Tan′ning.
Tanning is an operation which combines with the substance of the skin any other compound which has the property of rendering it imputrescible and elastic.
The agent most generally employed is a soluble vegetable extract termed
tannin, which forms insoluble compounds with the albumen, gluten, gelatine, and other components of the skin.
Tannin is yielded by the bark of oak, willow, and many other trees (see list, page 2493). The bark of oak is by far the most usual source of tannin.
Catechu, valonia, and many other inspissated vegetable extracts are also used.
Another class of agents which fortify the fibrous portions of skins against the joint attack of warmth, air, and moisture are minerals which seem to act as preservative salts on the gelatinofibrous structure of the skin.
Such are alum and salt, and copperas.
See tawing.
It is difficult to determine the origin of this art, and it is somewhat confused by translating words referring to hides as if they meant
tanned hides, that is,
leather.
Skins and raw hides were first used and were afterward softened by means probably substantially similar to those adopted by the
North American Indians.
The art was reduced to a scientific basis by
Sir H. Davy.
The pounding of skins, and sewing them up and inflating them, or filling them with
tanning or
tawing liquor, is shown in the ancient paintings of Kourna,
Thebes.
Simeon of
Joppa was a tanner, and dwelt by the seaside.
Elijah and
John Baptist wore leather girdles, perhaps
rawe hide.
Strabo refers to the dresses of the Massaytan islanders as being of bark, owing to their having no cattle, and also to the skins used by other people.
Pliny's statement of the materials used in treating leather shows that both the
tanning and
tawing operations were practiced.
Alum, sulphates of iron and copper, gall-nuts, bark of pomegranate, lotus, wild-vine roots, leaves of sumac, erythron, Rhus coriaria, and many inspissated juices.
The
Saracens used alum.
Du Cange mentions bark-mills and ground bark.
The art of tanning, though practiced immemorially in
Europe, undoubtedly originated in the
East, which, until very recent times, had almost a monopoly of the finer kinds of leather.
In 1730, a man was sent from
France to the
Levant to learn the process of morocco manufacture, and in 1749 the first
European morocco manufactory was established at St. Hippolyte, in Alsace; the art was not fairly developed in
France before 1797.
This manufacture was subsequently introduced into
England and
Germany.
In 1761,
McBride of
Dublin, and, in 1770,
Johnson, introduced the use of dilute sulphuric acid for swelling the hides.
Sumac was used in the first half of the eighteenth century,
divi-divi, from Caraccas in 1768.
Catechu at a much later period.
Steam-heating vats seem to have originated in
America, but formed the subject of a French patent of 1822.
The quick process was proposed by
McBride in 1759, but he extracted the tanning material with lime-water.
It was not until 1793-95 that the active principle requisite to the success of the process — tannic acid — was recognized by Deyeux and
Seguin of
Paris.
It was rendered practical by
Fay in
England, 1790, and
Seguin in
France, 1795, and improved by
Desmond, Brewin, Cant, and
Miller.
In 1839 the use of lime from gas-purifying works, previously suggested by
Professor Boettger of
Frankfort, was introduced into
Berlin.
Half-dried sole-leather was formerly rendered compact and, to some extent, flexible, by being beaten by hand with hammers.
In
Switzerland, as early as 1800, water-power hammers, and, subsequently, stamps were employed.
In 1842, Berendorf
[
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of
Paris invented pressing-stamps, which were supplemented by
Harvey and Debergue with a roller, which effected the same purpose by its being rolled back and forth over the leather.
Fresh-slaughter hides are washed and scraped on the flesh side; salted hides are sometimes scraped, but dry hides do not require this treatment.
Each of the latter kinds is soaked in water 10 or 14 days, and occasionally rubbed or beaten to supple them.
The skins are then placed in pits containing lime-water of 3 or 4 different degrees of strength; they are gradually transferred from the weaker to the stronger solutions, until, in the course of two or three weeks, the lime has dissolved the hairsheath and, by combining with the fat, formed an insoluble soap.
They are
handled, that is, taken from the pits, and allowed to drain for an hour or two each day. When the hair is readily separable, they are removed from the pit and scraped upon the
beam, a stand having a rounded upper surface, with the
unhairing-knife (this is a curved, two-handled scraper, fitting the convexity of the beam); the hair comes off easily, its removal leaving a
grain. Flesh and fat remaining on the other side are cut off with the
fleshing-knife, which has a sharp convex edge; this process is termed
fleshing. The hides are then washed in water, scraped, to get rid of adhering lime, the ears and projecting parts cut off, when they are ready for the tanpit.
The use of lime is objectionable, as it dissolves portions of the skin which would make good leather, makes the surface unequal, and interferes with the action of the tan. Various plans have been tried to avoid its use, among others smoking, causing an incipient fermentation which loosens the hair; piling the hides together and covering them with spent tan or litter, and allowing partial putrefaction to take place; and exposing them to air, kept constantly damp by the spray of water.
