110 Chapter 5
those described in the raising or drenching processes used by tanners and dried
without further processing. The organic acids and enzymes helped to remove
the non-collagenousinterfibrillary materials and give a thinner, softer prod-
uct. These drenches did not tan the pelts but made them acidic enough to pre-
vent bacterial attack.
4 The Deterioration of Leather
Leather and other skin products, in common with all other materials, organic or
inorganic, are subject to change and decay. The rate of this change is dependent
on the product itself (raw material, type of tannage, etc.) and the environment
to which it is subjected. Deterioration may be caused by physical, biological
or chemical agents or a combination of these. This section will highlight some
of the major factors leading to decay in leather and, where they are known,
will discuss the mechanisms involved.
4.1 Physical Deterioration
Leather objects are likely to show evidence of the wear and tear that they
were subjected to before they entered the museum environment. This will
include splits, tears, scratches and holes, as well as damage resulting from the
effects of perspiration, urine and accidental spillage of other undesirable liquids.
This type of mechanical deterioration does not automatically cease when the
object is placed in a collection, as damage caused by inappropriate handling
is not unknown.
Leather and other skin products have the ability to absorb and desorb large
amounts of water vapour from the surrounding atmosphere without appear-
ing wet. This property is associated with the amount of hydrogen and water
bonding within the triple-helical collagen structure. As leather takes up
moisture from a humid atmosphere, it increases both its thickness and, par-
ticularly, its area. In some cases these can increase by over 7% if they are
moved from an environment with a relative humidity of 20% to one of 80%.
If a piece of leather has been placed under restraint, for example, by fixing it
to a wooden frame, and it is moved to a low humidity environment, it will
attempt to shrink. This will set up tensions that could result in splitting and
tearing.
A related phenomenon is termed age hardening. This, as the name suggests,
exhibits itself by a progressive stiffening and shrinkage of the leather. In more
severe cases the stresses caused by differential shrinkage of the outer and
inner layers of the skin result in cracking and, eventually, loss of the whole
grain layer. The main cause of this condition is thought to be excessively high,