The Manufacture of French Leather.

Scientific American 18, 2.5.1868

For the Scientific American.

To learn the secret of making French leather; or what is popularly. known as fine fancy leather, has been a source of anxiety to the tanners of other nations for many years. Schemes have been thought of for the discovery of the mystery by leather manufacturers and others but they have been hitherto unsuccessful. Chemists have been called in to solve the question and though analyzing air, water, food, and the mysteries of nature, their researches to discover the wished. for process of tanning leather, have been as ineffectual as those of the most unscientific tanner.

That the leather manufacture is indeed of national importance, witness the statement of Mr. Smull, an extensive manufacturer of this city, who at a late meeting of the Polytechnic Association, asserted that more hides were Tanned in New York alone than in the cities of Paris and London combined. A description of the process employed in preparing the fancy and fine kinds of French and Russia leather may throw some light upon the subject under consideration.

The best kinds of kid leather are made from goat skins, on account of their lightness and smoothness, but cow hides and sheep skins are also used for the purpose. The first operation in preparing the leather is to put the skins into running water, where they are kept for one week, being taken out daily and thoroughly beaten with a wooden brake, a work of skill and patience, which effects the breaking up of the nerve and softens the fiber to a pulpy condition. Next they spend a month in a lye made of lime or ashes, of which the exact quality must be left to the judgment and experience of the operator. The hair is now removed and the alkaline properties are got rid of by soaking the skins in an infusion of white gentian in fresh water for twenty-four hours. The swelling of the skins is a matter of particular care, for which they are soaked four or five days in a mixture of oatmeal and water. They are now ready for the tannin, which is extracted from the bark of the willow. In the first solution the skins remain but three days, and are again beaten with the brake. The second solution, which is stronger than the first, retains them eight or ten days. After being taken out they are dried with the flesh side upward, again beaten, then greased, dried, and finished, using logwood and alum, and alum and green Vitriol for the dark coloring. The mode of dyeing is peculiar. A number of skins are sewed up in the form of a sack, closed all around except a small opening at one end to admit the dyeing liquid. When the dye has reached all parts they are hung up to drain, then to dry, and again dyed with asparge, the whole process being repeated two or three times. Again they are greased on the flesh side and grained with a notched stick passing through the length and breadth of the skins until small furrows are gradually produced. After graining, another greasing is necessary, this time with birch or linseed oil, and they are put on the wooden horse to be smoothed. The birch oil gives the leather a peculiar smell which distinguishes it from that prepared by any other process.

There is no article of manufacture in the United States or in the world of more importance than that of leather, and some process for converting raw hides into upper or sole leather in a short number of days or a few weeks would be of the greatest national importance. The present mode of preparing leather necessitates a long and tedious process, which makes serious inroads on the profits of the tanner, and consequently the necessary time is not allowed for properly converting the raw skins into leather, and thus the community have the sad experience that neither sole nor upper leather is impervious to water, and the wearer of the shoe made from it suffers from damp feet, and finally goes into a decline, a practical view of the case that alone should be sufficient to arouse the inventive talent of the American people for the discovery of a quick, trustworthy tanning process, which would speedily bring a fortune to the inventor.

The French tanners use valonia and oak bark with either caustic soda, carbonate of soda, ammonia, or carbonate of ammonia. By the use of these substances a considerable saving in time in the preparation of the skins is effected, and the leather is said to be of superior quality. The tanning is facilitated by means of a roller to which a slow motion is given by steam. The moving of the hides in the bath, the usual process of liming, fleshing, and unliming is carried out, and the skins are then submitted to the action of a bath composed of a solution of extract of valonia or other tanning material, Carbonate of soda is to be added in such quantity that the bath shall be raised 1° on Raumer's hydrometer, the bath then marking 2° on the hydrometer. After three days the skins are removed to a second bath, composed of a solution of valonia of 3°, strengthened one degree as before by adding caustic soda or carbonate of soda. After lying in this bath for four days, being turned several times a day, the skins are transferred to a third solution of valonia, marking originally 7°, but by the means as above, increased one degree in strength. In this liquor the skins are immersed for seven days, when the coarsest kinds must be changed to still another bath, marking, with the added carbonate of soda, 10°, wherein they are to remain for nine days, being turned three or four times during that period.

