Scientific American 47, 9.8.1851
We request the attention of our readers to the following letter of Dr. Reid, of Rochester, N. Y., to Dr. Gate, of the Patent Office, respecting the At of Tanning, and the woods or vegetable substances which are used, or might be used in the processes.
Dr. GALE — Dear Sir: As. a chemist interested in the discovery of now chemical facts, and as an American citizen in the developement of all branches of industry of our common country, permit me to call your attention to the following remarks and suggestions.
For two years and a half put I have been engaged, more or less of the time, in the investigation and developement of an improved system of tanning, founded, an I flatter myself, upon a more correct knowledge of the chemical affinities and qualities of the various substances used and of the processes employed is making leather.
The art of making leather embraces two species of operation, viz., the chemical and mechanical the first includes all the changes produced in the raw hide, by means of other substances applied to it, till it becomes leather. The second, all the physical labor expended upon it, whether by manual tools or machinery. The first is by far the moat essential and important, and petit is that which is least understood by practical tanners. For the want of chemical knowledge they are, in a great degree, incapable of understanding and appreciating the chemical phenomena daily passing before their eyes; hence improvement in the art of leather-making has been very slow; and those improvements which have been attempted belong chiefly to the tools and machinery employed. Very few tanners have ever ventured upon an improvement in the chemical branch of their art; and when they have, their supposed inventions or discoveries were in direct contradiction of chemical laws, and of course were impracticable and soon abandoned: as, for instance, patents have been taken out for the use of potash and soda ash, dissolved in the tan liquor or ooze. One man, a few years since, actually obtained s patent for the suspension of bags of ashes in the tan vays. If he were a tanner be must have known, what every practical tanner knows, that lime, remaining in the hide, prevents the process of tanning, besides making bad leather; but he did riot know that time and potash were both alkalies, and that tannin was an acid, and that alkalies and acids neutralize each other, and therefore, for his purpose, incompatible, or he never would have made such an absurd mistake.
For the last fifty yeare, nearly all the improvements, real or supposed, that have been patented, were chiefly for tools or machinery, for the purpose of expediting the mechanical labor necessarily employed, but the discovery and improvements which I have been investigating appertain solely to the chemical processes of tanning. They were first proposed by Harmon Hibbard, to whom Letters Patent were granted, as you are already aware; and with which improvements, and the chemical principles on which they are founded, you are familiar, having given them s careful and patient examination pending his application for a patent. But it is not my purpose to discuss, these topics now and I will diemiss this part of my subject by a quotation from Dr. Ure, and by offering a remark or two thereon.
In his Dictionary of the Arts, Dr. Ure says, "Various menstrua have been proposed for the purpose of expediting and improving the process of tanning; among others, limewater and a solution of pearlash; but these two substances form compounds with tannin, which are not decomposable by gelatine; it follows that their effects most be prejudicial. There is very little reason to suppose that any bodies will be found, which, at the same time that they increase the solubility of tannin in water, will not likewise diminish its attraction for skin."
Now the very objects here supposed by Dr. Ure to be unattainable, are literally and perfectly accomplished by Hibbard's method, viz., a menstruum has been found "for expediting and improving the process of tanning," and that, too, by "increasing at the same time both the solubility of tannin and its attraction for gelatine or skin;" by means, also, so simple, direct, and obvious, that it is wonderful that so learned a chemist as Dr. Ure should not himself have made the discovery.
But I come now to the principle object in view in this communication.
During the experiments and investigations above alluded to, my attention has been dirested to two important branches of the manufacture of leather.
First. The chemical principles involved in the several processes of making the various kinds of leather, whether it be in "tawing," as in making kid-glove leather, or in oil dressing, as in making buck-skin and chamois; leather, or in tanning proper, at in making morocco, upper, and sole leather.
Second. The various species and qualities of the tannin materials ueed, viz., the bark of hemlock, several varieties of oak, American and Sicily sumac, and terra japonica: these embrace the chief kinds used in this country.
It is to this latter — the materials for tanning — that I wink more particularly to call your attention.
We greatly need both a qualitive and quantative analysis, of the several kinds of substances used for tanning, especially of the hemlock bark — of the white, black, red, Spanish chestnut, oak, and other varieties of the Quercus; also of the American and Sicily sumacs, and of cstechu or terra japonica. We have many native trees and shrubs, of whose barks an analysis might prove to be something more than mere scientific curiosities.
A writer in one of our scientific journals asserts that the bark of the chesnut contains more tannin than oak, and more coloring matter than logwood of equal weights and qualities. On what authority he makes this statement, I know not, but if the fact be so, it should be established and known.
I am not ignorant that Sir Humphrey Davy and other distinguished foreign chemists have investigated this subject to considerable extent, but the barks and substances examined by them were not our indigenous products; besides, since their day, better and more sccurate methods of analysis have been discovered, so that even their experiments need revision, and many of their conclusions may need correction.
