The customary method of liming is simply to lay the hides horizontally one at once in milk of lime in large pits, taking care that each hide is completely immersed before the next is put into the pit, so as to ensure a sufficiency of liquor between them. 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, not only for various classes of leather, but for the same kinds in different yards. Lime, as we have seen, is only soluble to the extent of about 1·25 grm. per litre, or (as 1 cub. foot of water weighs about 1000 oz.) say 11⁄4 oz. per cub. foot, 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 or absorbed by the hide; and this is especially the case where, as is generally customary, the hides are laid flat in pits, so that no circulation of liquor is possible. Where hides are suspended in lime-water, which is constantly circulated and kept up to its full strength by agitation in another vessel with solid lime, they unhair as quickly as with milk of lime, but the method seems, in the case of lime, to present no special advantage over the ordinary one, if in the latter the hides are hauled sufficiently often to keep the lime uniformly distributed. The case is otherwise in dealing with more soluble depilatories. Various patents have been taken for methods of liming by suspending in liquors, but the idea is now public property, and is largely used on the Continent. It is necessary that the lime which settles to the bottom of the pit should be agitated and kept in suspension, which may be effected either by moving the hides on a frame as in “suspenders” ([p. 221]), or by agitators acting on the principle of pumps, and raising the liquor and sludge from the bottom. Such agitators have been patented in Germany, but had been in use much earlier in the Author’s tanyard. An agitator on the principle of the screw-propeller of a steamship, placed near the bottom of the pit, and protected by a lattice, may also be usefully employed ([Fig. 26]). Skins are frequently limed in paddles, or stirred up by blowing air into the pit. The latter method is neither effective nor economical in power.
Fig. 26.—Suspension Lime-Pit.
As has been noted, the solubility of lime, and consequently the strength of the lime-liquor, is diminished by rise of temperature, but its solvent action on hide-substance is much increased. As a consequence, the loosening of the hair proceeds much more rapidly in warm limes, but the hides do not plump well, and become loose, hollow and inclined to “pipe” in the grain, and to weigh out badly, and for sole leather the method is therefore in every way disastrous. In the few cases among the lighter leathers where a decided softening and loosening of the texture of the skin is required, it is possible that useful advantage may be taken of this effect; but it would be exceedingly difficult to regulate the temperature of an ordinary lime-pit with accuracy, and better results could probably be obtained with suspenders in which the liquor could be constantly circulated. When limes are very cold, in spite of the greater strength of solution, the action is very much checked, and where goods are frozen into pits in severe weather, there is but little danger of overliming, although the usual time may be much exceeded. It is generally best to work limes at about the ordinary summer temperature, and this is better done in winter by warming the limeyard than by any direct heating of the limes. If lime which has cooled after slaking is used, the water with which limes are made may safely be warmed in midwinter to a temperature not exceeding 20° C.
The quantity of lime used by different tanners, and for different sorts of hides and skins, is very variable, not only according to the effect which it is desired to produce, and the way in which it is used, but from the arbitrary fancy of the user, since its limited solubility renders an excess comparatively innocuous. For sole-leather, the amount recommended varies from under 1 per cent. to 10 or 12 per cent. on the green weight of the hide; but probably 2-3 per cent. is all that can be really utilised, the remainder being wasted. In order, however, to utilise the whole of the lime, very frequent handling or agitation is required to ensure its uniform distribution. It must also be borne in mind that the strength of commercial limes varies from above 80 down to 30 per cent. of available calcium oxide.
Von Schroeder has found that a strength of 6 grams of calcium oxide (CaO) per liter was sufficient, but, in practice, much more is generally added. It is also noteworthy that a perfectly fresh milk of lime must be made much stronger than one which has been used. This is partially due to the fact that some bacterial action takes place in an old lime and that ammonia is formed which assists unhairing, in addition to the effect of the lime itself, and partially because the lime in old liquors remains in suspension for a much longer time, and is thus more evenly distributed.
