H. R. Procter

After the softening and cleansing of the hide or skin is completed, and before proceeding to tan it, it is usually necessary to remove the hair or wool. The earliest method of accomplishing this was by means of incipient putrefaction, which attacks in the first instance the soft mucous matter of the epidermis, and thus loosens the hair without materially injuring the true skin. This loosening of the hair often takes place accidentally in hides which have been kept too long without salting, and is known as “slipping,” and is apt to be accompanied by some degree of injury to the grain. The old method of loosening the hair by putrefaction, or, as it is generally called, “sweating,” was to lay the hides in piles, usually in some warm and damp place. Occasionally a slight preliminary salting was given to prevent too much putrefaction of the hide. The action in this case, however, was very irregular, and it has been quite abandoned in all civilised countries.

Fig. 25.—Sweat-Pit.

The method which is now used is to hang the hides in a closed chamber, generally called a “sweat-pit,” Fig. 25, but usually constructed above the ground-level and protected from sudden changes of temperature by double walls, or by mounds of earth. The hides are hung in the sweat-pit, in small chambers each capable of holding 50 or 100 hides. The temperature is kept at about 15° to 20° C., the air being warmed, if necessary, by the admission of steam below a perforated floor, or cooled by a shower of water from sprinklers, so arranged as not to play directly on the skins, and is thus always kept saturated with moisture. Little if any ventilation is allowed, and a large quantity of ammonia is given off from the decomposition of the organic matter, and no doubt contributes to the solution of the epidermis and the loosening of the hair, as the writer has found that ammoniacal vapours alone very speedily produce this effect.

After 4-6 days of this treatment, the hair is sufficiently loosened to be removed by working the skin over the beam with a blunt knife, or by means of the stocks or hide-mill (see p. 116). Great care and watchfulness are required to avoid injury to the grain by putrefaction.

The hide is in a slimy and completely flaccid and “fallen” condition, and some trouble is occasioned by the hair being worked into the flesh by the hide-mill, to obviate which, a slight liming is frequently given after the sweating. Hides which have been unhaired in this way require to be swollen by acid in the liquors in order to produce a satisfactory sole-leather, as the sweating process does not swell or split up the fibres.

In some European tanneries a similar process, but at a higher temperature, is employed, and it is also largely used for sheep-skins under the name of “staling,” but in this case is sometimes conducted in a very rude and primitive manner, and frequently with the result of considerable injury to the pelt.

The great objection to the sweating process, however carefully conducted, is the liability of putrefaction to attack the skin itself, causing “weak grain.” Its most advantageous use is for sole leather, as, although the solution of the hide-substance may not be very much less than in the case of liming, the dissolved matter remains in the hide instead of being washed out, and being fixed by the tannin, contributes to the solidity of the leather.

In England, lime is the agent almost universally employed for unhairing, though every tanner admits its deficiencies and disadvantages. It is hard, however, to recommend a substitute which is free from the same or greater evils, and lime has one or two valuable qualities which will make it very difficult to supersede. One of these is that, though it inevitably causes loss of substance and weight, it is also impossible, with any reasonable care, totally to destroy a pack of hides by its use; which is by no means the case with some of its rivals. Another advantage is that, owing to the very limited solubility of lime in water, it is of comparatively small consequence whether much or little is used; and even if the hides are left in a few days longer than necessary, the mischief, though certain, is only to be detected by careful and accurate observation. With all other methods, exact time and quantity are of primary importance, and it is not easy to get ordinary workmen to pay the necessary attention to such details. Again, the qualities of lime, its virtues and failings, have been matter of experience for hundreds of years, and so far as such experience can teach, we know exactly how to deal with it. A new method, on the other hand, brings new and unlooked-for difficulties, and often requires changes in other parts of the process, as well as in the mere unhairing, to make it successful. As our knowledge of the chemical and physical changes involved becomes greater, we may look to overcoming these obstacles more readily.

The universal source of lime is chalk or limestone, which consists of calcium carbonate, and from which the carbon dioxide is driven off by burning in a kiln. Many limestones, however, are far from being pure calcium carbonate, but contain large proportions of magnesia, iron and alumina, the latter perhaps originally deposited in the form of clay with the sediment from which the stone was formed. Such clay limestones when burnt yield natural cements, like oolite and other “hydraulic” limes, which are capable of setting even under water. The presence of magnesia and clay is injurious not only by diminishing the amount of lime present, but by making the lime much more difficult to slake; and iron oxide, though quite insoluble, may become mechanically fixed in the grain of the hide, and may be the cause of subsequent stains. The burning of lime in the kiln is probably not quite so simple an operation as the equations of the text-books would suggest. By mere heating, the carbonate can, it is true, be decomposed, but to do this completely a good white heat is required, which is rarely attained in practical burning, and it is probable that at least a part of the carbon dioxide present is reduced to carbon monoxide by the combustible fuel-gases, and so separated from the lime, for which it has no affinity. Carbon monoxide is the cause of the intensely poisonous character of limekiln gases, the pure dioxide being irrespirable, but not strictly poisonous.

