While the chemical properties of a leather are indicative of the mechanism of tanning, the mechanical and physical properties of a leather are the result not only of the method of tanning but also of changes induced in the collagen during its preparation for tanning and of the subsequent lubricating and mechanical working of the tanned fibres. It is possible to produce leathers of quite markedly different structure and properties from skin of the same animal type by varying the degree of opening up of the fibre network and the angle at which the fibres interweave.
Pre-tanning processes, used to remove the epidermal layer and non-collagen proteins, lead to opening up of spaces between and within fibre bundles in the network. Mechanical manipulations such as pummelling (or chewing) can break down large fibre bundles into smaller units with more freedom of movement that will be reflected in greater flexibility of the final product. The angle of the fibres changes with the extent of swelling of the collagen and according to whether skin is under tension or not. By choosing the angle during tanning and the tension during drying the tanner can to some extent control the properties of the leather produced. In sole leathers for shoes, for example, large fibre bundles and a high angle of weave give a high degree of resistance to abrasion. In a flexible belting leather the large amount of splitting up of the fibre bundles gives greater flexibility. The lower angle allows the direction of pull to be more closely aligned with the long axis of the fibres giving increased tensile strength and reduced extensibility.
Post-tanning processing may be carried out to improve durability, mechanical properties or appearance. Sole leathers may be hammered or harness leathers stuffed with grease (curried) to improve durability. Leather may be oiled and/or worked, for example by rubbing it over a blunt metal blade or by pounding with wooden mallets, to soften it. Goat skins are folded, hair side in, and rubbed back and forth over a curved board causing them to develop the typical pebbly or grainy appearance of morocco leather. The original shagreen was made by trampling seeds into the surface of moist leather and shaking them out when dry. Thick hides may be split into two or more thickness and surfaces may be dyed or painted, sized waxed, or buffed to finish. In the United States, under rulings of the Federal Trade Commission, a split must be so marked and cannot be called 'genuine leather' or 'genuine cowhide' (Tanners Council of America, 1983).
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