The removal of hair from hides is most commonly effected in England by the agency of lime, or by sweating processes demanding no further description. But besides lime many other substances have been proposed or adopted as depilatories, and these will now be noticed.
(1) An unhairing process, largely coming into use on the Continent, depends on the action of alkaline sulphides, and particularly sulphide of sodium, upon the hair. While the methods already spoken of involve the softening or destruction of the hair-sheaths, either by lime or by putrefaction, the sulphides are peculiar in attacking the hair itself; when strong, they disintegrate it rapidly and completely into a sort of paste. From very 2 early times, sulphide of arsenic mixed with lime has been used in unhairing skins. About 1840 Bottger concluded that the efficacy of arsenic sulphide was due simply to the sulphydrate of lime formed by combination of the sulphur with the lime, and proposed sulphydrate of lime, formed by passing sulphuretted hydrogen into milk of lime, as a substitute for the poisonous and expensive arsenic compound. This proved a most effective depilatory, but has never obtained much hold in practice. This is probably due to the fact that it will not keep, oxidizing rapidly on exposure to the air; hence it must be prepared as it is required, which is both troublesome and expensive.
A minor objection is the unpleasant smell of sulphuretted hydrogen, which is inseparable from its use.
(2) It was proposed to replace it by sulphide of sodium, which, though at first said to be only effective when mixed with lime, so as to produce calcic sulphide, has since proved a powerful depilatory alone. Its use has been greatly extended on the one hand by its production on a large scale, and in the crystallized form (presumably by reduction of sulphate by heating with small coal), and on the other, by the great interest which Wilhelm Eitner, the able director of the Austrian Imperial Research Station for the Leather Trades, has taken in its introduction. The substance, as manufactured by De Haen, of List, Hanover, is in small crystals, coloured deep greenish-black by sulphide of iron, which must have been held in suspension at the time of crystallization. If the salt be dissolved in water, and the solution be allowed to stand, this is gradually deposited as a black sediment, leaving the supernatant liquor perfectly clear and colourless.
For sole-leather, the method recommended by Eitner is to dissolve 4 to 5 lb. sulphide per gal. of water, making the solution into a thin paste (of soupy consistence) with lime or pipeclay. This is spread liberally on the hair side of the hides, one man pouring it down the middle of the hide from a pail, while another, with a mop or cane broom, rubs it into every part. The hide is then folded into a cushion, and in 15 to 20 hours will be ready for un-hairing, the hair being reduced to a paste. In H. R. Procter's experience, the concentrated solution here prescribed will completely destroy all hair wetted with it in 2 to 3 hours, and if left on longer, will produce bluish patches, and render the grain very tender. The hides should be thrown into water before unhairing, to enable them to plump, and to wash off the sulphide, which is very caustic, attacking the skin and nails of the workmen. There is no doubt that this process gives good weight, and tough and solid leather; but there are several difficulties attending its use. Unless the mopping is done with great care, it will fail to completely destroy the hair, and the patches of short hair left are very difficult to remove. The expense of the material and the loss of hair are also important considerations.
The hides will be very difficult to flesh, unless previously plumped by a light liming, and it is generally considered necessary to swell the hides with acid before tanning, as the sulphide has but little plumping effect.
Another method, which is more generally adopted for dressing hides, is to suspend in a solution of sulphide of sodium containing about 3/4 lb. a hide; the hide is said to unhair in 24 hours. Very weak solutions loosen the hair without destroying it; but it is always injured, as the specific action of the sulphides is on the hair itself. After unhairing, the hides may receive a light liming, to plump them, or lime may be added to the solution of sulphide.
Various other depilatories have been proposed, but as they have not come into general use, brief mention of the most important will suffice.
(3) Anderson, in 1871, patented the use of wood-charcoal, applied in a similar manner to lime in the ordinary process. The hair was probably loosened simply by putrefaction, as in sweating, while the charcoal acted as a deodorizer.
(4) Caustic potash and soda will loosen hair, but seem to have no decided advantage over lime. They are more costly, and their corroding action on the hide-substance is more powerful.
(5) Squire, Claus, and J. Palmer have all taken out patents for the use of tank-waste as a depilatory. It consists of impure sulphides of calcium, and when brought into the form of soluble sulphydrate, either by boiling in water, or, it is said, by the oxidizing of the air, it will unhair hides. The conversion is, however, very imperfect in either case, and its action is uncertain and slow; while the iron present is apt to cause unsightly stains. It is probable that the weights obtained may somewhat exceed those by liming.
