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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 IO gallons linseed oil for one hour with 2 lb htharge
at 600° F. to jellify the oil, and then add 2 Th prussian blue and boil
the whole for half an hour longer. Before application the mixture is
tliinned with 10 gallons light petroleum. For the second coat, boil
IO 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 I hour, then add I lb prussian blue, and boil for
another hour; thin with IO 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.
Towing.-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 IOO Tb 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 drummcd
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 finishin
g.
Wool Rug DV6SSi1Zg.~'“7OOl 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 leather ed by
rubbing into the iiesh 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 leather ed. 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
througha 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 T077/I'Lf11g.*'l'l'1C 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,
35 lb of hydrochloric acid to each Ioo 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.<noinclude><references/></div></noinclude>