In some cases weak acids are used, as very dilute sulphuric acid, sour milk, pyroligneous acid, fermented barley, rye-water, and bran; the two latter are sometimes used after liming.
The effect of weak acids is to swell the pores, enabling the tanning liquor to penetrate them more readily.
This process is termed
raising: the liquid commonly employed consists of 1 part sulphuric acid to 1,000 parts water.
A moderate heat is applied, and the process is completed in 24 hours.
A new English process to open the pores and render the tanning by bark more expeditious, —
1. Remove hair and particles of flesh.
2. Cleanse from the action of lime.
3. Place in a vat, flesh sides up, and cover each hide with bran, in quantity varying from 6 to 14 oz. to each hide, according to size, and cover with water.
4. Ferment. This will take two days or more, according to the weather.
5. Remove and scrape.
6. Steep in a vat, with 5 pounds ground
Italian mustard and 5 pounds barley-meal to each 100 weight of hides.
Here they remain from 24 to 48 hours, according to size.
7. Hang up to partially dry, and then proceed with the process of tanning by bark.
The tan-yard contains a number of wooden-lined vats, whose tops are level with the surface of the ground.
Into these the hides and the ground bark, or ooze previously extracted therefrom, are put. In the old method the alternate layers of hides and of bark were placed in the pits, which were then filled up with water.
When the strength of this appeared exhausted, the vat was emptied and supplied with fresh bark and water; this was repeated many times, the process occupying as much as 15 months. It, however, produced superior leather.
It is now customary to prepare the oozes and conduct them to the vats by pipes.
The process consists in passing water through a stratum of the ground bark, until all its soluble matters are removed.
Usually cold, but sometimes hot or tepid water is employed.
Steam is conducted by a pipe beneath a pit containing the tanning material and water, and provided with a perforated false bottom, through which the extract percolates and is drawn off. In another method nearly spent bark is digested in water, at a moderate heat, and the weak ooze is transferred to a pit containing bark which is less spent, and so on until it is finally pumped into a pit filled with fresh bark.
It is common to introduce the skins into nearly spent ooze and transfer them successively to those which are stronger.
Those in which the tanning is effected are called
handler-liquor; stronger oozes, used for giving the
bloom on the surface, are termed
layer-liquor.
The skins are usually placed in horizontal layers, but are sometimes suspended vertically.
In the process of
handling, the hides are taken out with blunt-pointed, long-handled hooks, placed one over another, on a sloping rack over an adjacent pit, and permitted to drain for one or two hours.
When ooze instead of bark is employed, the hides are handled frequently.
By one plan, the skins are handled twice a day in the first liquor; once in two days in the second liquor; and once a month in the third liquor.
To save the trouble of handling by hooks in and out of the pit, various devices have been introduced.
Several are illustrated under tanning-apparatus.
1. (
Keasley, 1845.) The hides are suspended from the bars of a frame which is periodically lowered into the liquor and raised therefrom.
2. The hides are temporarily tacked together, so as to form a chain, which is passed down into the vat and then up over a roller, by which they are kept in continuous motion.
3. (
Cagswell.) The hides are arranged in a vat, laid flatly, except that the edges are a little raised to give the hide a dish form.
The hides are packed in sawdust, and the ooze admitted above is allowed to percolate through the hides and sawdust, passing off at the bottom.
4. (
Spilsbury, 1831.) Each hide is clamped in a rectangular frame, its edges being so nipped as to make it water-tight.
Two frames are so clamped together as to make a thin watertight box into which the ooze is admitted, to percolate through the hides, by hydrostatic pressure.
5. (
Drake. ) The hide is sewed up to form a bag, into which the ooze is admitted from an elevated cistern.
The bags may be suspended in a vat of ooze.
6. (
Knowles and Duesbury.) Hides are placed in a vessel from which the air is withdrawn by an air-pump.
The ooze is then admitted under hydrostatic pressure and forced into the pores.
The process is repeated with ooze of constantly increasing strength.
7. (
Herapath, 1837.) Hides are sewed together into an endless band, and are passed between rollers, as they are removed from one pit to another.
The object is to press out all of the ooze and open the pores for the reception of the ooze of the next pit.
8. (
Squire.) Hides are placed in a horizontal cylinder, four fifths full of hides and hot ooze.
The cylinder rotates at the rate of 6 or 8 revolutions per minute, and has interior ledges to tumble the hides as it rotates.
The ooze is renewed from time to time, but fresh bodies of atmospheric air are excluded.
9. (
Nossiter, 1844,
English.) Each hide is contained in a frame which occupies a horizontal position in the pit of ooze.
The frames are laid in, one over another, and circulation for the ooze is permitted all around them.
The object is to keep them from contact, and the time is said to be shortened one half.
10. (
Berenger and Sterlingue, French.) A series of 8 vats are made to communicate, so that the space between the false bottom and the bottom of No. 1 discharges, by a pipe, into the top of No. 2, No. 2 in the same manner to No. 3 and so on throughout the series, the contents of No. 8 being pumped into No. 1.
No. 1 is first charged with bark and hides and filled up with water.
