TANNING AND THE TANNING WOODS OF AMERICA.
The following letter in 1851 was addressed by Dr. Reid, of Rochester,
New York, to Dr. Gale, of the United States Patent office, respecting
the art of Tanning, and the various substances which are, or may be used
in the processes. It is a very able and elaborate document, and will be
found to contain not only facts, but important suggestions which must be
valuable to all interested in tanning.
DR. GALE—_Dear Sir_: As a chemist interested in the discovery of new
chemical facts, and as an American citizen in the development 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 past I have been engaged, more or less of the
time, in the investigation and development of an improved system of
tanning, founded, as I flatter myself, upon a more correct knowledge of
the chemical affinities and qualities of the various substances used,
and of the processes employed in 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 most
essential and important, and yet it 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 a patent
for the suspension of bags of ashes in the tan vats. If he were a tanner
he 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 not know that lime 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 years, 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 a careful and
patient examination pending his application for a patent. But it is not
my purpose to discuss these topics now, and I will dismiss 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, lime-water and a solution of pearlash; but these
two substances form compounds with tannin, which are not decomposable by
gelatine; it follows that their effects must 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
menstrum 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 directed 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 buckskin and
chamois leather, or in tanning proper, as in making morocco, upper and
sole leather.
Second. The various species and qualities of the tannin materials used,
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 wish more
particularly to call your attention.
We greatly need both a qualitative and quantitative 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
catechu 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
chestnut 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 accurate 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 tannin properties, to 6 or 7 lbs. of English oak
bark, and to three 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 these 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 as producing the most inferior
quality of leather. So hemlock, which, excepting the Sicily sumac,
possesses the next highest quantity, produce 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 sumacs 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 great
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 gums, resins, acids, 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 as the tannin, and become fixed, although none of them could alone
be made to unite thus permanently with the hide. It becomes, therefore,
a matter of much importance to the tanner to know what these several
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
Tinetosia which is so 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 hydrochloric 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 is 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 first 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 those 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 quantity of tan barks are far inferior to those
of the Middle, Southern, and South-western. The facilities and natural
resources of the South for manufacturing 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 best varieties of
sumac, viz., the Rhus Coriana and the Rus 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.