MINERAL-TANNED LEATHER.
The invention of the earliest form of mineral tanning, that with alum
and salt, dates from remote antiquity; but as it is in large measure
the type of all that has been since done, it deserves examination in
some detail, at least as regards principles. In practice it is used
alone in curing skins with the hair on, and for making white leather
for laces and other purposes; and, in combination with oil and albumen,
which, as we have seen, are the tanning agents in the case of "crown
leather," it forms the process for producing calf and glove kids, as
will be described under those headings (pp. 223, 225).
Careful researches by Reimer (Ding. Polyt. Jour., 205, p. 143 _et
seq._) show (what has long been known in practice) that alum alone
is not capable of making a pliable leather. The salt, nevertheless,
does not enter into combination with the alum, or even with the hide.
Its function is partially physical, increasing the diffusion of the
solution, and partially chemical, as in the presence of acids (and
salts of acid reaction) it precipitates the coriin, and prevents it
from gluing the fibres into a horny mass as it dries. Prof. Knapp
has shown that this is the first essential in producing leather, and
that raw hide may be converted into a pliable material with all the
properties of white leather by simply withdrawing the water with
alcohol, in which coriin is not soluble, and by which it is therefore
precipitated. This leather, containing when dried no added constituent,
is of course at once reconverted into raw hide by soaking in water.
Both the salt and a portion of the alumina is removed from tawed
leather by soaking in water, and it then dries hard and horny, and
by boiling in water will yield a considerable percentage of gelatin.
The alum is not absorbed as a whole. It is a double salt (alumina and
potash sulphate or alumina and ammonia sulphate), and only the alumina
sulphate is absorbed, potash (or ammonia) sulphate accumulating in the
liquor. The alumina salt retained by the hide, especially in presence
of much salt, contains slightly more than its normal proportion of
alumina to acid, or in chemical language is to some extent basic. This
is caused partly by the lime remaining in the skin from the unhairing
process, which neutralises a portion of sulphuric acid, but in part is
the result of the affinity of the hide-fibres for alumina, a certain
small proportion of free sulphuric acid being left in the liquor. The
accumulation of this and of potash sulphate is the reason why such
liquors cannot be used perpetually by mere strengthening with alum, but
must be frequently renewed. The attraction of hide-fibre for alumina
sulphate is so strong, that in presence of a sufficient excess of
hide it may be completely removed even from dilute solutions. Alumina
acetate or sulphate may be substituted for alum with equally good
results in practice, the only advantage of the latter being its easier
preparation. Ferric and chromic salts and iron or chrome alum, may be
substituted for common alum, and are absorbed in a similar manner, and
in presence of common salt give equally pliable leathers, of a buff
and pale greenish tint respectively. Without salt, the leathers are
hard and brittle. In all these cases, the tanning agent may be to a
large extent removed by simple washing with water. The tannage may be
rendered more durable by passing the leather before drying through a
weak bath of sodic carbonate or even lime-water, which precipitates the
alumina, iron, or chrome in a basic form on the hide-fibres. Soap baths
may also be used, by which aluminic, ferric, or chromic stearates and
oleates are formed, possessing considerable toughness and resistance to
water. So far as the writer is aware, no mineral tannage has yet been
produced which will not yield gelatin when treated, first with dilute
acid and then with boiling water; but this is rather a gain than
otherwise, as leather scraps might be utilised for glue. There seems
no reason why good and durable leather, for boot-uppers and for many
mechanical purposes, should not be fabricated with salts of iron and
chromium in conjunction with salt. If eggs and flour were also used,
products similar to calf-kid would be obtained. Iron-leathers may of
course be blacked with infusions of galls or many tanning materials, or
with logwood. Ferrous salts have no tanning properties.
