THE DRESSING, DYEING, AND FINISHING OF LEATHER
The dressing and finishing of leathers of all kinds seems to be of
growing importance every year, despite the fact that the durability
of leather is often impaired by the chemicals and heavy machinery
used in order to get a clear and bright colour or a highly-glazed
finish. One exception to this rule, however, is japanned and
enamelled leathers made by the new collodion-amyl acetate process,
which not only produces the so-called "patent" finish, but also adds
to the strength of the leather.
Dressing and finishing may be conveniently classified in four
sections--
1. Finishing of boot sole leather.
2. Dyeing and finishing of machine belting, strap, harness, and other
heavy greasy leathers.
3. Dyeing and finishing of boot, portmanteau, case, bookbinding, hat,
and upholstery leathers, and of the numerous fancy leathers.
4. Dressing, dyeing, and finishing of glove and chamois leathers.
There are also a few special kinds of leather which do not come
within the scope of this list. The limited size of this book makes it
impossible to outline the finishing of all kinds of leather, and only
a few of the important varieties can be referred to in the following
pages.
SOLE LEATHER
Formerly, the finishing of sole leather was a fairly easy matter, as
the slow process of tanning with oak bark which was in general use
gave the latter a nice fawn colour. The modern tendency, however,
is to demand even lighter-coloured leather than that produced by an
oak bark tannage, and, as most of the mixed tannages impart a darker
colour, the practice of bleaching has been universally adopted. This
process is to be greatly deprecated, since it reduces the strength
of the leather. Further, it has the serious drawback of removing
matters which would add to the waterproof quality of the leather, and
yet the boot manufacturers demand light-coloured sole leather. As in
many other articles of modern manufacture, quality is sacrificed for
appearance. The colour of sole leather is of no importance whatever
to the wearer of the boot, and the public should make a strong
protest against bleached sole leather, and also against leather
weighted with adulterants, if they wish to get boots of good wearing
quality. It is difficult to know where the finishing of some of the
modern sole leather begins. Formerly, the line of demarcation between
tanning and finishing was distinct, for the finishing processes were
begun as soon as the hides were sufficiently tanned. This is by no
means the case to-day, as, in many cases, the finishing may be said
to begin with a supplementary but unnecessary tannage in very strong
tanning extracts, with the object of making the leather firm and
plump, and, incidentally, imparting additional weight. This practice
would be discontinued if sole leather were sold by superficial
measurement. On the other hand, those who favour the continuance of
the present system of selling by weight assert that the adoption of
the measurement system would indubitably result in the production of
a lot of under-tanned leather, that mechanical means would be devised
to stretch the leather and thus give it an artificial area, and that
a fair quantity of the leather produced does not lie flat, and
would, therefore, be difficult to measure correctly. A great point in
favour of selling by measurement is that there would be no necessity
to weight the leather, and a mere visual examination would suffice
to determine the quality, whereas it is often difficult to state
whether leather has been treated with injurious artificial matters
or not, and certainly impossible to determine the extent of this
fraud, without a careful and somewhat elaborate analysis. Of course,
there are some rough practical tests for ascertaining the quality of
sole leather, especially in regard to the extent of its resistance
to water. A weighed quantity of the leather is soaked in water for
twenty-four hours, taken out, squeezed, dried, and reweighed. If
the loss of weight does not exceed 5 per cent., the leather is a
good sample and well tanned; on the other hand, it does not follow
that a loss, say, of 40 per cent., would indicate adulteration with
injurious materials, as the leather may have been tanned with an
excess of strong extracts which have not properly combined with the
fibres, with the result that the excess is easily removed in contact
with water. However, methods of fixing tanning extracts on the fibres
of the leather have been devised (see p. 117). A careful note should
be made of the time required to soak the leather through; if water is
absorbed rapidly, the leather is, naturally, unfit for soles.
After the tanner has taken great pains to produce the paleness of
tint required, the shoemaker buffs away the grain, thereby reducing
the resistance of the leather to wear, and then generally covers it
with a black, tan, or white dressing. In its turn, this finish is
likewise spoiled the first time the leather is worn. There could be
nothing more futile than the elaborate finishing of sole leather to
make it pale in colour, as it means that fully 80 per cent. of the
leather produced has to be bleached in some way or other; and the
manner in which this process is effected has a direct influence on
reducing the wearing quality of the leather. If, for example, the
colour of the leather is brightened with myrobalans, or sumach,
or with extracts of these materials, the wearing quality is not
affected; unfortunately, this method is not largely used, as it
does not increase the weight of the leather and would tend to make
it soft. A large proportion of sole leather is treated with strong
sulphited extracts, which both bleach and increase the weight of the
leather. In some cases, the leather is placed in vats containing
hot tanning extracts to increase the bleaching effect and add to
the weight. The gain of weight by this surplus tannage may be, and
often is, from 8 per cent. to 10 per cent., and, whereas the tanner a
century ago used to get only slightly above 50 per cent. of leather
on the weight of the raw hide, it is no uncommon thing nowadays to
get a yield of nearly 70 per cent.
The tanners are hardly to be blamed for this practice, which really
ought to be stopped; it is almost entirely due to the demand of
buyers for leather at a low price per lb., irrespective of its
superficial area compared with a higher-priced and often cheaper
article. It is only fair to state that those boot manufacturers who
have taken careful costings of their leather have not suffered great
loss in this way, but most repairers buy common leather in order
to be able to do their work at fixed prices usual in this trade. A
large proportion of this leather is under-tanned or weighted, and,
in either case, absorbs water like a sponge. This kind of leather
is quite unfit for soles, and ought to be condemned, as it not only
absorbs moisture but also retains it for a long time, so that it is
frequently a cause of colds and other complaints arising from chills.
If the repairer does not cost his leather as accurately as the
wholesale boot manufacturer, the latter often neglects to take into
account the quality of the leather, so long as cheap soles can be cut
from it.
The continual demand for leather which will yield low-priced
soles has brought about another practice which is much worse than
the use of hot extracts. In this case, the hides are not tanned
thoroughly, but, instead of completing the process with tanning
extracts, the leather is treated with cheap chemicals, such as epsom
salts (magnesium sulphate), glucose, and barium salts. These add
considerable weight and save the cost of expensive tanning materials;
further, it is claimed that they give increased firmness and
substance to the thinner parts of the leather, which otherwise could
not be used for soles. It is conceivable, therefore, that leather
of this kind could be cut to greater advantage than that properly
tanned, but, since this artificial filling is washed out on the first
contact with water and the leather rapidly deteriorates through
attracting and retaining moisture, the fraud of the system is at once
apparent.
Although the use of leather artificially weighted with hygroscopic
chemicals is extremely detrimental to health, it was only during last
year that laws were passed forbidding the adulteration of leather,
and even these are inadequate. Strange to relate, the two countries
where these legal measures have been taken, Australia and South
Africa, enjoy for the most part a hot and dry climate, while it is
in wet weather that adulterated leather is most harmful. However, to
Australia belongs the honour of initiating legal measures against
frauds in leather manufacture, and their praiseworthy action must
soon be copied by other civilised countries.
On the other hand, it would be inadvisable to prohibit the addition
of every other material except those with tanning properties. There
are undoubtedly a few substances which, used in combination with the
tanning materials, add to the strength and value of the leather, and
it is the opinion of some tanners that great developments will take
place in this direction. Certain oils, sulphonated in order to render
them easily miscible with water, and known commercially as "soluble"
oils, exercise a beneficial effect when mixed with the tan liquors,
for they lubricate the fibres of the leather, assist penetration of
and fix the tannin. The time required for the process is, therefore,
much reduced, as strong liquors may be used without harmful effect.
The oil also serves as a lubricant for the fibres, obviates the
harshness and brittleness usually associated with rapidly-tanned
leather, and fixed to the extent of about 3 per cent., must increase
its durability. Best of all, from the tanner's point of view, there
is a moderate increase of weight. There are two or three special
tanning oils on the market.
A vegetable gummy product has lately been introduced to the trade
which has been found to give remarkable results when used in
conjunction with tanning materials. This gummy matter is extracted
from vegetable seeds and is placed on the market under the name of
Tragasol. Its composition is somewhat similar to that of tannins,
the Tragasol Co.'s analysis showing it to contain 43.51 per cent, of
carbon, 6.23 per cent. of hydrogen, 48.38 per cent. of oxygen, 0.39
per cent. of nitrogen, and 1.49 per cent. of ash. It is very largely
used for strengthening and sizing textile fabrics, and seems destined
to play an important part in the leather trade, for it has tanning
properties, increases the strength of the leather, and considerably
hastens the process. Its most important advantage is that it permits
successful tannage in the drum, thus effecting a great saving of time
and labour.
