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.