Page images

with the tannin, into chemical combination with the skin. In no case is there any reason to believe that gallic acid is absorbed in this process; and M. Seguin's ingenious theory of the agency of this substance, in producing the de-oxygenation of skin, seems supported by no proofs. Even in the formation of glue from skin, there is no evidence which ought to induce us to suppose that it loses a portion of oxygen ; and the effect appears to be owing merely to the separation of the gelatine, from the small quantity of albumen with which it was combined in the organized form, by the sol. vent powers of water.

“ The different qualities of leather made with the same kind of skin, seem to depend very much upon the different quantities of extractive matter it contains. The leather obtained by means of infusions of galls, is generally found harder, and more liable to crack, than the leather obtained from the infusion of barks; and in all cases it contains a a much larger proportion of tannin, and a smaller proportion of extractive matter.

“ When skin is very slowly tanned in weak solutions of the barks, or of catechu, it combines with a considerable proportion of extractive matter ; and in these cases, though the increase of weight of the skin is comparatively small, yet it is rendered perfectly insoluble in water, and is found soft, and at the same time strong. The saturated astringent infusions of barks contain much less extractive matter, in proportion to their tannin, than the weak infusions; and when skin is quickly tanned in them, coinmon experience shows that it produces leather less durable than the leather slowly formed.

Besides, in the case of quick tanning by means of infusions of barks, a quantity of vegetable ex

tractive matter is lost to the manufacturer, which might have been made to enter into the composition of his leather. These observations show, that there is some foundation for the vulgar opinion of workmen, concerning what is technically called the feeding of leather in the slow method of tanning; and though the processes of the art may in some cases be protracted for an unnecessary length of time, yet, in general, they appear to have arrived, in consequence of repeated practical experiments, at a degree of perfeetion which cannot be very far extended by means of any elucidations of theory that have as yet been known.”

As a vast quantity of bark may easily be obtained in countries that are covered with natural forests, such as many parts of America, New Holland, &c. it has been suggested, as a method of lessening the expense of freight in bringing it over, to make an extract from the bark, which might be very easily transported, and which would serve the purpose

of the tanner as well as the bark itself. It was first suspected by Sir Joseph Banks, and afterwards confirmed by the experiments of Sir Humphry Davy, that a substance called catechu, or terra Japonica, brought from the East Indies, contained a vast quantity of tannin ; so much so, that it far excels every other known substance in this respect. One pound of catechu contains as much tannin as eight or ten pounds of common oak bark, and would consequently tan proportionately as much more leather. It is an extract made from the wood of a species of mimosa, by decoction and subsequent evaporation.

Oak bark being a very expensive article in the process of tanning, various substances have been proposed as substitutes for it. All the parts of' ve



getables which are of an astringent nature, contain tannin (which may be known by their giving precipitates with gelatine, insoluble in water), and will answer this purpose. The leaves, branches, fruit, flowers, of a vast number of plants; every part of the oak, as the leaves and acorns, oak saw-dust, and the barks of almost all trees, contain more or less tannin.

Mr. Biggins made a great many experiments upon the quantity of tanning principle in various barks, from which he constructed the following table.

Tanning principle (in grains),

from half a pint of infusion and an ounce of solution of glue.

Bark of elm,

oak, cut in winter,
willow (boughs)
willow (trunk),
Spanish chesnut,
smooth oak,
oak, cut in spring,
Huntingdon, or Leices-

tershire willow,

28 30 30 31 31 41 58 52 53 54 59 59 60 76 79 82 98 104 108

[ocr errors]



The art of currying consists in rendering tanned skins supple and of uniform density, and impregnating them with oil, so as to render them in a great degree impervious to water.

The stronger and thicker hides are usually employed for making the sales of boots and shoes, and these are rendered fit for their several purposes by the shoemakers after they are tanned; but such skins as are intended for the upper leathers and quarters of shoes, for the legs of boots, for coach and harness leather, saddles, and other things, must be subjected to the process of currying.

These skins after coming from the tanners, having many fleshy fibres on them, are well soaked in common water. They are then taken out and stretched upon a very even wooden horse ; where with a paring knife all the superfluous flesh is scraped off, and they are again put into soak. After the soaking is completed the currier takes them again out of the water, and having stretched them out, presses them with his feet, or a flat stone fixed in a handle, to make them more supple, and to press out all the filth that the leather may have acquired in tanning, and also the water it has absorbed in soaking

The skins are next to be oiled, to render them pliant and impervious to wet. After they are half dried, they are laid upon tables, and first the grain side of the leather is rubbed over with a mixture of fish oil and tallow; then the flesh side is impregnated with a large proportion of oil. After having been hung up à sufficient time to dry, they are taken down and rubbed, pressed, and folded in various directions, and then spread out, when they

are rolled with considerable pressure upon both sides with a fluted board fastened to the operator's hand by a strap; by this means, and by repeating the rolling, a grain is given to the leather.

After the skins are curried, it may be required to colour them. The colours usually given to them are black, white, red, green, yellow, &c.

If the skins are to be blacked, the process varies according to the side of the skin to be coloured. Leather that is to be blacked on the flesh side, which is the case with most of the finer leather intended for shoes and boots, is coloured with a mixture of lamp black, oil, and tallow, rubbed into the leather. And what is to be coloured on the grain side is done over with chamber lye, and then with a solution of sulphate of iron, which turns it black.


Soda, or the mineral alcali, (described above, under Chemistry) is sometimes found in a native state, as in the lakes of Natron in Egypt, which are dry. in the summer season; the water leaving after evaporation a bed of soda, or, as it is there called, natron, of two feet in thickness.

A marine plant, called the Salsola soda, which grows among the cliffs on the sea coast, seems to be endowed by nature with the property of decomposing the salt water, that is, of separating the muriatic acid from the soda, which latter it absorbs. This plant is collected by the Spaniards with great care, and burnt for the manufacture of barilla, which is a carbonate of soda mixed with various impurities.

Soda is also procured in a still more impure state, by the burning of the sea weeds on our own

« PreviousContinue »