CROWN LEATHER, OR PRELLER'S LEATHER.
The process of manufacture of this leather, which has obtained a
firm position as the most suitable material for certain classes of
belting, picker-straps, &c., was discovered about 35 years since by
Theodor Klemm, a cabinet-maker in Wurtemburg and founder of the present
well-known firm of leather manufacturers, Gebrüder Klemm of Pfullingen.
Klemm, at that time in poor circumstances, sold his patent in Paris to
an Englishman, Preller, who started a manufacture of it in Southwark
and adopted a crown as his trade-mark. Since this time the manufacture
has spread, first to Switzerland and then through Germany; but in
England, to the writer's knowledge, it is confined to one or two firms.
The process of manufacture of crown leather is in principle
intermediate between that of calf-kid (see p. 223), and the pure
oil-tanning, if we may call it so, of which the chamois leather (see
p. 210) is typical. It depends on impregnating the raw hide with a
mixture of fats and albumens, to which salt or saltpetre is added to
prevent putrefaction. The process as described in the original patent
was as follows,--The hides were unhaired by liming or painting (with
sulphides), and cleansed as usual, no plumping lime being given. After
unhairing they were allowed to dry some little time in the air till no
longer plump, and were then worked in a tumbler drum, without water,
till uniformly soft. They were then spread on a table and brushed
over on the flesh-sides with a mixture of 23 parts of ox-brain, 6-1/2
of butter, 28 of soft fat, and 4 of salt or saltpetre, with 26 of
barley-flour and 12-1/2 of milk, of which the leading 4 ingredients
were first to be mixed and the flour stirred in, the milk being last
added. The hides were then returned to the tumbler, which was provided
with tubular axes, through which a portion of exhaust steam was
admitted to warm the drum. After tumbling some hours, the drum was
opened, and the hides were examined. If the tanning was not complete,
the hides were hung in the air for a time to dry, and the process was
repeated till a cut showed that the mixture had completely penetrated
the hide.
From Eitner's researches it appears that the essential tanning
ingredients of the mixture above described are the fat (and butter
which acts simply as fat) and the albuminous matter of the milk
(casein), brains (albumen, &c.), and flour (gluten); the starch serving
at most to assist in the emulsification of the fats. Eitner treated
crown leather with dilute potash solution to remove the albumen
and fats, and after washing and drying obtained a material like an
insufficiently stocked chamois leather. On again stuffing with a
quantity of fat equal to that removed, but without the albumen, the
leather became dark and quite greasy, so that by sharp bending oil
could be pressed out. Good results may be obtained in crown leather
manufactured with fats and flour only, without the use of milk or
brains, so that it is obvious that the same purpose is served by either
vegetable or animal albumenoids. The most important point in the
purposes for which crown leather is employed is toughness, and this is
given by the unaltered hide-fibres, which are merely preserved by the
coating of oily matter with which, like those of chamois leather, they
are surrounded. The albumen serves the purpose of filling the spaces
between the fibres, and giving solidity and firmness, so that the belts
may keep their shape, and not stretch inordinately. It also serves to
make the leather waterproof, and fit it for water-bags for military
purposes (as it gives no taste to the water) and for hose-pipes. The
albumen, which much resembles the hide-fibres in composition, is like
them preserved by the fats.
For the modern process of manufacture, good, even and well-flayed
hides are selected, and unhaired either by sweating, or by a very short
liming, which must be assisted by rockers or some mechanical mode of
moving the hides, so as to get them unhaired in the shortest possible
time and with the least injury to the fibre. Sodium sulphide (see p.
147) may be employed with great advantage. The fleshing and scudding
are performed as usual, according to the mode of unhairing adopted. The
hides are then very commonly rounded, and the bellies tanned in the
usual manner; but sometimes the whole hide is made into crown leather.