Quicklime, CaO, on coming in contact with water, combines with it with the evolution of considerable heat, becoming slaked or converted into hydrate, Ca(OH)₂. This change takes place rapidly and easily when the lime is light and porous, such as is obtained by the burning of chalk or good limestone at a low temperature; but if it has been too intensely heated or “over-burnt,” or contains silicates or other salts which fuse at the temperature of the kiln, a compact lime is formed which slakes with difficulty and extreme slowness, thus being lost to the tanner, or leading to the still more serious result of burning holes in the hides by the heat produced by slaking in contact with them. It is stated by Le Chatelier[69] that for dense limes 24-48 hours is frequently required for complete slaking in the cold, while magnesia is still more obstinate, months being sometimes necessary for the complete hydration of hard-burnt samples; and mixtures of lime and magnesia are intermediate in their character. Slaking is greatly assisted by heat, even heavily burnt magnesia being hydrated in about six hours at 100° C. Slaking is also much more rapid in a dilute solution (2 per cent.) of calcium or magnesium chloride. From these facts it is easy to deduce the reason why a suitable quantity of water, neither too much nor too little, is desirable for the rapid and effectual slaking of lime. If too little is used, the lime is only partially slaked, and it is not easy for further portions of water to gain access to the interior of the powdery mass. On the other hand, if it is “drowned” by excess, the temperature is lowered, the process goes on slowly, and the mass does not readily fall into powder, and so fails to be utilised in the liming process. Of all methods of slaking lime, the ordinary one of tipping it direct into the lime-pits is perhaps the most irrational, leading to the formation of unslaked lumps which may burn the hides, and which, together with stones and dirt, rapidly choke the pits with useless matter. The best process is that adopted by builders and in many Continental yards, in which a large quantity of lime is slaked in a shallow tank by throwing on it sufficient water to thoroughly wet it, and after allowing it to heat and fall for 24 hours, adding enough water to convert it into a stiff paste. In this form it may be kept for months without material deterioration. When required for use, a suitable quantity of the paste is dug out, and well stirred with water in a tub or tank before running into the pit when the stones and sand remain in the tank. In this way all nuisance from dust is also avoided. If lime is stored unslaked, it gradually absorbs moisture from the air, falling, and soon becoming dusty and difficult to slake completely, while the traces of carbon dioxide in the air gradually convert it into useless carbonate.

[69] Bull. de la Soc. d’Encouragement, 1895, x. pp. 52-62; Journ. Soc. Chem. Ind., 1895, p. 575.

The solubility of lime in water is very limited, and the figures determined by different chemists do not agree very satisfactorily. The following table gives the result of determinations made by Mr. A. Guthrie in the Author’s laboratory, and is probably one of the most accurate[70]:—

100 c.c. of saturated	lime water at	5°	C. contain	0·1350	grm. of CaO.
„	„	10°	„	0·1342	„
„	„	15°	„	0·1320	„
„	„	20°	„	0·1293	„
„	„	25°	„	0·1254	„
„	„	30°	„	0·1219	„
„	„	35°	„	0·1161	„
„	„	40°	„	0·1119	„
„	„	50°	„	0·0981	„
„	„	60°	„	0·0879	„
„	„	70°	„	0·0781	„
„	„	80°	„	0·0740	„
„	„	90°	„	0·0696	„
„	„	100°	„	0·0597	„

[70] Journ. Soc. Chem. Ind., 1901, p. 224.

It will be noticed that unlike that of most substances, the solubility of lime in water diminishes as the temperature is raised. It is therefore necessary in employing lime-water as a standard solution to take care that it is saturated at a constant temperature. The results given in the above table are those from pure marble lime. Where the ordinary impure limes from limestone are employed, a somewhat stronger lime-water is often obtained. This is difficult to explain, but possibly some double hydrate of lime and magnesia is formed which is more soluble than either hydrate alone. The results harmonise with the old belief of tanners that chalk-lime is milder in its action on skin than that made from less pure limestones. The solubility of any given lime is easily determined by adding it in excess to water in a stoppered flask, and shaking frequently until a solution of constant strength is obtained. A known volume of this solution (which must be clear and free from undissolved lime) is then titrated with ^N/₁₀ hydrochloric acid, using phenolphthalein as the indicator.

