board; S, slicker.]
For waxing, the first operation is to black the goods. In England this
is generally done by hand, but machinery is much more used in the
United States. The process consists of well brushing into the flesh
side of the skins a black preparation made in one of two ways. The
older recipe is a mixture of lampblack, oil and perhaps a little
tallow; the newer recipe consists of soap, lampblack, logwood extract
and water. Either of these is brushed well into the flesh side, which
is then glassed up by means of a thick slab of glass, the smooth
rounded edges being used with a slicking motion, and the goods are
hung up to dry. When dry they are oiled with cod oil, and are ready
for sizing. Goods blacked with soap blacking are sized once, those
prepared with oil blacking are sized twice. The size used for soap
black skins may consist of a mixture of beeswax, pitch, linseed oil,
tallow, soap, glue and logwood extract. For oil blacked skins the
"bottom sizing" may be glue, soap, logwood extract and water, after
the application of which the goods are dried and the "top sizing"
applied; this consists of glue, cod oil, beeswax, tallow, venice
turps, black dye and water. The sizings having been applied with a
sponge or soft brush, thoroughly rubbed in with a glass slicker, crush
marks are removed by padding with a soft leather pad, and the goods,
after being dried out, are ready for the market.
In the dressing of waxed grain leathers, such as French calf, satin
leather, &c., the preparatory processes are much the same as for waxed
leathers described above as far as stuffing, after which the grain is
prepared to take the colour by light hand scouring with weak soap and
borax solution. The dye is now applied, and so that it may take well
on the grain of the greasy leather, a quantity of either soap, turkey
red oil or methylated spirit is added to the solution. Acid colours
are preferably used, and three coats are given to the dry leather,
which is then grained with an arm board, and finished by the
application of hard buck tallow to the grain and brushing. The dye or
stain may consist of aniline colours for coloured leathers, or, in the
case of blacks, consecutive applications of logwood and iron solutions
are given.
_Finishing dressing Hides for Bag and Portmanteau Work._--The hides as
received from the tanner are soaked down, piled to sammy, and shaved,
generally by machine, after which they are scoured, as under waxed
leather, sumached and hung up to dry; when just damp they are set out
with a brass slicker and dried right out. The grain is now filled by
applying a solution of either Irish moss, linseed mucilage or any other
mucilaginous filling material, and the flesh is sized with a mixture of
mucilage and French chalk, after which the goods are brush-stained with
an aniline dye, to which has been added linseed mucilage to give it
body; two coats are applied to the sammied leather. When the goods have
sammied, after the last coat of stain, they are "printed" with a brass
roller in a "jigger," or by means of a machine embosser. This process
consists of imprinting the grain by pressure from a brass roller, on
which the pattern is deeply etched. After printing, the flesh side is
sponged with a weak milk solution, lightly glassed and dried, when the
grain is sponged with weak linseed mucilage, almost dried, and brushed
by machine. The hides are now finished, by the application either of
pure buck tallow or of a mixture of carnauba wax and soap; this is
rubbed up into a slight gloss with a flannel.
_Light Leathers._--So far only the heavier leathers have been dealt
with; we will now proceed to discuss lighter calf, goat, sheep, seal,
&c.
In tanning light leathers everything must tend towards suppleness and
pliability in the finished leather, in contrast to the firmness and
solidity required in heavy leathers. Consequently, the liming is longer
and mellower; puering, bating or some bacterial substitute always
follows; the tannage is much shorter; and mellow materials are used. A
deposition of bloom in the goods is not often required, so that very
soon after they are struck through they are removed as tanned. The
materials largely used are sumach, oak bark, gambier, myrobalans, mimosa
bark, willow, birch and larch barks.
As with heavy leathers, so also with light leathers, there are various
ways of tanning; and quality has much to do with the elaboration or
modification of the methods employed. The tanning of all leathers will
be dealt with first, dyeing and finishing operations being treated
later.
The vegetable-tanned leather _de luxe_ is a bottle-tanned skin. It is
superior to every other class of vegetable-tanned leather in every way,
but owing to competition not a great deal is now produced, as it is
perhaps the most expensive leather ever put on the market. The method of
preparation is as follows.
[Illustration: FIG. 10.--Dash Wheel.]
The skins are usually hard and dry when received, so they are at once
soaked down, and when sufficiently soft are either milled in the stocks,
drummed in a lattice drum (American dash wheel, fig. 10), or "broken
down" over the beam by working on the flesh with a blunt unhairing
knife. They are next mellow limed (about 3 weeks), sulphide being used
if convenient, unhaired and fleshed as described under heavy leathers,
and are then ready for puering. This process is carried through at about
80° F., when the goods are worked on the beam, rinsed, drenched in a
bran drench, scudded, and are ready for tanning. The skins are now
folded down the centre of the back from neck to butt (tail end), flesh
outwards, and the edges are tightly stitched all round to form bags,
leaving an aperture at one of the shanks for filling; they are now
turned grain outwards and filled with strong sumach liquor and some
quantity of solid sumach to fill up the interstices and prevent leakage,
after which the open shank is tied up, and they are thrown into warm
sumach liquor, where they float about like so many pigs, being
continually pushed under the surface with a dole. When struck through
they are piled on a shelf above the vat, and by their own weight the
liquor is forced through the skins. The tannage takes about 24 hours,
and when finished the stitching is ripped up, the skins are slicked out,
"strained" on frames and dried. "Straining" consists of nailing the
skins out on boards in a stretched condition, or the stretching in
frames by means of strings laced in the edge of the frame and attached
to the edge of the skin.
The commoner sumach-tanned skins (but still of very good quality) are
tanned in paddle wheels, a series of three being most conveniently used
in the same manner as the three-pit system of liming, each wheel having
three packs of skins through it before being thrown away. This paddling
tends to make a bolder grain, as the skins are kept in continual motion,
and work over one another. Some manufacturers finish the tannage with a
mixture of sumach and oak bark; this treatment yields a less porous
product. Others, when the skins are strained and in a semi-dry
condition, apply neatsfoot or other oil, or a mixture of glycerine and
oil, to the grain to lubricate it and make it more supple; the glycerine
mixture is generally used for "chrome" leather, and will be discussed
later under that head.
The skins tanned as above are largely dressed as _morocco_. Originally
"morocco" was produced by the Moors in southern Spain and Morocco,
whence the industry spread to the Levant, Turkey and the Mediterranean
coast of Africa generally, where the leather was made from a species of
sumach. Peculiarly enough, the dyeing was carried out before the
tanning, with Roman alum as "mordant" and kermes, which with the alum
produced a fine red colour. Such leather was peculiarly clear in colour,
elastic and soft, yet firm and fine in grain and texture, and has long
been much prized for bindings, being the material in which most of the
artistic work of the 16th-century binders was executed. Now, in addition
to the genuine morocco made from goat skins, we have imitation or French
moroccos, for which split calf and especially sheep skins are employed,
and as the appearance of morocco is the result of the style of graining
and finish, which can now be imitated by printing or embossing machines,
morocco can be made from all varieties of thin leather.