Hitherto, rapid tannages have not been very successful, as they
generally made the leather harsh and brittle, and lessened its
durability. Leather tanned by previous rapid methods was also less
waterproof than that tanned in pits by slow methods. All these
defects are avoided by using Tragasol, which, when combined with
tanning extracts, forms a curdy precipitate, described by the makers
of Tragasol as cutiloid (contraction of cutis and colloid, cutis
meaning "skin," and the Tragasol being, chemically, a colloid). The
cutiloid tannage increases the water-resistant properties of leather,
and prevents the oxidation and consequent darkening of the colour of
the tanning material used with it. Tests have been made which show
that cutiloid-tanned leather will withstand a fall of water 12 in.
high for six weeks before complete penetration takes place; ordinary
tannages do not often resist water for more than a few hours.
For the tannage of sole leather in the drum by the cutiloid process,
about 120 per cent. of Tragasol and 40 per cent. of chestnut or
oakwood extract are required; this quantity is divided into four
equal portions, which are added to the drum successively at intervals
of one hour. The hides should then be nearly tanned through, and the
process is completed with strong extract (chestnut and quebracho,
or myrobalans, according to the kind of leather required). After
drumming the hides for ten hours, they should remain at rest for two
hours, then drummed another hour, and so on, alternately, until the
completion of the process, which may require from twenty-four to
thirty-six hours, according to the substance of the hides. By this
process, the yield of leather may exceed 60 per cent. of the weight
of the raw pelts. This percentage of yield may be raised to 70 per
cent. or more in the case of sole leather by steeping the hides in
hot extract.
The action of acids in swelling gelatine is sometimes abused,
particularly on the Continent and in America. As pointed out before,
tan liquors must be mildly acid in character before the process can
proceed, and, if the organic acids of the tanning materials are
insufficient, the addition of a small quantity of formic, or lactic,
or acetic acid is necessary; but, in many of the American acid
tannages, an excessive quantity of mineral acid is used, with the
object of swelling the hides. When in this condition, they rapidly
absorb an abnormal quantity of tannin, and the finished leather is
unnaturally thick. To make matters worse, the tannage is sometimes
not completed, but, instead, the hides are treated ("doped") with
hygroscopic chemicals, which not only increase the weight of the
leather but also endanger the health of those who have to wear
it. Most of the American tanneries make no secret of their use of
these unnecessary chemicals, but assert that they are obliged to
use them in order to meet competition, and because a large number
of bootmakers insist on buying sole leather at a low price per lb.
The Leather and Paper Department of the U.S.A. Bureau of Industry
analysed several American leathers in 1913 and found that a large
majority were artificially weighted with glucose and Epsom salts
(magnesium sulphate). Barium compounds are also used for the same
purpose, but the American tannages are apparently free of this
adulterant. The results were published in a pamphlet by the United
States Government, together with a list of the firms whose leathers
were tested.
While the European tanners do not weight their leather so freely as
the Americans with these chemicals, the practice of swelling the
hides unduly is not unknown, with the difference that, especially
on the Continent, the swollen fibres are fixed by treating the
hides with a weak solution of formaldehyde, which also acts as a
preliminary tanning agent. The tannage can then be rapidly completed
with fairly strong extracts, as the formaldehyde prevents the
contraction of the grain which would ensue if untreated raw pelts
were placed in strong tan liquor. The acid-formaldehyde process is
risky, and cannot be recommended. The use of the artificial tannin,
neradol, has replaced it in many Continental sole-leather tanneries.
A preliminary tannage with neradol forms a good mordant for the use
of the strong extracts.
[Illustration: FIG. 26
PINNING SOLE BUTTS]
Whatever the method used to improve or depreciate the natural
tannage, the surplus material left on the surface is raised or
brushed off, and, after the leather has been left to drain for a
few hours, it is sent to the finishing department, which is usually
the drying shed. There, it is left piled up for a day or two, until
it is in the right condition for oiling. The leather must be oiled,
otherwise the grain would be harsh and brittle when dry, and would
darken in colour, especially at the edges. Cod oil is generally used,
and sometimes a little mineral oil is added to it to prevent possible
impurities in the fish oil rising to the surface of the leather or
causing damage in other ways. A cod oil purified by distillation
would, however, be quite safe to use, and there seems to be no
reason why tanners should use crude oils so frequently as they do,
now that a large number of oils specially prepared for the leather
trade are available. Linseed oil is also used in admixture with cod
oil, its oxidising properties hastening the process of drying. The
leather is liberally oiled with a brush or swab on the grain side and
immediately hung up in the drying shed, where it is left until it
reaches what is technically known as a "sammed" condition (_i.e._,
just damp enough to exude no water when the leather is doubled over).
It is then struck out ("pinned") on the grain side, either with a
hand tool (Fig. 26) or by machine. In the former case, the leather
is placed on a long wooden beam and the pinner works from the left
side, keeping two of the three edges of his knife flat on the leather
while making heavy forward strokes. As the scum is worked out, the
operator mops it up with a wet cloth. Machinery has replaced hand
labour in this operation in all modern tanneries. The pressure of
the tool removes scud and dirt, varying in quantity according to the
amount of "bloom" contained in the particular tanning materials
used. The leather is then laid in pile for a few hours, when it is in
a fit condition for the first rolling, which is known in the trade as
"rolling on." This operation is generally done by machine (Fig. 27)
nowadays, but the old method of using a hand roller heavily weighted
with an iron box (Fig. 28) is still practised in a few yards. After
rolling, the leather is hung up again until nearly dry, when it is
taken down and sized with a weak colouring matter, made up of annatto
or tumeric, with chalk or whitening dissolved in white vinegar
(acetic acid) and diluted. It is then rolled again ("rolling off")
and polished with a piece of flannel. Finally, it is hung up to air
off, and is then ready for the warehouse.
[Illustration: FIG. 27
ROLLING MACHINE]
In many yards the finishing consists merely in striking out the
leather, rolling it when properly tempered, hanging up until nearly
dry, and re-rolling. An American machine for giving the final polish
to sole leather has lately been introduced.
Instead of the rolling machine, some French tanners use the hammering
machine, which gives good results, but is necessarily slow and
frightfully noisy in action. The steel hammer moves up and down with
remarkable rapidity, but only a small area is covered by each blow.
[Illustration: FIG. 28
OLD METHOD OF ROLLING SOLE LEATHER]
The artificial drying of sole leather, or of any other kind, is a
matter that needs careful attention, for it is obvious that great
delay would occur, and a large amount of business would be lost,
if tanners relied solely on the climatic conditions of the British
Isles; in fact, the weather is so rarely fit for drying leather that
every up-to-date tannery contains some artificial aids, the need
for which has been more pressing since a large proportion of leather
contains a surplus of tanning matter which is liable to oxidise in
contact with strong light and air, thereby darkening the leather and
making it brittle.
Among the satisfactory drying plants are those of Howard-Smith &
Co., and the Sutcliffe Ventilating and Drying Co., Ltd. Both work on
the theory of fanning a continuous current of hot air through the
drying-room, so that the moisture evaporating from the leather is
constantly drawn off while the apparatus is at work.
[Illustration: FIG. 29
FAN
(Howard-Smith system)]
The Howard-Smith plant consists of a series of steel tubes into which
hot water or steam is pumped. These pipes are enclosed in a chamber
in which the air naturally acquires great heat. A fan (Fig. 29) is
fixed in one side of this chamber (Fig. 30) and, when in motion,
drives the hot air through a wide tube which leads to the drying
chamber.
[Illustration: FIG. 30
FAN FIXED IN CHAMBER
(Howard-Smith system)]
Figure 31 shows the Sutcliffe Fan and Heater installed in a tannery.
At least two hygrometers should be hung up in a drying-room to
determine the amount of moisture in the air.
CHROME SOLE LEATHER
The manufacture of chrome sole leather suitable for ordinary walking
boots is of comparatively recent origin, although natural chrome
hides were dressed more than twenty years ago for tennis boot soles
and other athletic shoes. There can be no doubt of the superior
wearing and waterproof qualities of well-dressed chrome sole, but its
high price, compared with that of vegetable-tanned sole leather, is
against its general use.
