The peculiarities which were shown by the hide pieces on removal from the water were maintained throughout the tanning, which was conducted in imitation of the German method, the hide being swollen and coloured through in weak birch-bark liquors, made with distilled water and acidified in each case with equal quantities of lactic acid, and finally laid away, till tanned, in a mixture of oak bark and valonia. No. 6, from magnesium sulphate, was the best; then No. 2; No. 3 was less good, but all the pieces from 1 to 6 were firm, close and of good substance and texture, No. 1 having swelled well in the sour liquor. On the other hand, 7 and 8 scarcely swelled in liquor, but remained flat throughout, and were looser, thinner and of finer fibre. From this experiment it is clear that while sulphates and carbonates exert a favourable influence on plumping, chlorides do the reverse, as they themselves not only do not plump, but they place the hides in an unfavourable condition for the plumping action of acids in the liquors. These experiments are quite borne out by the writer’s experience in practice. The water at the Lowlights Tannery, which in dry weather was mostly obtained from beds of what was originally sea-sand, and which consequently contained a very abnormal proportion of chlorides (up to 68 pts. NaCl per 100,000), required special and very careful management to make thick leather, notwithstanding the fact that it contained a considerable quantity of calcium and magnesium sulphates. These facts also indicate the importance of the thorough removal of salt from hides intended for sole-leather. Plumping is not a desirable thing in leather intended for dressing purposes, and it is possible that the use of a small percentage of salt in the liquors or wash waters might in some cases enable bating to be dispensed with. Like a bate, salt would dissolve a small proportion of hide substance (see p. 65). There is no practicable means of removing chlorides from water, but Eitner suggests the addition of a small quantity of sulphuric acid to water containing much temporary hardness (bicarbonates), in order to convert it into permanent hardness (sulphates), which, as stated above, plumps better. The amount required may be calculated from an acidimetric determination of temporary hardness (see L.I.L.B., p. 19). A simple but not very accurate guide, is to add enough acid to purple, but not to redden litmus paper even after moving the latter about in the water for some minutes. In practice the acid must of course be very thoroughly mixed with the water by stirring and plunging. It must be borne in mind that Eitner’s experiment was on sweated hides, and that with limed hide, which is kept plump by the dissolved lime retained in the hide, different results as regards carbonic acid and bicarbonates would be obtained. Both these would convert the lime in the hide into chalk, which is insoluble and inert, and the hide would fall, at any rate when the lime was completely carbonated, while hides would remain plumpest in waters most free from substances capable of neutralising lime. From this we may conclude, what may be a priori expected, that the purer the water, the plumper limed hides remain in it. In soft but peaty waters, hides fall rapidly, from the neutralisation of the lime by the weak organic acids of the peat. Such waters are dangerous for domestic use from their solvent action on lead, but this danger can be entirely removed by storing the water in limestone reservoirs, or allowing it to flow slowly through a limestone culvert before use. In some towns in the north of England a small quantity of lime is added so as to neutralise the water as it leaves the reservoir and before it enters the mains.

Wherever the conditions of putrefaction or decaying organic matter are present, as in a bate, hides fall rapidly, and in extreme cases even the presence of the stronger acids will not maintain plumpness. Eitner mentions the case of a stream at Vissoko in Bosnia, which was in special repute among the tanners from its power of pulling down hides rapidly, and which took its rise in a common on which the pigs of the town were pastured. The causes of this action are no doubt due to the products of putrefaction, but are somewhat obscure. Bacteria present in water are a frequent source of injury in the soaks, and probably in other stages of the tanning process.

Rain water and the water of streams in mountain districts of hard igneous rock are generally nearly free from mineral constituents. This is the case with the Glasgow water from Loch Katrine, and the Thirlmere water which supplies Manchester. Such water, if cold enough, and free from mud and organic impurity, is the best for almost every purpose in the tannery. Most river-water contains material quantities of mineral matter, though it is usually softer than that of springs or wells.

For further details as to the chemical examination of water, and the methods of determining the amounts of its different constituents, see L.I.L.B., pp. 18 et seq.

CHAPTER XI.
SOAKING AND SOFTENING OF HIDES AND SKINS.

