Leather 95
it shrinks dramatically to about one-third of its original area. The temperature
at which this change occurs is called the shrinkage temperature, and the phe-
nomenon has been likened to melting. It is, though, fundamentally different
in that it is irreversible. The shrinkage temperature of any given samplewill
depend on a large number of factors. These include the species of animal
from which the skin was obtained, what pre-tanning and tanning processes it
had undergone, the moisture content of the sample and the exact procedures
employed for the measurement.
The amount by which a process increases the shrinkage temperature of a skin
has been considered as a measure of its leathering ability. Standard methods
have been drawn up for the determination of shrinkage temperature and, if
they are adhered to, duplicate results within 1° or 2°C can be obtained. Using
standard methods, the following shrinkage temperatures are exhibited by typical
commercial products:
Raw mammalian skin 58–64°C
Limed, unhaired cattle hide 53–57°C
Parchment 55–60°C
Oil-tanned chamois leather 53–56°C
Alum-tawed skins 55–60°C
Formaldehyde-tanned leather 65–70°C
Aluminium-tanned skins 70–80°C
Vegetable-tanned leather (hydrolysable) 75–80°C
Vegetable-tanned leather (condensed) 80–85°C
Chrome-tanned leather 100–120°CThe majority of these results confirm that while true leathers have higher
shrinkage temperatures than raw or pre-tanned skins, materials such as parch-
ment or alum-tawed pelts do not. There is, though, a major exception. Oil-
tanned leathers have all the characteristics of true leathers and retain these
even after repeated wetting and drying. Their shrinkage temperatures are not,
however, increased during the tanning procedures. Oil-tanned leather also
exhibits another significant difference in its hydrothermal properties. When
other leathers or raw skins shrink in hot water they are converted into a rubbery
material, which dries to a hard, brittle product. When oil-tanned skins shrink
they retain their leathery feel to a large extent and dry to give a relatively soft
and flexible product. Furthermore, if a wet, oil-tanned leather is heated above
its shrinkage temperature and then immediately immersed in cold water, it
can be stretched back to almost its original size.
The exceptions to the various criteria put forward to define what is, and
what is not, a true leather indicate that there is no simple explanation for the
tanning effect. It will therefore be necessary to consider the make-up of the