Either the one bath or two bath tannage (described on p. 108) may be
used, but, on the whole, the one bath method is preferred for the
following reasons: (1) Its application is easier; (2) it produces
a firmer and less elastic leather; and (3) there is practically no
danger of weakening the hide fibres. On the other hand, the two-bath
process produces a plumper leather which can, naturally, carry more
dressing and command a correspondingly better price. In skilled
hands, it is also under better control than the one-bath process,
while it is absolutely necessary for the popular pale bluish-grey
tint of chrome leather tennis soles.
[Illustration: FIG. 31
SUTCLIFFE SYSTEM OF DRYING]
The natural chrome sole requires very little dressing. The chromed
hides (usually rounded into butts or bends, shoulders and bellies)
are allowed to remain on a horse two or three days after the
completion of the tannage, so that the oxide of chromium may combine
more firmly with the fibres. They are then placed in the drum
tumbler again and washed in cold water for several hours. A constant
stream is conveyed to the drum through a rubber pipe fixed to the
tap, and escapes through small holes in the drum. The acid salts
are not thoroughly removed by this washing and must, therefore,
be neutralised with an alkali. Borax is the best for the purpose,
owing to its mild action on leather and antiseptic properties. From
1 to 2 per cent. borax on the weight of the leather is generally
sufficient, while, if soda be used for economical reasons, only
half the quantity is required. The drum is revolved for about an
hour; the liquor is then run off, and the leather receives a further
washing in cold water for half an hour, when it is in a condition to
receive the dressing, which is chiefly composed of materials which
help to fill and lubricate the interstices of the fibres so that
the substance is maintained and the natural strength of the fibres
preserved. Gelatine, pure glue, Tragasol, paraffin wax, carnauba wax,
spermaceti, Marseilles soap, are among the various ingredients used;
while the delicacy of the tint may be improved with china clay or
French chalk. Recipes vary considerably, each manufacturer claiming
to have secret methods. Solutions are made of the ingredients, or,
where one or more of the materials are insoluble, they are atomised
in a mixing pan before use. To induce thorough penetration, the
dressing liquid may be applied to the leather at a fairly high
temperature, as chrome leather is unaffected by heat which would
destroy the fibres of ordinary vegetable-tanned leather. Even so, it
is inadvisable to use a higher temperature than 170° F. The leather
is thoroughly impregnated with the dressing in about an hour's time,
and is then piled on a wooden horse for several hours, or overnight,
to drain and permit thorough incorporation of the dressing with the
leather. When well-drained, the grain of the hides is smoothed by
pressure with a special tool (Fig. 32) or by machine, the operation
being known technically as "striking out." The machine (Fig. 34)
gives quite satisfactory results and has replaced hand labour in all
modern works. The hides are then "strained" (_i.e._, extended and
nailed to wooden frames), or they are simply hung up to dry. In the
latter case, there is a small shrinkage of the size of the hide, but
the substance is maintained. With good straining, a satisfactory
increase of surface measurement can be obtained.
[Illustration: FIG. 32
SLEEKER]
When the leather is completely dried, it may be sold at once, but
if, as is generally the case, a special "nap" finish is required on
the flesh side, the hides are placed in damped deal sawdust until
they are in the right condition of flexibility for the next process,
known as "fluffing." This is done on a rapidly revolving wheel (Fig.
33) covered with emery powder varying in the size of grain according
to the effect desired on the finished leather. The abrasion of the
leather on the flesh (_i.e._, the under side) raises a soft nap.
Carborundum, a special abrasive made in America, by the fusion of
sand, coke, and salt at a very high temperature, has to some extent
supplanted the use of emery. As a final finish, the flesh side may
be rubbed over with chalk or china clay to give it a clear saleable
appearance.
[Illustration: FIG. 33
FLUFFING MACHINE]
The chrome sole leather used for ordinary boots is quite a different
product, and it is only in the last two or three years that it has
been used to any extent. The "Dri-ped" is the best known make of
this leather. The tannage is effected by the one or two-bath methods
already described. The processes preparatory to chroming are very
important, as it is essential to get the maximum yield of pelt. The
hides are, therefore, often treated with sulphide of sodium, which
loosens the hair in twenty-four hours or less. They are then placed
in fairly strong limes for two or three days to swell the fibres and
saponify the grease. After being washed in cold water, the hides are
treated with a weak solution of lactic, boracic, or butyric acid,
or butyrate of ammonia, to remove all trace of lime, when they are
ready to be chrome-tanned. After chroming them, neutralisation of
the acid chrome salts is effected with borax, and the leather is
ready for dressing. The principal objects of this process are to
make the leather firm and waterproof, and to remedy its tendency
to slip easily on wet pavements. The last-named is the greatest
difficulty, and there are only about two or three firms who have
really surmounted it. Each manufacturer keeps his recipe secret, but
the ingredients that go to make the various dressings are drawn from
the following: Stearines, mineral oil, Tragasol, resin, carnauba
wax, beeswax, paraffin wax, glue. Resin, or asphalt, or both, may be
used to prevent the leather slipping in contact with a wet pavement.
One patent describes a method of dressing chrome sole leather with a
mixture of asphalt, resin, and paraffin wax.
[Illustration: FIG. 34
STRIKING-OUT AND SCOURING MACHINE]
The waterproofing ingredients are preferably applied to the leather
in the tumbler at a high temperature, say, 160-170° F. After
running the drum for about three-quarters of an hour, the leather
is impregnated with the mixture, and is then horsed up for at least
twenty-four hours to allow complete incorporation of the fatty
ingredients with the fibres of the leather. A suitable waterproof
dressing and filling increases the strength of the leather.
The finishing of the leather is simple. The butts or bends are
smoothed and stretched out by machine (Fig. 34), oiled on the grain,
nailed or extended with special metal grips on square wooden frames,
and dried. In some cases, they are hung up until nearly dry, rolled
by machine instead of being put on boards and aired off in a drying
stove.
A patented method of making waterproof chrome sole leather to
prevent it slipping and losing its shape in wear consists in first
treating the leather in a solution of glue, gelatine, agar-agar,
or other colloidal substance, with the addition of formaldehyde,
and then submitting it to strong pressure under the influence of
a high temperature, in order to fix the colloidal matters. The
pressure is made between heated plates. The leather is afterwards
impregnated with a mixture of wool-grease, pitch, asphalt, resin,
wax, gutta-percha, etc. A recipe given in the specification consists
of wool grease (60 parts), asphalt (10 parts), soft pitch (25 parts)
and gutta-percha (5 parts). The mass is fused and kept at 90° C. The
hides are placed flat in this mixture, which they quickly absorb.
If the British Government should ever introduce a Bill to stop the
adulteration of leather, it may be found somewhat difficult to deal
with chrome sole, which must have some filling material to make its
use practicable. No exception can be taken to the dressing of leather
with materials that make it waterproof and increase its strength; in
fact, much more will be done in this direction than has ever been
attempted in the past, as waterproof sole leather is a necessity in
Great Britain. Any measure to check the adulteration of leather must,
therefore, clearly define the chemicals which are known to be harmful
in leather and which may not be used. This has not been overlooked
by the Australian Government, which has forbidden the use of barium
salts in leather, and likewise the importation of leather boots or
any goods made of leather containing this chemical. Such goods sent
to Australia are liable to confiscation, and it is reported that a
few consignments of boots have already been condemned.
Leather, either exported or imported, must not contain more than
3 per cent. of glucose unless the percentage is marked on each
consignment, in which case, it is presumed, a tanner may use as
much of the adulterant as he likes, although the fact that he
has to disclose this practice acts as a deterrent in most cases.
Unfortunately, the Australian Act leaves an important loophole, since
the use of Epsom salts, which can be made to increase the weight of
leather more than any other chemical, is not prohibited. In any case,
none of these hygroscopic materials is of any use to chrome sole, for
they would attract moisture which it is desirable to resist.
VACHE SOLE LEATHER
As its name implies, vache sole leather is a Continental product,
being made chiefly in France, Belgium, and Germany. The raw material
consists of light and medium cow hides. The details of working are
somewhat similar to those used in tanning English sole leather,
the main difference being that the hides are suspended in the lime
liquors instead of throwing them in flat. The system of tannage is
not so varied as the English, since the materials used generally
consist of oak and pine barks, myrobalans, quebracho and oak
extracts; and the finishing materials are of quite another character.
The Continental tanners generally use the Réaumur thermometer, while
the Beaumé hydrometer is used to determine the density of the tan
liquors in place of the English barkometer (Fig. 24). One degree
Beaumé is equal to 6.9° barkometer.
Vache leather is more pliable than English sole leather, but the
addition of valonia extract towards the end of the tanning process
would greatly increase its firmness.
