The customary method of liming is simply to lay the hides flat in milk of lime in large pits. Every day, or even twice a day, the hides are drawn out ("hauled"), and the pit is well plunged up, to distribute the undissolved lime through the liquor. The hides are then drawn in again ("set"), care being taken that they are fully spread out. How much lime is required is doubtful, but owing to its limited solubility, an excess, if well slaked, is rather wasteful than injurious. Great differences exist in the quantity of the lime used, the time given, and the method of working. Lime, as we have seen ([p. 140]), is only soluble to the extent of about 1·25 grm. per litre, or (as 1 cub. ft. of water weighs about 1000 oz.) say 11/4 oz. per cub. ft., or, in an ordinary lime-pit, not more than 1/4 lb. per hide. Only the lime in solution acts on the hide, but it is necessary to provide a surplus of solid lime which dissolves as that in the liquor is consumed. Jackson Schultz prescribes 1 bush. (56 lb.) of fresh lime to 60-70 hides, and 3-4 days as sufficient time to unhair and plump them; while a well-known English tanner states that, after working for 6-10 days through a series of old limes, the hides (presumably wet-salted South Americans) should have 4 days in a fresh lime, made with 3-12 lb. of lime per hide. It is obvious that if the American authority is right, the English process is wasteful in the extreme, both in hide-substance and lime. Much depends on the amount of hauling which the hides receive, and the more frequently they are moved the better. It is probable, however, that it would be found impossible to unhair and flesh hides, to suit the English market, in cold limes with the quantity and time mentioned, and if the limes are steamed, it is quite likely that the destructive action on the pelt may be even greater than by the longer and slower process in the cold. Most likely a compromise between the two is the most desirable, but about 2-4 lb. of lime per hide, according to weight, should be amply sufficient; while a week for market hides, and 14 days for heavy salted, will loosen the hair and plump the pelt as much as is requisite. This is on the supposition that the limes are kept at a uniform average temperature of about 60° F. (15° C.) in winter and summer. If they are heated to 80°-90° F. (27°-32° C.), of course much less time is required; but there are no published experiments showing the relative weights made by the two processes, and, from the fact that warmed limes are principally used for descriptions of leather where weight and solidity are not of primary importance, it may be concluded that, in this direction, the results are unsatisfactory. Hides do not plump in warm limes.
Another undecided point is whether the best results are obtained by making fresh limes for every pack, or by strengthening up the old ones. An old lime becomes charged with decomposing animal matter and with ammonia, and, within limits, loosens the hair more effectually than a new one. An experienced tanner states that, by using old limes, better weights are obtained, but that the leather is thinner than when a fresh portion of lime is used; and this is quite possible. If, however, the old lime-liquor be retained too long, it ceases to swell the hides as it should, and, in warm weather, the liming proper is complicated by a putrefactive process allied in principle to sweating.
Beside considerable quantities of ammonia, old limes contain tyrosin, leucin or amidocaproic acid, and some caproic acid, the disagreeable goaty odour of which is very obvious on acidifying an old lime-liquor with sulphuric acid, by which considerable quantities of a partially altered gelatin are at the same time precipitated. Very old limes, especially in hot weather, often contain active bacteria, which may be seen in the microscope under a good 1/4-in. objective. Their presence is always an indication that putrefaction is going forward, and leather out of such limes will generally prove loose and hollow-grained. Spherical concretions of calcium carbonate may also be seen under the microscope, resembling on a smaller scale those found in Permian limestone, and caused perhaps in both cases by crystallisation from a liquid containing much organic matter. It is probable that in many tanneries the ammonia would pay for recovery from the lime-liquors, which would be easily done by steaming the old limes in suitable vessels, and condensing the ammoniacal vapours in dilute sulphuric acid. (Some appliances suitable for this purpose are described in the Journal of the Soc. of Chem. Industry, iii. 630.) For methods of estimation of ammonia, see [p. 103].