Saturated lime-water may be conveniently used as an alkaline standard solution for many purposes, and if kept on excess of lime is always caustic, and varies very little in strength at ordinary laboratory temperatures. The solution is nearly ¹/₂₀ normal, but for accurate work its strength should be exactly determined with ^N/₁₀ acid. 1 liter of pure lime-water at 15° C. should require 471·4 c.c. of ^N/₁₀ acid for neutralisation.

Lime is much more soluble in sugar solutions than in water. Such solutions have been used as standard solutions, and sugar has been added to limes to increase the action on the hides.

The following is the analysis of a lime used in a Leeds tannery, which was made by Mr. G. W. Flower, B.Sc., in the Leather Industries Laboratory of the Yorkshire College[71]:—

The sample only contained 31·02 per cent. of available lime, the remainder being probably combined with the silica. It also contained an appreciable quantity of iron oxide, which might lodge mechanically in the pores of the skin and become dissolved in later processes, darkening the colour of the leather. The lime was also under-burnt, judging from the amount of carbonate it contained.

For comparison with this, the analysis of a good specimen of carboniferous-limestone lime from Buxton may be given:—

Determination of “Available” Lime.—The practical value of lime for the tanner is easily determined by drawing a sample by breaking off small pieces from a number of lumps of the bulk, coarsely pulverising them in a mortar, and then rapidly grinding a portion as fine as possible, and transferring it at once to a stoppered bottle for weighing. A portion of this, not exceeding 1 grm., is shaken into a stoppered liter flask, which is filled up roughly to the mark with hot and well-boiled distilled water, and allowed to stand for some hours with occasional shaking. When cold it is filled exactly to the mark with cold distilled water, well shaken again and allowed to settle, or rapidly filtered, and 25 or 50 c.c. of the clear liquid withdrawn with a pipette and titrated with ^N/₁₀ hydrochloric or sulphuric acid and phenolphthalein. Each cubic centimeter of ^N/₁₀ acid equals ·0028 grm. CaO. It is generally a very mistaken economy to make use of an inferior lime for tanning purposes, as any saving in cost is discounted by the larger quantity required, the more frequent cleaning of the pits, and the danger of stains and of burns from imperfect slaking.

The action of lime on the hide has already been spoken of to some extent. It is throughout a solvent one. The hardened cells of the epidermis swell up and soften, the mucous or growing layer and the hair-sheaths are loosened and dissolved, so that, on scraping with a blunt knife, both come away more or less completely with the hair (constituting “scud” or “scurf,” Ger. Gneist or Grund). The hair itself is very slightly altered, except at its soft and growing root-bulb, but the true skin is vigorously acted on. The fibres swell and absorb water, so that the hides become plump and swollen, and, at the same time, the “cement-substance” of the fibres is dissolved, and they become split-up into finer fibrils: the fibrils themselves become first swollen and transparent, and finally corroded, and even dissolved. A similar swelling of the fibres is produced by both alkalies and acids, and is probably due to weak combinations formed with the fibre-substance, which have greater affinities for water than the unaltered hide.[72] This swelling is useful to the tanner, since it renders the hide easier to “flesh” (i.e. to free from the adhering flesh) on account of the greater firmness which it gives to the true skin. It also assists the tanning, by splitting up the fibre into its individual fibrils, and so exposing a greater surface to the action of the liquors. This is advantageous in dressing-leather which is afterwards tanned in sweet liquors, and which must have the cement-substance of the fibres dissolved and removed for the sake of flexibility; and, in the case of sole-leather, it is necessary for sake of weight and firmness that the hide be plumped at some stage of the process; but it is probable that this effect is produced with less loss of substance and solidity by suitable acidity of the tanning-liquors. Another advantage of lime is that it acts on the fat of the hide, converting it more or less completely into an insoluble soap,[73] and so hindering its injurious effects on the after tanning process, and on the finished leather. If strong acids whether mineral or organic are used later on, this lime soap is decomposed, and the grease is again set free. In sweated or very low-limed hides this grease is a formidable evil, causing darkening or grease spots on the finished leather.

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.

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.)

For details of the analytical methods employed, Eitner’s original paper must be consulted, but the annexed table (see next page) summarises his results. The letters heading the columns have the following meanings.

