A Manual of Shoemaking and Leather and Rubber Products. William H. Dooley
some processes of tanning, calfskins, goatskins, and cattle hides are also pickled the same as sheepskins; for other processes they are not pickled, but are thoroughly bated or delimed, washed, and cleansed. Heavy hides are sometimes split out of the lime; more frequently, however, they are not split until after they have been tanned.
To capitulate, the preparatory processes may be briefly described as follows:—
Soaking, which dissolves the salt, removes the dirt and makes the hides soft and comparatively clean.
Liming and unhairing, which swell the hides and dissolve the perishable animal portion, loosen the hair, and put the hides into proper condition for tanning. Hides tanned without liming, even if the hair is removed by some chemical, do not make pliable leather, but are stiff and hard.
Bating, which removes the lime from the hides.
Pickling, which helps in the tanning later, and keeps the hides and skins from spoiling if they are not tanned at once.
The lumps of fat and flesh that may be on the hides are removed by machinery or by placing the hide over a beam and scraping it with a knife. The hair, when it is loosened by the lime, is removed by a machine or by hand.
CHAPTER THREE
PROCESSES OF TANNING
The various processes of tanning may be roughly divided into two classes, vegetable chemical and mineral chemical. The first class is often spoken of in tanneries simply as the “vegetable” while the second is called “chemical” process. In the vegetable processes the tanning is accomplished by tannin, which is found in various barks and woods of trees and leaves of plants. In the so-called chemical processes the tanning is done with mineral salts and acids which produce an entirely different kind of leather from that procured by vegetable tanning.
There is also a method of tanning, or, more properly speaking, tawing, in which alum and salt are used. This process makes white leather that is used for many purposes; it is also colored and used in the manufacture of fine gloves. Leather is also made by tanning skins with oil. Chamois skins are made in this way.
The materials that are used to tan hides and skins act upon the hide fibers in such a way that the hides are rendered proof against decay and become pliable and strong. There are many vegetable tans; they are used for sole leather, upper leather, and colored leather for numerous purposes. The bark of hemlock trees is one of the principal tans. The woods and barks of oak, chestnut, and quebracho trees are often used. Palmetto roots yield a good tan. Large quantities of leather are treated with gambier and various other tanning materials that come from foreign countries. Sumac leaves, which are imported from Sicily, contain tannin that makes soft leather suitable for hat sweatbands, suspender trimmings, etc. Sumac is also obtained from the State of Virginia, but the foreign leaves contain more tannin and make better leather than the American.
To a large extent the so-called chemical processes have supplanted the vegetable processes, that is, old tan bark and sumac processes; but in some tanneries both methods are used on different kinds of skins.
In the old bark process the tan bark is ground coarse and is then treated in leaches with hot water until the tanning quality is drawn out. The liquor so obtained is used at various strengths as needed.
In the newer method the tan liquor is displaced by a solution of potassium bichromate, which produces its results with much less expenditure of time.
When the hides or skins are ready for the tanning process, they are put into a revolving drum, known as a “pinwheel,” or into a pit in which are revolving paddles, with a dilute solution of potassium dichromate or sodium dichromate, acidified with hydrochloric or sulphuric acid. If the pinwheel is employed, it is revolved for seven hours or longer; after which time the liquor is drawn off and replaced by an acidified solution of sodium thiosulphate or bisulphite, and then the revolution is continued several hours longer. If the pit is used, the skins are removed to another drum containing the second solution, and kept at rest or overturned for a like period.
In removing the skins from the pinwheel or vat, and in handling them after treatment with lime for the loosening of the hair, the hands and arms of the workmen are seriously injured, becoming raw if not protected by rubber gloves; even with gloves it is difficult to prevent injury, and in some establishments the workmen are relieved by the substitution of a single-bath process, in which the liquor is less harmful to the skin.
Tanning Process
Showing the vats, the unhairing and liming processes. Page 24.
The hides are then removed from the pits, washed and brushed, followed by slow drying in the air. When partly dried, they are placed in a pile and covered until heating is induced. They are then dampened and rolled with brass rollers to give the leather solidity. Sole leather is oiled but little. Weight is increased by adding glucose and salt.
Various rapid processes of tanning have been devised in which the hides are suspended in strong liquors or are tanned in large revolving drums. It is claimed that this hastens the process, but the product has been criticized as lacking substance or being brittle.
Chrome tannage has been chiefly developed in this country during the last twenty years and is now in general use. It consists in throwing an insoluble chromium hydroxide or oxide on the fibers of a skin which has been impregnated with a soluble chromium salt—potassium bichromate. Other salts like basic chromium chloride, chromium chromate, and chromic alum are also used. The hydrochloric or sulphuric acid acts by setting free chromic acid.
After several hours, the skin shows a uniform yellow when cut through its thickest part. It is then drained and the skin worked in a solution of sodium bisulphite and mineral acid (to free sulphur dioxide). The chromic acid is absorbed by the fiber and later reduced by sulphur dioxide.
In the making of chrome black leather each tanner has his own method. Contrary to the general belief, there are many different methods of chrome tannage. No two tanneries employ just the same process.
Tanners of chrome leather seek to produce leather suitable for the particular demands made upon it by the peculiarities or characteristics of the varying seasons. Summer shoes require a cool, light leather; at other times a heavier tannage is essential, with some call for a practically waterproof product.
All leathers, whether vegetable-or chrome-tanned, must be “fat liquored.” That is to say, a certain amount of fatty material must be put into the skin in order that it may be mellow, workable, and serviceable. This is very essential in producing calf leather. Fat liquors usually contain oil and soap, which have been boiled in water and made into a thin liquor. The leather is put into a drum with the hot fat liquor; the drum is set in motion, and as it revolves the leather tumbles about in the drum and absorbs the oil and soap from the water. It is the fat liquor that makes the leather soft and strong.
Leather used in shoes is divided into two classes: sole leather and upper leather.
Sole leather is a heavy, solid, stiff leather and may be bent without cracking. It is the foundation of the shoe, and therefore should be of the best material. The hides of bulls and oxen yield the best leather for this purpose.
The hide that is tanned for sole leather is soaked for several days in a weak solution (which is gradually made stronger) of oak or hemlock tan made from the bark. Oak-tanned hide is preferred and may be known by its light color. A chemical change takes place in the fiber of the hide. This is a high-grade tannage, and is distinguished principally by its fine fibers and close, compact texture.
Oak sole leather, by reason of its tough character, and its close, fibrous texture, resists water and will wear well down before cracking. It is