Great quantities of "Persian" (East India tanned) sheep and goat are
now dressed as moroccos and for innumerable other purposes, the method
being as follows: The goods are tanned with turwar bark and cassia
bark, besides being impregnated with sesame oil, even to the extent of
30%. The first operation is to "strip" them of the oil and original
tannage as far as possible, by drumming in a solution of soda; the
soap thus formed is got rid of by thoroughly washing the goods, when
they are "soured" in a weak bath of sulphuric acid to brighten the
colour and remove iron stains, after which they are washed up and
re-tanned by drumming in warm sumach, allowing about 4 oz. per skin.
They are then slicked out, dried and are ready for dyeing.
The tanning of sheep and lamb skins differs very essentially from the
tanning of goat and other leathers, mainly in the preparatory
processes. As the wool is completely destroyed by lime, other methods
have to be resorted to. The process usually practised is known as
"sweating"; this consists of hanging the moist skins up in a warm,
badly-ventilated chamber and allowing incipient putrefaction to set
in. The chamber is always kept warm and saturated with moisture,
either by means of a steam jet or water sprinklers. During the process
large quantities of ammoniacal vapours are given off, and after two or
three days the skins become slimy to the touch, and the wool slips
easily; at this stage the goods are removed, for if the putrefaction
goes too far the grain of the skin is irretrievably ruined. The wool
is now "pulled" by pullers, who throw it into bins arranged to receive
the different qualities; for one pelt may have three different grades
of wool on it.
Other methods of dewooling are to paint the flesh with a solution of
sodium sulphide, or cream of lime made with a solution of sodium
sulphide; in either case the goods are piled flesh to flesh for an
hour or so, and care is taken that the dewooling agent does not touch
the wool. The pelt is then pulled and rapidly swilled in a stream of
running water. The goods are now, in some yards, lightly limed to
plump them superficially, by paddling in a milk of lime, and at this
stage, or when the goods have been "struck through" with tan liquor,
they are "degreased" either by hydraulic pressure or by benzene
degreasing. This is to expel the oleaginous or fatty matter with which
sheep skins are richly impregnated; the average yield is about 4 oz.
per skin. The tannage is carried out in much the same way as for goat
skins, the goods being started in old acid bark liquors; the general
tannage consists of sumach and bark.
_Basils_ are sheep skins tanned in various ways. English basils are
tanned with oak bark, although, as in all other leathers, inferior
tannages are now common; Scotch basils are tanned with larch bark,
Australian and New Zealand basils with mimosa bark and Turkish basils
with galls. The last are the commonest kind of skins imported into Great
Britain, and are usually only semi-tanned. _Roans_ are sumach-tanned
sheep skins.
_Skivers_ are the grain splits of sheep skins, the fleshes of which are
finished for chamois leather. The goods are split in the limed state,
just as the grains are ready for tanning, and are subsequently treated
much as sumach-tanned goat skins, or in any other convenient way; the
fleshes, on the other hand, go back into the limes, as it is necessary
to get a large quantity of lime into leather which is to be finished as
chamois.
_Russia Leather_ was originally a speciality of Russia, where it was
made from the hides of young cattle, and dressed either a brownish red
or black colour for upper leather, bookbinding, dressing-cases, purses,
&c. It is now made throughout Europe and America, the best qualities
being obtained from Austria. The empyreumatic odour of the old genuine
"Russia" leather was derived from a long-continued contact with willow
and the bark of the _white_ birch, which contains the odorous betulin
oil. Horse hides, calf, goat, sheep skins and even splits are now
dressed as "Russia leather," but most of these are of a decidedly
inferior quality, and as they are merely treated with birch bark oil to
give them something of the odour by which Russia leather is ordinarily
recognized, they scarcely deserve the name under which they pass. The
present-day genuine Russia leather is tanned like other light leathers,
but properly in willow bark, although poplar and spruce fir barks are
used. After tanning and setting out the goods are treated with the
empyreumatic oil obtained by the dry distillation of birch bark. The red
colour commonly seen in Russia leather is now produced by aniline
colours, but was originally gained by the application of an infusion of
Brazil wood, which was rubbed over the grain with a brush or sponge.
Some time ago Russia leather got into disrepute because of its rapid
decay; this was owing to its being dyed with a very acid solution of tin
salts and cochineal, the acid completely destroying the leather in a
year or two. The black leather is obtained by staining with logwood
infusion and iron acetate. The leather, if genuine quality, is very
watertight and strong, and owing to its impregnation with the
empyreumatic oil, it wards off the attacks of insects.
_Seal Leathers, &c._--The tannage of seal skins is now an important
department of the leather industry of the United Kingdom. The skins form
one of the items of the whaling industry which principally centres in
Dundee, and at that port, as well as at Hull and Peterhead, they are
received in large quantities from the Arctic regions. This skin is that
of the white hair seal, and must not be confused with the expensive seal
fur obtained from Russian and Japanese waters. These white hair seal
skins are light but exceedingly close in texture, yielding a very strong
tough leather of large area and fine bold grain, known as _Levant
morocco_. The area of the skins renders them suitable for upholstery
work, and the flesh splits are dressed in considerable quantity for
"japanned" ("patent") leather and "bolsters," which are used to grain
other skins on, the raised buff affording a grip on the skin being
grained and thus preventing slipping. When the skins arrive in the
tanyard (generally lightly salted) they are drummed in old drench
liquors until soft, dipped into warm water and "blubbered" with a sharp
knife; they are then alternately dipped in warm water and drummed
several times to remove fat, after which they are heavily limed, as they
are still very greasy, and after unhairing and fleshing they are heavily
puered for the same reason. The tannage takes about a month, and is much
the same as for other leathers, the skins being split when "struck
through."
Alligator leather is now produced to some extent both in the United
States and India. The belly and flanks alone are useful. There are no
special tanneries or processes for dressing the skins. Layers are not
given. The leather is used mostly for small fancy goods, and is much
imitated on sheepskin by embossing.
Snake and frog skins are also dressed to some extent, the latter
having formed a considerable item in the exports of Japan; they are
dressed mostly for cigar cases and pocket books. The general procedure
is first to lime the goods and then to remove any scales (in the case
of snake skins) by scraping with an unhairing knife on a small beam,
after which the skins are bated and tanned in sumach by paddling.
