PRACTICAL HINTS ON TANNING.
The Hon. Zadock Pratt, one of the largest Tanners in the State of New
York, has given some very valuable information concerning the various
tanning processes which his experience and knowledge have afforded him.
We shall select a few of the most important as furnished by him to
Morfit’s valuable work.
Skins with the hair on are first soaked in cold water for forty-eight
hours, and are then subjected to the action of the machine (Monier &
Ray’s Mill) for an hour and a half; exposure for which time is generally
sufficient to render them pliable. Eight or ten skins, according to
their size and thickness, are generally contained in the apparatus. The
water is then allowed to drain off, a sufficient quantity of cream of
lime is poured in and the skins are again beaten for four hours, when
they are taken out and piled up. After having been left to drain for
five hours in this position, they are again fulled for a time in the
machine, and then are deprived of their hair and fleshed by the workmen.
They are then beaten for an hour and a half in the machine, and cleaned
and scraped with the slate. In order to remove the lime, some of which
remains attached to the skins, they are now soaked in water containing
one hundredth part of sulphuric acid, and, after being constantly
stirred in this liquid for an hour, are washed and rinsed in running
water.
This method of preparing skins for tanning dispenses with the laborious
manipulations to which they are commonly subjected, and preserves their
quality, not injured as they are in the old way, by the hands of the
workmen. It also presents the additional advantage, that they do not
require the long exposure to the action of lime which is so apt to
injure their tissue.
Since I first commenced business, the gain of weight in converting hides
into leather, has been increased nearly 50 per cent. That is, from a
quarter to a third more leather can now be obtained from a given
quantity of hides, than at the time when I learned my trade at my
father’s tannery, conducted in the old fashioned way, some forty years
ago. The great improvement in weight seems to have been gained by the
judicious use of strong liquors, or “ooze,” obtained from finely ground
bark, and by skilful tanning.
The loss and wastage upon hides, from hair, flesh, &c., may be estimated
at from 12 to 15 per cent.
To green hides, particularly, nothing can be more injurious than to
suffer them to remain too long in weak ooze. On the other hand, however,
the effects of an early application of ooze, that is too strong and too
warm, to green hides, is very injurious. It contracts the surface fibres
of the skin, tanning at once the external layers so “dead,” as it is
termed, as to shut up the pores, and prevent the tannin from penetrating
the interior. In the impossibility of adapting fixed rules to the
innumerable variety of cases, nothing can be depended upon but the
judgment of the practical tanner. In softening hides, and preparing them
for the process of tanning, a great deal depends upon the judgment of
the one superintending the operation.
In “sweating,” the character of the hides, and the temperature, are
essential, but ever-varying considerations. As a general rule, however,
the milder the process for preparing the hides for the bark, the better.
Too high a temperature is particularly to be avoided. Hides that are
very stiff and hard, resisting all the ordinary modes of softening, are
assisted by a solution of ashes, potash, or even common salt will be
found beneficial; and particularly so in hot weather.
The following table may be found useful in conveying an approximation to
a definite idea of the practice in my tannery:
40° 50° 60° 70°
SOAKING. Days. Days. Days. Days.
Buenos Ayres hides 10 to 12 8 to 12 6 to 8 3 to 6.
Carthagena and Laguaira 8 to 12 7 to 9 5 to 7 2 to 3.
SWEATING.
Buenos Ayres hides 15 to 20 12 to 16 8 to 12 2 to 3.
Carthagena and Laguaira 15 to 20 10 to 15 6 to 8 2 to 3.
I would here remark that I changed the process from liming to sweating
in the sole leather in 1836—the only change I have made in tanning for
twenty years; and for sole leather, it has been proved to be quite as
good as liming, if not better, and somewhat cheaper; besides yielding a
greater gain of weight, and when well tanned, making leather more
impervious to water. Liming and “bating,” however, for light leather, is
preferable. Salted hides do not require more than two-thirds the time to
soak; but generally rather longer to sweat. After the hides are prepared
for tanning, the next process is what is commonly called “handling,”
which should be performed two or three times a day in a weak “ooze”
until the grain is colored. New liquors, or a mixture of new and old,
are preferable for Spanish or dry hides—old liquors for slaughter. They
are then, after a fortnight, laid away in bark, and changed once in two
or four weeks, until tanned. Much care and judgment are necessary in
proportioning the continually increasing strength of the liquors to the
requirements of the leather in the different stages of this process. The
liquors should be kept as cool as possible, within certain limits, but
ought never to exceed a temperature of eighty degrees; in fact, a much
lower temperature is the maximum point, if the liquor is very strong;
too high a heat, with a liquor too strongly charged with the tanning
principle, being invariably injurious to the life and color of the
leather. From this it would seem that time is an essential element in
the process of tanning, and that we cannot make up for the want of it by
increasing the strength of the liquor, or raising the temperature at
which the process is conducted, any more than we can fatten an ox or
horse by giving him more than he can eat.