The finish is applied to the flesh side of the leather, and a typical
recipe consists of a size such as Irish moss or Tragasol (1 part),
flour (20 parts), china clay (20 parts), and pure gambier (2 parts)
in 200 parts of water. The size is dissolved and cooled, and the
other ingredients are then added.
MACHINE BELTING, HARNESS, AND SADDLERY LEATHERS
These leathers are rarely made by one firm; usually, strap butts and
most kinds of belting leathers are dressed by curriers, whose trade
is quite distinct from that of currying shoe leather. The tanning and
currying of harness and saddlery leathers is another special branch.
Besides these three important trades, there is the dressing of
hydraulic, mechanical, and other industrial leathers, which likewise
form special sections of the heavy leather industry.
Belting leather is an important item in the trade, and, as a good
proportion is made from the finest hides, this class of leather is
among the most expensive made. At present, there is much controversy
in the trade as to whether the vegetable-tanned or the chrome-tanned
article is the more economical in use. Briefly, chrome belting is
superior in works where there is much steam or mineral acids, as it
is not affected by these conditions to the same extent as bark-tanned
belting. Its tensile strength is also greater, but against these
advantages must be set its higher cost and tendency to stretch
unduly in comparison with the vegetable-tanned product. It can be
safely said, however, that the use of chrome belting is on the
increase, although its production is not nearly on such a big scale
as that of the old type of belting.
The tannage of belting leather is similar to that of sole leather,
except that there is no necessity to continue the process, after the
tannin has struck through, with the object of increasing its firmness
and weight. Attention must also be paid to the class of tanning
materials used, especially for belting of the best quality, as it
must have great tensile strength and stand a severe strain in the
dynamometric test. Tannins that rapidly oxidise in contact with light
and air have, therefore, to be avoided. The oak bark tannage, with
a little chestnut and oakwood extracts to finish the process, is a
good method of imparting to the hide the right degree of firmness and
flexibility.
As in the case of sole leather, belting may be artificially weighted
during the tannage, although it is usually done in the currying
process, if at all. There can be no doubt that a quantity of
adulterated belting leather is made, and will continue to be made
while the practice of selling by weight is in vogue. As a rule,
sophisticated belting leather has a much weaker tensile strength than
the pure product and is of relatively poor value, since the reduction
of price is usually a matter of only a few pence. If users of belting
bought on the principle of value instead of price, they would never
buy adulterated leather.
[Illustration: FIG. 35
LEATHER-STUFFING DRUM]
In tanning hides for belting, the shoulders and bellies are cut off
and usually dressed for sole or harness leather, leaving the butt,
the prime part of the hide, 4-1/2 ft. in length (Fig. 23). The
tanned butt is first shaved to level the substance, then washed in
a solution of borax, and sumached in the drum. After smoothing the
leather in the striking-out machine, it is partially dried ("sammed")
and then rubbed on both sides with a dubbin of cod oil and tallow. In
this condition the butts are left piled up for a few days, so that
the dubbin penetrates the leather, which is then extended with the
sleeker (Fig. 32) to make it smooth and to remove surplus grease,
which is sold to soap-makers under the currier's name of "moisings."
The leather is finally dried, rolled (optional), stretched by
machine, and cut up in the width of belting required. Instead of
"hand-stuffing" the leather with cod oil and tallow, which is still
the best method, a large quantity of butts are now "drum-stuffed,"
the object being to make use of stearines, which are harder and
heavier greases than tallow. The type of drum shown in Figure 35, in
which the grease is incorporated with the leather by means of live
steam, gives satisfactory results, although hot air apparatus is now
replacing the steam injector.
A patent has lately been granted to an American inventor for the
production of a strong and cheap belting. Flesh splits of hides are
simply stuck together with a collodion or nitrocellulose solution.
Any number of layers may be used, according to the substance
required, the only essential quality being that the leather should
be free from grease. The adhesive property of nitrocellulose
solutions is probably stronger than that of any other material, but,
unfortunately, the tough film they form on drying does not grip
a greasy surface. Before the solution used to join the pieces of
leather together is dry, the leather is rolled under heavy pressure,
and, when thoroughly dried, can be cut up for belting.
The dressing of harness leather is similar to the manual process of
making belting, with the exception that the butts or bends (half
butts) are stained either black or a pale straw colour, the latter
being known to the trade as the "London colour." Annatto is often
used to get this colour. The bright, greasy finish on harness leather
is obtained by rubbing buck tallow over the grain and polishing with
a glass sleeker or a piece of flannel. The operation is sometimes
done by machine, in which the working cylinder is covered with pieces
of cotton rag.
Saddlery leathers are dressed on similar lines, but the finish is not
so greasy as harness. Pig skins make the best and toughest leather
for this purpose. They are dressed in a similar manner to harness
hides, but special attention has to be given to pig skins to get rid
of the large quantity of natural grease they contain.
BOOT UPPER LEATHERS
Whereas the number of different kinds of boot upper leathers in use
less than three decades ago was limited to five or six (waxed calf,
calf kid, French kid, mock kid, levant, coloured calf) it is now
almost legion. Although the variety is almost bewildering, however,
the popular demand is confined to about half-a-dozen sorts, which
include real and imitation box calf, box hide, _glacé_ or glazed kid,
patent leather, willow calf, dull-finished chrome leather, and waxed
kip butts and waxed splits for workmen's boots.
Box leathers and glazed kid are the most popular of all. Generally
speaking, glazed kid is more suitable for wear in the summer and
autumn; while box leather, being thicker and stronger, is preferable
for the winter. Imitation willow calf (_i.e._, calf skin tanned in
vegetable and chrome liquors, either separately or combined) is very
suitable for easy-wearing boots, but is not so durable and resistant
to water as the pure chrome-tanned article.
The tannage of box and willow leathers may be effected by the one
or two-bath process already described (p. 108). Before the tannage,
it is advisable to pickle the hides or skins in a solution of alum
and salt, with the object of preventing contraction of the tissues
of the hide, and providing a mordant for the more rapid absorption
and fixation of the chrome salts. The tannage completed, the acid
salts in the leathers are neutralised with borax, and after the
leather has been washed in warm water it is ready for dyeing. The
dyeing of box calf is generally done in the drum tumbler, but there
are several other methods. One system used on the Continent is to
dip the skins, a pair at a time and placed flesh to flesh, in a very
strong dye liquor. This method is slow, and not in accordance with
modern ideas; used for producing browns and fancy shades, it is very
difficult to avoid irregularity of colour where large parcels are
manipulated. Another method is vat dyeing, the process being carried
out in a wooden vat (Fig. 25) and the dye liquor circulated by means
of a paddle. The one advantage of this system is that the leather
can be easily examined during the process. A useful method of dyeing
is carried out by brushing the colouring materials on the grain side
of the leather. This process is economical, and it has the further
advantage that the finished leather can be used for unlined boots and
other purposes where an undyed flesh is necessary.
The original "box" leather was dyed a dark-blue shade in the drum
before it was shaved, and, as the dye does not penetrate very far
into chrome leather, unless a strong mordant of tannin be previously
used, the subsequent shaving left the flesh side of the leather a
very pale-bluish tint.
The process of dyeing is least troublesome and most effective when
done in the drum tumbler. The leather is run in warm water until the
temperature reaches 140°-150° F (60°-66° C.) and the dyeing materials
are added gradually in a box at the side of the drum whence it passes
through the axle or journal into the tumbler. Although chrome leather
is not materially affected by boiling water in contradistinction
to vegetable-tanned leather, which cannot be treated with water
above 60° C. without injury, it is inadvisable to dye it in
boiling solutions, the above-mentioned temperature being the most
satisfactory. There are two methods of dyeing blacks in the leather
trade: (1) the logwood-ammonia, and (2) the aniline black. The former
is the cheaper of the two and quite satisfactory, although many
dyers seem to prefer the aniline process. The latter certainly gives
a deeper black, but it is not at all necessary to make the flesh side
of box calf black, and many buyers prefer the back blue, although, of
course, the grain side of the leather must be finished a jet black.
The crude logwood is generally subjected to a process of fermentation
or "ageing." The new wood has to be placed on stone or cemented
flooring in a warm room and frequently turned over until fermentation
has ceased, which usually takes about a week. It is then cut into
small chips by machinery and packed in bags. This is the form in
which it always used to enter the tannery, but it is now generally
prepared in paste or crystal extract by makers of tanning and
dye-wood extracts, a business which grows in importance every year.
