SOLE-LEATHER:--Preparing the Hides.
The principal sources of hides for sole-leather are:--
(I.) Market hides, from the cattle slaughtered for food in the United
Kingdom. These are received by the tanner, fresh, or slightly salted,
and are either bought directly from the butcher, or, now more commonly,
through the auction markets established in all large towns. The latter
system, while it perhaps slightly enhances the price of the hides to
the tanner, ensures him a better classification according to weight,
and, in some cases, as notably in that of Glasgow, a better flaying,
through an organised system of inspection and sorting. The Scotch
hides, being mostly from Highland cattle, are many of them small and
very plump, for, as a rule, the hides are thickest on those animals
which are exposed to cold and the hardships of out-door life. On the
other hand, the hides of highly-bred cattle are apt to be thin and
spreading; and, if they have been kept much indoors, and negligently
managed, the grain of the hide is injured by the dung which adheres to
it. The Irish hides are usually somewhat roughly flayed.
(II.) South American hides are from the River Plate, Uruguay, and Rio
Grande. Those from the River Plate are considered the best, as being
stoutest and finest in texture. They are usually cured by salting,
and are known as "saladeros," "estancias," and "mataderos," according
to the slaughter and cure. The saladeros are the best, and are from
cattle killed at large slaughtering establishments on the coast. The
estancias are from cattle killed in the interior, and are worse in
flaying than the saladeros, but free from the objectionable dark cure
of the mataderos, which are killed by the city butchers. Many hides
are brought from Brazil, and are generally both salted and sun-dried,
or simply stretched out and dried. Hides are also imported from
Valparaiso, both dry and wet-salted.
Chinese and West Indian hides are mostly dried. Chinese hides are
occasionally infected with _Bacillus anthracis_, which produces the
dangerous "malignant-pustule," or "wool-sorters' disease." Hence any
pimple appearing after working with such hides should have immediate
medical attention. French market hides have been of recent years
largely imported; they are mostly well flayed, and some of them
very heavy, but are sold at original butchers' weight, and, in the
experience of some tanners, the result in leather is 5-6 per cent. less
than from English market hides. They usually lose about 25 per cent.
in skulling and salting. Lisbon hides are often well flayed, but are
frequently branded, and the grain is injured by insects. They yield
considerably more leather than market hides in proportion to weight.
Hambro' hides are salted, but mostly wet and ill-flayed. Very heavy
hides are produced in the Rhine district and in Switzerland.
For further information about hides, see the Commercial Section.
_Preparation for Tanning._--Market hides should be well washed in
fresh water, to remove blood and dirt, before unhairing; but prolonged
soaking dissolves a portion of hide-substance, and probably reduces
weight, though it facilitates the action of the lime. It is very
advantageous if grease and flesh, and also dung can be removed before
liming, and if hand-labour is too costly machinery might be employed.
Salted hides should be soaked somewhat longer, and in clean water, so
as to remove the salt before liming. This water should be frequently
changed, since 10 per cent. brine dissolves coriin freely (see p.
19). Dried hides require more lengthened treatment. Before they are
prepared for tanning, they must be brought back as far as possible
to the condition of fresh hides, and, for this purpose, must be
thoroughly soaked and softened in water. There are many ways of doing
this: sometimes hides are suspended in running water; sometimes laid in
soaks, which may be either renewed, or allowed to putrefy; sometimes in
water to which salt, borax, or carbolic acid has been added, to prevent
putrefaction.
The first of these methods, were it desirable, is rarely possible in
these days of River Pollution Acts; of the others, it is difficult to
say which is better, since the treatment desirable varies with the
hardness of the hide and the temperature at which it has been dried.
The great object is to thoroughly soften the hide, without allowing
putrefaction to injure it. As dried hides are often damaged already
from this cause, either before drying, or from becoming moist and
heated on ship-board, it is frequently no easy matter to accomplish
this. The fresh hide, as has been seen, contains considerable portions
of albumen, and if the hide is dried at a high temperature, this
becomes wholly or partially coagulated and insoluble. The gelatinous
fibre and the coriin (if indeed the latter exists ready formed in the
fresh hide) do not coagulate by heat, but also become less readily
soluble. Gelatin dried at 266° F. (130° C.) can only be redissolved by
acids, or water at 248° F. (120° C.). Eitner experimented with pieces
of green calf-skin of equal thickness, which were dried at different
temperatures, with results given in the following table:--
───────┬────────────┬───────────┬──────────┬────────────┬─────────────────┐
│Temperature │ │ Time of │ │ Coriin Dissolved│
Sample.│ of Drying.│ Remarks. │ Softening│ Remarks. │ by Salt │
│ │ │ in Water.│ │ Solution. │
───────┼────────────┼───────────┼──────────┼────────────┼─────────────────┤
I. │ 59° F. │ In vacuo │ 24 hours│{Without }│ 1·68 per cent. │
│ (15° C.) │ │ │{mechanical}│ │
II. │ 71-1/2° F. │ In sun │ 2 days │{ work }│ 1·62 " │
│ (22° C.) │ │ │ │ │
│ │ │ │ │ │
III. │ 95° F. │{In drying-│} 5 " │twice worked│ 0·15 " │
│ (35° C.) │{closet │} │ │ │
│ │ │ │ │ │
IV. │ 140° F. │ " │ {Refused to soften }│ Traces. │
│ (60° C.) │ │ { sufficiently for }│ │
│ │ │ { tanning }│ │
───────┴────────────┴───────────┴──────────┴────────────┴─────────────────┘
Hence it is evident that, for hides dried at low temperatures, short
soaking in fresh and cold water is sufficient, and, except in warm
weather, there would be little danger of putrefaction. With harder
drying, longer time is required, and it may be necessary to use brine
instead of water. A well-known tanner recommends a solution of 30°-35°
barkometer (sp. gr. 1·035, or about 5 per cent. of NaCl). This will
have a double action, not only preserving from putrefaction, but
dissolving a portion of the hide-substance in the form of coriin.