Several variations in the above-described method of liming have been proposed. The hides may be suspended on laths, or by strings attached to pegs or notches, and the liquor agitated by plunging in place of hauling. Probably this is an actual improvement, especially if some mechanical agitating contrivance be substituted for hand plunging. It has, however, the drawback that much room is required, though this may be, to some extent, compensated by the hides liming more quickly. The method has been long in use in America, and had been tried in several places in England before the patent of Messrs. Conyers and Pullein was obtained. Two other American labour-saving methods in connection with liming may be mentioned here. One is to have the liming-vat double the ordinary size, and, instead of hauling the hides, to simply draw them from one side to the other by two strings, which are attached to the fore and hind shank of each hide, either by sharp iron hooks or by loops. The strings are looped over iron rods at the four corners of the pit, or have simple knots, which are placed in notches sawn in wood. Of course, while the hides are at one side of the pit, the other side may be plunged or warmed. The other method ([Fig. 24]) is to have a spindle sunk below the surface of the liquor, and with discs A, at each end, to which the hides or sides are attached by hooks set round the edges. The hides are turned over by revolving the spindle with a handspike inserted in the holes C, at the ends of the cross-arms B, and the whole spindle is also capable of being raised and lowered in the liquor, in the slot D. In Germany, hides are frequently suspended on laths radiating from a central upright revolving spindle in a round vat (Drehkalk).
An American plan, sometimes known as the "Buffalo method," is described by Jackson Schultz. The hide is prepared in the usual way, and is then thrown into a strong lime for 8-10 hours, when it is taken out and immersed in water heated up to 110° F. (43° C.), in which it remains 24-48 hours. The warm water soaks, softens, and swells the roots of the hair, and much the same result is obtained as in "scalding" pigs. So little lime really permeates the inner fibre that, after a slight wheeling, the hides may be thrown into cold water, and allowed to cool and plump, preparatory to taking their places in the handlers. The process is strongly recommended for sole-leather, particularly where great firmness of fibre is desired. The tanner who tries it must be satisfied if he gets 20-30 sides a man unhaired and fully ready for the liquor per diem. Of course this process may be varied to any extent by giving more liming, and less hot water, and this is frequently done in America. About 3-4 days' cold liming in good limes, and with hauling if possible twice daily, followed by 12-24 hours in water at 86°-95° F. (30°-35° C.), which should be changed at least once, will give good results. The hides are of course less plump than usual, but if properly managed in the handlers will swell well in the tan-house. Grease is obviously less thoroughly "killed" than in the ordinary method, and especial care must be used that the hides are well worked on the beam, both on grain and flesh. In this method, and indeed in all liming processes, much is gained if the fat can be fleshed off green.
Fig. 24.
On the Continent and in America, the prevalent mode of loosening the hair, at least for sole-leather purposes, is called "sweating," and consists in inducing an incipient putrefaction, which attacks the soft parts of the epidermis and root-sheaths, before materially injuring the hide-substance proper. The old European method of "warm-sweating" consisted simply in laying the hides in pile, and, if necessary, in supplying heat by covering them with fermenting tan; but as this crude and dangerous process is everywhere being supplanted by the American plan, where sweating at all is adhered to, it is not necessary to do more than describe the latter. This is called "cold sweating," but really consists in hanging the hides in a moist chamber, kept at a uniform temperature of 60°-70° F. (15°-21° C.); or in some cases slightly warmer.
The "sweating-pit" now in use is sometimes of wood, but usually consists of a building of brick or stone, protected from changes of temperature, both above, and at the sides, by thick banks of soil or spent tan. If soil be used, it will form an excellent bed for vines, &c., which are fertilised by the ammonia penetrating from below, which is evolved in large quantities and which assists the unhairing process by its action on the epidermis.[S] Though called a "pit," it is undesirable that it should be actually below the level of the ground, but should be arranged so that the hides can be wheeled in and out in barrows. It is lighted and ventilated by a lantern roof above a central passage, and should be divided into chambers, each capable of suspending a pack of hides. By means of sprinklers above and steam-pipes below, the chambers may be cooled or warmed, as required, and the air kept so moist that globules of condensed water collect on all parts of the hides, which are suspended from tenterhooks.