The colouring matter of logwood is haematoxylin, which is converted
into haematin by oxidation; hence, the extract is often referred to
as haematin crystals. The paste may be used where it is desirable
to increase the substance of the leather, but most dyers prefer
the crystals, which are easily dissolved in hot water. One lb. of
crystals, to which is added just enough ammonia to change the brown
colour to violet blue, suffices for each 100 lb. of leather dyed. The
colouring matter is absorbed by the time the drum has been running
half an hour, leaving clear water behind. The next process is known
technically as "fat-liquoring," which, as its name implies, consists
in lubricating the fibres of the leather with fatty or oily matters.
This is a very important process, for, when suitable ingredients
are used, the pliability, strength, and waterproof quality of the
leather are greatly increased. The making of special fat-liquors for
different kinds of leather has become quite an important business,
and most makes can be depended upon for the specified purposes. It
is a debatable point as to whether fat-liquoring should precede or
follow dyeing. Generally speaking, it is better to adopt the latter
course, as the fatty ingredients help to fix the colouring matter on
the fibres of the leather. The dye is also less liable to fade when
fat liquoring follows the colouring.
The number of materials that may be used for the process of
fat-liquoring is almost legion, but the principal are soaps, oils,
egg-yolk, tallow, flour, Tragasol, Irish moss, china clay, and
starch. Unless the special preparations made by leather trade
chemical firms be used, much care has to be exercised in selecting
the most suitable ingredients for each class of leather. For example,
the mixture that would give good results on vegetable-tanned leather
might be quite unsuitable for chrome leather. Then, again, the
selection of material is based on the style of finish required, a
dull finish necessitating heavy fats and greases which would be
quite unsuitable for bright leathers, and particularly for those of
the chrome-tannage. Among the hundreds of recipes that have been
published from time to time, tallow has only found a place in one or
two mixtures, yet it is one of the finest materials for strengthening
and lubricating the fibres of leather. If its splendid properties
were fully known, it would be very largely used for fat-liquoring
purposes. Of course, it is well known to the currying trade, having
for several centuries formed the basis, together with cod oil, of the
dubbin used in stuffing waxed leather and belting butts.
The following is a useful recipe for a fat-liquor for box calf: 2 lb.
Marseilles soap and 3 lb. neatsfoot oil for every 100 lb. of leather.
The soap is cut into small pieces and dissolved in hot water, and
the oil is added slowly and thoroughly mixed by stirring vigorously.
Where possible, emulsification should be done in a machine, in order
to atomise the ingredients. The finer they can be brought into a
state of division, the better they will be absorbed by the leather.
Other suitable fat-liquors for box leathers are the following: (1)
Turkey red oil, 2 per cent. of weight of leather and neutral soap 1
per cent.: (2) neatsfoot oil, 2 lb.; tallow, 1 lb.; and Tragasol,
1 lb.: (3) cod oil, 2 lbs.; Marseilles soap, 1 lb.: (4) neatsfoot
oil, 2 lbs.; egg-yolk, 1/2 lb., or six fresh yolks. The process is
completed in 20-30 minutes.
In the case of fancy colours or browns, the leather should generally
be well-drained and warmed up again in the drum in hot water (170°
F.) before fat-liquoring, but the precaution is unnecessary for
black leathers. After the fat-liquoring process, the leather is
removed from the drum, and piled up on the horse to drain, and left
for several hours or overnight, to allow it to assimilate the fatty
ingredients. It is then submitted to an operation known as sleeking
or striking out, with the object of removing surplus water and so
facilitating drying, and making the grain side smooth. Machinery is
now almost universally employed, especially for chrome leather. A
good type of striking out machine is shown in Figure 34.
If done by hand, the skins are placed on a sloping glass or slate
table, with the flesh side uppermost, which is then stretched out by
strong pressure with a tool known as a sleeker, the strokes being
made downward in the direction of the hair growth. The flesh side
done, the whole skin is turned over and the grain side is similarly
treated. Sometimes only the flesh side is struck out, especially
if the grain is soft and tender. The sleeker consists of an iron,
steel, or copper blade, about 6 in. square, fitted in a wood handle
(Fig. 32). The skins are hung up in the drying-room immediately they
are extended and smoothed by sleeking, and are then either allowed
to dry completely, or, as is generally the case with best leather,
they are hung up until a great deal of the moisture is evaporated
and the skins are left in a slightly moist, or, as it is generally
termed, "sammed" condition. The skins are then taken down, damped in
dry parts, folded over, laid in piles to equalise the moisture, and
finally smoothed out and stretched again by hand or machine. This
second operation after the dyeing is known as "setting."
Chrome-tanned skins are usually stretched and nailed on square boards
after setting, as, owing to their elasticity, they are liable to
shrink considerably in drying. The superficial area can be increased
by fully extending the skins. This operation is technically described
as "straining." It may be stated here that vegetable-tanned leathers
are not generally "strained" on boards, as such leather is better in
quality and substance when hung up and dried. Leather of combined
tannage (_i.e._, tanned with vegetable and chrome products) is,
however, frequently "strained," as the astringent property of chrome
would otherwise cause contraction of the fibres.
When dried on the boards the leather is somewhat stiff, and has,
therefore, to be softened before any finishing ingredients are
applied. Chrome leather is softened by placing it for a few days in
damp sawdust, which, for brown or fancy coloured leather, must be of
deal or white pine, and, therefore, free from objectionable colouring
matter; but, for black leather, the sawdust of any wood is suitable.
The skins must be systematically piled one above the other, and a
little damped sawdust scattered over each skin.
When properly "seasoned," the skins are brushed free of the sawdust
and "staked," either by hand or machine, usually by the latter
means. The staking knife may be fixed in a wood crutch (arm stake),
or fixed upright in a vertical wooden stand (knee stake). In the
former case, the skins to be staked must be secured in a horizontal
wooden groove fixed to two uprights, the workman then pressing
heavily in a downward direction with the staking knife, of which the
crutch is held under the armpit. In using the upright stake, the
leather is moved to and fro over the edge of the fixed knife. Both
of these manual operations are arduous and somewhat dangerous, and
should be displaced by machinery wherever possible (Fig. 36).
[Illustration: FIG. 36
STAKING AND GLAZING MACHINE]
The thoroughly softened leather is then dried, mordanted with a
solution of logwood, or haematin crystals, blacked with levant ink,
a specially prepared black dye, or with a suitable aniline dye, and
dried again. It is finally seasoned with a glutinous or albuminous
mixture (blood albumen, milk, and a little aniline black dye make a
good mixture), dried, glazed, seasoned a second time, and reglazed.
A light coating of mineral oil turns the finish into a jet black,
and the leather is ready for the warehouse. There are several types
of glazing machines, but the "grasshopper" (so called owing to its
action) is the most popular for box calf. This machine is the same as
shown in Figure 36, except that the working tool is replaced with a
smooth glass or agate roller.
The term "box" applied to this leather was invented by a prominent
American tanner, and had he registered the name and patented his
process he would have reaped a very rich harvest, for the leather
is used in enormous quantities throughout the world. As it was, the
American firm tried to obtain an injunction against English firms
manufacturing the leather, but had to withdraw their claim, as they
were not the inventors of chrome leather, which was made in Scotland
before it was introduced to the States. Moreover, the word "box," as
applied to leather, was not registered in the United Kingdom.
Willow calf is exactly the same kind of leather as box calf, except
that it is dyed brown instead of black. The seasoning mixture
naturally differs and may consist of a mixture of egg albumen, milk,
and a little of the same, or similar, dye solution as that used in
the dyeing process. In order to get clear colouring, the dyes should
be well dissolved and strained through fine muslin.
CHAMOIS LEATHER
This kind of leather is well known to the general public under the
name of wash-leather, but it is, perhaps, not so widely known that
there is practically no real chamois leather available, since the
species of animal bearing this name is almost extinct. Nowadays,
"chamois," or "shamoy" is made from the flesh split of sheep skin,
and the method whereby the leather is produced is described as the
oil tannage. The chamois leather dresser may also do the preliminary
work of fellmongering, but more often he receives the pelts, or
"fells," from the fellmonger. Although the pelts have been in a lime
liquor known as the fellmonger's "gathering limes," the process of
liming has to be continued and carefully regulated. Too much liming
makes the pelts loose, owing to the development of bacteria. This
effect would cause the finished leather to be soft and spongy. On the
other hand, under-liming fails to remove sufficient of the cement
substance which binds the fibres of the pelts; consequently, the
leather produced from these pelts is thin and somewhat gristly. After
being limed, the skins are "cobbed" (_i.e._, the bits of wool or hair
left on by the fellmonger are removed) and the pelts are then fleshed
by machinery. The next operation, splitting, is very important and
requires skilful attention to get good results. The Reeder machine
is largely in use, but the band-knife machine is also suitable. The
top half of the sheep skin (_i.e._, the grain side) is utilised for
the manufacture of skivers, while the under portion, or flesh side,
technically called "lining," serves as the raw material for chamois
leather. The best linings are generally sorted out for making into
parchment, which, of course, commands a much higher price than
chamois. Linings for chamois are then submitted to the operation of
either re-splitting or frizing, the object being to remove the loose
tissue lying between the grain and flesh. The pelts are re-split in
the case of cheap chamois, but frized if intended for choice finish.
