Leather 93
nature of the skin chosen and on the processes employed. It has been the skill
of the tanner throughout the ages to make a product with just the combination
of properties required by the end user.
An examination through the thickness of a skin reveals that it consists pri-
marily of long fibres and fibre bundles interwoven in three dimensions within
a jelly-like ground substance. Other features are present, which are vital for
the functioning of the skin in life but which are generally removed during pro-
cessing.These include a keratinous epidermal layer in which hairs and hair
root structures are embedded, and muscles, blood vessels and fat cells. It is, how-
ever, the intricate three-dimensional woven fibrous structure which predom-
inates, and it is this that imparts many of the unique physical properties
characteristic of the wide variety of materials that can be made from skin. These
include a relatively high tensile strength with particular resistance to shock
loads; low bulk density; flexibility where required; resistance to abrasion,
tearing and puncturing; good heat insulation and an ability to be stretched and
compressed without distorting the surface.
2.1 Criteria which Define Tannage
The above physical characteristics are common to most skin-based products.
Linguistic studies show, however, that even from early times, materials such as
rawhide, oil-tanned pelts, parchment, alum-tawed skins and vegetable-tanned
leathers were distinguished from each other. The question arose as to what
are true leathers and how they can be differentiated from other materials
made from skin without undergoing a tanning process. A number of criteria have
been suggested. These include resistance to microbiological decay, physical
properties such as flexibility and a “leathery handle” and elevated shrinkage
temperatures.
Resistance to Microbiological Attack. Under normal ambient temperatures,
a wet raw skin will decay rapidly due, primarily, to the action of bacterial pro-
teolytic enzymes. Leather, on the other hand, resists such microbiological
attack even if it remains wet. This fundamental difference has been used to
define tannage.
There are, however, a number of techniques employed in the leather-making
industry to inhibit bacterial action and prevent the degradation of freshly flayed
pelts. These include drying the skin, salt curing and acid pickling. These enable
raw materials to be transported from source to production unit, and to be held
temporarily in a safe condition until they are required for processing. This
resistanceto decay is, however, lost if the skins are wetted. In a similar manner,
alum-tawed pelts and parchment, both renowned for their longevity, degrade
rapidly if they become wet for any appreciable time.