A. Hide substance precipitated by neutralisation of the lime with carbonic acid.

B. A further precipitate obtained by slight acidification with hydrochloric acid.

C. Soluble peptones precipitated by hypochlorous acid or mercuric nitrate.

It is obvious that none of these figures represent the total dissolved organic matter, and it is to be regretted that this was not determined. It is, however, fairly safe to assume that the table correctly represents the relative solubility in the different liquors. In each case 2 liters of liquor were used for each kilo of green hide. When old liquors were employed, the hide-substance they originally contained was determined, and deducted from the final result.

Taking into account the liming necessary for unhairing only, as shown in the table, it will be noted that the percentage of loss is invariably greater in old limes than in new ones, and less in limes sharpened with sulphide of sodium than where lime alone is used. The only exception to this rule is in No. 15, where a sulphide lime 4 weeks old shows the least loss of any in the time required for unhairing; and indeed sulphide limes if kept strengthened with the requisite addition of sulphide, seem to deteriorate very slowly, No. 8, with a lime 5 months old, showing a result which may still be considered good. Another point especially noted by Eitner is the slight action of old limes during the first stages of liming, as compared with their rapid solvent effect as the hair becomes loosened. The loss in any case does not appear to be so great as the advocates of other unhairing processes have often claimed. If we assume that all the dissolved hide-substance might have made leather, the worst loss on oxhide only limed to the point of unhairing amounts to less than 3¹/₂ per cent. on the possible total; and it must be remembered that at least a part of this consists of dissolved epidermis matter, which could not by any possible method have been converted into leather. It will be noted in Nos. 2, 4, 10, 12 and 16, what considerable losses are produced by plumping limes after unhairing, but it must be borne in mind that, in the case of dressing-leather, solution of at least a part of the cementing matter is essential to produce the necessary softness and flexibility. Eitner calculates the dry pelt-weight from that of the green hide on the assumption, based on experiment, that 100 parts of the original skin corresponds to 32 parts of dry pure pelt in green oxhide, 25 parts in green calf-skins, and 56 parts in dried calf-skins. In some of the smaller skins, such as kid worked for glove leather, where great softness and stretch is required, the loss is necessarily much greater than in ordinary dressing-leathers, amounting, in the case of kid, to from 20 to 27 per cent.

The parts taken by the purely chemical activity of the lime, and by the action of bacteria and bacterial ferments in the unhairing process must still be regarded as uncertain. The late Professor von Schroeder[77] carried out a series of experiments on liming and sweating which were characterised by his usual care and thoroughness, and which tend to prove that the chemical action is far more important than the bacterial. He had fresh hides well washed in a tannery immediately after slaughter, and fleshed. The butts were then cut into pieces of about 10 cm. (4 inches) square, and salted in brine repeatedly changed, and finally preserved for use in glass jars in saturated salt solution. He found that when washed free from salt, and placed in a moist chamber at a temperature of 16° C., the hair was sufficiently loosened by bacterial action in four to five days. Pieces placed in the moist chamber without previous removal of the salt only showed signs of sweating after about ten weeks’ exposure. Liming experiments were made with similar pieces of salted hide, both after three days’ washing to free them from salt, and unwashed, and in both cases the pieces unhaired freely in three to four days. These experiments were varied by using 6, 18 and 30 grms. of lime per liter of water in which about 200 grms. of hide were placed, but neither in the washed, nor unwashed portions was there any material difference in the time required to loosen the hair. Addition of 1 vol. of used lime-liquor to 3 vols. of water in making up the limes was equally without perceptible influence, and careful bacteriological examination of hide and liquors showed that the former was almost sterilised by the intense salting, and that the lime-liquors were practically free from bacteria.

Von Schroeder’s conclusion that no gain arises from the use of excessive quantities of lime, so long as the solution is kept saturated, is fully justified both by experience and scientific reasoning, but his results with regard to the effect of old liquors and bacteria contradict the conclusions both of practical tanners and of other scientific experimenters.

