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 Wein 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 st the time of crystal lization. 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 4-5 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 bair 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 will be very difficult to flesh, unless previously plumped by alight limeing, 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 lb. a hide ; the hide is said to unhair in 24 hours. Very weak solutions loosen tho 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 limeing, 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. 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. Caustic potash and soda will loosen hair, but seem to have no decided advantage over limo. They are more costly, and their corroding action on the hide-substance is more powerful. 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 aulphydrate, either by boiling in water, or, it is said, by the oxidizing action of the air, it will unhair hides. The conversion is, however, very imperfect in either ease, 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 limeing.
Palmer employe 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 liido for rapid and heavy tanning, but the swelling and subsequent reduction almost certainly entail loss of weight and quality.
Whatever method of loosening the hair may he 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. 902), 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. 903.
When a bide is lightly limed, it is often easy to remove the long hair, but excessively difficult to get rid of the short under-coat of young hairs, which are found in spring, and which can sometimes only be removed by the dangerous expedient of shaving with a sharp knife. The reason of this difficulty is obvious : not only do the short hairs offer very little hold to the unhairing knife, but, as has been explained in describing the anatomical structure of the skin, their roots are actually deeper seated than those of the old hairs they replace. - Several attempts have been made to unhair by machinery, but so far without such success as to lead to their general adoption. The fleabing-machine invented by Garrie and Terson, and manufae ured in this country by T. Haley and Co., of Bromley (Fig. 904), is furnished with a special wheel for unhairing. An American machine for the purpose, invented by J. W.