tanning

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tanning,

process by which skins and hides are converted into leatherleather,
skin or hide of animals, cured by tanning to prevent decay and to impart flexibility and toughness. Prehistoric and primitive peoples preserved pelts with grease and smoke and used them chiefly for shoes, garments, coverings, tents, and containers.
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. Vegetable tanning, a method requiring more than a month even with modern machinery and tanning liquors, employs tannin; its use is shown in Egyptian tomb paintings dating from 3000 B.C. Mineral tanning includes tawing, or alum tanning, another ancient method, and chrome tanning, the process most common today, based on the use of chrome salts and requiring only a few hours. Known as early as 1856, chrome tanning was first patented in the United States by Augustus Schultz in 1884. In oil tanning, or chamoising, the pelts are treated with fats and hung to dry; the leather is commonly napped on both sides and is very absorbent. The most recently developed tanning process employs artificial agents (syntans). Most heavy leathers, such as sole leather, are vegetable tanned; many light leathers are chrome tanned. The Native Americans of North America used the chamois method, employing the fat, livers, and brains of animals. Their tanned white buckskin was highly esteemed, especially for clothing, both by Native Americans and by colonial pioneers. In the tanyards of European settlers tanners used oak and hemlock bark; gallnuts; the wood, nuts, and leaves of the chestnut tree; and the leaves of sumac.

Tanning

 

one of the main processes in the production of leather and furs. In tanning, chemical bonds are formed between the molecules of the tannin and the albumin (collagen) of the dermis (in leather production) and of the hair (in fur production), as a result of which irreversible changes occur in the properties of the dermis or hair. Tanning promotes an increase in the fusion temperature of the semifinished product, a decrease in shrinkage during drying, increased porosity after drying, resistance to tearing after swelling (when wet), and resistance to the action of fermentation and various hydrolyzing agents. It also reduces swelling in water and improves the quality of the hairs (resiliency and wettability).

The evenness and rate of penetration of the semifinished products by the tanning agents depend on a number of factors: the type of raw material, the method of preparation of the rawhide (the degree of separation of the fibrous structure and the degree of swelling, or the water content), the pH of the rawhide, the nature and means of preparing the tanning solution (the degree of dispersion of the tanning particles), the concentration of tannins, the temperature and pH of the tanning solution, the tanning time, and the nature of mechanical influences. The penetration of the semifinished product by tannins in most cases causes a change in its color (dyeing of the rawhide) and thickness. The relationship of the dyed part to the total thickness of the semifinished product, expressed in percent, is called the degree of tanning penetration.

The fixing of tannins by the semifinished product can vary in nature. The adsorption of tannins depends on their concentration on the surface of the structural elements of the albumin and on the temperature. Coagulation is caused by the mutual neutralization of charges and therefore depends on the nature of the tanning agent, its concentration, and its pH. The bonding of the tannins with the albumin through chemical interaction depends above all on the degree of separation of the structure of the albumin (its receptivity to particles of the tanning substance), the nature of the tannin, its concentration in the solution, the introduction of neutral salts, the pH, the temperature, and the type of solvent in which tanning is taking place. The basicity of the tanning solution and the presence of masking substances also have a major influence on the bonding of the mineral tannins.

Various technological plans are followed, depending on the type of tanning: tanning with tannides, mineral tannins, fat, or formaldehyde, and combination tanning. For tanning with tannins, widespread use is made of a drum or rotary screw apparatus. The rawhide is pretreated in concentrated solutions of tanning extracts in a drum and is usually chromed (combination tanning). To hasten tanning, tannin solutions are changed once or twice to higher concentrations (phase tanning). Tanning by this process takes three to four days.

In tanning with mineral tannins, a distinction is made between single-vat and double-vat processes and between various combinations of them. Single-vat tanning is done in a solution of mineral compounds (usually chromium compounds) that already have tanning properties. By this method, the tanning is done in a single stage, in which the entire tannin is introduced at the same time in a single solution, or in two stages—first in a solution using one-third to one-half of the total amount of tannin, and then, after a number of intermediate operations, in a solution of the rest of the tannin. Compounds of hexavalent chromium are used in double-vat tanning. The semifinished product is first treated in a solution of bichromate and a mineral acid. The bichromate acid thus formed, which has no tanning properties, colors the rawhide evenly. The semifinished product is then treated in a solution of hyposulfate to reduce the hexavalent chromium to trivalent chromium and thus to convert it into a tannin that bonds with the albumin, causing the tanning effect.

Formaldehyde tanning imparts specific properties to leather and furs: white color and resistance to alkalies, oxidizers, and the effects of perspiration. Formaldehyde tanning is seldom used independently. Fat-liquoring is used to produce natural suede.

Combination tanning is used to produce a wide assortment of leathers. It involves several tannins, such as compounds of chromium and tannides (chrome-tannide tanning), or a combination of natural and synthetic and artificial tanning agents, compounds of zirconium and aluminum, tanning amino resins, Formalin, and other tannins.

REFERENCES

Mikhailov, A. N. Khimiia dubiashchikh veshchestv iprotsessov dubleniia. Moscow, 1953.
Khimiia i tekhnologiia kozhi i mekha. Moscow, 1970.

L. P. GAIDAROV

tanning

[′tan·iŋ]
(engineering)
A process of preserving animal hides by chemical treatment (using vegetable tannins, metallic sulfates, and sulfurized phenol compounds, or syntans) to make them immune to bacterial attack, and subsequent treatment with fats and greases to make them pliable.