A considerable amount of leather is now produced in Australia from the
skins of kangaroo, wallaby and other marsupials. These skins are both
tanned and "tawed," the principal tanning agents being mimosa bark,
mallet bark and sugar bush, which abound in Australia. The leather
produced is of excellent quality, strong and pliable, and rivals in
texture and appearance the kid of Europe; but the circumstance that
the animals exist only in the wild state renders them a limited and
insecure source of leather.
_Japan and Enamel Leathers._--Japanning is usually done on flesh splits,
whereas enamelling is done on the grain, and if splits are used they are
printed and boarded. The leather should be mellow, soft, free from
grease, with a firm grain and no inclination to stretch. It is first
shaved very smooth, thoroughly scoured with a stone, sumached, washed,
slicked out tight and dried; when "sammied," the grain is buffed to
remove scratches and oiled, the goods are then whitened or fluffed, and
if too hard, bruised by boarding; enamel goods are now grained. The
skins are now tightly nailed on boards and any holes patched up with
brown paper, so that the japan shall not touch the flesh when the first
thick coat of japan or the "daub" is put on. This is applied so thickly
that it cannot soak in, with fine-toothed slicker, and then placed in a
hot stove for twenty-four hours until quite dry; the coating is then
pumiced smooth and the second thinner coat, termed "blanback," is
applied. This is dried and pumiced, and a fine coating of japan or copal
varnish is finally given. This is dried and cooled, and if the goods are
for enamel they are boarded.
English japans sometimes contain light petroleum, but no turps. The
secret of successful japanning lies in the age of the oil used; the
older the linseed oil is, the better the result. To prepare the ground
coat, boil 10 gallons linseed oil for one hour with 2 lb. litharge at
600° F. to jellify the oil, and then add 2 lb. prussian blue and boil
the whole for half an hour longer. Before application the mixture is
thinned with 10 gallons light petroleum. For the second coat, boil 10
gallons linseed oil for 2 hours with 2 lb. prussian blue and 2 lb.
lampblack; when of a thin jelly consistency thin with 5 gallons of
benzine or light petroleum. For the finishing coat, boil 5 gallons of
linseed oil for 1 hour, then add 1 lb. prussian blue, and boil for
another hour; thin with 10 gallons petroleum and apply with a brush in
a warm room. After drying, the goods are mellowed by exposure to the
sun for at least three days.
_Tawing._--Wool rugs are, after the preliminary processes, sometimes
tanned in oak bark liquors by paddling, but are generally "tawed," that
is, dressed with alum and salt, and are therefore more suitably dealt
with under that head. Tawing implies that the conversion of skins into
leather is carried out by means of a mixture of which the more important
constituents are mineral salts, such as alum, chrome and iron, which may
or may not be supplemented with fatty and albuminous matter, both animal
and vegetable.
As an example of alum tawing, calf kid may be taken as characteristic of
the process; glove kid is also treated on similar lines. The goods are
prepared for tawing in a manner similar to the preparation of tanned
leathers, arsenical limes being used to ensure a fine grain. After being
well drenched and washed the goods are ready for the tawing process. On
the continent of Europe it is usual for the goods to be thrown into a
tub with the tawing paste and trodden with the bare feet, although this
old-fashioned method is gradually being driven out, and the drum or
tumbler is being used.
The tawing paste consists of a mixture of alum, salt, flour, egg yolk
and water; the quantities of each constituent diverge widely, every
dresser having his own recipe. The following has been used, but cannot
well be classed as typical: For 100 lb. skin take 9 lb. alum, 5 lb.
salt, dissolve in water, and mix to a thin paste with from 5 to 13 lb.
flour, using 4 to 6 egg yolks for every pound of flour used. Olive oil
is also mixed in sometimes. The skins are drummed or trodden, at
intervals, in the warm paste for some hours, removed, allowed to
drain, and dried rapidly, damped down or "sammied" and "staked" by
drawing them to and fro over a blunt knife fixed in the top of a post,
and known as a knee stake; this process softens them very
considerably. After staking, the goods are wet back and shaved smooth,
either with a moon knife, i.e. a circular concave convex knife, the
centre of which has been cut out, a piece of wood bridging the cavity
forming the grip, or with an ordinary currier's shaving knife; the
skins are now ready for dyeing and finishing.
_Wool Rug Dressing._--Wool rugs are first thoroughly soaked, well washed
and clean-fleshed, scoured well by rubbing into the wool a solution of
soft soap and soda, and then leathered by rubbing into the flesh of the
wet skins a mixture consisting of three parts of alum and two parts of
salt until they are practically dry; they are now piled up over-night,
and the mixture is again applied. After the second or third application
the goods should be quite leathered. Other methods consist of stretching
the skins in frames and painting the flesh with a solution of alum and
salt, or, better, with a solution of basic alum and salt, the alum
being made basic by the gradual addition of soda until a permanent
precipitate is produced.
The goods are now bleached, for even the most vigorous scouring will
not remove the yellow tint of the wool, especially at the tips. There
are several methods of bleaching, viz. by hydrogen peroxide, following
up with a weak vitriol bath; by potassium permanganate, following up
with a bath of sulphurous acid; or by fumigating in an air-tight
chamber with burning sulphur. The last-named method is the more
general; the wet skins are hung in the chamber, an iron pot containing
burning sulphur is introduced, and the exposure is continued for
several hours.
If the goods are to be finished white, they are now given a vitriol
sour, scoured, washed, retanned, dried, and when dry softened by
working with a moon knife. If they are to be dyed, they must be
prepared for the dye solution by "chloring," which consists of
immersion in a cold solution of bleaching powder for some hours, and
then souring in vitriol.
The next step is dyeing. If basic dyes are to be used, it is necessary
to neutralize the acidity of the skins by careful addition of soda,
and to prevent the tips from being dyed a darker colour than the
roots. Glauber salts and acetic acid are added to the dye-bath. The
tendency of basic colours to rub off may be overcome by passing the
goods through a solution of tannin in the form of cutch, sumach,
quebracho, &c.; in fact, some of the darker-coloured materials may be
used as a ground colour, thus economizing dyestuff and serving two
purposes. If acid colours are used, it is necessary to add sulphuric
acid to the dye bath, and in either case colours which will strike
below 50° C. must be used, as at that temperature alum leather
perishes.
After being dyed, the goods are washed up, drained, and if necessary
retanned, the glossing finish is then produced by passing them through
a weak emulsion or "fat liquor" of oil, soap and water, after which
they are dried, softened by working with a moon knife and beating,
when they are combed out, and are ready for the market.
