[CHAPTER X.]
SOLE-LEATHER.—Treatment in the Tan-house.
On first coming into the yard, the butts are usually suspended by the shoulder or butt ends from sticks placed across the pits. They should be kept in almost constant movement, either by raising and shaking them by hand, or by supporting them on frames, which are rocked, or otherwise worked. Perhaps the best device for this purpose is the "travelling handler" of W. N. Evans, which consists of a frame supported on wheels, and worked slowly backwards and forwards by power. This frame should extend the length of a range of pits sufficient to take in at least a 3 days' stock of butts, which should be tied to sticks resting crossways upon it. It should have a stroke of 1-2 ft., repeated, say 6 times a minute. The power required is very small.
The American rocker consists of a wooden frame balanced on its centre, and made to oscillate by power. It is a cheap and efficient machine, its defects being that the butts at the ends are much more moved than those in the centre, and that their upper parts, being lifted out of the liquor, are liable to become blackened.
The suspender pits should be supplied with old handler liquors, which, if the tannage is a mixed one, may range from 12° to 20° barkometer, as a large proportion of the weight consists only of lime-salts, gallic acid, and other worthless products. It must here be explained that the barkometer (also called "barkrometer" or "barktrometer") is a hydrometer, graduated to show the sp. gr. thus—20° Bark. = 1·020 sp. gr. In using it the temperature of the liquor must be at or near 60° F. (15° C.). It is, of course, affected by any other matters in solution, precisely the same as by tannins. In the Lowlights Tannery the waste liquors are constantly about 12° Bark., and contain tannin equal to less than 0·2 per cent. (expressed as crystal oxalic acid), and gallic acid and similar matters equal to O·6 to 0·7 per cent. If the tannage is pure bark, it may perhaps be advisable to let the strength be somewhat less, but something depends on whether the exhausted liquors are returned with all their impurities to the "taps" or liquor-brewing pits, or whether the liquors are made with water, and hence purer. In any case, the free acid in the suspenders should always be sufficient in quantity to neutralise the lime brought in by the butts, or bad colour will certainly result, making itself visible in the shed, or as the tanning proceeds. If the butts, when first brought into liquor, take a lemon-yellow colour, especially in places that have been imperfectly exposed to it, this is an indication of danger which must not be disregarded. It may be met either by cleansing the butts more thoroughly before bringing into the yard, or by adding acid (acetic, hydrochloric, or sulphuric) to the liquor. If this be done, great care must be taken not to over-do it, and an acid free from iron must be used. The use of sulphurous acid for the purpose has been patented, and presents some advantages. Sulphites have been observed by the writer to give a pink or purple reaction even with very dilute infusions of valonia (see [p. 112]); but any coloration from this cause would probably disappear as the tannage proceeds. The difficulty can, however, often be remedied, either by altering the way of working the liquors, so as to bring more sour liquor down to the suspenders, or by using a larger proportion of materials capable of yielding acetic acid by fermentation, such as myrobalans. It is a common error to call all the free acid of sour liquors "gallic," as this is scarcely present in pure bark-yards, and at the best is a very feeble acid. The most abundant acid is usually acetic, though butyric, lactic, and other acids are frequently present in varying proportions, according to the tanning materials employed. In the English process, with its comparatively short layers, in which the butts almost float in strong liquors, but little souring takes place, and we have nothing comparable to the German "sour bark" and "sour liquor" from long layers with weak liquor, and much dusty material. These contain large quantities of acetic and lactic acids, and plump almost like vitriol. Though the American tanners generally use the latter, their hemlock liquors sour much more intensely than those of English yards. It must always be borne in mind, in comparing English with American and Continental tanning, that, in the first, the opening up of the fibre is effected by lime, and the swelling is maintained in the liquors, not so much by acids, which are only present in very small proportion, as by the careful and gradual working forward into infusions stronger and stronger in tannin; while in the two latter, lime, if used at all, is simply employed to loosen the hair, and the swelling and differentiation of the fibre is first accomplished in the liquors either by vegetable or mineral acids. Hence good results cannot be expected in English yards from such processes as sweating or painting with sodium sulphide, which does not plump, without a radical modification of the whole tanning process. This point has been ably treated by Eitner in a series of papers on Extract-gerberei, published during the last few years in 'Der Gerber,' which will well repay attentive perusal by English as well as German tanners.
The butts should at first be brought into the weakest liquor; a circulation system, by which the liquors are all pumped in at one end of a set of suspenders, and run out at the other, the butts being moved forward in the opposite direction, seems to have much to recommend it. In this case, the top of one pit should be connected by a wooden box with the bottom of the next.
It is usually advisable to run away the first liquor into which butts are brought from the lime-yard, as it is very completely spent, and highly charged with lime salts and impurities. Whether other exhausted liquors are to be retained or rejected is largely a question of climate, and mode of working. In hot weather, such liquors, charged with organised ferments (moulds, bacilli, and bacteria), are apt to cause ropiness, and other fermentive diseases of the liquors. This danger may be lessened by boiling all spent liquors, so as to kill the ferments, before running on the taps, or prevented by the free use of antiseptics, such as carbolic acid. Small doses of carbolic acid, however, are useless; at least 1/10 per cent. must be employed; and it must be borne in mind that antiseptics prevent souring as well as other fermentations, and hence, where they are employed, other means must be adopted to maintain the necessary acidity. Such liquors are very liable to darken if boiled.
The suspender liquors should be acid enough freely to redden litmus-paper. The present author has published a simple volumetric method for the determination of the free acid; 10 cc. of the carefully filtered liquor is placed in a beaker, and clear lime-water is run in from a burette till permanent cloudiness is produced. The quantity of lime-water employed is that which the acid is capable of neutralising, without producing discoloration of the leather, and care must be taken that the lime introduced with the butts does not exceed this proportion. The explanation of the reaction is that dark-coloured tannates of lime are formed, which are dissolved by the free acid so long as it remains in excess. It must be remembered that this process estimates all acids capable of retaining tannates of lime in solution, including some so feeble as to have practically no plumping effect. A liquor may have acidity equal to several cc. of lime-water, and yet react absolutely alkaline to methyl-orange (see [p. 9]), a colour which is distinctly reddened by small excess of acids, even so weak as gallic, which is barely acid to the taste. Hence, the acidity of a liquor available for plumping may be taken as represented by the lime-water required to change the red of methyl-orange to yellow, and if the liquor does not redden methyl-orange it is incapable of plumping. If 5 or 10 drops of orange solution be added to the pale filtered liquor from suspenders, there is no difficulty in approximately hitting the point of change, but great accuracy is not to be expected. If the liquor will not filter clear, kaolin (see [p. 119]) may be used to clear it. It is well to test the lime-water occasionally on 10 cc. of decinormal sulphuric or oxalic acid ([p. 96]), to make certain of its constancy. Lime-water should be kept in a bottle with excess of lime, shaken occasionally, and a small quantity filtered off as required. Liquors are frequently miscalled "sour" which are not acid, but putrid. Such liquors will not plump, but reduce and soften hides placed in them. (Compare also [p. 185]). Suspender liquors usually consist mainly of liquors from the handler shift. If liquors be used direct from the leaches, they generally produce harsh grain and bad colour.