(6) Palmer employs sulphuric acid to plump the hide and remove stains, and then reduces it by a bate of whiting and water. He claims that this prepares the hide for rapid and heavy tanning, but the swelling and subsequent reduction almost certainly entail loss of weight and quality.
(7) Instead of the offensive bate generally used, a composition holding 2 to 3 1/2 oz. of realgar to 11 lb. of well-causticised potash is employed. It is applied, at a strength of 8° to 14° B., by means of a mop, to the flesh side of hides. It is also used in a vat for unhairing, at a strength of 1° to 4° B. For preserving hides for transport, he simply keeps the hides in a solution of American (caustic) potash, 8° to 15° B., with the addition of 30 gr. of salicylate of soda to every cwt. of composition, at 10° B. The hides are immersed, and then left wet, or dried. If dried, by steeping in water for 12 hours (the inventor states) they are brought back to their natural condition. (Moret.)
(8) The Carter Process
Writers on tanning, theoretical and practical, all recognise the damage done to the hide by the use of lime, which, while decomposing the grain and roots of the hair, also injures the animal fibre. The caustic nature of the lime alters the texture of the skin more or less; and as it penetrates it in one or another form, it destroys much of the gluten or gelatine, and is so tenacious of its hold that traces of the lime can be found in the best leather, notwithstanding the bating by hen or other manures - a most filthy process - to remove it. Processes known as sweating hare been introduced, and are largely used, as being less injurious than liming; with these, fermentation and incipient decomposition are a necessity, the stench of a sweat-pit being enough to disgust any one with this process, even if there were no other objectionable features - one of the worst of which is the danger of destroying the softer portions of the hide before the firmer portions have been sufficiently softened for unhairing. The slimy condition of hides, unhaired by sweating, is so objectionable, however, that some tanners treat them with lime to overcome the evil.
The sweating process is more objectionable than liming, but it is preferred because of being less destructive to the gelatine, the presence of which is so essential to making good leather, as it alone is the absorbent of tannin. Decomposition is alike the case in both processes, and carelessness in handling would result in complete destruction; while decomposition, no matter however slight, corrodes the gelatine or natural filling of the skin. After decomposition comes bating, to arrest decay, and acids; when these have done their work, they too must be washed out before the hide is in a fit condition for the tanning liquor. The Carter process for unhairing averts all the evils of liming and sweating. By it the hide is unhaired by being placed in a liquid, which in a few hours loosens the hair and plumps the hide without robbing the skin of a particle of gluten. Yet this unhairing liquor is of so harmless a character that no more injury results to the hands from working in it than though it were the purest of water.
The subtle preserving characteristics of this unhairing liquor are shown by the fact that a hide can be fitted for unhairing as easily as by any other known process; yet the hair can be again reset as firmly as it was before being put into the unhairing liquor, provided the hair is perfect. But if there be a spot that has been sweated, or where decomposition has taken place, no matter how slight, the decay will be arrested, but the hair cannot be reset. Fur skins treated by this method can be tanned with the fur on, leaving the leather very fine and the fur firmly set. The fact that the hair can be set as firmly as before tanning commenced, establishes the other fact that this liquor does not decompose the slightest particle of the substance of the hide. The result attained by this process of unhairing is the abolition of decomposition by lime or sweat-pits, and the use of bates, acids, hot liquors, etc. Hides can remain for weeks and months in this liquor without injury. The process is simplicity itself. If dry hides are used, they are softened by being soaked in clean water; the purer the water the better. They are then "broke" over the beam, and thrown into the unhairing liquor. Green hides are washed to remove blood and dirt, and then put into the liquor.
The hides are allowed to remain in the unhairing liquor 24 hours, after which they are in a condition to be unhaired, though no harm would result by the hides remaining in liquor for months. The hides are unhaired and fleshed in the usual way, and then thrown into weak liquor in the wheel pit, where they are allowed to remain 24 hours, after which they are tanned in pure liquors in the regular way. There are no destructive lime-pits, no filthy bating, no putrescent sweating, no absorption or corroding of gluten, no acids, no hot liquors - nothing, in fact, that can injure the hide or disgust the workmen. The putrid smell so prevalent with other methods is unnoticed in this, simply because the first act is to cleanse the hide of filth, and put it into a liquor which at once arrests decay, and commences the process of converting raw hides into leather, (Chem, Rev.)