Bourne Archive:
Muspratt: Leather
http://boar.org.uk/aaiwxw3MusprattL1Leather.htm Latest edit 27 Dec
2010
The Bourne Archive
Muspratt’s Chemistry, Theoretical,
Practical & Analytical (ca. 1859)
Extracts Concerning Leather, 1: Leather.
The web pages linked from this
introduction are from an article on tanning, under the heading ‘Leather’. The
original article is presented here in several web pages, respectively dealing: 1,
with leather; 2,
with tanning materials; 3, Sources of
tannin; 4,
crushing mills; 5,
varieties of skin; 6,
hide preparation; 7,
the common tanning process; 8, finishing
processes; 9, fancy
and speciality leathers.
Vol. 2. pp. 491-3
Leather.—Cuir, French; leder,
German.—Leather is the compound which the fibrous matter of the skin of
warm-blooded animals forms with various substances of an organic and inorganic
nature. It remains in a great measure unaltered under the joint influences of
air and moisture; and in this behaviour differs considerably from the skin, per se, since the latter, if subjected
to the action of the forementioned bodies, readily undergoes a species of
fermentation that resolves itself into the putrefactive kind, and quickly
destroys the integument of the skin.
Historical
Notice.—Of the early
history of leather little is known; still the meagre allusions met with in old
records are sufficient to show that it is of high antiquity. Like many other
manufacturing arts of long standing, the preparation of leather from the skins
of animals originated with the Oriental people, but of the methods adopted
nothing is stated to inform the reader of the present day, beyond the remark that
alum was used as a tanning agent by the Saracens.
It is recorded that
leather was employed among the Romans, long before they acquired the knowledge
necessary for its preparation. In those and subsequent epochs it was in request
for sandals and other articles appertaining to dress. In the middle ages, it is
stated that several uses were made of this article, both personal and domestic,
and although the manufacture must have experienced a corresponding impetus,
still the mode of tanning or treating skins at that period is in oblivion.
According to Fosbrooke1,
the Britons in early times exported the skins of their slain animals,
but afterwards learned the methods by which they were converted into leather.
This is evidenced by the accounts extant of the large tanning establishments
which they had, and which were usually erected on the banks of some rivulet,
across which they constructed dams for the purpose of intercepting the water
for their use, and frequently to the detriment of a large tract of country,
which the water occasionally submerged.
Travellers have
related that the conversion of the skins of animals into leather is practised
by the aborigines of savage nations. Thus, Sir
Robert Southwell2 describes the methods by which the savage
tribes of
On the skin being
taken off the animal, the cerebral matter is extracted from the skull, and both
it and the skin are dried in the sun’s rays. During the exposure, the skin is
kept stretched, so that no shrivelling of the hide takes place. When the
hunting season terminates, the dried skins are steeped in water, and the hair
detached by use of an old knife; after this, they are placed in an earthen pot with
the powdered brains, and afterwards heated to 95°, or thereabouts. By this
treatment the cerebrous matter is converted into a kind of soap, which forms a
lather with the water remaining in contact with the skins, and it makes the
latter very clean and pliable. After remaining immersed for some time, the
skins are taken out and stretched in every direction by means of thongs, and a
frame formed of upright stakes and cross-pieces. During the exsiccation upon
the frame, they are rubbed with a smooth stone, or hard piece of wood, so as to
expel the water and fat from the pores of the hide.
Whether the leather
employed by the ancient Hebrews and their contemporaries, and of which girdles et cetera, were made as stated in the
book of Kings, was prepared in a similar manner, is unknown. Whatever methods
were pursued, it is evident that the making of skins into leather was a
business confined to a few who made it their trade, in the beginning of the
Christian era, and that the manufacture has been since progressing slowly.
