in the form of a bag of nutrient material for bacteria to thrive on
and a bottle of bacterial culture. The nutrient material is dissolved
in water and the bacterial culture added, and after allowing the
mixture to get working it is ready for use. Many tons of this bate
are now being used per annum. Its advantages are: (1) that it is
clean, (2) that it is under perfect control, and (3) that stains and
bate burns, which so often accompany the dung bate, are absolutely
absent. Bate burns are caused by not filtering the dung bate
through coarse sacking before use. The accumulation of useless
solid matter settles on the skins if they are not kept well in motion,
causing excessive action in these places.
After pulling down the goods to a soft, silky condition by bating or puering, it is necessary, after scudding, to plump them up again and bring them into a clean and fit condition for receiving the tan. This is done by “drenching” in a bran drench. A quantity of bran is scalded and allowed to ferment. When the fermentation has reached the proper stage the goods are placed, together with the bran liquor, in a suitable pit or vat, and are allowed to remain until they have risen three times; this rising to the surface is caused by the gaseous products of the fermentation being caught by the skin. The plumping action of the bran is due to the acids produced during fermentation and also in part to the gases, and the cleansing action is due to the mechanical action of the particles of bran rubbing against the grain of the skins. After drenching, the goods are washed free from bran, and are ready for the tanning process.
Drenching, now that all kinds of acids are available, is not so much used for heavy hides as for light skins, it being found much more convenient and cheaper to use acids. In fact, bating and puering are being gradually replaced by acid baths in the case of heavy leathers, the process being carried out as deliming for sole leather, only much more thoroughly in the case of dressing leather.
The tanning of dressing hides, which are not rounded into butts and offal, is briefly as follows. They first enter a series of colouring pits or suspenders, and then a series of handlers, by which time they should be plump and coloured through; in this condition they are split either by means of a union or band-knife splitting machine (fig. 7).
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| Fig. 7.—Band Knife Splitting Machine. |
This latter is the most popular machine, and consists essentially of an endless band knife a, which revolves at considerable speed with its cutting edges close to the sides of a pair of rollers through which the leather is fed and pressed against the knife. The lower of these rollers is made of short segments or rings, each separately capable of yielding so as to accommodate itself to the unequal thicknesses of various parts of a hide. The thickness of the leather to be cut is gauged to the utmost minuteness by means of the hand screws b b which raise or lower the upper roller. The knife edge of the cutter is kept keen by rubbing against revolving emery wheels c as it passes round. So delicately can this machine effect its work that slices of leather uniform throughout and as thin as paper can be easily prepared by it, and by its aid it is quite common to split hides into as many as three useful splits.
The dressing hides are usually split in two. Here we will leave the split (flesh) for a time and continue with the treatment of the grain. After splitting, they enter another series of handlers, are then piled up for a day or two, and thrown into a large drum with sumach mixed to a paste with hot water and a light-coloured extract. They are drummed in this for one hour to brighten and mellow the grain, washed up in tepid liquor, piled for two days, and drummed with cod oil or some other suitable oil or mixture; they are now piled for a day or two to absorb, dried out, flattened on the grain, and flesh folded.
The splits are rinsed up in old sumach liquor and drummed with cheap extracts and adulterants, such as size, glucose, barium chloride, epsom salts, &c. after which they are piled up to drain, dried to a “sammied” condition, rolled to make firm, and dried right out.
In the dressing hide tannage very mellow materials are used. Gambier and myrobalans form the main body of the tannage, together with a little quebracho extract, mimosa bark, sumach and extracts.
Upper Leather.—Under the head of upper leather are included the thin, soft and pliable leathers, which find their principal, but by no means exclusive, application in making the uppers of boots and shoes, which may be taken as a type of a class of leathers. They are made from such skins as East Indian kips, light cow and horse hides, thin split hides, such as those described under dressing leather, but split rather thinner, and calf. The preparatory dressing of such skins and the tanning operations do not differ essentially from those already described. In proportion to the thinness of the skin treated, the processes are more rapidly finished and less complex, the tannage is a little lighter, heavy materials such as valonia being used sparsely if at all. Generally speaking, the goods have a longer and mellower liming and bating, the lime being more thoroughly removed than for the leathers previously described, to produce greater pliability, and everything must tend in this direction. The heavier hides and kips are split as described under dressing leather, and then tanned right out.
Currying of the Lighter Leathers.—The duty of the currier is not solely directed towards heavier leathers; he is also entrusted with the dressing and fitting of the lighter leathers for the shoemaker, coachbuilder, saddler, &c. He has to pare the leather down and reduce inequalities in thickness, to impregnate it with fatty matter in order to render it soft and pliable, and to give it such a surface dressing, colour and finish as will please the eye and suit the purposes of its consumers. The fact that machinery is used by some curriers for nearly every mechanical operation, while others adhere to the manual system, renders it almost impossible to give in brief an outline of operations which will be consistent with any considerable number of curriers.
The following may be taken as a typical modern dressing of
waxed calf or waxed kips. The goods are first of all soaked down
and brought to a “sammied” condition for shaving. In the better-class
leathers hand-shaving is still adhered to, as it is maintained
that the drag of the shaving machine on the leather causes the
“nap” finish to be coarser. Hand-shaving is carried out on a
beam or strong frame of wood, supporting a stout plank faced with
lignum vitae, and set vertically, or nearly so. The knife (fig. 8) is
a double-edged rectangular blade about 12 in. by 5 in., girded on
either side along its whole length and down the centre with two
bars 3 in. wide, leaving each
Fig 8.—Currying Knife.
blade protruding 1 in. beyond
them; it has a straight
handle at one end and a cross
handle at the other in the
plane of the blade. The edges
of this knife are first made
very keen, and are then turned
over so as to form a wire edge,
by means of the thicker of the
two straight steel tools shown
in fig. 9. The wire edge is
preserved by drawing the thinner of the two steel tools along
the interior angle of the wire edge and then along the outside
of the turnover edge. The skin being thrown flesh uppermost
over the vertical beam, the shaver presses his body against it,
and leaning over the top holds the knife by its two handles almost
at right angles to the leather, and proceeds to shave it by a
scraping stroke downwards which the wire edge, being set at right
angles to the knife and almost parallel with the skin, turns into a
cut. The skin is shifted so as to bring all parts under the action of
the knife, the shaver frequently passing a fold between his finger
to test the progress of his work. After shaving, the goods are
thoroughly soaked, allowed to drip, and are ready for “scouring.”
This operation has for its object the removal of bloom (ellagic acid)
and any other superfluous adherent matter. The scouring solution
consists of a weak solution of soft soap and borax. This is first
well brushed into the flesh of the leather, which is then “sleeked”
(slicked) out with a steel slicker shown at S fig. 9. The upper part