As crown leather is naturally almost white, it is usual at this stage
to colour the hide with bark or other liquors. As in this case simply
colouring and not tanning of the grain is required, high-coloured
liquors, made by steaming materials with much colour and little tannin,
are preferable. For this purpose wood extracts, such as chestnut,
quebracho, or oak-wood are said to be very suitable, and beech, pine
or alder bark may also be used. In practice, chestnut and hemlock
extracts, and occasionally cutch are employed; but the last named is
not to be recommended. A chestnut liquor of 7-1/2° Tw. or 5° B. (34°
Bark.), with constant handling or in a paddle-tumbler, will give a
satisfactory colour and grain in 1-2 hours. This rapid colouring is
preferable to the slower process, which occupies 24 hours in weaker
liquors. If sweated, the hides are now plumped with sulphuric acid,
but only to a very moderate extent. This process is best performed
in a paddle-tumbler; about 3-1/2 oz. of sulphuric acid are required
per hide, and a time of 6-12 hours according to the water employed.
The liquor may be several times used, strengthened with the necessary
quantity of acid. Limed hides do not require further swelling. The
hides are washed through clean water, and hung up to dry somewhat.
The hides are next spread on a table, flesh-side uppermost, and
covered with a layer of the tanning paste nearly 1/4 in. thick. The
composition of this paste may be varied according to the relative
prices of different materials, and the amount of hard fats must be
regulated according to whether or not appliances are provided for
heating the tumbler. A good mixture is 7 parts common wheat-flour, 7 of
horse-grease, 1 of salt, and 1-2 of tallow. If too soft, more tallow
may be employed. The salt is first added to the horse-grease, then
the melted tallow, These fats are added little by little to the flour
till a uniform paste is obtained. Another good mixture is 27 parts
wheat-flour, 25 of bone-grease, 4 of tallow, and 4 of salt. Another
recipe gives 28 lb. fine white flour made to a paste with 13-14 pints
water and then worked up to a uniform mass with a tepid mixture of 28
lb. beef tallow and 28 lb. hard horse-fat (_Pferdekammfett_). These
mixtures are all for use in warmed drums; a specimen of one used in a
factory where the mixture was simply trodden in cold into the leather
in open tubs is as follows:--7 parts flour, 9·4 of horse-fat, 2·8 of
fish-oil, 7 of ox-brains and 0·7 of salt. The hides are next folded in
bundles and placed in the drum; or in stocks, which are occasionally
used for the purpose. If a drum be used, it must be of large
diameter, 8-9 ft., provided with pegs inside, and should make about
25 revolutions per minute, so as to work the hides with considerable
force. Much more care is needed in warming the drum, than is required
in ordinary stuffing, and this is best accomplished by warmed damp air.
This may be arranged by the use of an air-pump, which draws air through
water warmed by exhaust steam, and forces it through the hollow axles
of the drum (or drums); or a simple aspirator consisting of a cask
filled with water may be connected to one axle, so that as the water
runs out it will draw air through the drum from the opposite axle,
which is connected with a cask half filled with hot water through which
air is allowed to bubble. Probably the same effect could be reached in
a still simpler and cheaper manner by the use of a steam-jet blower,
such as Körting's. In any case the drum must be warmed to a temperature
of 82°-104° F. (28°-40° C.). Warm dry air may also be used, but is not
so suitable, as it dries the hides too much. The hides are tumbled
8-12 hours, hung up till half-dry, and the process is repeated. For
very heavy hides, 4 tumblings may be required. In the later tumblings,
a lower temperature, 95° F. (35° C.), may be employed, and the time
extended to 15 hours.
The currying of crown leather is very simple. It is set out on flesh
and grain, and boarded to raise the grain. Mossner, before currying,
washes 2 hours in water and brushes with tepid soda solution (1 in
60). The yield of weight is small, only amounting to about 30-40 per
cent. of the raw hide employed, and hence the price per lb. must be
considerably higher than that of tanned leather to yield a profit. The
above information is mostly drawn from articles by W. Eitner ('Der
Gerber,' iv. 1 _et seq._) and Franz Kathreiner ('Gerber Zeitung,' 21st
December, 1875).