94 Chapter 5
Many indigenous peoples treat skins by impregnating them with fats. They are
then allowed to dry under controlled conditions while being worked mechan-
ically. 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 pro-
duce 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.
It is true that if the skin is simply dried in an uncontrolled manner the result
is likely to be as described. If, however, it is impregnated with fats as in the
production of pseudo-leather the product will have all the physical character-
istics of a properly tanned skin.
While parchment and vellum are also untanned skin products, they have phys-
ical properties very different to rawhide. These are made by stretching unhaired
skins under tension and drying them carefully. As the moisture is removed, they
shrink to form an opaque, white, relatively flexible product, which has been
employed for millennia as a stable writing or bookbinding material.
Conversely, when the technique of chrome tanning was being developed in the
late nineteenth century, it was found that the leather produced would dry out as a
hard, stiff, inflexible material. It was only by the addition of fatty lubricating
products in the form of an emulsion, in what became known as the fat liquoring
process, and by mechanically working the skin that a useable product could be
made. In a similar way, the production of alum-tawed skins involved the use of
such fatty materials as egg yolk or olive oil and mechanical softening procedures.
It can be seen then that the development of a leathery feel cannot be used
on its own as evidence that tannage has taken place.
Shrinkage Temperature. If a piece of skin, tanned or untanned, is wettedthor-
oughly, placed in water and heated slowly it will reach a temperature at which