After a period, say from 15 to 21 days, No. 2 is similarly filled, and a quantity of strong ooze is introduced into No. 1, which displaces its former liquid contents and causes them to overflow into No. 2.
After a similar interval of time (15 to 21 days), the pit No. 3 is charged with bark and hides, and strong ooze being poured into No. 1, its liquid contents are driven into No. 2, which overflow into No. 3.
Again there is an interval, when No. 4 is charged, and so on until No. 8 has been charged and has laid its allotted time under the first liquid.
No. 1 is by this time ready to be drawn, and is recharged with fresh bark and hides, the filling of No. 2 with ooze acting throughout the series of pits, the contents of No. 8 being pumped into No. 1, which has now become the last of the series.
11. (
Turnbull, 1845.) The hides are tanned by endosmotic and exosmotic action, being sewed into the form of bags, filled with weak ooze and suspended in vats of strong ooze.
The inequality of density causes a circulation of both liquids through the tissues.
The weakness of one and the strength of the other solution must be maintained to preserve energy of action.
12. (
Schynder.) The hide is indented by means of an instrument having 200 to 300 needles to the square inch.
It is then exposed to the action of tan-liquor, which is led by the perforations to the interior portions of the hide.
13. The method employed by the Baskirs and Kirguises of
Asia is a
substitute for tanning, but may be here mentioned.
The hair is detached by a knife.
The skins are suspended in a pit, from parallel cords.
Smoke is introduced into this pit by a tunnel from another pit, in which a fire is made.
The fire is a smudge of dry and decayed wood, and both pits are covered in so far as is consistent with a draft sufficient to keep the smoke in effective action.
Two or three weeks are required to put the hides into a tough and lasting condition, impermeable to water.
14. The
Pampas Indians stretch the hides, so that they shall not shrivel while drying in the sun's rays.
The brains are also dried.
At the end of the hunting season the hides are steeped, the hair shaved off: the wet hides and powdered brains being placed in an earthen pot and heated to
about 95°. The villains have no thermometers.
The cerebral matter is converted into a kind of soap, and forms a lather which renders the skin pliable.
The skins are then stretched by thongs, and rubbed frequently during the process of exsiccation.
The
Esquimaux use in tanning the urine of man and beast.
The skins are prepared in the fur, and softened and tanned in urine, which is usually kept in tubs in the porches of their huts for use in dressing deer, seal, and other skins, in the preparation of which they show great skill.
The boots worn by the Esquimaux are generally made from seal or walrus hides, and resist water.
A number of distinguished chemists and practical tanners have attempted, and, to some degree, succeeded, in tanning leather by the application of minerals, obviating the use of the vegetable extract,
tannin.
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Of these may be cited the methods of Bordier and Cavalin, and the alum process.
1.
Bordier's Process. The hides, being
unhaired and
bated, are steeped in a solution prepared as follows: —
224 pounds of bruised copperas are dissolved in 15 gallons of boiling water, in a copper kettle.
This being transferred to a vat of 44 gallons capacity, 44 pounds of sulphuric acid (sp.
gr. 1,848) are added, and to this gradually added 44 pounds of black oxide of manganese, in powder, the solution being constantly stirred.
The mixture is thinned with water and the hides steeped therein.
The effect is to impregnate them with an insoluble sub-sulphate of
peroxide of iron, and render the animal fiber imputrescible.
From 3 to 8 days are required for the process.
2.
Cavalin's process is to impregnate the cleaned and unhaired hides with a solution of
| Bichromate of potassa | 10 pounds. |
| Alum | 20 pounds. |
| Water | 180 pounds. |
They are immersed 4 days, being handled and rubbed every day.
They are next steeped in a solution of
| Protosulphate of iron | 1 pound. |
| Water | 1 gallon. |
The hides are not allowed to touch in the vat, but are taken out every 12 hours and drained, the process being repeated till the leather is formed.
The iron base is peroxidized in the hide by means of the chromic acid, which is itself reduced to the state of sesquioxide, and remains with the iron and a portion of the alumina base, firmly united with the tissue.
 |
|
Oscillating tan-vat. |
3. The alum process consists in applying to the skins a saturated solution of alum and salt, followed by dressings of flour, yolk of eggs, oil, etc. See tawing.
 |
|
Plunging-vat. |
For tanning
in vacuo, see patents: —
| No. | Name. | Date. | No. | Name. | Date. |
| 23,360. | Fergusson. | Mar. 29, 1859 | 60,524. | Johnston. | Dec. 18, 1866 |
| 29,656. | Aldrich | Aug 21, 1860 | 75,391 | Doty | Mar 10, 1868 |
| 48,361. | Brewer et al. | June 27, 1865 | 84,190. | Hosmer | Nov. 17, 1868 |
 |
|
Rotary-movement tan-vat. |
Symonds' process for utilizing the useful matters which are not withdrawn from the bark by steeping, consists in burning the spent bark and conducting the products of combustion into a trough filled with water, where the solid and soluble portions are retained.