Blacks are dyed by immersing the goods alternately in solutions of
logwood and iron, or a one-solution method is used, consisting of a
mixture of these two, with, in either case, varying additions of
lactic acid and sumach, copper salts, potassium bichromate, &c.; the
time of immersion varies from hours to days. After striking, the goods
are exposed to the air for some hours in order to oxidize to a good
black; they are then well scoured, washed, drained, retanned, dried,
softened and combed.
_Chrome Tanning._--The first chrome tanning process was described by
Professor Knapp in 1858 in a paper on "Die Natur und Wesen der
Gerberie," but was first brought into commercial prominence by Dr
Heinzerling about 1878, and was worked in a most persevering way by the
Eglinton Chemical Company, who owned the English patents, though all
their efforts failed to produce any lasting effects. Now chrome tanning
is almost the most important method of light leather dressing, and has
also taken a prominent place in the heavy department, more especially in
curried leathers and cases where greater tensile strength is needed. The
leather produced is much stronger than any other leather, and will also
stand boiling water, whereas vegetable-tanned leather is completely
destroyed at 70° C. and alum leather at 50° C.
The theory of chrome tanning is not perfectly understood, but in
general terms it consists of a partial chemical combination between
the hide fibre and the chrome salts, and a partial mechanical
deposition of chromium oxide in and on the fibre. The wet work, or
preparation for tanning, may be taken as much the same as for any
other leather.
There are two distinct methods of chrome tanning, and several
different methods of making the solutions. The "two bath process"
consists of treating the skins with a bichromate in which the chromium
is in the acidic state, and afterwards reducing it to the basic state
by some reducing agent. The exact process is as follows: To prevent
wrinkled or "drawn" grain the goods are first paddled for half an hour
in a solution of vitriol and salt, when they are piled or "horsed" up
over night, and then, without washing, placed in a solution consisting
of 7 lb. of potassium bichromate, 3½ lb. of hydrochloric acid to each
100 lb. of pelts, with sufficient water to conveniently paddle in; it
is recommended that 5% of salt be added to this mixture. The goods are
run in this for about 3 hours, or until struck through, when they are
horsed up for some hours, care being taken to cover them up, and are
then ready for the reducing bath. This consists of a 14% solution of
plain "hypo," or hyposulphite of soda, to which, during the process of
reduction, frequent additions of hydrochloric acid are made to free
the sulphurous and thiosulphuric acids, which are the active reducing
agents. After about 3 hours' immersion, during which time the goods
will have changed in colour from bright yellow to bright green, one or
two skins are cut in the thickest part, and if the green has struck
right through, the pack is removed as tanned, washed up, and allowed
to drain.
The "single-bath process" consists of paddling, drumming, or otherwise
introducing into the skins a solution of a chrome salt, usually chrome
alum, which is already in the basic condition, and therefore does not
require reducing. The basic solutions are made as follows: For 100 lb.
of pelts 9 lb. of chrome alum are dissolved in 9 gallons of water, and
2½ lb. of washing soda already dissolved in 1 gallon of water are
gradually added, with constant stirring. One-third of the solution is
added to 80 gallons of water, to which is added 7 lb. of salt, and the
skins are introduced; the other two-thirds are introduced at intervals
in two successive portions. Another liquor, used in the same way, is
made by dissolving 3 lb. of potassium bichromate in hot water, adding
½ gallon strong hydrochloric acid and then, gradually, about 1½ lb. of
glucose or grape sugar; this reduces the acidic chrome salt, vigorous
effervescence ensuing. The whole is made up to 2 gallons and 5% to 15%
of salt is added. In yet another method a chrome alum solution is
rendered basic by boiling with "hypo," and after the reaction has
ceased the solution is allowed to settle and the clear portion used.
After tanning, which takes from 8 hours to as many, and even more,
days, depending upon the method used and the class of skin being
dressed, the skins tanned by both methods are treated in a similar
manner, and are neutralized by drumming in borax solution, when they
are washed free from borax by drumming in warm water, and are ready
for dyeing, a process which will be dealt with further on. The goods
are sometimes tanned by suspension, but this method is generally
reserved for the tanning of the heavier leathers, which are treated in
much the same way, the several processes taking longer.
_Iron Tannage._--Before leaving mineral tanning, mention may be made
of iron tannage, although this has gained no prominent position in
commerce. Ferric salts possess powerful tanning properties, and were
thoroughly investigated by Professor Knapp, who took out several
patents, but the tendency to produce a brittle leather has never been
entirely overcome, although it has been greatly modified by the
incorporation of organic matter, such as blood, rosin, paraffin,
urine, &c. Knapp's basic tanning liquor is made as follows: A strong
solution of ferrous sulphate is boiled and then oxidized to the ferric
state by the careful addition of nitric acid. Next, to destroy excess
of nitric acid, ferrous sulphate is added until effervescence ceases
and the resulting clear orange-coloured solution is concentrated to a
varnish-like consistency. It does not crystallize or decompose on
concentration. The hides or skins are prepared for tanning in the
usual way, and then handled or otherwise worked in solutions of the
above iron salt, the solutions, which are at first weak, being
gradually strengthened.
The tannage occupies from 2 to 8 days, and the goods are then stuffed
in a ventilated drum with greases or soap. If the latter is used, an
insoluble iron soap is precipitated on the fibres of the leather,
which may then be finally impregnated with stearin and paraffin, and
finished in the usual manner as described under Curried Leathers. A
very fair leather may also be manufactured by using iron alum and salt
in the same manner as described under ordinary alum and salt.
_Combination Tannages._--Leathers tanned by mixtures or separate baths
of both mineral and vegetable tanning agents have now taken an important
position in commerce. Such leathers are the Swedish and Danish glove
leathers, the United States "dongola leather," and French glazed kid.
The usefulness of such a combination will be evident, for while
vegetable tanning produces fullness, plumpness and resistance to water,
the mineral dressing produces a softness unnatural to vegetable tannages
without the use of large quantities of oils and fats. It may also be
noted that once a leather has been thoroughly tanned with either mineral
or vegetable materials, although it will absorb large quantities of the
material which has not been first used, it will retain in the main the
characteristics of the tannage first applied. The principle had long
been used in the manufacture of such tough and flexible leathers as
"green leather," "combing leather" and "picker bands," but was first
applied to the manufacture of imitation glazed kid by Kent in America,
who, about 1878, discovered the principle of "fatliquoring," and named
his product "dongola leather." The discovery of this process
revolutionized the manufacture of combination leathers.