According to Sir H. Davy, terra japonica contains about 54 per cent. of tannin, and is equal, in tanning properties, to 6 and 7 lbs. of English oak bark and to 3 lbs. of Sicily sumac. The tanners of this country consider American sumac as possessing only half the amount of tannin of the foreign and imported article; and it is worth only half as much per ton hence it would require 6 lbs. of it to equal 1 lb. of terra japonica or catechu, and is, therefore, equal in tanning to English oak bark. But the hemlock of this country has probably double the amount of tannin that the white oak of the Northern States has; hence it holds a middle rank between Sicily sumac and terra japonica, and would consequently require 4 or 5 lbs. of it to equal one of the latter.
But the quality of the tannin, or rather the quality of the leather produced by three different kinds of tanning materials, is a matter of quite as much importance as the relative or absolute quantity of tanning contained in each of them. While terra japonica possesses the greatest quantity of tannin, it is considered Le producing the most inferior quality of leather. So hemlock, which, excepting the Sicily sumac, possesses the next highest quantity, produces the next worst quality of leather; while the oaks, which are the lowest in the scale of quantity, afford the most superior in quality. And although the American and Sicily sumachs may be considered to be on a par with the oaks, as to quality, yet the same law, seems to hold with respect to each other, that is, the American sumac, which possesses only about half the amount of tannin, makes a better quality of leather than the Sicily sumac.
Now pure tannin is probably the same in all cases, then why this grest diversity of quality in the leather? A careful chemical analysis of the substances; used, would determine the question; but, in the absence of such analysis, we readily and perhaps correctly conjecture, that very different vegetable gume, reeine, aside, extracts, &c., must be combined with the tannin in these several tanning materials, which being also soluble in water, combine in some way with the gelatine of the hide as well ae the tannin, and become fixed, although none of them could alone be made to dolts thus permanently with the hide. It becomes, therefore, a matter of much importance to the tanner to know what these aeveral vegetable products are which are combined with the tannin of each species of bark, or substance used for tanning, and, as they are not merely useless, but injurious, to know how, if possible, he may get rid of them. Among these products, there is in hemlock bark a large amount of resin or pitch, a small portion of which, however, is soluble, unless very hot water is used in leeching the bark; but in all barks there is, besides extractive or coloring matter, a large amount of acetite of potash, which is nearly as soluble as tannin itself, and which is always leeched out of the bark and forms a part of the tan liquor or ooze in which the tanner steeps his hides. That the potash, which abounds in all barks, is leeched out, is evident from the fact, that ashes, obtained from burning the leeched bark of tan yards, will not afford a ley sufficiently strong to make soap. The same thing is true of wood that has been long soaked in water. The black oak or Quercitron — the Quercus Tinctoria which is no valuable for its coloring properties, is among the richest of barks in tannin, and makes the best quality of leather, but it is generally abhorred by tanners, and avoided in the first stages of tanning. It abounds in a rich, deep yellow precipitate, which attaches itself, like paint, so tenaciously to the surface of the hides, that the tannin penetrates very slowly. But by the Hibbard process of tanning, the hydro-chloric acid used decomposes and neutralizes both the potash and coloring matters leeched out of the bark, in a great degree., so that the process of tanning ia more rapid, and the color of the leather much fairer and more beautiful, besides it, the leather, being tougher and more pliable.
Here then, in the analysis of our indigenous barks, is a field large enough to give useful and honorable employment to all the Brat chemists of the country. Not possessing, myself, either the time, skill, or requisite means to pursue this subject, but believing that you possessed them all, in addition to a taste and zeal for such pursuits, I have taken the liberty to present these views and suggestions for your consideration.
There are other matters connected with this subject which belong rather to the commercial and agricultural business of the country, but are not wholly devoid of interest to the naturalist and chemist. I allude to the quality and quantity of tanning materials as produced and influenced by latitude, locality, and climate. In the Eastern, Northern, and Western States the quality and qnantity of tan barks are far inferior to those of the Middle, Southern and Southwestern. The facilities and natural resources of the South for manufcturing leather, over those of the North, as far exceeds those of the latter, as the actual amount of leather and shoes manufactured by the North exceeds those manufactured by the South. The South, in fact, ought to furnish the North with leather; and should, moreover, produce all the sumac needed for home consumption, both for dyeing and tanning, of which we now import large quantities. By procuring from the coast of the Mediterranean the beet varieties of sumac, viz., the Rhus Coriana and the Rhus Cotinus — the former used chiefly in tanning, and the latter in dyeing, the South might grow enough in a few years for export, and find it a profitable branch of industry.
But having extended this communication much beyond the limits first designed, I close by expressing the hope that you will find it of sufficient interest to secure your good wishes and efforts to aid in the developement of the great unexplored resources of our country.
With much esteem I am, respectfully yours,
W. W. REID.
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