“Inoffensive” unhairing solution contains a large quantity of arsenic sulphide apparently dissolved in caustic soda, although Moret’s original patent claimed the use of wool-sweat potash only!
W. R. Earp[84] has suggested the use of compounds of sulphur and arsenic (thio-arsenates, thio-arsenites, etc.), in 5 per cent. alkaline solution. He prefers to add the compounds to the ordinary lime-liquors, or to manufacture them in situ by adding the proper quantities of arsenious or arsenic acid mixed with one-third of its weight of sulphur to a solution of an alkaline sulphide in lime-liquor. The pelt is not bated or drenched in the ordinary way, but, after unhairing, is passed directly into the tanning liquor to which sulphurous acid has been previously added.
[84] Eng Pat., No. 2052, Feb. 12, 1886.
There is more danger of injury to the hide from the very prolonged action of weak solutions of sulphides, which tend ultimately to destroy the structure and reduce the fibre to a gelatinous condition, than there is from too concentrated solutions. No danger need, however, be apprehended in the course of any ordinary liming. Arsenical limes are not suited for tainted skins, and they should not be made so strong as to destroy the hair or wool.
For methods of analysis of both old and new lime-liquors, see L.I.L.B., pp. 27 to 34.
Whichever method of loosening the hair be adopted, the actual removal must be effected by placing the hide on a sloping beam with a convex surface, and then scraping it with a blunt two-handled knife ([Fig. 27]), the workman pushing the hair downward and away from himself. The beam may be either of cast iron or of wood, usually covered with zinc to increase its wearing capacity. The hides after being removed from the lime-pits, are allowed to drain for half an hour or so before the hair is removed, and immediately this operation has been completed, they should be placed in soft water. It is of great importance that the limed hides should not be exposed to the air longer than is absolutely necessary for the removal of the hair, as the carbonic acid present in the atmosphere quickly carbonates any lime contained in the surface of the skin, forming chalk, and leading to uneven tanning at a later stage.
When hide has been insufficiently limed it is often easy to remove the longer hair but excessively difficult to get rid of the short under-growth of the young hairs, which even in properly limed skins can often only be removed by shaving them with a sharp handknife. This difficulty is caused partly by the small resistance which the short hairs offer to the unhairing-knife, and partly by their being more deeply rooted in the skin than the older hairs (see [p. 49]).
Fig. 27.—Unhairing (Penketh Tannery).
Various machines have been devised to accomplish the removal of the hair, but owing to the rapidity with which it may be worked off by hand, and the fact that the work is not difficult, no machine has as yet come into general use. Hand-work has the further advantage that in those portions of skin where the hair is tighter than usual it may be removed by greater pressure of the knife or by hand-shaving, whereas after goods have been unhaired by machine they must always be examined and any patches of hair removed by hand on the beam. The edges invariably require to be gone over by hand.