CHAMOIS OR WASH-LEATHER.
This leather, which is remarkable for its soft felty texture, which it
retains even after wetting, although perfectly porous and free from
greasiness in its finished state, is prepared by the action of oil on
the raw skin. Wash-leather was formerly manufactured from sheep- and
calf-skins, and from those of the chamois, and various deer (hence the
name), from which, after liming, the grain was removed (frized) with a
sharp knife, either with the hair, or after unhairing. The flesh-splits
of sheep-skins are now generally employed for ordinary wash-leather,
and of course no such process is needed, though buff-leather for belts
and military purposes is still so manufactured. The skins receive a
thorough liming, which, where softness is desired, is so conducted as
very thoroughly to remove the cement-substance (coriin) from between
the fibres; and this removal is frequently carried still further by a
short bran-drench, which also secures the complete absence of lime.
After the usual beam-work, the skins are pressed or wrung out to remove
surplus water, and while still moist are oiled on a table and folded in
cushions. Fish-, seal-, or whale-oil is generally used, and vegetable
oils do not seem to answer even in mixture, with the exception perhaps
of olive-oil. The skins are next stocked for 2-3 hours, shaken out,
and hung up for 1/2-1 hour to cool and partially dry. They are then
again folded in bundles, and stocked for a short time, taken out,
oiled again, and returned to the stocks; and this process is repeated,
until the skins lose their original smell of limed hide, and acquire a
peculiar mustard-like odour, and the water at first present has been
entirely replaced by oil. The later dryings are frequently conducted
in a heated room, and when the oiling is complete, the skins are piled
on the floor, and the oxidation of the oil, which has already commenced
during the fullings and dryings above described, is completed by a sort
of fermentation, in which the skins heat very considerably. During this
process, they are carefully watched, and if the heat rises so high as
to endanger the quality of the leather, the pile must be turned over,
so as to cool the skins, and bring those which were originally outside
to the centre. When the fermentation comes to an end, the skins are
no longer susceptible of heating, and are of the well-known yellow or
chamois colour. Where this colour is objectionable, the oxidation is
sometimes completed by hanging the leather in a heated room instead
of by piling. It is now necessary to remove the surplus oil, and this
in France is done by oiling with any sort of oil, throwing into hot
water, and wringing or squeezing. The oil obtained in this way forms
the _moëllon_ or _dégras_ so much prized for currying purposes. The
unoxidised oil still retained by the skins is removed by washing
with soda or potash lye. In England and Germany, the whole of the
uncombined oil is removed in this way, and is recovered from the lye,
in which it exists in a partially saponified state, by neutralisation
with sulphuric acid. It forms the "sod" oil of commerce. About half
the oil employed is obstinately retained by the skin, and cannot be
removed even by boiling with alkalies, while no gelatin is obtained
by boiling water, to which the chamoised skin is much more resistant
than ordinary leather. The nature of the tanning process does not seem
to be well understood. It is generally stated that the fibres of the
skin are unaltered, but are merely coated with the oxidised products of
the oil. It is hard, however, on this hypothesis to understand their
extraordinary indifference to water, even at a boiling temperature,
which speedily converts kid and other tawed leathers into a solution
which gelatinises on cooling; and it seems more probable to the present
writer that some actual chemical combination is formed. Lietzman
('Herstellung der Leder,' p. 164) supposes that the whole of the
gelatigenous tissue has been removed by liming and bating, and that
only the very indifferent yellow elastic fibres (see p. 21) remain.
This view, however, is quite untenable, in consideration of the very
small proportion of these fibres originally present in the skin. Müntz,
in his researches (see p. 17), showed that the fibres insoluble in
boiling water scarcely exceeded 3 per cent, of the dried pelt. Dry
gelatigenous fibre has a considerable resistance to heat, and it is
possible that the action of the oil may consist in preventing the
absorption of water. This, however, will not explain its resistance to
alkalies. Cotton or other vegetable fibres moistened with oil, readily
undergo oxidation, with so much evolution of heat as sometimes to
cause spontaneous combustion; but the oxidation products are easily
and completely removed by alkaline solutions, leaving the fibre in its
original state, as indeed is noted by Lietzman (_loc. cit._).
The finishing processes consist in staking during drying to retain
softness, and in whitening and smoothing the flesh (or sometimes both
sides) on the fluffing wheel. Skins for gloves, &c., are bleached
like linen, by sprinkling and exposure to the sun; or more rapidly by
treatment with a weak solution of potash permanganate, and subsequently
with sulphurous, or very dilute sulphuric acid, to remove the brown
manganous oxide formed (Barreswil, Dingl. Polyt. Jour., 161, 312).
Gaseous sulphurous acid from burning sulphur may also be used for
bleaching. The "dyeing" of chamois leather is generally done with
ochres and similar colouring matters, and may be removed by washing.
Treatment with egg-yolk in water, or with an emulsion of olive-oil with
a little soap, and rubbing, or stretching, will restore softness to
chamois leather which has become stiff by washing.