The use of hides and skins, tanned or untanned, as useful articles has been with us for thousands of years. The Oxford Dictionary refers to leather as "material made from the hide or skin of an animal by tanning". Tanning, in simple terms, refers to the treatment of raw hides and skins with tanning substances to render the material immune to bacterial attack, ie to produce leather. Additional changes introduced in the process of tanning are secondary and are related to the tanning and retanning chemicals used.
There are hundreds of different leather types and tens of thousands of different chemicals to choose from when producing these leathers. The most important chemicals in the tanning process are the tanning agents as they define the process of leather manufacture as a whole. In this modern day and age, tanners will choose tanning chemicals based on price, convenience of use, environmental issues, and by matching the physical and aesthetic properties introduced by the tanning chemicals to the desired leather properties of the end product. A basic knowledge of the general processes involved in leather production, the tanner's true raw material ie collagen, the pretanning, tanning and retanning chemicals used in the production of leather, and the mechanistic interaction of tanning chemicals, are all factors which are important in order to appreciate just part of the intricate process of leather manufacture.
The Tanning Process
Hides and skins are primarily composed of water, protein and fatty materials. The most important protein in the production of leather is collagen, which makes up approximately 29% of the mass of a freshly flayed hide. The collagen desirable for tanning is found in the grain and reticular layers where it is "intimately woven" in a three-dimensional mesh that is think and tightly woven in the grain and coarser and stronger in the reticular layer.
After slaughter, hides and skins must be temporarily preserved for shipment and storage prior to tanning. The most common commercial method of preservation is to cure the hides and skins using salt to produce a dehydrated wet-salted or dry-salted material.
The majority of leather (almost 90%) is tanned with chromium and is therefore commonly used as the basic format when trying to understand leather production. Other methods of tanning will have significantly different production flow although if the material to be produced is without hair, wool or scales, the preparation and isolation of the collagen in the beamhouse processes will remain largely similar. The use of drums as process vessels but paddles, pits and mixers are also common. Leather manufacture is usually divided into three or four zones but this may vary slightly according to processing and the perspective from which it is viewed. Preparation of the Raw Pelt for Tanning
Tanning primarily involves the reaction of tanning chemicals with collagen, raw hides contain a number of undesirable components, which are commonly removed prior to tanning. Approximately 75% of the mass purchased as raw material is removed as polluting solid or liquid waste. Preparation of the raw pelt includes the processes from soaking to pickling. Soaking
Soaking is usually the first process encountered in the tannery, as the previously dehydrated raw material must be carefully rehydrated before it can be subjected to extended mechanical action. Water is also the vehicle for both chemical penetration and removal, and is a necessary prerequisite for most of the processes to follow. Soaking also assists in the removal of curing agents, non-structural proteins and fats. The flesh layer is removed mechanically to aid an even and full penetration of the chemicals to follow. Fleshing is commonly done after slaughter, after soaking, or after liming. Unhairing and Liming
The majority of leathers produced are treated in order to remove the hair or wool to leave the characteristic pattern of the grain surface...
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