The different effects of old and new limes are too well known to practical tanners to be discounted by laboratory experiments, even if they were not confirmed not only by Eitner’s results, but by a considerable amount of work done in the Author’s laboratory and elsewhere; while the necessity of bacterial action is at least rendered probable by the fact that soda solutions, which are completely sterile to bacteria, fail to unhair hides which have not previously undergone some putrefaction (see p. 137). In some experiments undertaken at the suggestion of the Author it was found that a perfectly fresh and sterilised calf-skin which was not unhaired after ten days’ liming in sterilised lime-liquor unhaired rapidly on the addition of a bacterial culture to the lime. It is extremely difficult to exclude bacteria, and even where perfectly fresh skins treated with chloroform or carbon disulphide were employed, bacteria were always to be recognised when the skin was ready for unhairing. Von Schroeder’s work, is, however, so painstaking and reliable, that these divergent results must be explained as other than experimental errors. With regard to old liquors, it is known that ammonia is a powerful aid to the unhairing process, and it is not certain to what extent the liquors he used were charged with it. It is also certain that old limes containing much organic matter, support bacterial life freely, while 25 per cent. of a possibly not very old liquor would probably be sterilised by the addition of lime and 75 per cent. water. In order to test the matter fairly under exact tannery conditions, the lime should have been made up entirely with old lime-liquor well charged with ammonia and organic matters, instead of with water. It is also probable that the hides had undergone a sufficient amount of bacterial change in the tannery before they came into Von Schroeder’s salt solutions, and it is not at all unlikely that the salt solution itself exercised some specific effect on the unhairing. It is also possible that his bacterial cultures were made on gelatine media unsuitable for the growth of alkaline bacteria, and therefore gave blank results. Under these circumstances it is scarcely possible to arrive at any very definite conclusions, and it is obvious that further experiments on these points are extremely desirable.

Sodium and Potassium Hydrates.—From the earliest antiquity, wood-ashes, consisting mainly of potassium carbonate, have been used for unhairing, either alone or in conjunction with lime, and indeed the German name of the process (Aeschern) is derived from the fact. In more recent times, caustic soda, either ready formed, or causticised on the spot by the addition of lime, has often been recommended as a substitute for lime. Its action is very similar to lime, but, from its greater solubility, is far more powerful, and probably this has hitherto formed one of the greatest obstacles to its use, since a solution of the strength of lime-water is almost immediately exhausted, while a much stronger one is too violent in its action on the hides. Experiments made in the Author’s laboratory show that caustic soda, in solutions of the same strength as lime-water, dissolve considerably less hide substance than the latter, but it is more antiseptic than lime, and does not unhair readily without the aid of bacterial action (cp. p. 137). It also swells more violently, and it is difficult to keep the grain smooth and unwrinkled.

Caustic soda has the great advantage that from its solubility, and that of its carbonates in water, it is much more easily and completely removed by washing than is the case with lime. It has been successfully applied in some instances to soften skins of which the texture is naturally too compact for moroccos and the softer leathers; and is usefully employed in softening dried goods (p. 115). Where caustic soda is required merely to “sharpen” limes, it is best added in the form of sodium carbonate (soda-ash or crystals), which are causticised by the lime in the pits. One-quarter or one-half per cent. on the weight of hides added in this way decidedly increases the plumping power of the lime. It may be noted that in the use of sodium sulphide in conjunction with lime, caustic soda is one of the products of its decomposition,[78] and is probably one great cause of the difference of effect of this material for sharpening limes as compared with red arsenic.

An indirect method of liming has recently been patented by Messrs. Payne and Pullman of Godalming,[79] which is of both scientific and practical interest. From the difficult solubility of lime, and the consequently weak solutions which must be employed, the ordinary process of liming is a slow one. Caustic soda, however, can be used in much stronger solutions without producing injury to the hide, or larger solution of hide substance, and from its great diffusibility, it penetrates very rapidly. Used alone, however, the hide becomes too much swollen for most purposes, and for certain classes of leather at least (e.g. buff and chamois leather) the presence of a portion of lime in the hide appears to be necessary for successful work. If a hide which has been swollen with caustic soda be afterwards treated with a solution of calcium chloride, double decomposition takes place, and caustic lime is formed actually in the interior of the fibre of the hide, while the sodium unites with the chlorine to form common salt. Both solutions may be used in any convenient way, and by the employment of drums, the whole liming process may be accomplished in five or six hours. It is found, however, that perfectly fresh hides treated in this way cannot be unhaired, and the explanation appears to be that in the ordinary liming process, the epidermis is made soluble by the joint action of bacterial ferments and of the alkaline solutions. If sodium sulphide be added to the caustic soda used for unhairing, the goods will unhair without the use of putrefactive means, but the process is difficult to manage without destruction of the hair, and Messrs. Pullman now recommend that all hides or skins for unhairing by their process should be soaked for forty-eight hours in winter, and twenty-four hours in summer in a really putrid stale soak. This necessity constitutes for very many purposes a serious weakness in the method, as putrid soaking is always extremely dangerous to the grain of the hide, and especially so in hot weather. For certain purposes, however, advantage may be taken of the fact that the hide or skin can be fully limed by Pullman’s process and the fibres swollen so as to be prepared for tanning without any loosening of the hair, and the Author has seen deerskins which have been treated in this way, on which the hair was perfectly firm, while they possessed a softness and fulness which could not be attained without liming.

[79] Eng. Pat. 2873, 1898.