This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
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.