CHAPTER I.

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.”

[5] There is a copy in the British Museum and also in the Patent Office Library.

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 | Per cent. CaO on | Per cent. lime washing | dry skin | removed -----------+------------------+--------------- 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 -----------+------------------+---------------

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]

[6] Some further analyses of grains at Trent Bridge gave the following results:--

Water CaO CaO on dry skin Grain from splitting machines 76·3 1·14 4·8 The same after washing 24 hrs. 86·0 0·22 1·57 The same after puering -- -- 0·80 Another grain 79·7 1·06 5·20 The same after deliming as } completely as possible with} -- -- 0·45 HCl } Skin in natural condition 64·0 -- 0·125

M. C. Lamb has found in sheep grains split for skivers the following amounts:--

Ash CaO CaO in Ash Grain from splitting machine 9·3 3·2 34·8 The same after washing 4·1 1·5 35·1 The same after puering 3·1 0·9 29·1 The same after bran drench 1·7 0·55 32·4

The figures are all percentages calculated on the dry weight of the skin.

Munro Payne (Tanners’ Year Book, 1905, p. 75) gives the following amounts of lime as Ca(OH)_{2} in limed hides, calculated on dry weight at 212° F.:--

Direct from limes max. 3·859 per cent. " " min. 2·836 " Limed for buff 4·621 " Limed for tanning 3·7659 " Ditto after bating 0·689 " Calf limed 2·601 " Calf bated 0·1215 " Goat limed 5·613 " Goat bated 1·268 "

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 CO_{2}, 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.

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 = CaCl_{2} + 2H_{2}O 74 + 73·1 = 110·9 + 36[7]

[7] From the molecular weights it will be seen that 74 grm. of calcium hydrate, equivalent to 56 grm. of lime (CaO), require 73 grm. of HCl gas for neutralization. This quantity of gas is contained in 265 grm. of commercial hydrochloric acid of 18° Bé., or about 230 c.c.--that is, for 100 grm. CaO, 410 c.c. of 18° Bé. acid are required; hence for the 100 kilos wet grain containing 400 grm. CaO, 1640 c.c. of acid are required for _complete neutralization_. It is impracticable to use this amount of acid, for the reasons stated above.

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.

[8] Tanners’ Year Book, 1911, p. 59.

[9] See Chapter III.

COST OF ACIDS TO REMOVE 1 LB. LIME.

------------+------+-----------+----------+----------+---------- | | | Cost | Strength | Cost for Acid | Eq. | K | per cwt. | per cent.| 1 lb. CaO ------------+------+-----------+----------+----------+---------- | | | _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,”