The Swedish and Danish glove leathers were first given a dressing of
alum and salt, with or without the addition of flour and egg, and were
then finished and coloured with vegetable materials, generally with
willow bark, although, in cases of scarcity, sumach, oak bark, madder
and larch were resorted to. The "green leathers" manufactured in
England generally receive about a week's tannage in gambier liquors,
and are finished off in hot alum and salt liquors, after which they
are dried, have the crystallized salts slicked off, are damped back,
and heavily stuffed with moellon, degras or sod oil. Kent, in the
manufacture of his dongola leather, used mixed liquors of gambier
alum and salt, and when tanned, washed the goods in warm water to
remove excess of tanning agent, piled up to samm, and fatliquored. In
making alum combinations it must be borne in mind that alum leather
will not glaze, and if a glazed finish is required, a fairly heavy
vegetable tannage should be first applied. For dull finishes the
mineral tannage may advantageously precede the vegetable.
Very excellent chrome combination leather is also manufactured by the
application of the above principles, gambier always being in great
favour as the vegetable agent. The use of other materials deprives the
leather of its stretch, although they may be advantageously used where
the latter property is objectionable.
_Oil Tanning._--Under the head of oil tanning is included "buff
leather," "buck leather," "piano leather," "chamois leather," and to a
greater or lesser extent, "Preller's crown or helvetia leather." The
process of oil tanning dates back to antiquity, and was known as
"shamoying," now spelt "chamoising." Chamoising yields an exceedingly
tough, strong and durable leather, and forms an important branch of the
leather industry. The theory of the process is the same as the theory of
currying, which is nothing more or less than chamoising, viz. the
lubrication of the fibres by the oil itself and the aldehyde tanning
which takes place, due to the oxidation and decomposition of the esters
of the fatty acids contained in the oil. The fact that an aldehyde
tannage takes place seems to have been first discovered by Payne and
Pullman, who took out a patent in 1898, covering formaldehyde and other
aldehydes used in alkaline solutions. Their product, "Kaspine" leather,
found considerable application in the way of military accoutrements.
Chamois, buff, buck and piano leathers are all manufactured by the same
process slightly modified to suit the class of hide used, the last three
being heavy leathers, the first light.
As regards the process used for chamois leather, the reader will
remember, from the account of the vegetable tannage of sheep skins,
that after splitting from the limes, the fleshes were thrown back into
the pits for another three weeks' liming (six weeks in all)
preparatory to being dressed as chamois leather. It is necessary to
lime the goods for oil dressing very thoroughly, and if the grain has
not been removed by splitting, as in the case of sheep skins, it is
"frized" off with a sharp knife over the beam. The goods are now
rinsed, scudded and drenched, dried out until stiff, and stocked in
the faller stocks with plenty of cod oil for 2 to 3 hours until they
show signs of heating, when they are hung up in a cool shed. This
process is repeated several times during a period of from 4 to 6 days,
the heat driving the water out of the skins and the oil replacing it.
At the end of this time the goods, which will have changed to a brown
colour, are hung up and allowed to become as dry as possible, when
they are hung in a warm stove for some hours, after which they are
piled to heat off, thrown into tepid water and put through a wringing
machine. The grease which is recovered from the wringing machine is
known commercially as "degras" or "moellon," and fetches a good price,
as it is unrivalled for fatliquoring and related processes, such as
stuffing, producing a very soft product. They next receive a warm soda
lye bath, and are again wrung; this removes more grease, which forms
soap with the lye, and is recovered by treatment with vitriol, which
decomposes the soap. The grease which floats on top of the liquor is
sold under the name of "sod oil." This also is a valuable material for
fatliquoring, &c., but not so good as degras.
After being wrung out, the goods are bleached by one of the processes
mentioned in the section on wool rug dressing, the permanganate method
being in general use in England. In countries where a fine climate
prevails the soap bleach or "sun bleach" is adopted; this consists of
dipping the goods in soap solution and exposing them to the sun's
rays, the process being repeated three or more times as necessary.
The next step is fatliquoring to induce softness, after which they are
dried out slowly, staked or "perched" with a moon knife, fluffed on a
revolving wheel covered with fine emery to produce the fine "nap" or
surface, brushed over with french chalk, fuller's earth or china clay,
and finally finished on a very fine emery wheel.
_Preller's Helvetia or Crown Leather._--This process of leather
manufacture was discovered in 1850 by Theodor Klemm, a cabinetmaker of
Württemberg, who being then in poor circumstances, sold his patent to an
Englishman named Preller, who manufactured it in Southwark, and adopted
a crown as his trade mark. Hence the name "crown" leather. The
manufacture then spread through Switzerland and Germany, the product
being used in the main for picker straps, belting and purposes where
waterproof goods were required, such as hose pipes and military water
bags. No taste is imparted to the water by this leather.
The process of manufacture is as follows: The hides are unhaired by
short liming, painting with lime and sulphide, or sweating, and
cleansed by scudding and washing, after which they are coloured in
bark liquors, washed up through clean water, and hung up to dry
partially. When in a sammied condition the goods are placed on a table
and a thick layer of the tanning paste spread on the flesh side. The
tanning paste varies with each manufacturer, but the following is the
mixture originally used by Preller: 100 parts flour, 100 parts soft
fat or horse tallow, 35 parts butter, 88 parts ox brains, 50 parts
milk, 15 parts salt or saltpetre.
The hides are now rolled in bundles, placed in a warm drum and worked
for 8 to 10 hours, after which they are removed and hung up until half
dry, when the process is repeated. Thus they are tumbled 3 to 4 times,
set out flesh and grain, rinsed through tepid water, set out, sammied,
and curried by coating with glycerin, oil, tallow and degras. The
table grease is now slicked off, and the goods are set out in grease,
grained and dried.
_Transparent Leather._--Transparent leather is a rather horny product,
somewhat like raw hide, and has been used for stitching belts and
picker bands. The goods to be dressed are limed, unhaired, very
thoroughly delimed with acids, washed in water, scudded and
clean-fleshed right to the veins; they are now stretched in frames,
clean-fleshed with a moon knife, and brushed with warm water, when
several coats of glycerin, to which has been added some antiseptic
such as salicylic or picric acid, are applied; the goods are then
dried out, and another coat is applied, and when semi-dry they are
drummed in a mixture of glycerin, boracic acid, alum and salt, with
the addition of a little bichromate of potash to stain them a yellow
colour. After drumming for 2 to 3 hours they are removed, washed up,
lightly set out, and stretched in frames to dry, when they are ready
for cutting into convenient lengths for use.
