CHAPTER I.

TANNING.

The science or art of Tanning is the process by which the animal hide or skin is converted into _leather_, thus rendering it subservient to the use of man. The art of tanning in its most simple form was known to antiquity, and consisted simply in the drying and cleansing the skins, which were then converted into articles of clothing. Thus used, they answered but a temporary purpose, as the re-absorption of moisture would speedily cause them to decay. The Egyptians were among the earliest tanners of whom we have any definite account, and Thebes was justly celebrated for its artificers in leather. But unlike most of the arts in which the Egyptians excelled, they do not seem to have reached an equal degree of skill in the preparation of leather, although it was wrought by them into tapestry, and made to subserve various useful purposes. In later times this useful art has been reduced to scientific principles, owing to the investigations of Sir H. Davy, Sequin Proust, and others. Leather considered chemically is a compound of tannin and gelatine, possessing the qualities of durability, pliability, and insolubility in water.

The important elements in the manufacture of leather are, first, _cleansing and softening the skins_. Second, the depilation or removal of the hair. Lime is principally used for the latter purpose, although any agent which possesses the power to soften or destroy the roots of the hair, and facilitate its removal is equally desirable.

The process of depilation with lime is slow, and tanners have yet to learn that practically there are other chemical processes which will greatly facilitate it. Hydrosulphuret of calcium is recommended.

The great secret of tanning consists in so preparing the cuticle or porous surface of the hide as that the gluey or gelatinous tissue shall unite with the tannin. This is produced by immersing the skins in an infusion of oak bark, or other substances containing tannin. Therefore any process which will open the pores of the unhaired skins so that they become light and spongy and thus facilitate the union of the tannin with the tissue of the skin, is desirable. The name of _tan_ takes its origin from a coarsely powdered bark, which contains the active principle in the tanning process. The component is called _tannin_.

To Seguin, a celebrated French chemist, the discovery of a property in nut-galls of combining with the albumen and gelatine of skins, thus forming an insoluble and unalterable compound, is due. This is the key to the theory of tanning.

Morfit’s definition of the principle is, “to saturate a skin with tannin in such a manner as to promote the slow combination of this principle with the gelatine, albumen and fibrine contained in the former, so as to form with them a new compound. This reaction, in the operation of tanning, does not proceed spontaneously, but is the result of a slow process, requiring great care and skilful manipulation.”

Tannin exists in the bark of the trunks of nearly all perennial trees, and especially that portion of the bark next to the fibre and sap wood. It is found in the leaves of trees in small quantities. The oak-tree bark, leaves and fruit, contain a larger amount of tannin than any other tree species in North America. The bark is usually gathered from May to July, as during those and the intervening months they contain more tannin than during the other portions of the season. There are various kinds of oak in the United States, all of which yield a large amount of tannin. There is a species of oak which grows in Delaware, Maryland and Virginia, known by the name of Spanish Oak, which is superior for its tannin properties. The rock-chestnut oak, the black and white oaks, and Spanish oak, are considered the most valuable, and are used almost exclusively by the manufacturers of leather in the United States. The wood of almost every tree, the bark of which contains tannin, is also useful for the same purpose. There are several varieties of tannin. The pure tannin and the impure tannin is a generic term for the different varieties. It was a long time before tannin was separated in a pure state. It is so intimately combined with other extractive matter that it was with difficulty separated.

Chemists give us four varieties.

_Natural tannin_, comprising several species, originating from the different proportions of extractive and gallic acids. Also three kinds of artificial tannin, obtained by the action of nitric acid upon charcoal; also upon indigo and the various resins; also that obtained by the action of sulphuric acid upon the resins and camphor.

The best kinds of hides suitable for tanning into leather are those of heifers. The large ox hides are those which are used chiefly for conversion into sole leather. The skins of horses, cows, bulls, and buffaloes, are used for thick sole leather. For upper, thin, and the various fancy leathers, the skins of small animals, dogs, sheep, goats, seals, &c.

The quality of the leather depends upon several circumstances; the nature of the skin, its condition in curing, upon the method of tanning, also the nature of the food upon which the animal fed, and its healthiness. Hides from cattle slaughtered in cold weather are said to produce five per cent. more leather than when taken in the warm months. Dry hides produce more leather than salted hides. For the production of 40 pounds of leather, the average is estimated 30 pounds of dry hide—60 pounds of salted, or 74 pounds of market hide. Large supplies of skins are imported from Buenos Ayres, Brazil, California, and Texas. Sheep skins are received from the Cape of Good Hope and Buenos Ayres; goat skins from Barbary; lamb and kid from Italy. Very few hides, comparatively, are obtained at home. In order to form some idea of the extent of the leather trade, as represented in the tanning establishments, a few figures will enable the reader to judge.

There are, according to Morfit, 6,263 Tanneries in the United States, the largest number being in Pennsylvania. These unitedly employ 20,909 persons, and in which business is invested the sum of $18,900,557. The value of the products is $32,861,796. About 6,000,000 skins of sheep, goats and other small animals, are tanned and dressed annually, which are not included in the above estimate.

The _Scientific American_, in alluding to Morfit’s excellent work on the “Arts of Tanning,” had the following notice which we here insert:

“In looking over this book, and reading the different plans for improving leather, and for reducing the time occupied in tanning a skin or hide, we are more and more convinced of the important fact that the tanning art has been greatly improved by modern discoveries and application—a contrary opinion to that held by the universal mass of the people. We know it is very difficult to introduce new plans of tanning, for tanners are, like others, wedded to old things; thus the rolling of leather—an operation now generally practiced—was opposed with much bitterness by some of our most experienced tanners, one of whom said ‘he never would roll a hide while he lived,’ an assertion which he wisely lived either to forget or repudiate.

“The whole science of tanning depends on two principles, one the removal of the hair from the skin, with the least injury to the gelatinous matter of which it is composed, and the other is the rendering of the skin insoluble in water, and to resist the action of the atmosphere, and yet be flexible. The hair can be removed by lime, sweating, and other means, but the employment of a substance or substances that will combine with the gelatine of the skin to form a new substance, insoluble in water and incapable of being injuriously acted upon by the atmosphere, offers a wide field for the historian of the tanning art, and presents a subject for the study of every tanner at least in our country. The art of tanning was known, we suppose, before the flood; it is practised among all nations, civilized and savage, and the gist of it lies in soaking the skins in different solutions of various vegetable substances of an astringent character, until the tanning juices of those substances have combined with the whole skin and rendered it a new substance named leather. Oak and hemlock barks, sumac, willow, blackberries, catechu, kino, &c., are employed. Those who wish to get an account of the various processes and substances employed, must consult this book.

“We have only another remark to make, it is this, we have never known any of the metallic solutions to be employed in tanning, and from their nature, in rendering some vegetable substances insoluble, we believe that it would be worth the trouble for some of our tanners to make a few experiments. We would recommend the chloride of tin; it is made by feeding drop-tin into muriatic acid until effervescence ceases. The way to try it would be to make up a solution of it in a tub of cold water, until it stood about 1½° in the hydrometer; the skin should have undergone through the whole tanning process before it is placed in this solution, in which it should lie about two hours, and be stirred up two or three times. After this it should be well washed in cold water, and then finished in a milk-warm water bath, when it will be ready for drying. It is our opinion that a superior leather would be produced by this addition to any of the present processes.”