As has been explained in the last chapter, hides and skins come into the hands of the tanner either uncured (“green”), as they are taken off the animal, preserved with salt or some other antiseptic, dried, or “drysalted” in which both methods are combined. His object in each case is to remove blood and dirt, and to restore the hide to its soft and natural condition; but the treatment required varies much with the state of the hides.

Fresh hides merely require cleansing from blood and dirt. This is necessary because the blood causes bad colour, and both blood, lymph and adhering dung are sources of putrefaction, which ultimately attacks the grain and fibrous structure of the hide. Hence washed hides keep better than unwashed. Cold water is most desirable, as checking putrefaction. If the water is much over 10° C., or if it is charged with organic matter and ferment-germs; or if, as is too generally the case, the hides are in a partially putrid state when received, the time of soaking must be reduced as much as possible, and it may be necessary to sterilise the water with carbolic acid or creolin (pp. 26, 28). In such cases the use of a wash-wheel, or tumbler, is very desirable, rapidly cleansing the hides and removing adhering dung, which interferes with the liming, and is a serious cause of damaged grain. The American pattern of wash-wheel shown in Fig. 21 is very suitable for the purpose. In no case is it desirable to allow green hides to lie for more than a few hours in water; and unwise treatment at this time is the cause of many troubles, which are only detected at later stages, and which are very difficult to trace to their source. “Weak grain,” in which the hyaline layer (p. 50) is destroyed, and which tans a whitish colour; “pricking,” or perforation of the grain with small pinholes, which may go on to “pitting” with larger holes, and a general weakening of fibre, with softening and needless loss of weight, are among these results. An instructive instance may be quoted. A large tanner found that his curried leather was affected with small spots and rings of darker colour, which rendered it quite unfit for staining, and which reappeared even when the leather was buffed. When finished as black grain, these spots had a tendency to “spue,” or rise as little pimples of resinous matter. Before the leather was stuffed no defect was noticeable to the eye, but either then, or on stripping the grease by a solvent, they could be seen under the microscope as lighter patches of open and porous grain which absorbed more than their share of fat. During the tanning process they could hardly be detected, but in the first colouring they appeared for a few hours as blackish specks almost exactly like those caused by particles of iron or iron-rust. By careful observation they were traced back to the limes; specimens of the limed hide were submitted to Director Eitner, who identified the defect as “Stippen,” caused by a species of bacteria, which cannot subsist in limes, and which therefore must have been in the soaks. These, which had been somewhat neglected from pressure of work, were cleaned out and sterilised with creolin solution, and the mischief ceased. It is worth noting that the tanner dated the beginning of the trouble from the soaking of some “Spanish” horse-hides, which may have introduced the infection. Several very similar cases have come under the writer’s notice.

It is not absolutely necessary to soak fresh hides or skins at all before liming, and where the water is scarce or unfavourable, or the skins tainted or “slipping” hair, it is best to pass straight into a weak lime. In this case the limes must be worked in shifts (see p. 131) and the whole of the oldest liquor run away and the hides rapidly changed into a fresh lime, or the limes will become so charged with organic matter and bacteria that the hides will cease to plump, and may even putrefy.

Salted hides and skins require more soaking and more thorough washing than fresh ones, as it is not only necessary to remove the salt, but to soften and plump the fibre which has been dehydrated and contracted by salting. If goods with salt in them are taken into limes, they will not plump properly,[67] and creases and wrinkles (drawn grain) are formed which no after-treatment will remove. This is especially important in sole leather. In deciding on a method, we must bear in mind that salt is easily soluble, and diffuses rapidly into water or weaker solutions, and that weak salt solutions tend to prevent the plumping of the fibre, while those of about 10 per cent. have considerable power of dissolving the cementing matter of the fibres (p. 65) and so lessening weight and firmness. It may also be noted that though salt is not a true disinfectant (p. 22), salted hides are much less prone to putrefaction than fresh ones, and therefore a longer soaking may be safely given.