_Parchment._--A certain class of sheep skin known as Hampshires is
generally used in the manufacture of this speciality. The skins as
received are first very carefully washed to remove all dirt, dewooled,
limed for 3 to 4 weeks, they are then cleanly fleshed, unhaired,
rinsed up in water, and thickly split, the poorer hides being utilized
for chamois; they are now re-split at the fatty strata so that all fat
may be easily removed, and while the grains are dressed as skivers,
the fleshes are tied in frames, watered with hot water, scraped and
coated on both sides with a cream consisting of whiting, soda and
water, after which they are dried out in a hot stove. In the drying
the whiting mixture absorbs the grease from the skins; in fact, this
method of degreasing is often employed in the manufacture of wool
rugs. When dry, both sides of the skins are flooded to remove the
whiting, and are then well rubbed over with a flat piece of
pumice-stone, swilled, dried, re-pumiced, again swilled, and when
sammied are rolled off with a wooden roller and dried out.
_Tar and Peat Tanning._--Tar tanning was discovered by a French
chemist named Philippi, who started with the idea that, if coal was a
decomposition product of forests, it must still necessarily possess
the tanning properties originally present in the trees. However
far-fetched such an argument may seem, Philippi succeeded in producing
a leather from wood and coal tar at a fairly cheap rate, the product
being of excellent texture and strength, but rather below the average
in the finish, which was inclined to be patchy, showing oily spots.
His method consisted of impregnating the goods with refined tar and
some organic acid, but the product does not seem to have taken any
hold upon the market, and is not much heard of now.
Peat tanning was discovered by Payne, an English chemist, who was also
the co-discoverer of the Payne-Pullman formaldehyde tanning process.
His peat or humic acid tannage was patented by him about 1905, and is
now worked on a commercial scale. The humic acid is first extracted
from the peat by means of alkalis, and the hides are treated with this
solution, the humic acid being afterwards precipitated in the hides by
treatment with some stronger organic or mineral acid.
_Dyeing, Staining and Finishing._--These operations are practised almost
exclusively on the lighter leathers. Heavy leathers, except coloured and
black harness and split hides for bag work, are not often dyed, and
their finishing is generally considered to be part of the tannage. In
light leathers a great business is done in buying up "crust" stock, i.e.
rough tanned stock, and then dyeing and finishing to suit the needs and
demands of the various markets. The carrying out of these operations is
a distinct and separate business from tanning, although where possible
the two businesses are carried on in the same works.
Whatever the goods are and whatever their ultimate finish, the first
operation, upon receipt by the dyer of the crust stock, is sorting, an
operation requiring much skill. The sorter must be familiar with the why
and wherefore of all subsequent processes through which the leather must
go, so as to judge of the suitability of the various qualities of
leather for these processes, and to know where any flaws that may exist
will be sufficiently suppressed or hidden to produce a saleable
product, or will be rendered entirely unnoticeable. The points to be
considered in the sorting are coarseness or fineness of texture,
boldness or fineness of grain, colour, flaws including stains and
scratches, substance, &c. Light-coloured and flawless goods are
parcelled out for fine and delicate shades, those of darker hue and few
flaws are parcelled out for the darker shades, such as maroons, greens
(sage and olive), dark blues, &c., and those which are so badly stained
as to be unsuitable for colours go for blacks. After sorting, the goods
are soaked back to a limp condition by immersion in warm water, and are
then horsed up to drip, having been given, perhaps, a preliminary
slicking out.
Up to this point all goods are treated alike, but the subsequent
processes now diverge according to the class of leather being treated
and the finish required.
Persian goods for glacés, moroccos, &c., require special preparation for
dyeing, being first re-tanned. As received, they are sorted and soaked
as above, piled to samm, and shaved. Shaving consists of rendering the
flesh side of the skins smooth by shaving off irregularities, the skin,
which is supported on a rubber roller actuated by a foot lever, being
pressed against a series of spiral blades set on a steel roller, which
is caused to revolve rapidly. When shaved, the goods are stripped,
washed up, soured, sweetened and re-tanned in sumach, washed up, and
slicked out, and are then ready for dyeing.
There are three distinct methods of dyeing, with several minor
modifications. Tray dyeing consists of immersing the goods, from 2 to 4
dozen at a time, in two separate piles, in the dye solution at 60° C,
contained in a flat wooden tray about 5 ft. × 4 ft. × 1 ft., and keeping
them constantly moving by continually turning them from one pile to the
other. The disadvantages of this method are that the bath rapidly cools,
thus dyeing rapidly at the beginning and slowly at the termination of
the operation; hence a large excess of dye is wasted, much labour is
required, and the shades obtained are not so level as those obtained by
the other methods. But the goods are under observation the whole time, a
very distinct advantage when matching shades, and a white flesh may be
preserved. The paddle method of dyeing consists of paddling the goods in
a large volume of liquor contained in a semi-circular wooden paddle for
from half to three-quarters of an hour. The disadvantages are that the
liquor cools fairly rapidly, more dye is wasted than in the tray method,
and a white flesh cannot be preserved. But larger packs can be dyed at
the one operation, the goods are under observation the whole time, and
little labour is required.
The drum method of dyeing is perhaps best, a drum somewhat similar to
that used by curriers being preferable. The goods are placed on the
shelves inside the dry drum, the lid of which is then fastened on, and
the machinery is started; when the drum is revolving at full speed,
which should be about 12 to 15 revolutions per minute, the dye solution
is added through the hollow axle, and the dyeing continued for half an
hour, when, without stopping the drum, if desired, the goods may be
fatliquored by running in the fatliquor through the hollow axle. The
disadvantages are that the flesh is dyed and the goods cannot be seen.
The advantages are that little labour is required, a large pack of skins
may be treated, level shades are produced, heat is retained, almost
complete exhaustion of the dye-bath is effected, and subsequent
processes, such as fatliquoring, may be carried out without stopping the
drum.
Of the great number of coal-tar dyes on the market comparatively few
can be used in leather manufacture. The four chief classes are: (1)
acid dyes; (2) basic or tannin dyes; (3) direct or cotton dyes; (4)
mordant (alizarine) dyes.
Acid dyes are not so termed because they have acid characteristics;
the name simply denotes that for the development of the full shade of
colour it is necessary to add acid to the dye-bath. These dyes are
generally sodium salts of sulphonic acids, and need the addition of an
acid to free the dye, which is the sulphonic acid. Although
theoretically any acid (stronger than the sulphonic acid present) will
do for this purpose, it is found in practice that only sulphuric and
formic acids may be employed, because others, such as acetic, lactic,
&c., do not develop the full shade of colour. Acid sodium sulphate may
also be successfully used.
Acid colours produce a full level shade without bronzing, and do not
accentuate any defects in the leather, such as bad grain, &c. They are
also moderately fast to light and rubbing. They are generally applied
to leather at a temperature between 50° and 60° C., with an equal
weight of sulphuric acid. The quantity of dye used varies, but
generally, for goat, persians, &c., from 25 to 30 oz. are used per ten
dozen skins, and for calf half as much again, dissolved in such an
amount of water as is most convenient according to the method being
used. If sodium bisulphate is substituted for sulphuric acid twice as
much must be used, and if formic acid three times as much (by weight).
