SOLE-LEATHER.--Tanning Materials.
Before describing the management of the hides in the tan-house, it
is necessary to say a few words about one or two of the principal
materials used, and the methods of preparing them for use. Further
details of their nature and origin have been given in the section on
Tannins, p. 23.
Oak-bark is one of the oldest of tanning materials, and the leather
produced by its aid is still considered for many purposes the best. For
sole-leather, its weakness in tannin (8-12 per cent.), the slowness
of its action, and the light weight of the leather produced, render
it unavailable alone except for the very finest class of work. It
is, however, generally used in admixture with stronger and cheaper
materials, such as valonia.
Valonia, the acorn-cup of an evergreen oak growing in Greece and the
Levant, is perhaps the most important of materials to the English
sole-leather tanner. It contains 25-35 per cent. of a tannin somewhat
similar to oak-bark, and, like it, communicating a light-coloured bloom
to the leather, but giving much greater firmness and weight, and a
browner colour.
Myrabolanes or myrobalans, the fruit of an Indian shrub, contains about
as large a percentage of tannin as valonia, and gives a similar bloom,
and excellent colour; but it can only be used very sparingly on butts,
since it produces a soft and porous leather.
Divi-divi is a South American bean, which contains much of a brown
tannin in the pod, being considerably stronger than valonia. It makes
a heavy and solid, but somewhat horny leather. Its great danger arises
from a tendency to sudden fermentation in thundery weather, which,
produces brown or red stains on the leather. At all times it is liable
to give a bluish or violet colour, which is most obvious in the
interior of the leather, and which resists both acids and alkalies.
Mimosa-bark is the product of several Australian acacias, and is
probably nearly as strong as valonia. It gives a hard and heavy
leather, but of a dark-red colour.
Hemlock-extract is a deep-red syrupy extract of the bark of the hemlock
pine of America.
Chestnut-extract is a similar product from the rasped wood of the
Spanish chestnut. Its colour is paler and yellower than that of the
hemlock, and hence it is often employed to correct the red tone
produced by the latter.
Oakwood extract is an analogous preparation from oak saw-dust.
Grinding and Exhaustion of Tanning Materials.
Before tanning materials can be exhausted, it is almost invariably
necessary to crush or grind them, so as to enable the water to get
freely at the tannin, which, in most cases, is enclosed in the cellular
tissue of the plant. It may be thought that for this purpose it would
scarcely be possible to crush too finely, but in practice, a very
fine powder is extremely difficult to spend, as it cakes into compact
and clay-like masses, through which liquor will not percolate. The
object, therefore, is to grind finely enough to allow the liquor
ready access to the interior, but not so finely as to prevent liquids
running through the mass. The mill most usually employed for this
purpose consists of a toothed cone, working inside another cone, also
toothed on its interior, precisely like those of a coffee-mill. As
bark is frequently delivered "unhatched," or in long pieces, it is
necessary to crush it preparatory to grinding, and this is usually
accomplished by rollers composed of toothed discs, called breakers. In
Fig. 31 is illustrated such a mill, as made by Newall and Barker, of
Warrington, combining both utensils. Fig. 32 shows a section of the
well-known American "keystone" mill, in which the preliminary breaking
is accomplished by the arms A; the bark is then finely ground by the
toothed cones N, and discharged at the spout R by the revolving shover
M. Fig. 33 shows a somewhat similar mill, made by Gläser of Vienna,
in which the axis is horizontal, and driven directly by a belt. It is
better to drive bark-mills by a belt than by toothed gearing, as in
event of iron getting into them there is less danger of breakage. In
America, a cheap cast-iron coupling is frequently used, weak enough to
give way before serious damage is done. Safety "friction" clutches are
generally ineffective. American bark-mills are run faster than English,
up to about 80 rev. per minute, and where the bark is to be used
immediately it is frequently damped by a small jet of steam below the
mill, which lays dust, and prevents danger of fire. Bark which is damp
before grinding can scarcely be ground in these toothed mills, but must
be dried, or a disintegrator used.
[Illustration: Fig. 31.]
[Illustration: Fig. 32.]
[Illustration: Fig. 33.]
Now that a large variety of other materials besides bark are required
by tanners, the mills just described are not always sufficient for the
purpose. Myrobalans and mimosa-bark have proved specially troublesome,
the former from its very hard stones and clogging character, and the
latter from its combined hardness and toughness. "Disintegrators" of
various makes have proved admirably adapted for grinding both of these
materials, their advantage being the universality of their reducing
powers, ranging from oak-bark to bones or brick-dust, and their
disadvantages, the somewhat considerable power they consume, and the
rather large portion of fine dust they make. Their principle is that
of knocking the material to powder by rapidly revolving beaters,
which, in the smaller mills, are driven at so high a speed as 2500-3000
rev. a minute. Wilson's is shown in Fig. 34, as an example. It is one
of the oldest tanners' disintegrators, and probably still one of the
best. In the figure, it is opened, showing the disc with its steel
beaters attached. When myrobalans are only required roughly crushed,
a machine with fluted or toothed rollers (Fig. 35) acts better than
a disintegrator, making less dust, and requiring less power. Such a
machine also crushes valonia very satisfactorily.
[Illustration: Fig. 34.]
[Illustration: Fig. 35.]
In England, the tanning material is generally carried from the mill, to
the pits where it is exhausted, in baskets or barrows; in America, this
is frequently accomplished by a "conductor," or horizontal spout, in
which a double belt, or malleable iron "drive chain,"[U] with wooden
cross-pieces, carries the bark forward, on the same principle as the
elevators of corn-mills. Fig. 36 shows the conveyors used in a Chicago
tannery. Another American plan is to use circular tubs for extraction.
