Leather 111
or excessively low, moisture content in the leather and, in particular, cycling
between these two conditions.
The mechanism of deterioration appears to be that, during periods of high
humidity, the excess moisture present in the leather dissolves soluble mater-
ials. As the leather dries out, the moisture moves to the surface, transporting
these materials with it, leaving them as a deposit on the surface structure as it
evaporates. Over time, the removal of the tannins leads to a detanning of the
inner layers as is evidenced by a drop in shrinkage temperature. This causes
shrinkage and stiffening. The presence of excess material in the grain structure
reduces flexibility and increases the tendency to grain crack.
4.2 Biological Deterioration
By definition, true leathers are resistant to bacterial attack. If, however, rawhide,
parchment or pseudo-leathers become wet, putrefaction sets in and they will
decay rapidly due to the action of proteolytic enzymes secreted by the bacteria.
Even prolonged exposure in very damp atmospheres can render parchment
open to bacterial attack.
While leathers are resistant to bacteria, the same cannot be said about
fungi. More than 50 species of fungi have been reported to have been found
growing on leather, and much work has been carried out by the leather indus-
try to determine the factors favouring mould growth. It has been reported
that: leathers tanned with vegetable extracts are much more liable to mould
growth than chrome-tanned materials; vegetable-tanned leathers processed
with hydrolysable tannins are more susceptible to attack by fungi than those
tanned with condensed tannins; and leathers with a high fat content are more
open to fungal attack than those containing only small amounts of fatty
materials.
It has also been determined that the loss of strength, tendency to crack and
other signs of physical deterioration associated with mould growth are not
due to proteolytic attack. The fungal enzymes tend to digest fats, sugars and
other carbohydrates present in the leather, liberating various organic acids.
It is these acids that react with the vegetable tannins and break down the
tannin–protein complex.
Common names such as the hide beetle, the bookworm and the leather
beetle suggest that leather is particularly susceptible to attack by insects.
This, however, is not the case. Untanned collagen is a rich source of protein for
a large number of insects but, once it is tanned, collagen is not broken down
by most of the proteolytic enzymes secreted in the insects’ digestive systems.
Insect damage to leather objects is, therefore, usually coincidental as the insect
passes through the leather structure or along the surface in search of some-
thing more nourishing. The bookworm or booklice, for instance, feed on