Criteria which Define Tannage

Resistance to Microbiological Attack. Under normal ambient temperatures, a wet raw skin will decay rapidly due, primarily, to the action of bacterial proteolytic enzymes. Leather, on the other hand, resists such microbiological attack even if it remains wet. This fundamental difference has been used to define tannage. 

Many indigenous peoples treat skins by impregnating them with fats. They are then allowed to dry under controlled conditions while being worked mechanically. This procedure both coats the individual fibres and fills the spaces between them with the fatty material. This renders the skins water-resistant and even if they are subjected to wet conditions, the fibres themselves remain too dry for bacterial action to take place. These materials therefore appear to satisfy the criteria of resistance to microbiological attack. Such products are found widely in ethnographic collections and have been called pseudo-leathers. 

These should not be confused with oil-tanned skins such as chamois wash-leathers or the buff leathers employed widely by seventeenth century armies to make protective jerkins. These are not impregnated with stable, water-repellent fats but treated with reactive, oxidisible oils, which undergo chemical reactions with the skin during processing to give a proteolytic enzyme-resistant product. 

It can be seen that methods can be employed to produce materials, which are apparently resistant to microbiological attack, but which are not truly leathers. 

Physical Properties. It is generally considered that if a raw skin is allowed to dry it will become hard, horny, translucent and relatively inflexible. If the hair has been removed first, these characteristics have been exploited to produce such diverse objects as rawhide mallet heads and dog chews. Leather on the other hand is expected to dry to give a soft, flexible, opaque product with a characteristic feel. This has been cited as evidence that tannage has occurred. 

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