These conditions point to the desirability of free exposure to water, attained by suspending, handling frequently, or tumbling, and repeated changes to remove the salt. The degree of removal of salt is easily determined by the estimation of Cl in the last wash-water (L.I.L.B., p. 18). American tanners universally soak wet-salted hides three or four days with as many changes of water, and frequently finish by a few minutes in a wash-wheel. Any washing tumbler may be used; but the cheap and simple construction of the American wash-wheel will be easily understood from Fig. 21. The sides are open, so that hides can be put in or removed between the spokes. The rim of the wheel is generally perforated, for the escape of water which is supplied by a pipe passing through the axis; and the wheel is often driven by a chain or rope round its circumference. No severe mechanical treatment, such as “stocking,” is necessary or desirable for green or salted hides.

Dry and dry-salted hides require much longer soaking than wet-salted, the amount naturally depending on the thickness of the hide and the character of drying. Even thin skins when strongly dried require considerable time to soften and swell the fibres, although they soon become wet-through and flexible. Many different methods of soaking have been employed. Sometimes hides are suspended in running water; sometimes laid in soaks which may be either renewed, or allowed to putrefy; sometimes in water to which salt, borax or carbolic acid has been added, to prevent putrefaction; and more recently weak solutions of caustic soda, sulphide of sodium or sulphurous acid have been used with much success.

The first of these methods, were it desirable, is rarely possible in these days of River Pollution Acts; of the others, it is difficult to say which is better, since the treatment desirable varies with the hardness of the hide and the temperature at which it has been dried. The great object is to thoroughly soften the hide without allowing putrefaction to injure it. As dried hides are often damaged already from this cause, either before drying, or from becoming moist and heated on shipboard, it is frequently no easy matter to accomplish this. The fresh hide, as has been seen, contains considerable portions of albumin, and if the hide is dried at a high temperature, this may become wholly or partially coagulated and insoluble. The gelatinous fibre and the coriin (if indeed the latter exists ready formed in the fresh hide) do not coagulate by heat, but also become less readily soluble. Gelatin dried at 130° C. can only be redissolved by acids, or water at 120° C. Eitner[68] experimented with pieces of green calf-skin of equal thickness, which were dried at different temperatures, with results given in the following table:—

Hence it is evident that, for hides dried at low temperatures, short soaking in fresh and cold water is sufficient, and, except in warm weather, there would be little danger of putrefaction. With harder drying, longer time is required, and more vigorous measures may be necessary. A well-known tanner recommended a brine of 30°-35° barkometer (sp. gr. 1·035, or about 5 per cent. of NaCl). This has a double action, not only preserving from putrefaction, but dissolving a portion of the hide-substance in the form of coriin, which is undoubtedly a loss to the tanner, though it is questionable if there is any process which will soften overdried hides without loss of weight; since even prolonged soaking in cold water at a temperature which is too low to allow of putrefaction taking place will dissolve a serious amount of hide-substance. Chlorides, however, do not seem well adapted for the purpose in view, from their weak antiseptic power and tendency to prevent swelling. To prevent this Jackson Schulz advised the use of water at 80° F. for soaking during the winter months. Water containing a small quantity (0·1 per cent.) of carbolic acid has been recommended for the purpose, and will prevent putrefaction, while it has no solvent power on the hide, but, on the contrary, tends to coagulate and render insoluble albuminous matters. Borax has been proposed for the same purpose, and, in 1 per cent. solution, certainly prevents putrefaction, and has considerable softening power, but is far too costly. Other methods of chemical softening are described on p. 115.

For some descriptions of hides, and notably for India kips, putrid soaks were formerly much employed, the putrefactive action softening and rendering soluble the hardened tissue. In India the native tanners soften their hides in very few hours by plunging them in putrid pools, into which every description of tannery refuse is allowed to run. Putrefactive processes, however, are always dangerous, as the action, through changes of temperature, or variation in the previous state of the liquor, is apt to be irregular, and either to attack one portion of the hide before another, or to proceed faster than was expected. Hides are also frequently more or less damaged by putrefaction and heating during the process of cure, and these damages are accentuated in a putrid soak. Hence hides in the soaks require constant and careful watching, and the goods must be withdrawn as soon as they are thoroughly softened, for the putrefaction is constantly destroying as well as softening the hides. It is possible that putrefactive softening is less injurious to kips, and such goods as are intended for upper-leather, than to those for sole purposes, as it is generally considered necessary in the former case that a good deal of the albumen and interfibrillary matter be removed, and that the fibre be well divided into its constituent fibrils for the sake of softness and pliability; and thus the putrid soak, if acting rightly, accomplishes part of the work which would afterwards have to be done by the lime and the bate, as the actual fibre of the hide seems less readily putrescible than the softer cementing substance.