A method of liming, sometimes known as the “Buffalo method,” has been largely adopted for sole-leather in America, and is now used in many Continental yards. It consists in a very short liming and the subsequent use of warm water. The limes are also often sharpened by the addition of a little sodium sulphide or of some other sulphide. Thus, in one large yard in the States, the hides for sole-leather (salted “packers”) are limed for 10 hours only with 2 lb. lime and 21⁄2 oz. of sulphide of sodium per side, and after lying overnight in water of a temperature of 35-45° C., are easily unhaired. A Continental firm lime two days in weak fresh limes with a little tank-waste, and then treat with water at 32° C. for 6-8 hours, when the hides are unhaired and returned to the warm water for two hours before scudding. All sorts of combinations between liming and hot water treatment can be employed. The longer and stronger the liming, the lower temperature or shorter time in the water will suffice. The method is much to be recommended for firm sole-leather, but it does not saponify grease or swell the fibres thoroughly, and usually vitriol is used for the latter purpose in a later stage. The hide goes into the liquors practically free of lime, and the loss of hide-substance is much less than in the ordinary method of liming.
A point of probably much greater importance than the quantity of lime used is the length of time during which a lime is worked without change of liquor. An old lime becomes charged with ammonia and other products of the action of lime upon the skin, such as tyrosin, leucin (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 partially altered gelatin are at the same time precipitated (compare [p. 64]). Lime has considerable antiseptic power, and a new lime is practically sterile, but very old limes, especially in hot weather, often contain large numbers of active bacteria, which may be seen in the microscope under a good 1⁄6-inch objective. Their presence is always an indication that putrefaction is going forward, and if their number be very excessive, the leather out of such limes will generally prove loose, hollow and dull-grained, and in extreme cases hides may be totally destroyed. 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 hardly probable that in many tanneries the ammonia would pay for recovery from the lime-liquors, though it could be easily done by steaming the old limes in suitable vessels, and condensing the ammoniacal vapours in dilute sulphuric acid. Its quantity rarely exceeds 0·1 per cent. of NH3. For methods of estimation of ammonia, see L.I.L.B., p. 30.
Up to a certain point, it is found that old limes unhair much more readily, and have a greater softening effect than new ones, which is often advantageous for dressing goods; though for sole leather, where weight and firmness are of primary importance, the use of stale limes must be kept within the narrowest limits. In the finer leathers also, such as kid and moroccos and coloured calf, where a sound and glossy grain is desired, the effects mentioned are generally better obtained in other ways, such as by the use of sulphides. On East India kips and other dried hides, which are difficult to soften, and which have great power of resistance to the action of lime, old limes are distinctly useful, but, even there, there are limits which should not be passed. Probably no lime ought to be allowed to go for more than three months at the outside limit without at least a partial change of liquor, and the system of allowing all the limes in a yard to run for twelve months, and then cleaning them all together, is almost the worst which can be planned. A very much better way is to clean the limes in regular rotation, using, if desired, a portion of the old liquor in making the new lime, so as to avoid a too sudden transition. The old liquor is valuable, if at all, for the ammonia and organic matter which it contains, as the amount of lime in solution is not worth considering. The ammonia considerably increases the solvent and unhairing power, while swelling the hide less than an equivalent amount of lime. In some cases it may be desirable to add ammonia artificially for this purpose. In this case it will be cheaper and more convenient to add it in the form of ammonium sulphate than as liquid ammonia. If it be desired to retain ammonia, the lime should be kept covered. Very old limes containing excess of ammonia and lime, sometimes in hot weather cause a transparent swelling of the goods, with destruction of the fibrous texture.[74] The writer has observed a similar phenomenon in very weak and old limes strengthened with sulphides, in which hide was left experimentally for several weeks. The principal effect of the dissolved animal matter is to enable bacteria to thrive in it, which they will not do in a fresh lime, but putrid limes probably also contain liquefying ferments produced by the bacteria present ([p. 17]), and which dissolve hide. Eitner has published researches on the amount of hide-substance dissolved by limes,[75] in which he shows that the loss of substance in liming sufficiently to unhair is materially greater in old limes than in fresh ones, although during the first two days of liming the new limes are decidedly the most active. As he remarks, this justifies the wisdom of the method, now largely adopted, of working limes in shifts, and beginning the operation in old limes and completing it in fresh ones. (See also [p. 131].)
[74] Gerber, 1884, pp. 150, 184.