Frizing is an operation peculiar to the making of chamois and glove
leather. It is done with a very sharp knife, similar to the fleshing
knife, and on a more upright beam than that used by tanners. The work
requires great skill, frizers being among the best paid workers in
the trade. Frizing done, the linings are freed from lime by washing
them in the drum tumbler, or paddle-vat, through which cold water is
allowed to flow continuously for two or three hours. When lamb skins,
which are too thin to split, are made into chamois leather, the grain
is removed by frizing.
A quick and effective method of deliming is to treat the skins
in a weak solution of lactic acid. Some dressers use a drench of
pea-flour or bran. The mild acids produced by the fermentation of
these materials not only neutralise the lime but also reduce the
gristly nature of the skins to a soft, supple condition. The bran
infusion is slightly warmed to hasten the process of fermentation,
but the temperature must not exceed 100° F. (32° C.). The linings
are then rinsed in cold water and sent to the stocking machines,
in which they are kneaded until they become quite soft. Either the
faller-stocks or the mechanical pushers (Fig. 14) may be used, the
latter being the more modern machine. The operation may require from
four to ten hours, the completion being determined by the condition
of the skins. "Samming" follows stocking, and for this purpose the
linings are hung up in the drying shed until thoroughly dripped, but
not dried. In this slightly damp condition they are then prepared
for the next process of oiling, which is the most important part of
chamois-dressing, as it converts the perishable raw linings into
leather. In the United Kingdom, cod oil (generally Newfoundland) is
used exclusively, and gives best results. Whale and shark liver, or
menhaden oils are often used abroad.
The linings are placed in a tub or vat, a few at a time, and oil is
poured over each layer until a sufficient number has been treated
to fill the stocking machine. The stocks are run for half-an-hour,
or until the oil has penetrated the linings, when they are put
back into the vat, where they remain for about an hour. They are
then re-stocked, taken into the shed to samm, re-oiled in the vat,
and stocked again. These processes are repeated until the skins
are thoroughly impregnated with the oil, when they are dried in a
warm stove. The skins are not yet converted into leather, which
only occurs in oil-dressing, after the oxidation of the oil. This
is effected by spontaneous heat, the dry oiled skins being heaped
in boxes and covered. Strict attention has to be paid to avoid
over-heating the skins, which are turned over and changed at
intervals. When the leather ceases to heat, the process is completed.
It is then dipped into hot water and mechanically pressed, to remove
surplus or uncombined oil, which is collected and sold under the
name of "sod oil." The leather is afterwards drummed in warm water
and finely cleansed in an alkaline solution, potash, soda, or borax,
chiefly the first named, being used for this purpose. They are then
rinsed in water, dried, damped, and softened by staking. At this
stage, the best skins are sorted out for the glove makers. The
others are finished for wash-leather by re-staking, paring with the
moon-knife, and smoothing both sides of the skin with a scurfer, or
fine pumice-stone. In the warehouse, they are damped, stretched out,
piled up, and kept fully extended by placing heavy weights near the
edges of each pile of skins.
GLOVING LEATHER
Progress in the art of making leather for gloves has been rapid
during the last few years; but further important developments are
expected, particularly in the process of tanning. Practically the
only method that has been used for many decades for converting kid,
lamb, and sheep skins into gloving leather is that known technically
as "tawing," which consists of treating skins with alum, salt,
egg-yolk, flour, and a vegetable oil. These substances change skins
into extremely supple and "stretchy" leather, but when this is
made into gloves it is far from ideal in wear, because it fails
to keep the hands warm in cold, wet weather, it is easily soiled
and cannot be cleaned without great expense, and it is not very
strong in texture. Combination tannages have lately been produced,
however, which remove the defects of alumed leather. By means of a
light chrome tannage after tawing, the leather is strengthened and
made more resistant to water, and can be cleaned with a damp rag
or sponge. By tanning skins with the formaldehyde process, or with
Neradol, the artificial tannin, for making into suède leather, the
finished article is not only washable but also resists the action
of alkalies and soap. The adaptation of combined tannages in the
manufacture of gloving leathers has only lately been developed, and
further improvements will doubtless be effected before long.
Lamb, kid, goat, and sheep skins constitute the raw material for
gloving leathers, although deer and antelope skins are also used
to a small extent. Real kid skins are the best wearing dress
gloves, but the great majority of so-called "kid" gloves are made
of lamb skins. The raw kid and lamb skins are chiefly of European,
Arabian, and Indian origin. Sheep skins from the Cape provide the
raw material for a large number of men's gloves, and leather of
very good quality can be produced from the best grades. Most of the
skins are preserved by drying, or by salting and drying, although
some kid skins are wet-salted and packed in barrels for export. It
would save leather-dressers a great deal of trouble if they could
always get wet-salted skins; but the object of drying them is to
reduce the weight and lower the cost of freight. Soaking is done by
methods already described (p. 65). Loose flesh and pieces of fat
are cut off in order to facilitate the action of the depilitant,
of which the best for glove leather is undoubtedly a paste of lime
and red arsenic. Seven or eight parts of lime to one of arsenic is
a satisfactory proportion, the quantities to be mixed depending on
the number of skins to be treated, as a fresh mixture should be made
for each lot, or "pack" as the tanner terms it. The lime should be
well broken up, or, better still, pure powdered lime should be used
and the red arsenic well mixed with it; a little water is then added
to slake the lime gradually, and the mixture is stirred to promote
chemical reaction. The compound is further diluted with water until
it has the right consistency and the colour has changed. The reaction
generates great heat, and the "paint" should, therefore, not be used
at once. The flesh side is mopped with the paint and the skins are
folded flesh to flesh. After a few hours, or as soon as the hair
or wool is loosened, the skins are dehaired or dewoolled. The hair
or wool is not allowed to come into contact with the depilitant,
otherwise it would be damaged. In large yards, the white hair is
separated from the coloured, as it is worth nearly twice as much.
Wool is sorted into different qualities, of which the number may vary
from four to eight, or even nine, according to the class of skins
treated. The pelts are then thoroughly washed and placed in lime
liquors, where they remain for one or two weeks, being hauled and set
in the usual manner. Fleshing and piecing or trimming are the next
operations, and then follows the very important process of puering,
which, in the case of glove leather, must be thoroughly done so as to
reduce the pelts to a very soft and flaccid condition. Success in the
making of glove leather depends largely on the "puering" process.
In most of the English tanneries a decoction of dog manure is used,
at a temperature not exceeding 90° F., but on the Continent the
artificial puer, oropon, is preferred. It is much safer to use and
more uniform in its action than excrement, which develops bacteria
rapidly in contact with gelatinous pelts, and could ultimately
destroy them entirely. After puering them, the pelts are well washed
and submitted to the process of drenching, which consists in putting
the skins into a warm infusion of bran or pea-flour and leaving them
covered until the following morning. The slightly acid fermentation
causes the pelts to rise to the top of the vat. They are pushed into
the liquor again with a pole and stirred round. This is repeated
three or four times to prevent damage to the grain. The process is
often done in the paddle-vat (Fig. 25), in which the bran liquor is
circulated for several hours before the pelts are allowed to remain
quiescent. Drenching thoroughly purges the pelts of the last traces
of lime, and puts them in suitable condition for being made into
leather. The pelts are then rinsed in tepid water and "scudded" on
the grain with a slate or vulcanite tool, shaped somewhat like a
dehairing knife. The scud removed consists of dirt, dissolved lime
salts, short hairs, and pigment. Machines are rapidly replacing
manual labour for this operation.
