains due to the alkaline polysulphides; it is also said to remove salt stains.
Fig. 3.—Puer Wheel, with Cover Removed.
Fig. 4.—Elevation of Puer Wheel (back).
A more interesting process, introduced by Dr. Gr. Eberle[12] as a bating process, but which is really a deliming process, consists in using organic acids in the form of their anhydrides, lactones, or lactides. During the process these anhydrides, lactones, or lactides, are gradually decomposed with formation of the free acids, which in a nascent condition unite directly with the lime in the skins. The anhydrides of acetic, propionic, butyric and lactic acid have been tried, the lactone of γ-oxybutyric acid
| CH2 CH2 |  | O |
| CH2 CO |
After washing and deliming, the goods, warmed to a temperature of 90° to 100° F., are now measured in the truck above described, and put as quickly as possible into the bating paddle. This is constructed to carry two trucks. The figures (Figs. [3], 4 and 5) show a modern paddle adapted for puering light skins, such as goat, sheep, splits (grains), either with dung or with an artificial bate.
Fig. 5.—End Elevation of Puer Wheel.
The inside dimensions are: length, 4 ft. 6 in.; width, 4 ft.; depth, 2 ft. 6 in. A copper pipe of two coils is provided at the back, through which steam is passed for maintaining the temperature of the wheel—the goods are prevented from coming in contact with this pipe by a perforated board. Another steam pipe, jointed so that it may be turned in and out of the paddle, is provided for heating up the wheel to the required temperature before beginning the bating. This pipe is not shown in the drawing. The thermometer can be read while the paddle is in motion. A suitable speed to keep the goods in motion is twenty revolutions per minute. The whole wheel is covered in, for the purpose of preserving the heat, and also to exclude light and air, which are prejudicial to the action of the bacteria. Such a paddle, or wheel, has a capacity of 200 gallons liquor, or 900 litres, and will bate 25 dozens of average grains, weighing in the wet state 1100 lb. or 500 kilos.
The dog dung, which is usually obtained from hunting kennels, and of which the composition is given in Chapter [II]., is simply diluted with water in the paddle;[13] five buckets, weighing about 165 lb., are required for the above quantity of goods.[14]
The amount used depends on the state of the goods, the prevailing weather, and, further, on the quality of the puer.[15] When the temperature is maintained at 40° C., ordinary limed goods, which have been washed, come down in from one to three hours.[16]
During puering, the lime soaps in the skin are decomposed, and the fat set free—in an estimation of the fat, 0·151 per cent. on the wet skin was found before puering, and 2·48 per cent. after puering. The fat thus set free can be removed to a great extent by scudding; but in modern practice this is not pushed to extremes, as the grain is liable to be damaged by excessive scudding. The fat is afterwards removed from the dry leather by means of benzine or other solvent, employed in a suitable apparatus.
Fig 6.—Sir John Turney’s Scudding Machine for Sheep Grains.
Scudding.—When the goods are “down,” they are ready for scudding. This operation in the case of grains is performed either by hand over the beam, or by means of a special scudding machine.
The scudding machine figured (Fig. 6) was invented by Sir John Turney in 1880,[17] and is the only type of machine which will safely scud split skins. It will readily be understood that the usual type of scudding machine, in which a spiral knife is used, would tear the delicate structure of the grain. The Turney scudding machine works with hard, circular brushes, which revolve against a wooden roller carrying the skin. At the same time a continuous stream of water, at a temperature of 110° F., is sprayed upon the skins from a perforated pipe.
After scudding, the skins are washed in cold water for about ten minutes. This checks the action of the puer, and also washes away any loose dirt. They are then ready to pass on to the drench (Chapter [IX].), or to the pickling process, or direct to the tan liquor, according to the kind of leather required.
The Bating of Hides for Harness or Dressing Purposes.—The hides intended for harness leather, or for dressing purposes, are usually bated with hen or pigeon manure. The exact process varies in different tanyards, being dependent upon the condition of the pelt prior to bating, as to whether it is bated directly after fleshing or receives a preliminary deliming. The process is also dependent upon the amount of bating action required for the specific purpose prior to tanning, but the following three methods are in actual use, and may be taken as typical methods:—
Harness Backs.—About a hundredweight of hen manure is put into a suitable tub or vat capable of holding about fifty gallons; some thirty gallons of water, at a temperature of about 100° F., is poured on to the manure, and the whole is thoroughly stirred up by means of a wooden stick or plunger, working the manure about in order to mix it to a consistence of thin soup. The vat is now stored in a warm place, so that the temperature is kept about 70° F.—it is customary to store this in the boiler-house, or some similar warm room. It should be stirred two or three times a day, until it begins to show signs of working; two days is generally sufficient for this.