Basic dyes are salts of organic colour bases with hydrochloric or some
other suitable acid. Basic colours precipitate the tannins, and thus,
because of their affinity for them, dye very rapidly, tending to
produce uneven shades, especially if the tannin on the skin is
unevenly distributed. They are much more intense in colour than the
acid dyes, have a strong tendency to bronze, and accentuate weak and
defective grain. They are also precipitated by hard waters, so that
the hardness should be first neutralized by the addition of acetic
acid, else the precipitated colour lake may produce streakily dyed
leather. To prevent rapid dyeing, acetic acid or sodium bisulphate
should always be added in small quantity to the dye-bath, preferably
the latter, as it prevents bronzing. The most important point about
the application of basic dyes to leather is the previous fixation of
the tannin on the surface of the leather to prevent its bleeding into
the dye-bath and precipitating the dye. All soluble salts of the heavy
metals will fix the tannin, but few are applicable, as they form
colour lakes, which are generally undesirable. Antimony and titanium
salts are generally used, the forms being tartar emetic (antimony
potassium tartrate), antimonine (antimony lactate), potassium titanium
oxalate, and titanium lactate. The titanium salts are economically
used when dyeing browns, as they produce a yellowish-brown shade; it
is therefore not necessary to use so much dye. About 2 oz. of tartar
emetic and 8 oz. of salt is a convenient quantity for 1 dozen goat
skins. The bath is used at 30° to 40° C., and the goods are immersed
for about 15 minutes, having been thoroughly washed before being dyed.
Iron salts are sometimes used by leather-stainers for saddening
(dulling) the shade of colour produced, iron tannate, a black salt,
being formed. It is often found economical to "bottom" goods with
acid, direct, or other colours, and then finish with basic colours;
this procedure forms a colour lake, and colour lakes are always faster
to light and rubbing than the colours themselves.
Direct cotton dyes produce shades of great delicacy, and are used for
the dyeing of pale and "art" shades. They are applied in neutral or
very slightly acid baths, formic and acetic acids being most suitable
with the addition of a quantity of sodium chloride or sulphate. After
dyeing, the goods are well washed to free from excess of salt. The
eosine colours, including erythrosine, phloxine, rose Bengal, &c., are
applied in a similar manner, and are specially used for the beautiful
fluorescent pink shades they produce; acid and basic colours and
mineral acids precipitate them.
The mordant colours, which include the alizarine and anthracene
colours, are extremely fast to light, and require a mordant to develop
the colour. They are specially applicable to chamois leather, although
a few may be used for chrome and alum leathers, and one or two are
successfully applied to vegetable-tanned leather without a mordant.
Sulphur or sulphide colours, the first of which to appear were the
famous Vidal colours, are applied in sodium sulphide solution, and are
most successfully used on chrome leather, as they produce a colour
lake with chrome salts, the resulting colour being very fast to light
and rubbing. A very serious disadvantage in connexion with them is
that they must necessarily be applied in alkaline solution, and the
alkali has a disintegrating effect upon the fibre of the leather,
which cannot be satisfactorily overcome, although formaldehyde and
glycerin mixtures have been patented for the purpose.
The Janus colours are perhaps worth mentioning as possessing both acid
and basic characteristics; they precipitate tannin, and are best
regarded as basic dyes from a leather-dyer's standpoint.
The goods after dyeing are washed up, slicked out on an inclined glass
table, nailed on boards, or hung up by the hind shanks to dry out.
Coal-tar dyes are not much used for the production of blacks, as they do
not give such a satisfactory result as logwood with an iron mordant. In
the dyeing of blacks the preliminary operation of souring is always
omitted and that of sumaching sometimes, but if much tan has been
removed it will be found necessary to use sumach, although cutch may be
advantageously and cheaply substituted. After shaving, the goods, if to
be dressed for "blue backs" (blue-coloured flesh), are dyed as already
described, with methyl violet or some other suitable dye; they are then
folded down the back and drawn through a hot solution of logwood and
fustic extracts, and then rapidly through a weak, cold iron sulphate and
copper acetate solution. Immediately afterwards they are rinsed up and
either drummed in a little neatsfoot oil or oiled over with a pad, flesh
and grain, and dried. When dry the goods are damped back and staked,
dried out and re-staked.
After dry-staking, the goods are "seasoned," i.e. some suitable mixture
is applied to the grain to enable it to take the glaze. The following is
typical: 3 quarts logwood liquor, ½ pint bullock's blood, ½ pint milk, ½
gill ammonia, ½ gill orchil and 3 quarts water. This season is brushed
well into the grain, and the goods are dried in a warm stove and glazed
by machine. The skins are glazed under considerable pressure, a polished
glass slab or roller being forced over the surface of the leather in a
series of rapid strokes, after which the goods are re-seasoned,
re-staked, fluffed, re-glazed, oiled over with a pad, dipped in linseed
oil and dried. They are now ready for market. If the goods are to be
finished dull they are seasoned with linseed mucilage, casein or milk
(many other materials are also used), and rolled, glassed with a
polished slab by hand, or ironed with a warm iron.
Coloured glacés are finished in a similar manner to black glacés, dye
(instead of logwood and iron) being added to the season, which usually
consists of a simple mixture of dye, albumen and milk.
Moroccos and grain leathers are boarded on the flesh side before and
after glazing, often being "tooth rolled" between the several
operations. Tooth rolling consists of forcing, under pressure, a toothed
roller over the grain; this cuts into the leather and helps to produce
many grains, which could not be produced naturally by boarding, besides
fixing them.
Many artificial grains and patterns are also given to leather by
printing and embossing, these processes being carried out by passing the
leather between two rollers, the top one upon which the pattern is
engraved being generally steam heated. This impresses the pattern upon
the grain of the leather.
The above methods will give a very general idea of the processes in
vogue for the dressing of goods for fancy work. The dressing of chrome
leathers for uppers is different in important particulars.
_Chrome Box and Willow Calf._--Willow calf is coloured calf, box calf
is dressed black and grained with a "box" grain. A large quantity of
kips is now dressed as box calf; these goods are the hides of yearling
Indian cattle, and are dressed in an exactly similar manner as calf.