Putrefaction is caused, as we have seen, by a great variety of living organisms, each of which has its own special products and modes of action. It is quite possible that, if we knew what precise form of putrefaction was most advantageous, we might by appropriate conditions be able to encourage it, to the exclusion of others, and obtain better results than at present. Putrid soaks (in the old sense) are, however, disused in the present day by all enlightened tanners, as it is recognised that the risks outbalance the advantages, and when drysalted hides are worked, the soluble salts of the cure accumulate to an injurious extent. The modern method, where no chemicals are used, is to give one fresh water at least to each pack of hides or skins. Even in this case considerable putrefaction takes place where the soaking occupies 7 to 14 days, as is the case with kips and hides, and it is probable that the use of chemical and antiseptic methods of soaking will ultimately be generally adopted, both on technical and sanitary grounds.

The use of dilute acids for softening has much to recommend it, their power of causing the fibre to swell and absorb water being quite equal to that of the alkalies, while few, if any, putrefactive bacteria can thrive in an acid liquid. Very dilute sulphuric acid has been used with success to dissolve the alkaline “plaster” of East India kips (p. 39). It has considerable disinfectant power (p. 23), but its action on the hide-fibre is undesirably strong.

Sulphurous acid is much more suitable. Its use for this purpose was patented by Maynard, along with a number of other possible uses, but the patent has now lapsed, and he does not seem to have succeeded in introducing it into practice. Experiments at the Yorkshire College, and also at a tannery on a manufacturing scale, have shown that the method is capable of excellent results. The hides are soaked for 24-48 hours in a solution of sulphurous acid containing about 2 per cent. of SO₂ (for manufacture, compare p. 24; for testing, L.I.L.B., pp. 16, 37), and are then transferred to water, where they swell freely to their full thickness. They may be either limed at once, or first neutralised with dilute caustic soda, ammonia, or sulphide of sodium, which, for dressing leather, is perhaps desirable. No putrefaction takes place, even if they are retained for a considerable time in water, and the acid has little or no solvent effect on the hide-fibre, the strength of which is well preserved. The liming, however, must either be conducted with the aid of sodium sulphide or in old limes, since the sterile condition of the hides renders liming in fresh lime very slow (cp. p. 137). For experimental purposes a ¹⁄₂ per cent. solution of Boakes’ “metabisulphite of soda” may be used, to which ¹⁄₄ per cent. of concentrated sulphuric acid previously diluted with water is gradually added during the soaking, the hides being first withdrawn. For permanent work it will be found much cheaper to manufacture the acid on the spot by burning sulphur.

The use of solutions of caustic soda (1 part per 1000), or of sodium sulphide (1¹⁄₂-3 parts per 1000) as suggested by Eitner, seems at present likely to supersede all other methods of softening from their simplicity and safety. Twenty-four to forty-eight hours in either of these solutions, which may if necessary be followed by a short soak in plain water, seem sufficient to soften either kips or hides. Experiments at the Yorkshire College have shown that solutions of this strength have little or no solvent action on the hide-fibre, but promote its swelling in water so effectively that no mechanical softening is needed (though a slight drumming is advantageous), while putrefaction is almost entirely prevented, so that the solution may be repeatedly used if kept up to its original strength, which is easily determined with standard acid and phenolphthalein (see L.I.L.B., p. 17). Neither caustic soda nor sodium sulphide have any injurious effect on liming, though it may prove somewhat slower than with the older methods, where the epidermis was partially destroyed by the action of putrid ferments. The dilute solutions used are not only less injurious to the hide than those of greater strength, but they are also more effective in softening. Eitner (Gerber, 1899, p. 584) states that when using a solution of caustic soda of 1 part in 1000 strength, the time required to soften some hides was only two days, as against three days for a sodium sulphide liquor, and four days for pure water, and that with the soda solution only about 0·6 per cent of the hide-substance of the skin was dissolved out, whilst when sodium sulphide was used it was 0·7 per cent., and with pure water alone no less than 1·9 per cent. was lost by solution.

