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 South America prepare skins to make leather:—

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, Seguin4, Macbride5, Proust, Sir Humphrey Davey, and several other chemists, explained the nature of the changes which took place, and attributed them to true chemical combinations. These researches, therefore led the tanners to the comprehension of the scientific principles upon which their operations were based, and likewise opened up new sources of improvement. Since then, however, the progress in this art has been much less marked than that of other branches of trade in which chemistry has been a handmaid; and though numerous experiments have been tried, and many patents granted for new processes and improvements, no very decided advantage has been gained to show that the modern system is superior to that practised prior to their introduction. This is the more unaccountable, considering that the growth of social refinements, and increased wants, as likewise the universal impetus which trade, commerce, and manufactures have received during the last half century, created so great a demand for leather as to render it one of the most important articles of manufacture. This is especially the case in England, where—apart from the consumption of this article in boots and shoes—so much of it has been used in connection with machinery, in the manufacture of carding and other apparatus employed in the cotton trade, for saddlery and carriages, in upholstery, bookbinding and a variety of other branches which are year by year springing up, that the consumption is almost outstripping the production.

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 County Antrim. He was a physician and chemist who studied anatomy in Edinburgh and midwifery in London. He was a medical officer in a Royal Navy hospital ship, before setting up in medical practice in Dublin. He was interested in finding a cure for scurvy and in 1774, he discovered that lime-water was more efficacious than plain water in part of the tanning process. For this, he was made an honorary member of the Dublin Royal Society (DNB).

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).


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