The aim when producing leather from skin is to preserve the fibrous structure from which the principal characteristics of strength and flexibility arise and to modify the proteins in the skin to avoid decay in damp conditions.
Early methods of preparation involved cleaning and scraping the skin to remove flesh and hair followed by sun drying, smoking or salting. By the eleventh century ad, three principal methods - oil chamoising, alum tawing and vegetable tanning - had evolved and remained in use little altered until the nineteenth century. The term tanning is now used generally to describe conversion of skin to leather rather than in the more limited original sense of treatment with vegetable materials rich in tannin.
There is no exact definition for tanning but modern criteria relate to shrinkage temperature, microbiological stability, chemical stability and retention in the dry state of the fibrous structure to produce an opaque flexible material.
If raw collagen is heated in water a point is reached at which it transforms from a regular coiled structure to a disordered random coil. This is accompanied by sudden shrinkage and the point at which this occurs is called the shrinkage temperature. For raw collagen this occurs at about 65 °C. Introducing materials into the skin that form chemical cross links in the protein structure alters the temperature at which this change takes place. Oil tans and alum tawed skins have shrinkage temperatures in the range 50-63 °C, vegetable tans 75-85 °C and modern chrome tans 95-105 °C (pre-treatment of skin with alkalis reduces shrinkage temperature).
Removal of excess bound water and of water soluble plasma proteins (such as albumins and globulins that surround collagen fibres) from the skin already gives leather increased resistance to micro-organisms when compared to untanned skin. Ultimately, however, the increased resistance of leather to biological deterioration in comparison to skin is a reflection of the tanned product to resist enzymatic attack. Digestibility of collagen diminishes as tanning proceeds, with chrome tans being more effective than vegetable tans and vegetable tans more effective than alum tawing. Alum tawing, however, does increase resistance to enzyme attack.
Once tanned, it is important that the leather should remain tanned to prevent deterioration in normal use. It is important for example that the tanning agent is not readily washed out on contact with water. Chrome and vegetable tanned leathers are very resistant to removal of the tan but alum tawed leather readily reverts to the untanned condition on washing.
Removal of excess water from the spaces between the fibres during tanning can lead to the collapse of the fibre network and the production of a glassy, hard, inflexible, translucent material rather than the opaque, soft, and flexible result that is leather. To prevent this, materials introduced during tanning, either the tanning agent itself (as with vegetable tans) or other material such emulsified oil (as with chrome tans) must be introduced before the leather is allowed to dry out. This renders the fibres hydrophobic and permanently non-adhesive thereby preserving the fibre structure.
Early methods of chamoising with oils and fats would have lead to separation and dehydration of the fibres, providing them with a water-resistant film and producing flexible and tough leathers though these would count as only partially tanned. Use of alum alone led to a stiff and imperfect leather which was improved by addition of salt to the tan but still lacked some important qualities such as water resistance and which would therefore qualify as a partial or pseudo tan. Alum tawing was often combined with treatments including egg, flour and oil to improve the result. Vegetable tanning, in which skins were immersed in successively stronger baths of vegetable matter (e.g. oak bark, oak galls, pomegranate rind or sumac) for periods of fifteen months or longer, produced a true tan.
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