Regarding the choice of materials for tanning, such as the barks of trees and
the like, it is unknown whether those now in use were then recognized as
possessing the virtues of converting skins into leather, or whether the
discovery is due to the tanners of the middle ages. It is certain, however that
the scientific principles of the business were not at all, or at best but very
imperfectly understood, before the end of the last and beginning of the present
century. The investigations of Deyeux3,
M’Culloch6 considers that the leather manufacture
ranks fourth in importance, being inferior in money value to those of cotton,
wool, and iron, whilst others are disposed to think that it is quite as
important as cotton. A glance at the extent of this branch of trade will show
at once how much it has merited the above rank. M’Culloch estimates the number of persons employed in the
tanneries alone as exceeding twenty-eight thousand three hundred; and those
engaged in the subsidiary trades to which leather gives rise, such as the
currier, boot-maker, saddler, et cetera,
average two hundred and twenty-five thousand, among whom there is expended
annually from seven and a half and eight millions sterling in wages alone. The
value of the manufactured goods reaches to no less than nineteen or twenty millions
annually.
It is plain,
however, that without some very great changes or modifications of the old slow
process of tanning, the extensive demands for leather, which may be inferred
from the preceding, could not have been met. Such modifications have been
introduced more especially during the last fifty years, in which time mechanism
has done far more than chemistry to expedite the operations and improve the
appearance of leather. The mission of chemistry has hitherto extended little
further than discovering tanning agents, and extracting the tanning principle,
which, when the hide is prepared by mechanical pressure and other treatment,
yields up the tannin more readily than the bark. It is nevertheless a fact, that
as the operation has been quickened thus far, the quality of the leather has
deteriorated proportionably, and hence when a fine kind of leather is required,
recourse must be had to the old methods for its preparation.
Proximate Principles of Leather.—As may be inferred from its
nature, leather is formed by the combination of the substance of the skin with
any other compound which has the property of rendering it imputrescible and
elastic. Many substances possessing these properties in relation to skins of
animals, are known to exist both in the organic and inorganic classes; but the
one most generally employed, as well as the most efficient, is tannin. That a
firm chemical union of the two bodies, such as that above referred to, exists,
is evidenced by the modified form of the materials, and the physical appearance
and chemical behaviour of leather, when subjected to microscopical and chemical
examinations. The principle in the skin esteemed by the tanner is, as will be
shown later on, a gelatino-fibrous compound which constitutes the basis of leather;
and the combining agent—tannin—partakes of an acid nature, so that it is no
longer subject to the ready putrefactive change which the skin undergoes under
the influence of air and moisture. As the gelatino-fibrous principle forms only
one of several others in the hide of the animal, so the tannin or tannic acid
constitutes only one among many other substances in the sources whence it is
extracted. In order to render the comprehension of the scientific nature of the
business of the tanner as easy as possible, it will be necessary to dwell
somewhat in detail upon the materials which he employs, so as to exhibit their
respective nature, and show how they mutually affect one another. And firstly,—
The Skin.—Strictly speaking, the skin of
animals is composed of two parts, the corium
or cutis, and the cuticle or epidermis: the former is the portion which enters into the
composition of leather, and forms the true skin, and the latter constitutes the
exterior covering in which the fur, wool, or hair of the animal is rooted. Some
anatomists, however, distinguish three distinct parts in the skin, namely the corium, the rete mucosum, and the cuticle.
These three several parts are illustrated in Figs. 339 and 340, the former
showing the position of the hair, and the latter the magnified layers of which
the skin is compounded. In both, A represents the epidermis; B, the rete
mucosum, in which the roots of the hair are embedded; C, the fibrous tissue of
the true skin; and D, the cellular tissue beneath the latter, showing the fat
cells, a, in Fig. 340, and sweat glands, b, with the follicles, h, through
which the this secretion passes out to the surface of the skin.
Behaviour of the Epidermis and Cutis with reagents.—When the fresh skin is immersed
and macerated repeatedly in water, the matter of the cuticle is separated from
the fibrous substance of the cutis, not by the solution of the former, but, as
it would appear, by dissolving an extractive matter amounting, according to Weinholt’s 7
analysis, to about eight and a half per cent., and which is probably to some
extent intermediate between the horny matter of the cuticle and the cutis; weak
acids also abstract it, but solutions of the alkalies and many of their salts
are more effectual for bringing about the separation, since these agents
dissolve it.