A pit is now prepared, with the necessary amount of water; and, by the aid of a steam pipe, or other similar means, the temperature of the water is raised to 70°, and the contents of the bate tub are now added. This should be poured into the pit through a sack, or a canvas filter, so as to keep back stones, solids, feathers, and other extraneous matters. The pit is now thoroughly plunged, and the goods entered; these should be handled up three to four times a day for two days. At the end of two days the goods are usually scudded, by working the back on an ordinary unhairing beam with an unhairing knife. They then go into a fresh pit containing a new solution of bate made up in the same manner, and, when sufficiently bated, which generally takes about three days, the goods are removed from the pit, scudded again by working them over with a slate scudding tool, when they are ready for tanning. Some tanners give them a bath of boracic acid prior to their going into the tanning liquor; this has the effect of materially brightening the colour. The second pack of harness backs go into the same liquor—which must be re-warmed to a temperature of 70°—in which they are handled as before for about two days, and are finished off in a new liquor made as above described.
It will be understood, therefore, that in a tanyard, working regularly, the pack of backs go first into an old bate for two days, and are then finished off in a freshly made one; so that each pack gets two days in an old pit, and one or more days in the new. The bate in the tub may also have a second water, or be strengthened.
Dressing Hides.—For these goods a somewhat stronger bating effect is required, and pigeon manure is therefore frequently used. The preparation of the bating liquor is as described above, both as regards quantity and temperature of the water and time of maceration.
The whole process may be carried out exactly as described for harness backs, but the following is an illustration of another method in practical use. Imagine four ordinary bating pits, worked in a round. The first pit is very old, having had three packs of hides through it; the second having had two packs, and the third one pack; and the fourth is made new. The goods, after unhairing and fleshing, and washing, are put into the first pit quite cold, are handled in this three times on the first day, and afterwards put into the second pit, the temperature of the second pit having been previously raised to 65°—the cold pelt going into this pit reduces the temperature to about 60°. They are handled in this pit twice, and remain overnight; and the third day they go into the third pit, the temperature of which is raised to 70° before the goods go in. They are raised, scudded thoroughly on the grain, and new liquor is made from new bating liquor prepared as described above; the goods are now entered, handled at least twice a day, and, if insufficiently bated, are left overnight. By the next day they will probably be sufficiently “down” to permit of them going on to the scudding beam and thence on to tanning.
In the event of any hide being insufficiently bated—in other words, if the “bone” has not completely disappeared in the inner part of the hide—they are returned once more for three or four hours, the temperature of the pit being raised to 70° with a steam coil.
By this system, which is known as a system of “sets,” it will be noticed that the pits are worked in a round of four; the first pit, or oldest, being used more or less as a wash pit, the actual bating taking place in the second, third and fourth pits. No pit has more than four packs passing through it, and the temperature never rises above 70°, or, at the outside, 75°. The reason of the comparatively low temperature is due to the fact that in pit bating, if the temperature is raised above 75°, the grain of the hide becomes affected before the internal “bone” is sufficiently soft, which results in a tender grain, which considerably affects the resulting staining or dyeing which the leather usually receives before it is put on the market.
The Third Process.—This process is suitable for either harness or dressing hides, and differs from the above-described process only in the fact that instead of bating the hides in pits, a latticed drum is used in order to keep the hides in constant motion. This consists of a large pit, in which a latticed drum revolves two-thirds immersed in the liquor. The bate liquor is made up as usual, and the temperature of the liquor raised to 75° or 80° F.; the hides are then placed in the latticed drum, which revolves at from four to six revolutions per minute, and, if the goods are put in the drum in the morning, and if the temperature is kept at about 70° F., the hides are generally sufficiently bated by evening. They may then be taken out of the drum, scudded, and left suspended in boracic acid overnight, and can then be taken to the liquor next morning.
Where there is considerable motion, even the heaviest hides will stand a temperature of 75°, and even 80°. Some tanners who use the latticed drum method, do not place their hides in the drum until the afternoon; they are drummed for about an hour in the cold bate, and left overnight; next morning the temperature of the bate is raised to 75°, and the drum started; they are then removed from the drum towards the afternoon, when sufficiently “down” to admit of them passing forward into the next process.
The above are three typical methods in common use in England, but many modifications of these processes exist. Everything depends upon the amount of lime in the hide, on the number of haulings or handlings given to the goods, and the temperature or the conditions under which the hides are bated, so that no hard-and-fast lines can be laid down. Much also depends upon the method of liming used, prior to the bating. It must be borne in mind that the bating of hides, and even of kips, differs materially from the bating of goat and sheep skins, as one has to deal with double or even treble the substance. If the bate is worked at a temperature of even 70° or 75°, there is the danger of the grain being seriously affected before the bate has penetrated to the interior of the pelt, unless the goods are kept in constant motion.
Instead of a latticed drum, large paddles are used in some factories. If the pit is large enough the paddle causes both hides and liquor to revolve, and thus gives the required movement necessary for a regular and even “bating effect” to be obtained.