If, instead of using neutral iron salts, basic ferric salts (which may
be obtained by dissolving ferric oxide in solution of neutral ferric
salts, or by oxidising ferrous sulphate with manganese black oxide,
or nitric acid) be employed, much larger quantities are absorbed by
the hide, and if this be fixed with soap baths and finished with a
moderate quantity of oil, a gain of weight--approaching 50 per cent.
of the finished leather, or about the same as that given by bark, may
be obtained. The leather, however, has by no means the same resistance
to wet and decay as bark-tanned leather, and invariably has a tendency
to crack when sharply bent. The process has been most carefully worked
out by Professor Knapp, and was patented and worked commercially for
a short time in Brunswick, but apparently without financial success.
Professor Knapp's method is as follows:--The iron solution is prepared
by adding nitric acid to a boiling solution of ferrous sulphate (green
vitriol) till the iron is completely oxidised to the ferric condition.
To this, ferrous sulphate is again added so long as it continues to
cause effervescence. The resulting solution is a clear orange, and
of more or less syrupy consistence, and may be evaporated without
decomposition or crystallisation to a transparent varnish. The hides
are unhaired and prepared for tanning in the usual way, and are then
handled in solutions of the iron salt, which are at first weak, and
are gradually strengthened. Skins are tanned in 2-3 days, and the
heaviest hides in a week. After tanning, the hides are stuffed in a
drum ventilated through the axes, very similar to that described
under "crown leather," p. 213, with an insoluble iron-soap made by
precipitating soap solution with the iron-liquor; or the iron-soaps may
be formed in the hide by the alternate use of iron and soap solutions,
as already described. The leather is finally saturated with a solution
of stearin and paraffin, to render it waterproof.
A process which has been worked on a larger scale, is that of Dr.
Heinzerling, introduced about 1878, with the usual promise of "complete
revolution" in the leather trade; but which, in spite of the most
determined and persevering efforts of the Eglinton Chemical Company,
who own the English patent, has failed to take any very prominent
position in commerce. The tanning materials employed are alum and salt,
with a varying proportion of potash, soda, or magnesia bichromate.
These salts have a very marked hardening effect on animal tissues, and,
when mixed with gelatin and exposed to light or acted on by acids,
become reduced, and at the same time render the gelatin insoluble even
in hot water, a property which is made useful in many photographic
processes. This is probably due to the formation of salts of chromium,
which, as has been stated (p. 219) have a similar tanning effect, but
perhaps more powerful, than those of alumina. However this may be, the
effect of potash bichromate when exposed to light with gelatin, differs
from that of the addition of chrome salts ready formed, the gelatin in
the first case becoming incapable of even swelling in hot water, while
in the second, though rendered insoluble, it becomes soft and swollen.
The use of potash bichromate in tanning had been previously patented by
Cavalin, who used it in conjunction with alum and salt, and with the
addition of a portion of green vitriol, to give the leather a colour
more similar to that of bark-tanned.
Dr. Heinzerling uses metallic zinc in the salt and alum solution to
assist in the precipitation of amorphous alumina on the hide-fibres.
The same material was used in a similar way by Jennings (No. 2295,
1861), but with the object of whitening the goods. Yellow or red
prussiates of potash (potassic ferrocyanide or ferricyanide), are also
sometimes mixed with the solution, in order to enable the leather to
be blacked with iron-liquor, with which they produce prussian blue. To
fix the tannage on the fibre, and prevent its washing out, the use of
barium chloride, lead acetate, and of soap solution is claimed; the
latter having been also patented for similar purposes by Knapp, and
subsequently by Jennings and others.
In order to render the leather waterproof, it is finally saturated with
solutions of paraffin, stearin, and other fats and hydrocarbons (resin
is employed, though not named in the patent), in petroleum spirit and
similar solvents.
Such is the original patent, which, it will be seen, is rather a
combination of older processes than an original discovery. Whether
it is still worked on the same lines the writer is unable to say,
but he is aware that considerable improvements have been made in
the finish and appearance of the goods. The leather in its present
form possesses considerable resistance to water, is free from the
brittleness so common in mineral tannages, and like other alumed
leathers, considerably exceeds bark-tanned leather in toughness and
elasticity. These make it valuable for many purposes, and among others,
for machine-belting, although it has the disadvantage of elongating
considerably while in use.