[S] Hides have been unhaired by the action of gaseous ammonia alone, but the method does not seem suited for technical use.
The process is principally used in America for dried hides, but may be employed either for wet or dry salted, after complete removal of the salt. It is imperatively necessary that dried hides should be completely softened before sweating. As the sweating process advances more rapidly in the upper than in the lower part of the pit, and as the thick portions are more resistant than the thin ones, the hides, after about 3 days' sweating, require constant attention in changing their positions, and in checking the forward ones by taking down and laying in piles on the bottom of the pit.
The usual treatment for sweated hides, when the hair is sufficiently loosened, is to throw them into the stocks, and work out in this way the slime and most of the hair. This has the disadvantage of working out too much of the dissolved gelatin, and of fulling the hair so firmly into the flesh, that it is difficult again to remove it. To overcome these evils, some American tanners now pass the hides, after sweating, through a weak lime. This, to a great extent, prevents the hair fixing itself in the flesh, and tends to counteract the injurious effect of the vitriol (which is almost invariably used in plumping sweat stock) on the colour of the leather. By this process, 10,000 Texas and New Orleans wet-salted hides gave an average yield of leather of 73 per cent. on their green weight, and the leather was excellent in quality (Schultz). If sweated or very lightly limed hides are imperfectly worked on the grain, greasy spots are apt to remain, which will not colour in the liquors ("white spots"). These may be made to colour by scraping and working the grain with a knife, or by the application of a solution of soda or soda ash, and would probably be avoided by the use of soda ash in the soaks on greasy parcels of hides.
It must be clearly understood that all sweating depends on partial putrefaction. This is proved both by the plentiful production of ammonia in the pits, and by the fact that antiseptics, such as salt or carbolic acid, entirely prevent sweating till they are removed. Although the process undoubtedly has advantages, and especially so in the treatment of dried hides, it is an open question whether it gives the extreme gains over liming in weight and firmness, which are claimed by some of its advocates.
An unhairing process, largely coming into use on the Continent, depends on the action of alkaline sulphides, and particularly sodium sulphide, upon the hair. While all the methods already spoken of involve the softening and 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 early times to the present day, arsenic sulphide ("rusma") mixed with lime has been used in unhairing skins for glove-leather and similar purposes. About 1840, Böttger 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 lime sulphydrate, formed by passing sulphuretted hydrogen into milk of lime, as a substitute for the poisonous and expensive arsenic compound. It 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, oxidising 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.
It was proposed to replace it by sodium sulphide, 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 crystallised form (at first 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 iron sulphide, which must have been held in suspension at the time of crystallisation. 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. Sodium sulphide is now manufactured from tank waste in a much purer form by Schaffner and Helbig's process, of which Messrs. Gamble of St. Helens are sole licencees. The crystallised salt is SNa210Aq, and therefore contains 69·8 per cent. of water.
For sole-leather, the method recommended by Eitner is to dissolve 4-5 lb. of sulphide per gal. of water, making the solution into a thin paste (of soupy consistence) with lime or pipe-clay. 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 15-20 hours will be ready for unhairing, the hair being reduced to a paste. In the writer's experience, the concentrated solution here prescribed will completely destroy all hair wetted with it in 2-3 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 are rather difficult to flesh, unless previously plumped by a light liming, and it is necessary to swell them with acid or sour liquor in the tanhouse, as the sulphide has but little plumping effect.
Another method, which is much cheaper in labour and easier in execution, is to suspend in a solution of sodium sulphide, containing 3/4 lb. a hide or upwards; the hide should unhair in 24 hours. Very weak solutions loosen the hair, without destroying it; but it is always weakened, as the specific action of the sulphides is on the hair itself. After or before unhairing, the hides may receive a light liming, to plump them, or lime may be added to the solution of sulphide, which by forming calcium sulphide, and liberating caustic soda, considerably increases the unhairing and plumping effect. The pit may be several times strengthened for successive packs, but the loosened hair must be fished out, or it will quickly spoil the solution. When hides have been suspended in sodium sulphide solution, the hair is very quickly loosened by a short liming. Squire, Claus, and J. Palmer have all taken out patents for the use of tank-waste as a depilatory. It consists of impure calcium sulphides, and when brought into the form of soluble sulphydrate, either by boiling in water, or by the oxidising action 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. 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, and to get good results the bate should at most only be allowed to have a superficial effect. Professor Lufkin proposed the use of a mixture of various sulphides of lime and soda, formed by mixing 10 lb. each of soda ash and sulphur, kneading to a paste with a little moist slaked and then mixing warm in a cask with 80 lb. stone lime slaked to a paste. This quantity will unhair 50 hides in the same way and in about the same time as an ordinary lime. The pelt is not much plumped and is easily reduced by a few minutes' wheeling in warm water. (J. S. Schultz.)
Various other depilatories have been proposed, but as they have not come into general use, brief mention of the most important will suffice. Anderson, in 1871, patented the use of wood-charcoal, applied in a similar manner to lime in the ordinary process. The hair is probably loosened simply by putrefaction, as in sweating, while the charcoal acts as a deodoriser, very little smell being produced, and the action proceeding with considerable uniformity. John Palmer has patented a process for unhairing, in which the hides are alternately steeped in water and exposed to the air till the hair loosens. In this, very similar principles to those of the charcoal method are involved. Caustic potash and soda will loosen hair, but seem to have no decided advantage over lime, though it is quite possible that in skilful hands good results might be obtained. They are more costly, and their corroding action on the hide-substance is more powerful, but they form soluble soaps with the grease of the hide. Unless used in very dilute solution, the pelt is so swollen as to fix the hair, and the leather is dark-coloured and spongy. Soda-ash or crystals (sodic carbonate) may be used to strengthen ordinary limes, in which caustic soda is formed. The time of liming is shortened, the hides are more swollen, and the grease is better "killed" than when lime alone is used. The patent for Moret's "Inoffensive" claimed the use of the carbonate or caustic potash formed from calcined wool-washings, for unhairing. This is more costly than, and has no advantage over soda. I am not aware whether "Inoffensive," as now sold, has other constituents.
Whatever method of loosening the hair may be adopted, the next step is to remove it by mechanical means. This is usually accomplished by throwing the hide over a sloping beam, and scraping it with a blunt two-handled knife ([Fig. 25]), the workman pushing the hair downwards and away from him. The beam is now usually made of metal. The knife employed is also shown at C, [Fig. 26].