Although this is undoubtedly a loss to the tanner, it is questionable
if there is any process which will soften overdried hides without loss
of weight: since even prolonged soaking in cold water at too low a
temperature to allow of putrefaction will dissolve a serious amount
of hide-substance. Water containing a small quantity of carbolic acid
has been recommended for the purpose, and will prevent putrefaction,
while it has no solvent power on the hide, but, on the contrary,
will coagulate and render insoluble albuminous matters. Concentrated
carbolic acid, however, tans the grain and renders it incapable of
colouring in the liquors. Borax has been proposed for the same purpose,
and, in strong solution, certainly prevents putrefaction, but is
probably too costly. Sodium sulphide and other sulphides seem to have
considerable effect in softening dried hides, from their property of
attacking hard albuminous matters, without injuring the true hide-fibre.
For some descriptions of hides, however, and notably for India kips,
putrid soaks seem actually to be an advantage, the putrefactive action
softening and rendering soluble the hardened tissue. In India the
native tanners soften their hides in very few hours by plunging them in
putrid pools, into which every description of tannery refuse is allowed
to run. Putrefactive processes are always dangerous, as the action,
through changes of temperature, or variation in the previous state of
the liquor, is apt to be irregular, and either to attack one portion of
the hide before another, or to proceed faster than was expected. Hence
hides in the soaks require constant and careful watching, and the goods
must be withdrawn as soon as they are thoroughly softened, for the
putrefaction is constantly destroying as well as softening the hides.
It is possible that putrefactive softening is less injurious to kips,
and such goods as are intended for upper-leather, than to those for
sole purposes, as it is generally considered necessary in the former
case that the albumen and interfibrillary matter be removed, and that
the fibre be well divided into its constituent fibrils for the sake of
softness and pliability; so that the putrid soak, if acting rightly,
only accomplishes a part of the work which would afterwards have to be
done by the lime and the bate. The actual fibre of the hide seems less
readily putrescible than the albuminoid parts; hence the putrefaction
may soften the latter better, and even at less expense of valuable
hide-substance, because more rapidly, than fresh water. On this point,
there is room for investigation. Putrefaction is a general name for a
class of decompositions which are caused by a great variety of living
organisms, each of which has its own special products and modes of
action. It is quite possible that, if we knew what precise form of
putrefaction was most advantageous, we might by appropriate conditions
be able to encourage it to the exclusion of others, and obtain better
results than at present. It will be necessary to revert to this subject
when speaking of the bates used in preparing dressing-leather, which
also owe their activity to putrid fermentation.
Beside merely soaking the hides, it is necessary to work them
mechanically, to promote their softening, which was formerly
accomplished by "breaking over" the hides on the beam with a blunt
knife. This process is now usually superseded or supplemented by the
use of the "stocks"; these consist of a wooden or metallic box, of
peculiar shape, wherein work 2 very heavy hammers, raised alternately
by pins in a wheel, and let fall upon the hides, which they force
up against the side of the box with a sort of kneading action. The
ordinary form of this machine is shown in Fig. 22. A more modern form,
which seems to possess some advantages, is the American double-shover,
seen in Fig. 23.
[Illustration: Fig. 22.]
[Illustration: Fig. 23.]
The number of hides which can be stocked at once naturally varies with
the size of both hides and stocks, but should be such that the hides
work regularly and steadily over and over. The whole number should not
be put in at once, but should be added one after another, as they get
into regular work. The duration of stocking is 10-30 min., according
to the condition and character of the hides. Hides should not be
stocked till they are so far softened that they can be doubled sharply,
without breaking or straining the fibre. After stocking, they must be
soaked again for a short time, and then be brought into an old lime. A
small quantity of sodium sulphide added to the soaks or in the stocks
has been recommended as of great value in softening obstinate hides,
and probably with justice, from its well-known softening action upon
cellular and horny tissues.
In Continental yards, another machine is in use for softening hides,
and which seems to present some advantages over stocks, as being less
severe on the thinner portions of the hide. It consists of a pair
of rollers, arranged like those of a wringing machine, and pressed
together by springs, but not allowed to come into actual contact. One
of them is studded with rounded pegs, which correspond in position to
grooves round the other, and the hide when passed between them is thus
subjected to a very thorough kneading and stretching. Tumbler drums of
various forms may also be used with good effect for softening purposes,
especially for skins.