The alum tannage, known technically as "tawing," is largely used for
kid and lamb gloves. The tawing mixture is composed of alum, salt,
egg-yolk, and wheaten flour. The proportions used vary considerably
in different tanneries, but the following is a typical recipe: 4 lb.
alum, 2 lb. salt, 1 lb. salted egg-yolk, or the yolks of twenty fresh
eggs, and 5 lb. flour for 100 lb. of pelts. The flour is made into
a paste, the egg yolk is diluted in warm water and mixed with the
flour, the salt and alum are dissolved and added, and the mixture
thoroughly stirred. A suitable quantity of water (about 2 gals. per
100 lb. pelts) is then placed in the drum tumbler, the tawing mixture
is added, and the drum revolved for a few minutes before putting the
pelts in. The process is completed in two or three hours in the case
of thin skins. It is a good plan, however, to leave them at rest
in the drum for a day, after which they are piled up overnight to
allow further combination of the tawing materials with the fibres
of the pelts. The leather is then dried out completely, damped in
clean sawdust, or by sprinkling with water, levelled by shaving if
necessary, staked over an upright knife fixed in a wooden stand
or by machine, and dried in a hot stove. In this condition, or in
the "crust," as dressers term it, the leather is allowed to remain
several weeks to "age," a most essential process for the production
of soft, and supple glove leather.
Dressing and dyeing are begun as soon as the leather is
satisfactorily aged. The skins are uniformly soaked in warm water,
dyed, and re-dressed with egg yolk ("re-egged"), to which a small
quantity of olive oil, or a sulphonated oil, is added. Some dressers
prefer to give the second tawing mixture before dyeing, but the
advantage of dressing the leather after dyeing is that the colour is
securely fixed. In "re-egging," many dressers use a similar mixture
to the first dressing. The dyeing process is of great importance,
since the colour must be fast. The leather is dyed either in the
drum or on a convex table. In the former case, the leather is
naturally coloured both sides, while, in the latter, it is stained
with a brush on the grain side only. Staining is the more difficult
method. Kid glove leather may be dyed with aniline colours, or,
as more generally practised, with natural dye-woods as a base and
aniline dye for top-colouring. The great advantages of the latter
method are economy in dye-stuffs and increased depth of colour. The
skins are first prepared for dyeing by brushing with, or drumming
them in, an alkaline solution. Stale urine was largely used for this
process, but ammonical salts are now generally preferred, if only
for sanitary reasons. The skins are then drummed or paddled in, or
brushed with, dye-wood liquids which have been carefully strained. A
large selection is available, including fustic, cuba wood, saffron,
peachwood, logwood, sappan wood, cutch, Persian berries, gambier or
terra japonica, and golden tan bark.
Light and medium brown can be obtained from these dye-woods without
the aid of aniline colours; but for dark shades, and to increase the
brilliancy of other colours, a top dye or coal-tar dye is often given.
The natural dyes are further developed with "strikers," which mainly
consist of metallic salts. Iron, copper, and zinc sulphates, nitrate
and acetate of iron, bichromate of potash, and titanium salts
(titanium lactate, titanium potassium oxalate, and tanno-titanium
oxalate) are the most important. The lactate, sold commercially under
the name of "corichrome," is especially suitable, as, unlike the
mineral acid salts, especially the sulphates, it has no destructive
effect on the fibres of the leather.
Dye-woods are now concentrated in the form of a paste, or dry
extract, the latter being the more reliable. They are also very
convenient to use and dissolve, while mixtures are, of course,
easily prepared. A good tan shade on Cape sheep can be obtained by
mordanting the leather with a solution of 1 lb. of bichromate of
potash for every 100 lb. of leather, drumming it in 8-9 lb. of pure
gambier, and then with a mixture of cuba wood extract, 1 lb.; fustic
extract, 3/4 lb.; Brazil wood extract, 1/2 lb.; and logwood extract,
1/2 oz. After drumming the leather in this dye liquor for about an
hour, the colour is developed with corichrome. If a darker shade be
required, the leather can be treated with a suitable basic dye. After
dyeing the leather, some dressers only fat-liquor it with egg-yolk
and a small quantity of olive oil, while others prefer to re-dress it
with a similar mixture to that used for tawing, namely, alum, salt,
egg-yolk, and flour; but, where titanium salts are used, the latter
method is not essential, because titanium has tanning properties.
When dry, the leather is ready for finishing, but it is advisable to
keep it in store for a few days before packing it in damp sawdust or
sprinkling it with water to prepare it for the operation of staking.
Anything more unlike leather would be difficult to imagine at this
stage, but, after stretching the skin in the staking machine, or by
drawing it over the upright stake, the dry, stiff, and shrivelled
leather is reduced to a very supple condition. The flesh side of the
leather is then pared with the moon-knife, or in the shaving machine,
to equalise the thickness. In some works, a special tool which pares
the leather on a flat table is preferred; this particular operation
is called "doling." The flesh side is finished by fluffing it on the
emery or carborundum wheel (Fig. 33). Finally, the grain is brushed
and polished with the glass sleeker, or ironed.
Chamois leather has been largely used for gloves of late years,
but this leather has the defect, in common with suède leathers,
of getting soiled much more quickly than grain leathers, such as
kid, lamb, or Cape sheep. Nevertheless, suède and chamois gloves
are likely to remain fashionable to a more or less extent. The
manufacture of chamois leather is described on page 144. Sun-bleached
skins are the best for dyeing, especially if delicate shades are
wanted. The frontispiece shows a field covered with skins bleaching
in the sun. Chemically bleached leather is likely to become
discoloured after dyeing. Defective skins are often dyed with
pigments (dust colours), and this system is also applied to skins
which have to be dyed such delicate shades as cannot be produced by
wood or aniline colours. Although it gives attractive results to the
eye, and certainly covers up any defects of the grain, this method
of dyeing is not altogether satisfactory, as the leather remains
unpleasantly dusty in wear for quite a long time.
The dyeing of chamois with wood-dyes or coal-tar colours is by no
means easy, but this method gives the best results when successful.
The grease must first be removed from the leather with a solution of
5 lb. of borax or 3-1/2 lb. of soda for every 100 lb. of leather.
If the leather is still greasy on the surface, a further quantity
of soda or borax is given, after which the leather is well washed
in warm water, sumached, rinsed to remove the particles of sumach,
and mordanted with titanium salts. The dyeing is then done with
anilines or wood-dyes, or a combination of both, and this is followed
by fat-liquoring with egg-yolk and a sulphonated oil. The finishing
operations are staking and fluffing.
To get a good, fast black on chamois and suède leathers is one
of the difficult processes in the leather trade, although it is
easier to get a good black on alumed or chromed leather than on
vegetable-tanned. Alumed leather is washed in a solution of borax
or carbonate of ammonia to remove uncombined dressing in order to
prepare it for dyeing. Chrome-tanned suède leather does not need this
preparation. The leather is first mordanted with dye-wood extract,
of which a suitable mixture is logwood and fustic, or logwood and
quercitron, in the proportion of 4 lb. and 2 lb. to every 100 lb. of
leather. After drumming the leather in this solution for about an
hour, a weak solution of copperas (ferrous sulphate) and bluestone
(copper sulphate) is added, and the milling is continued for twenty
minutes, when the leather is well prepared to receive the black dye.
Instead of the iron and copper salts, corichrome is often preferred,
as it is quite safe to use, whereas iron salts have a destructive
action on the fibres of the leather, unless the precaution be taken
to mordant the skins with a good quantity of dye-wood extract.
Following the application of the iron or corichrome striker, the
leather is dyed with suitable aniline black (leather black, or
corvoline) and finally fat-liquored to nourish the leather, and to
fix and intensify the black. This recipe also gives good results
where the skins are dyed only on the flesh side, the solutions being
applied with a brush.
WHITE WASHABLE LEATHER
Among the new kinds of leather for gloves, none is more remarkable
or more useful than the washable sheep or goat skins. The great
advantage of this leather is that it can be washed in warm water
and soap any number of times without injury, whereas gloves of
ordinary tawed kid and lamb skins have to be dry cleaned and cannot
be renovated many times. An additional advantage of washable leather
is its warmth. After being dehaired, puered and drenched, the skins
are drummed in a solution of formaldehyde and soda. In two or three
hours, the skins are tanned, and are then treated with a solution
of sulphate of ammonia. The quantities required are about 3 lb. of
formaldehyde (40 per cent.) and 8 lb. of sodium carbonate (80 per
cent.), and 1 lb. of sulphate for 100 lb. of pelts, using sufficient
water to cover the skins well in the drum. This tannage produces a
white but somewhat thin and empty leather, and the fat-liquoring
must, therefore, be filling and softening. An emulsion of white curd
soap and olive oil, or of egg-yolk and neatsfoot oil, is suitable.