In the case of ordinary hides they will not need to be subjected to the action of the bath in which thick hides are treated, but they should be transferred from their own bath and allowed to remain seven days, to the final bath, which is composed of the extract of valonia marking 9° on the hydrometer, the bath marking about 10°. Between each hide or skin as they are placed in the bath, about six pounds of oak bark and six pounds of valonia are strewn, and they are allowed to remain therein for fifteen days, when they are removed and finished in the usual way. Finally the process is hastened and the labor of handling the hides lessened by fixing over the bath a roller orwinch to which a slow motion is given by a steam engine. The hides are fastened together end to end, and then motion is given to the roller by means of the steam engine, no as to move the hides at the rate of four or five a minute. When the process of moving the hides and agitating the liqUor is employed, a stronger bath may be used, beginning at the first bath at 2° of the hydrometer, the hides being regularly moved in the daytime and remaining in the solution two or three days. They are then to be taken out and put into the bath No. 3, marking 7° of Rammer and 1½° of the alkane mixture and to remain in the bath from four to five days, being moved around as before, after which they are placed in the finishing bath with oak bark or valonia scattered between. An example has been made of the properties of the carbonate of soda to be employed in the different baths, but when the other alkalies or other alkaline carbonates are to be used, such as ammonia or carbonate of ammonia, they are used in the same proportions as carbonate of soda, but not marking the degrees given for carbonate of soda, as the density of the solution will vary with the different alkalies. The skill of the mechanic has done more to expedite the preparation of the leather than chemistry, but the great difficulty is, that in quickening the process the quality of the leather is not so good, no that when the best kind of leather is required the old slow method must be adopted.

Catechu will produce four or five times the quantity of leather that oak bark will. A considerable quantity of this tannin is used, but the quality of the leather from catechu is not equal to oak bark tanned leather. The process is much quicker and the tanner is able to save time by the use of catechu; nevertheless the action of this substance on the leather is not satisfactory, as the leather is soft and spongy and absorbs moisture.

Valonia is the fruit of a tree which is known by the name of "acorn cups;" it cornea from Italy, Turkey, and the East Indies. The leather tanned with valonia is not liable to absord moisture, and for this reason is preferred by many to oak bark, and presents the advantage of imparting to the leather a smooth, soft, and nice texture, which is thorougly impervious to water. Two pounds of this tannin will make one pound of leather.

Catechu is taken from a tree, acacia catechu, which grows mostly on the Malabar coast. The sap or bark of this tree is boiled, the solution evaporated, and the astringent matter is taken by this process. There is another kind of catechu brought from the East which is known by the name of gambir. This is collected on the shore of the Malacca; the wood, bark, and leaves are boiled in water, and when evaporated there is added sago to give it a body; it is then dried in the sun ready for use. Five thousand tins of this catechu, better known as gambir, are annually exported from Rhio by the Chinese. It yields forty per cent of tanning matter. This substance of catechu; or kassu, as the natives call it, has been introduced into Europe, but has not as tannin yielded satisfactory results. Sumac is used for the preparation of Spanish leather. It is said to harden the leather. It is quite expensive, its cost varying from $100 to $150 per tun, and is chiefly used by the glazed leather manufacturers. Devidivi is also used in tanning operations, but has the bad reputation that leather tanned by it is porous and consequently absorbs moisture.

Birch bark is used in Ireland for tanning bazils. It contains 7 per cent tanning matter. It is also used in France for making the fine red leather and other fine kinds known as Russian leather.

Hemlock is principally employed in tanning in this countsy, and such leather is porous and absorbs moisture. It is likewise stiff and hard, and presses on the feet.

Elm bark is very generally used in Norway for making leather, and it is said the fine Norway gloves are prepared from the elm bark, and that the softness and beauty of the leather are attributable to this bark. The white willow is used in Denmark for the manufacture of gloves. Russia also uses this bark in the manufacture of fancy leather, and the leather being impregnated with the oil of birch bark, which gives it a peculiar, agreeable smell. It is a noteworthy fact that the Norway tanners use birch and willow in preference to oak bark.

France uses the bark of a species of oak known as komes oak, a stunted shrub growing in the south of France. This species of oak is in clumps, and grows in Night to about three feet. The shrub which is called coppice oak has roots of a yellow brown hue, and is very rich in the tanning principle, and is used in France for tanning sole leather of first quality.

Vauguelin, by chemical analyses, found that kino contains 75 per cent of tanning property. Esanleck found that terrejaponica or gambir contains 40 per cent. White willow, according to Davy, contains 16 per cent; birch bark, 1.6 per cent; beech bark, 2 per cent; weeping willow, 16 per cent; sumac, 16 per cent; and sassafras root, 58 per cent of tanning matter.

For the removal of the hair and other extraneous matter from the skin, some of the French tanners use acids; others employ a bath of sour milk for the purpose. The acid ferment of milk and barley meal is acetic acid, and is found to be very efficacious for the separation of the hair and other substances from the skin. Sulphuric acid is.a good agent, but from its causticity is very likely to injure the leather if not used with great care. The process of sweating, which is adopted in the United States, is known to all experts in the trade, and it is needless to expatiate on it.

The process of oak tanning is of such general use and so familiar to the trade that it is of no interest, or there is no novelty in the process that is not familiar to all tanners.

The vapor of steam has been introduced for removing the hair, a method that finds great favor in France. The hides are hung up in a close room the floors of which are perforated with holes, through which steam is admitted. By this process the hair becomes soft and is easily scraped off with the hair knife, and a quick process is afforded, and one having the advantage that the hides cannot be injured by putrefaction, as with the ordinary tanning processes.