The use of moderately warm water (40° C.) in a drum is quite successful in rapidly softening sound hides after they have previously been soaked for some days in cold water; but if they are tainted in the cure, it is very apt to intensify the mischief. Hides which have partially putrefied internally, or which have been exposed to a hot sun while the interior is still moist, are very apt to appear sound while dry, but to blister or go to pieces from the destruction of the fibres as soon as they are limed, and this in spite of even the most careful treatment. For tainted hides, caustic soda is probably preferable to sodium sulphide.

Many chemicals have been patented for softening hides. Sulphide of arsenic is said to be in use, and if dissolved in caustic soda solution would differ little in its effect from ordinary sulphide of sodium. Saltpetre has also been employed, but its effect, if any, was probably merely antiseptic. Ordinary sodium carbonate has been used, but is less effective than caustic soda. Gas liquor and mixtures of this with tar and water were patented by Barron, and probably the first would soften by virtue of its ammonia and sulphides, while tar contains carbolic acid. Probably the most absurd mixture of all was patented by Berry, which consisted of ¹⁄₂ bucket of slaked lime, ¹⁄₂ bucket of wood-ashes, 12 lbs. of potash, 5 lbs. of oil of vitriol, and 4 lbs. of spirit of salt!

Beside merely soaking the hides, it is sometimes necessary to work them mechanically, to promote their softening; this was formerly accomplished by “breaking over” the hides on the beam with a blunt knife. This process is still in use for skins of many sorts, but for the heavier classes of leather is now usually superseded or supplemented by the use of “stocks,” or drums. The former consist of a wooden or metallic box, of peculiar shape, wherein work two very heavy hammers, raised alternately by pins or cams on a wheel, and let fall upon the hides, which they force up against the curved end of the box with a sort of kneading action. The ordinary form of this machine is shown in Fig. 22. A more modern form, which seems to possess some advantages, is the American “double-shover,” or “hide-mill,” seen in Fig. 23. “Crank stocks,” similar in form to the faller stocks, but driven by cranks, are sometimes used for softening, but are better adapted to lighter uses.

The number of hides which can be stocked at once naturally varies with the size of both hides and stocks, but should be such that the hides work regularly and steadily over and over. The whole number should not be put in at once, but should be added one after another, as they get into regular work. The duration of stocking is 10-30 min., according to the condition and character of the hides. Hides should not be stocked until they are so far softened that they can be doubled sharply, without breaking or straining the fibre. After stocking, they must be soaked again for a short time, and then be brought into an old lime. A small quantity of sodium sulphide added to the soaks or in the stocks has been recommended as of great value in softening obstinate hides, and probably with justice, from its well-known softening action upon cellular and horny tissues.

Tumbler drums of various forms may also be used with good effect for softening purposes, especially for skins, and are much less detrimental than stocking, both as regards the weight and quality of the goods.

For sole leather, and even for kips, the use of stocks has in recent years been entirely discarded by many of the more advanced tanners. If mechanical work is required at all, the drum is preferred, and is sometimes employed after a few days’ liming, the goods being first merely softened in fresh water. The use of caustic soda, sodium sulphide, or sulphurous acid renders mechanical softening almost unnecessary.

The drums employed are in principle like a barrel-churn, and are large cylindrical wooden chambers 6 to 12 feet in diameter, and fitted inside either with shelves like the floats of a water-wheel, or with rounded pegs on which the hides fall. The American wash-wheel figured on p. 111 is a machine of this kind, and one of a more elaborate description is shown in Fig. 24. Drums are not only used for softening, but for tanning, dyeing, and many other purposes in leather manufacture. It is advantageous to be able to reverse the direction of their rotation to prevent the rolling up of the hides.

CHAPTER XII.
DEPILATION.

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.

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.

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]:—

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]:—