These are mounted on wheels, and are worked on a railway, coming up to
the mill to be filled, and thence under a series of sprinklers like
those used by brewers, and finally "dumping" their contents before
the boilers, which are heated solely by wet bark, burnt in a peculiar
furnace with brick chambers. This furnace for burning wet bark seems
worthy of extended adoption in Europe, as spent tan is frequently
not only valueless, but costly to get rid of. Full details and scale
drawings may be found in Jackson S. Schultz's book on 'Leather
Manufacture' and in Fig. 37 is shown a modification of it, patented by
Huxham and Brown, which has been very successfully used in burning wet
tan, either alone or with a portion of coal. In American sole-leather
tanneries, where the bark is resinous and almost unlimited in quantity,
sufficient steam may be raised with tan wet from the leaches; but
in England, where material is more sparingly used, it is advisable
partially to dry it before burning. This is accomplished by powerful
roller-presses, as shown in Fig. 38. Gläser, of Vienna, constructs
tan-burning furnaces on a different principle from the American, the
essential point being the use of a "ladder-grate" (_Treppenrost_), on
which the burning tan is exposed to a draught of air playing over its
surface. Fig. 39 shows a portable stove of this construction. Gläser
also makes furnaces of larger size for heating air for drying-rooms,
and for boiler purposes. The essentials of successful tan-burning
are good draught, a large grate-surface, and a high temperature of
the combustion-chamber, and hence the ordinary Cornish or Lancashire
boiler, with its limited grate-area, surrounded by the comparatively
cool boiler-tube, is peculiarly ill-adapted for the purpose. The writer
has profitably burnt a mixture of wet tan and very small coal in such a
boiler by the aid of a steam-jet under-grate blower, but such a method
can only be regarded as a makeshift in default of better appliances.
[Footnote U: Such chains with attachments for elevators and conveyers,
are manufactured in this country by Ley's Malleable Casting Company, in
Derby.]
[Illustration: Fig. 36.]
[Illustration: Fig. 37]
In England, the tanning material is usually exhausted in pits called
"leaches," "latches," or "taps." These, in large yards, are made
capable of holding about 50 cwt. of material. The new material is first
flooded with a pretty strong liquor. When this has gained as much
strength as possible, it is pumped off, and is followed by a weaker
one, and so on till the material is exhausted. Much of the economy of
a tan-yard depends on the way, systematic or otherwise, in which this
is done. It is customary to complete the exhaustion with hot liquors,
or water, but opinions differ on the expediency of the practice. By
the use of heat, however, stronger liquors and more rapid spending are
attained; and with some materials, such as mimosa, complete exhaustion
is impossible in the cold.
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
The worst tap is frequently boiled by inserting a steam-pipe; but if
heat is used at all, it would probably be better to heat a strong
liquor by a steam-coil, and run it on the new material, which would
be softened and swollen, and yield a much larger proportion of its
strength to the first liquor; while it is stated by Eitner that the
colouring matters of tanning materials are much less soluble in strong
than in weak infusions. Boiling weak old liquors containing lime is
specially prejudicial, causing great darkening and discoloration.
Careful tanners also cast their material over from one pit into
another, before throwing away, so as to lighten it up, and allow
the liquor to penetrate to every part. In bark-yards, latches are
frequently worked in series, which are connected by pipes, so that the
liquor flows from the bottom of one upon the top of the next stronger.
This is an excellent plan for bark, which is open and porous, but is
scarcely adapted to such materials as valonia or myrabolans, which
have a tendency to form compact masses, through which the liquor does
not circulate. The same objection, in an almost higher degree, must be
urged against the Allen and Warren, or sprinkler leach, in which the
liquor, distributed on the surface by a rotary sprinkler, is allowed
to percolate downwards, and run freely away at the bottom. In this
case, it is almost sure to form channels, instead of flowing uniformly,
and, in addition, the material is constantly exposed to the action of
the air, which causes oxidation, with its attendant discoloration and
loss of tannin. Various attempts have been made to exhaust tanning
materials in closed vessels. Dr. Kohlrausch applied the _diffuseur_
used in extracting beet-root sugar, and which consists of a series of
closed copper vessels in which the coarsely ground material is placed,
of which the bottom of one is connected with the top of the next by
a pipe, through which the liquor is forced by steam pressure. This
apparatus is in use at the large tannery of Gerhardus, Flesch, and Co.,
of Vienna, and is said to give satisfaction, though it is very costly,
and the liquors produced are not of great strength. Gläser, of Vienna,
has patented an apparatus of which a model is illustrated in Fig. 40,
in which the materials are used finely powdered, and very rapidly
exhausted by the combined action of heat and mechanical agitation. Of
its mechanism I have not been able to obtain any detailed description,
but it is said to be capable of exhausting 9 tons of valonia _per
diem_, to 2 per cent., giving only 70° liquors, clear and of good
colour, while good bark is exhausted to 0·5 per cent. giving 30°
liquors. The cost of the apparatus is very heavy, but if the results
claimed are realised in practice it would pay well for an extensive
tannery. I have not been able to ascertain where it is to be seen in
use.
[Illustration: Fig. 40.]
It is one of the great attractions of extracts that they avoid almost
all the expense and labour inseparable from the exhaustion of other
tanning materials. It is usually necessary to dissolve the fluid
extracts in water or liquor of as high a temperature as has been
employed in their preparation, as otherwise, from some unexplained
chemical change, a large portion of the tannin is precipitated,
probably as an anhydride of the tannin. Gambier is usually dissolved by
boiling or steaming, but is said to give a better colour when dissolved
cold. This may be accomplished in a rotating latticed drum, sunk in a
pit of liquor.
Where circumstances permit, it is a great advantage to place the taps
either on a higher or a lower level than the layers and handlers, so
that liquors may be run one way without pumping.