The epidermis,
which is analogous to horn, does not combine with tannin or any other substance
by the agency of which leather is produced. Hence it becomes useless to the
tanner; and therefore, the first process to which hides and skins are subjected
by him, is that for removing the hair and epidermis, both being useless.
The corium,
deprived of the epidermis, is a substance organized of a number of fibres
ramifying and intersecting one another in every position; leaving, however,
interstices contracting in size as the reach the outer portion, and which are
more or less charged with fluid matter that serves to renew the cuticle and
keep the skin pliant and moist. On treating the skin with water these matters
are removed, and ultimately there remains but the fibrous portion saturated
with water. In this state it appears semitransparent, and, if the water be
expelled by a gentle heat, it assumes the physical appearance of horn,
constituting only about thirty-two and a half to thirty-three per cent. of the
raw hide. From Weinholt’s
researches, it appears that the skin deprived of the epidermis and its
subjacent fluid, as also of the mucous membrane and fat on the interior,
affords forty-three per cent. of solid matter, yielding—
|
|
Centesimally |
|
Fibrous matter |
74.42 |
|
Uncoagulated albumen |
3.49 |
|
Extractive matter—soluble in
water; insoluble in alcohol |
17.44 |
|
Extractive matter—soluble in
alcohol |
2.32 |
|
Fatty matter and loss |
2.33 |
|
|
100.00 |
Digestion in water removes the albumen and
extractive matter taken up by this menstruum8;
in alcohol, the further portion of extract dissolves, and ether separates the
fat with which the residue is impregnated. If the corium, deprived of the
epidermis, be treated with boiling water, it dissolves, with the exception of a
little fat and some nervous filaments; and when the liquid is evaporated
slowly, a gelatinous residue is left, which, when the entire skin is operated
upon, forms the glue of commerce.—See Glue,
ante, Vol. II., page 181.—This effect
of the water does not arise, however, from the solution of the gelatine which,
as might be supposed, the skin contains, but it modifies the components of the
fibrous tissue so as to bring them into this compound. The same property is
possessed by acids and alkalies in a more powerful degree, since the change is
effected by these agents at the ordinary temperature.
Fibrin, albumen,
and their modification, gluten, behave in a remarkable manner towards tannin or
tannic acid, and by the transformation of the fibrous matter of the corium
during the operations to which the skins are subjected in the tanning into a
semi-gelatinous mass, they conjointly produce leather.
Next Page: Tannin.
Commentary
1. ^ Thomas Fosbrooke (1770-1842), was a parish churchman who took to writing to eke out an inadequate income. An important part of his written work was concerned with the history of Gloucestershire (DNB).
2. ^ This may be Robert Southwell (1635-1702), a
widely knowledgeable man who spent some time on diplomatic duty in Lisbon,
where he may have heard about South American practices (DNB).
3. ^ Nicolas Deyeux (1745-1837) was Napoleon’s
pharmacist but he also took much interest in the wider aspects of chemistry.
See French Wikipedia.
4. ^ Marc Seguin (1786-1875) was a grand nephew of Joseph de Montgolfier. Although he was primarily notable for civil engineering, he had a wide range of technological business interests. It seems likely that he is the man in question. See Seguin on the www.art-et-histoire.com site (in French).
5. ^ John M’Culloch (1789-1864) published M'Culloch's
universal gazetteer: a dictionary, geographical, statistical, and historical,
of the various countries, places, and principal natural objects in the world,
in 1843. See a sample of
it.
6. ^ This is probably David Macbride
(1726-1778), from
7. ^ I am afraid I have not been able to track him
down.
8. ^ A solvent, a liquid by which a solid may be
dissolved. The use of this word derives from Alchemy. There, the base metal
intended for transmutation was likened to the seed within the womb in relation
to the blood of a woman’s monthly or menstrual, blood flow. Menstruum is the neuter nominative form
of the Latin adjective, menstruus,
monthly (OED).