"Crestanol," a special preparation, also gives satisfactory results,
since it is adapted for giving nourishment and resiliency to thin,
empty leather.
FANCY LEATHERS
The best known of the fancy leathers is "morocco." This variety has
been made for ages, and the name probably originated from the fact
that very fine leathers of this kind were manufactured in Morocco a
few centuries ago. History records that a similar leather, dyed red,
was made in the ninth century before the Christian era.
The best morocco leather is made from Continental goat skins, which
are mostly obtained from Central Europe and Spain. The Norwegian goat
skins are also said to be of good quality for the morocco finish. An
inferior morocco leather, which is produced in large quantities, is
manufactured from East India goat skins, while a cheaper grade still
can be produced from certain classes of East India sheep skins. The
real moroccos are tanned in sumach, but the cheaper sorts are tanned
in India with babool or turwar bark and re-tanned in sumach in the
countries to which they are exported, chiefly Great Britain, Germany,
France, and America.
One of the best methods of sumach-tanning goat and calf skins is
that known as the "bottle" tannage. Each skin is doubled over and
sewn by machine round the edge, leaving part of the neck unsewn. The
skins are then turned inside out and filled with a strong infusion of
sumach, and floated in a tub containing sumach liquor. After being a
few hours in the tub, the skins are heaped one above the other in a
large pile, where the pressure forces the tannin through the skins.
The process can be completed in twenty to twenty-four hours, after
which the skins are cut open, rinsed, and finished in the usual
manner. This method of tanning is now largely replaced by the use of
paddle-vats (Fig. 25).
The dyeing and finishing are somewhat similar to that of coloured
boot upper leather, except that the leather is slightly oiled
on the grain instead of being fat-liquored. There are several
different methods of graining morocco leather; some of the grains
are made naturally by pressing the leather, when folded over, with a
cork-covered board, while others are first embossed in various ways
and then boarded. The well-known "crushed morocco" is produced by
glazing the grained leather under heavy pressure.
Skivers, the grain splits of sheep skins (see p. 145) are extensively
used for fancy articles. The majority are finished with a smooth
grain for hat-bands, bookbinding, and linings. The grain of a
sheep skin is, naturally, soft, and not very strong. To stiffen
and strengthen the grain, an artificial layer, consisting of paste
finishes, is often applied. Skins finished in this manner are termed
"paste grain skivers." These are largely used for bookbinding and
cheap purses. Sheep grains are sometimes given a finish somewhat
similar to that of leather bags. This is produced in a printing
machine by means of a copper roller which is run over the damp (but
not too wet) leather. The skins are then described as "long-grain"
skivers. The grain surface of sheep skins is particularly suitable
for embossing, and wonderful imitations of all kinds of skins can be
reproduced on skivers.
The fancy leather trade is not confined to these imitations, however,
as real lizard, seal, ichneumon, alligators, crocodile, shark,
porpoise, snake, and even frog skins (Japanese) are utilised.
INDEX
Acid Swelling of Hides, 119
Adulterated Leather, 114
Ageing of Leather, 151
Algarobilla, 43
Alum Tannage, 150
Annatto, 136
Antelope Leather, 63
Anthrax, 27, 69
Arsenic Sulphide, 71
Bablah, 45, 107
Babool, 45, 107
Band-knife Splitting Machine, 54
Bark Mill, 93
Barkometer, 97
Bating, 86
Belting Leather, 133
Birch Bark, 106
---- Tar Oil, 107
Bleaching Leather in Sun--_Frontispiece_
Boot Leather, 137
"Bottle" Tannage, 157
Borax, 64
"Box" Leather, 137
Buffing Cylinder, 54
Butts, 95
Butyrate of Ammonia, 85, 129
Butyric Acid, 85, 129
Canaigre, 45
Cape Sheep, 153
Catechol Tannins, 46
Caustic Soda, 66
Celavina, 43
Chamois Leather, 18, 144
Chestnut Oak, 37
---- Tree, 36
Chrome-quebracho Tan Liquor, 32
Chrome Sole Leather, 125
---- Tanning, 108
Cod Oil, 100, 120, 146
Collodion, 136
Colouring Pits, 94
Combination Tannages, 111
"Crestanol," 156
"Crust" Leather, 151
Currying, 133, 135
Cutch, 46
Dehairing Hides, 71
---- Knife, 80
---- Machine, 82
Dehydration, 2
Depilatories, 78, 149
Disinfection of Hides, 2
Divi-divi, 43
Doling, 153
Dressing Leathers, 102
---- of Leather, 112
Drenching, 89
"Dri-ped" Leather, 129
Drum-tanned Leather, 103
Drum Tumbler, 66
Drying of Leather, 123
Dust Colours, 154
Dyeing, 112, 155
Dye-wood Extracts, 139, 152, 155
Ellagic Acid, 47, 121
Enzymes, 72
"Erodin," 88
Evolution of Leather Manufacture, 1
Extracts, 47
Faller Stocks, 66, 146
Fancy Leathers, 156
Fat Liquors, 139
Fellmongering, 145
Finishing, 112, 122, 127
Flaying: Mechanical method, 21
----: Pim's method, 24
Fleshing Knife, 83
---- Machine, 82
Fluffing, 128, 153
---- Machine, 129
Foot and Mouth Disease, 6
Formic Acid, 28, 66, 85, 95, 100
Freezing Hides, 71
Frizing, 145
Fustic, 152
Gallotannic Acid, 46
Gambier, 40
Glazed Kid, 137
Glazing Machine, 143
Gloving Leather, 147
Goat Skins, 15
Golden Tan Bark, 152
Greasy Hides, 12
---- Sole Leather, 75
Hammering Leather, 123
Handlers, 94
Harness Leather, 133
Hauling and Setting Limes, 77
Hemlock, 44
Hide Markets, 7
Hides and Skins, 5
----, Curing, 2, 26
----, Defects of, 19
----, Disinfection of, 2, 26
----, Horse, 17
----, Sources of Supply, 5
Hyaline Layer, 68
_Hypoderma bovis_, 30
---- _lineatum_, 30
Japanese White Leather, 3
Lactic Acid, 85, 95, 100, 129, 146
Larch Bark, 45
Layers, 94
Leather-working Machinery, 52
Liming, 71
Linings, 145
Logwood, 138
"Lysol," 84
Machinery, 52
Mangrove, 41
Marseilles Soap, 140
Mimosa, 42
Mineral Tannage, 108
Mixed Tannages, 101
Morocco Leather, 156
Motor Leather Bands, 108
Myrobalans, 38, 93
"Neradol," 50
Oak Bark, 34
Oakwood, 35
"Oropon," 87
Paddle-vat, 146
Parchment, 145
Pickled Sheep Skins, 10
Pigments, 154
Pig Skins, 18, 136
Pinning Leather, 121
---- Machine, 121
Porpoise Laces, 19
Preparation of Hides for Tanning, 63
"Puerine," 88
Puering, 150
Pyrogallol Tannins, 46
Quebracho, 32, 37
Red Arsenic, 71, 149
"Reeder" Splitting Machine, 145
Roller Leather, 13
Rolling Sole Leather, 122
Rounding of Sole Leather, 95
Saddlery Leather, 133
Samming, 146
Scudding, 86
Seal Skins, 19
Setting, 142
Shaving Cylinder, 53
---- Knife, 55, 56
---- Machine, 56, 61
Sheep Skins, 12
Skins and Hides, 5
Skivers, 145, 157
Sleeker, 128
Soaking Hides, 65
Sod Oil, 147
Sole Leather, 95
"Soluble" Oils, 117
Staking Machine, 143
Sterilising Hides: Seymour-Jones's method, 28
Straining, 128, 142
Striking-out, 128, 130
Stuffing, 135
Suède Leather, 154
Sulphide of Sodium, 66, 76
Sulphonated Oils, 117
Sumach, 38
Suspender Pits, 94
Synthetic Tannin, 49, 95
Tanner's Beam, 68
Tanning Extracts, 47, 92
Tanning Materials, 32
---- Processes, 92
Tan-yard, 94
Tawing, 150
_Terra japonica_, 40
Tick Fever, 27
Titanium Salts, 152
"Tragasol," 117, 130, 140
Turkey Red Oil, 141
Turwar Bark, 156
Vache Sole Leather, 132
Valonia, 35
Warble Fly, 28
Wattle Bark, 42
Weighted Leather, 114
White Washable Leather, 155
Willow Bark, 45, 106
---- Calf, 144
Wool Skin Dressing, 51
THE END
_Printed by Sir Isaac Pitman & Sons, Ltd., Bath, England_