Hides that are treated with liquor below the proper strength become much
relaxed in their texture, and lose a portion of their gelatine. The
leather necessarily loses in weight and compactness, and is much more
porous and pervious to water. The warmer these weak solutions are
applied, the greater this loss of gelatine. To ascertain whether a
portion of weak liquor contains any gelatine in solution, it is only
necessary to strain a little of it in a glass, and then add a small
quantity of a stronger liquor. The excess of tannin in the strong,
seizing upon the dissolved gelatine in the weak liquor, will combine
with it, and be precipitated in flakes, of a dark, curdled appearance,
to the bottom. At the Prattsville Tannery, the greatest strength of
liquor used for “handling,” as indicated by Pike’s _bark-ometer_, (an
instrument to test only FRESHLY made liquors,) is sixteen degrees; of
that employed in laying away, the greatest strength varies from
thirty-five to forty-five degrees.
After the leather has been thoroughly tanned and rinsed, or scrubbed by
a brush-machine or broom, it will tend very much to improve its color
and pliability to stack it up in piles, and allow it to sweat until it
becomes a little slippery from a kind of mucus that collects upon its
surface. A little oil added at this stage of the process, or just before
rolling, is found to be very useful.
Great caution is necessary in the admission of air, in drying when first
hung up to dry. No more air than is sufficient to keep the sides from
moulding, should be allowed. Too much air, or in other words, if dried
too rapidly, in a current of air, will injure the color, giving a darker
hue and rendering the leather harsh and brittle. * * * *
The average time of tanning in 1842, was five months and seventeen days;
of 1843, five months and twenty-two days; 1844, six months; 1845, six
months and eleven days; average of the whole time, five months and
twenty-seven days. The average weight of the leather was over eighteen
and one-half pounds per side. This, according to the best authorities we
have, is considerably below the time employed in England. There, it is
no uncommon thing for eight or ten months to be employed in tanning a
stock of leather, and some of the heaviest leather, it is said, takes
fourteen and eighteen months. Such deliberation undoubtedly insures a
fine quality of leather, but it may be questioned whether there is not a
great loss of weight—a loss of interest on capital, and in consequence
an unnecessary enhancement of price, which does not suit the American
market.
The tanning of leather, more than almost any other manufacture, is a
chemical process, the success of which depends almost wholly upon the
skill and judgment with which its complicated manipulations are
conducted. To attain the requisite skill in the laboratory of the
chemist, is evidently impossible; it can only be acquired in tanning
itself, by long and careful attention and observation.
The labor in the loft and peeling bark during the above five years was
8820 days. One man will work through the beam-house, in one year, 6260
sides. One man will tan and finish 2228 sides. One cord of bark tans 196
pounds.
The question has been frequently asked me, how long does it take to tan
sole leather. I answer, from four to six months, according to the
strength of the liquor and the number of sides in the vats; and the
quicker tanned the better.
If the hides are fresh, they are capable of being properly softened,
and, if so, the process of tanning may be completed much sooner than in
the case of old and hard hides, that cannot be softened with the same
facility.
If the hides have sufficient room in the vats, so as not to lay crowded,
they will tan much faster.
As the tanning advances, the liquor should be renewed seasonably, and
its strength increased in a ratio proportionate to each stage of
tanning.
The question, “Is the leather to be tanned so as to barely pass in
market, or to be well prepared, so as to make firm and solid leather?”
involves a consideration of much importance.
Every one interested in the science of tanning should purchase and study
the able and elaborate work upon the subject by Morfit. No portion of
this important art is left untouched, and the work gives abundant
evidence of laborious research, and intelligent compilation, combined
with a thorough knowledge of the subject.