After tanning and boraxing to neutralize the acidity of the chrome
liquor, the goods are washed up, sammied, shaved, and are ready for
mordanting previous to dyeing. Very few dyes will dye chrome leather
direct, i.e. without mordanting. Sulphide colours are not yet in great
demand, nor are the alizarines used as much as they might be. The
ordinary acid and basic dyes are more generally employed, and the
goods consequently require to be first mordanted. The mordanting is
carried out by drumming the goods in a solution containing tannin,
and, except for pale shades, some dyewood extract is used; for reds
peachwood extract, for browns fustic or gambier, and for dark browns a
little logwood is added. For all pale shades sumach is exclusively
used. After drumming in the warm tannin infusion for half an hour, if
the goods are to be dyed with basic colours the tannin is first fixed
by drumming in tartar emetic and salt, or titanium, as previously
described; the dyeing is also carried out as described for persians,
except that a slightly higher temperature may be maintained. If the
goods are to be dyed black they are passed through logwood and iron
solutions.
After dyeing and washing up, &c., the goods are fatliquored by placing
them in a previously heated drum and drumming them with a mixture
known as a "fatliquor," of which the following recipe is typical:
Dissolve 3 lb. of soft soap by boiling with 3 gallons of water, then
add 9 lb. of neatsfoot oil and boil for some minutes; now place the
mixture in an emulsifier and emulsify until cooled to 35° C., then add
the yolks of 5 fresh eggs and emulsify for a further half hour. The
fatliquor is added to the drum at 55° C., and the goods are drummed
for half an hour, when all the fatliquor should be absorbed; they are
then slicked out and dried. After drying, they are damped back,
staked, dried, re-staked and seasoned with materials similar to those
used for persians; when dry they are glazed, boarded on the flesh
("grained") from neck to butt and belly to belly to give them the box
grain, fluffed, reseasoned, reglazed and regrained.
_Finishing of Bag Hides._--The goods are first soaked back, piled to
samm, split or shaved, scoured by machine, finished off by hand,
washed up and retanned by drumming in warm sumach and extract, after
which they are washed up, struck out, hung up to samm, and "set."
"Setting" consists of laying the grain flat and smooth by striking out
with a steel or sharp brass slicker. They are then dried out, topped
with linseed mucilage, and again dried. This brushing over with
linseed mucilage prevents the dye from sinking too far into the
leather; gelatine, Irish moss, starch and gums are also used for the
same purpose. These materials are also added to the staining solution
to thicken it and further prevent its sinking in.
When dry, the goods are stained by applying a ½% (usually) solution of
a suitable basic dye, thickened with linseed, with a brush. Two men
are usually employed on this work; one starts at the right-hand flank
and the other at the left-hand shank, and they work towards each
other, staining in sections; much skill is needed to obviate markings
where the sections overlap. The goods may advantageously be bottomed
with an acid dye or a dye-wood extract, and then finished with basic
dyes. Whichever method is used, two to three coats are given, drying
between each. After the last coat of stain, and while the goods are
still in a sammied condition, a mixture of linseed mucilage and French
chalk is applied to the flesh and glassed off wet, to give it a white
appearance, and then the goods are printed with any of the usual bag
grains by machine or hand, and dried out. For a bright finish the
season may consist of a solution of 15 parts carnauba wax, 10 parts
curd soap and 100 parts water boiled together; this is sponged into
the grain, dried and the hides are finished by either glassing or
brushing. For a duller finish the grain is simply rubbed over with
buck tallow and brushed. Hide bellies for small work are treated in
much the same manner.
_Glove Leathers._--As these goods were tanned in alum, salt, flour and
egg, any undue immersion in water removes the tannage; for this reason
they are generally stained like bag hides, one man only being employed
on the same skin. The skins are first thoroughly soaked in warm water
and then drummed for some minutes in a fresh supply, when they are
re-egged to replace that which has been lost. This is best done by
drumming them for about 1½ hours in 40 to 50 egg yolks and 5 lb. of
salt for every hundred skins; they are then allowed to be in pile for
24 hours, and are set out on the table ready for mordanting. The
mordants universally used are ammonia or alkaline soft soap; 1 in 1000
of the former or a 1% solution of the latter. When the goods have
partially dried in, bottoming follows, and usually the natural wood
dyestuffs are used for this operation, such as fustic, Brazil wood,
peachwood, logwood and turmeric. After application of these colours
the goods are sammied and topped with a 1% solution of an acid dye, to
which has been added 20% of methylated spirit to prevent frothing with
the egg yolk; they are then dried out slowly, staked, pulled in shape,
fluffed and brushed by machine. The season, which is sponged on, may
consist of 1 part dye, 1 part albumen, 2 parts dextrine and ¼ part
glycerine, made up to 100 parts with water; when it has been applied,
the goods are sammied, brushed and ironed with a warm flat iron such
as is used in laundry work.
_Bookbinding Leathers._--A committee of the Society of Arts (London)
has investigated the question of leather for bookbinding, attention
having been drawn to this subject by the rotten and decayed condition
often observed in bindings less than fifty years old. This committee
engaged in research work extending over several years, and the report
in which its results were given was edited for the Society of Arts and
the Leathersellers' Company (which also did much important work in
connexion with it) by Lord Cobham, chairman of the committee, and Sir
Henry Trueman Wood, secretary of the society. The essence of the
report, so far as leather manufacture is concerned, is as follows: The
goods should be soaked and limed in fresh liquors, and bating and
puering should be avoided, weak organic acids or erodine being used;
they should also be tanned with pyrogallol tanning materials, and
preferably with sumach. In shaving, they should only be necked and
backed, i.e. only irregularities should be removed, as further shaving
has a considerable weakening effect on the fibre. The striking out
should not be heavy enough to lay the fibre. In dyeing, acid dyes and
a few direct colours only are permissible, and in connexion with the
former the use of sulphuric acid is strongly condemned, as it
absolutely disintegrates the fibre; the use of formic, acetic and
lactic acids is permitted. The use of salts of mineral acids is to be
avoided, and in finishing, tight setting out and damp glazing is not
to be recommended; oil may be advantageously used.
BIBLIOGRAPHY.--H. G. Bennett, _The Manufacture of Leather_ (1909); S.
R. Trotman, _Leather Trades Chemistry_ (1908); M. C. Lamb, _Leather
Dressing_ (1907); A. Watt, _Leather Manufacture_ (1906); H. R.
Procter, _Principles of Leather Manufacture_ (1903), and _Leather
Industries Laboratory Book_ (1908); L. A. Flemming, _Practical
Tanning_ (1910); A. M. Villon, _Practical Treatise on the Leather
Industry_ (1901); C. T. Davis, _Manufacture of Leather_ (1897). German
works include J. Borgman, _Die Rotlederfabrikation_ (Berlin,
1904-1905), and _Feinlederfabrikation_ (1901); J. Jettmar, _Handbuch
der Chromgerbung_ (Leipzig, 1900); J. von Schroeder, _Gerbereichemie_
(Berlin, 1898). (J. G. P.*)