The constituent parts of leather seem to be but little understood. The
opinions of those engaged in the manufacture of leather differ widely on
this question.
Some think that tannin assimilates itself with the hide and becomes
fixed there by reason of a special affinity. Others regard the hide as
a chemical combination of gelatine and tannin. We know that the hide
contains some matters which are not ineradicable, but only need a slight
washing to detach them.
We deem it advisable, in order to examine the hide properly so-called,
to dispense with those eradicable substances which may be regarded, to
some extent, as not germain to it, and confine our attention to the raw
stock, freed from these imperfections.
It is well known that a large number of vegetable substances are
employed as tanning agents. Our researches have been directed to leather
tanned by means of the most important of these agents.
Many questions present themselves in the course of such an examination.
Among others, that most important one, from a practical point of view,
of the weight the tanning agent gives to the hide, that is to say, the
result in leather of weight given to the raw material. The degree of
tannage is also to be considered; the length of time during which the
tanning agent is to be left with the hide; in short, the influence upon
the leather of the substances used in its production. That is why we
have made the completest possible analysis of different leathers.
Besides ordinary oak bark there are used at present very different
substances, such as laurel, chestnut, hemlock, quebracho and pine bark,
sumac, etc.
Water is an element that exists in all hides, and it is necessary to
take it into consideration in the analysis. It is present in perceptible
quantity even in dry hides. This water cannot be entirely eradicated
without injuring the leather, which will lose in suppleness and
appearance. Water should then be considered as one of the elements of
leather, but it must be understood that if it exceeds certain limits,
say 12 to 14 per cent., it becomes useless and even injurious. Moreover,
if there is any excess over the normal quantity, it becomes deceptive
and dishonest, as in such a case one sells for hides that which is
nothing but water. Supposing that a hide, instead of only 14 per cent.,
contained 18 per cent. of water, it is evident that in buying 100 pounds
of such a hide one would pay for four pounds of water at the rate for
which he purchased the hide.
There are, also, some matters soluble in air, which are formed to a
large extent from fat arising as much from the hide as from tanning
substances. The air dissolves at the same time a certain amount of
organic acid and resinous products which the hide has absorbed. After
treating with air, alcohol is used, which dissolves principally the
coloring matters, tannin which has not become assimilated, bodies
analogous to resin, and some extractive substances.
That which remains after these methods have been pursued ought to be
regarded as the hide proper, that is to say, as the animal tissue
saturated with tannic acid. In this remainder one is able to estimate
with close precision that which belongs to the hide. The hide being an
elementary tissue of unchangeable form, it is easy, in determining the
elementary portion, to find the amount of real hide remaining in the
product. With these elements one can arrive at a solution of some of the
questions we are discussing.
We give below, according to this method, a table showing the composition
of the different leathers exhibited at the Paris Exposition of 1878.
They are the results of careful research, and we have based our work
upon them:
Matter Soluble Fixed
in Air Tannin
| |
| Matter Solu- |
| ble in Alcohol |
| | |
Moisture | | Gelatine |
--+-- --+-- --+-- --+-- --+--
Steer hide, hemlock tanned (heavy leather) 10.95 4.15 19.77 39.1 26.03
Sheepskins, sumac " (Hungarian) 10.8 10.3 12.1 40.3 26.5
Finished calf, pine bark tanned (Hungarian) 11.2 1.7 7.4 41.6 38.1
Steer hide, quebracho tanned (heavy leather) 11.7 1.6 11.2 43.1 32.4
" " chestnut " " " 13.5 0.29 1.99 45.46 38.76
Finished calfskins,
oak tanned (Chateau Renault) 12.4 0.33 3.59 46.74 36.94
Steer hide, laurel tanned (heavy leather) 12.4 1.05 7.95 47.47 31.13
" " oak tanned after three years in
the vats (heavy leather) 11.45 0.37 3.31 49.85 35.02
The following table shows the amount of leather produced by different
tannages of 100 pounds of hides:
Pounds.
Hemlock 255.7
Sumac 248.1
Pine 240.3
Quebracho 232
Chestnut 219.9
Oak 213.9
Laurel 210.6
Oak, lasting three years 206
It is important to mention here the large proportion of resinous matter
hemlock-tanned leather contains. This resin is a very beautiful red
substance, which communicates its peculiar color to the leather.
We should mention here that in these calculations we assume that the
hide is in a perfectly dry state, water being a changeable element which
does not allow one to arrive at a precise result.
These figures show the enormous differences resulting from diverse
methods of tanning. Hemlock, which threatens to flood the markets of
Europe, distinguishes itself above all. The high results attributable to
the large proportion of resin that the hide assimilates, explain in part
the lowness of its price, which renders it so formidable a competitor.
One is also surprised at the large return from sumac-tanned hides when
it is remembered in how short a time the tanning was accomplished,
which, in the present case, only occupied half an hour.
The figures show us that the greatest return is obtained by means of
those tanning substances which are richest in resin. In short, hemlock,
sumac, and pine, which give the greatest return, are those containing
the largest amount of resin. Thus, hemlock bark gives 10.58 per cent.
of it, and sumac leaves 22.7 per cent., besides the tannin which they
contain. We know also that pine bark is very rich in resin. There is,
then, advantage to the tanner, so far as the question of result is
concerned, in using these materials. There is, however, another side to
the question, as the leather thus surcharged with resin is of inferior
quality, generally has a lower commercial value, and is often of a color
but little esteemed.
The percentage of tannin absorbed by the different methods of tannages
appears in the following table:
Hemlock 64.2
Sumac 61.4
Pine 90.8
Quebracho 75.3
Chestnut 85.2
Oak 76.9
Laurel 64.8
Oak, three years in the vat 70.2
The subjoined is a statement of the gelatine and tannin in leather of
different tannages, and also shows the amount of azote or elementary
matter contained in each:
Gelatine. Tannin. Azote.
Hemlock 60.4 39.6 10.88
Sumac 60.4 39.6 11
Pine bark 52.5 47.5 9.56
Quebracho 57.1 42.9 10.4
Chestnut 53.97 46.03 9.79
Oak 55.87 44.13 10.24
Laurel 60.4 39.6 10.94
Oak, 3 years in vat 58.75 41.25 10.65
It is not pretended that these figures are absolutely correct, as they
often vary in certain limits even for similar products. They form,
however, a fair basis of calculation.
As to whether leather is a veritable combination, it seems to us that
this question should be answered affirmatively. In fact, the resistance
of leather properly so-called to neutral dissolvents, argues in favor of
this opinion.
Furthermore, the perceptible proportion of tannin remaining absorbed by
a like amount of hide is another powerful argument. It remains for us to
say here that the differences observable in the quantity of fixed tannin
ought to arise chiefly from the different natures of these tannins,
which have properties differing as do those of one plant from another,
and which really have but one property in common, that of assimilating
themselves with animal tissues and rendering them imputrescible.
In conclusion, these researches determine the functions of resinous
matters which frequently accompany tannin; they show a very simple
method for estimating the results of one's work, as well as the degree
of tannage.--_Muntz & Schoen, in La Halle aux Cuirs_.--_Shoe & Leather
Reporter_.
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