Leather 107
them to move over each other. If required, the leather was coloured, using
natural dyestuffs, often in conjunction with an alum mordant. It was only
after all these operations had been carried out that the leather was ready
to be sold.
Nineteenth Century. While the changes that took place in processing during
the 19th century were considered to be improvements on previous techniques
aimed at giving a less expensive, more uniform product, at the same time
some had deleterious effects which only became apparent decades after they
were introduced. This was particularly significant in the case of leathers for
bookbinding which, unlike those for other purposes, are expected to last a
lifetime. This is also relevant to leather objects which are stored and dis-
played in museums and other collections, long after the time when they
would have normally been expected to have been discarded. These poten-
tially damaging changes in tanning techniques will be considered.
The rapid increase in population and improvements in standards of living
which took place in the nineteenth century led to a rise in demand for leathers
of all types, resulting in shortages of both indigenous tanning materials and
hides. This encouraged the introduction of a wider range of imported tanning
materials, including many species not previously employed in Europe. We
now know that leathers prepared using condensed tannins decay more rapidly
than those manufactured using hydrolysable tannins. It is fortuitous therefore
that the new imported materials which gave the best quality light leathers,
were mostly of the hydrolysable type. It is quite possible, however, that con-
densed tanning materials were employed by some tanners, resulting in leathers
with poor ageing properties.
A potentially more damaging improvement was the introduction of synthetic
dyestuffs from the middle of the century onwards. Initially, these were developed
for colouring textiles, but soon they were being used on a wide range of
leathers. The replacement of the traditional natural materials with these new
products adversely affected the ageing properties of the leathers in two ways.
First, the natural dyestuffs previously employed were almost invariably applied
in conjunction with an alum mordant whose beneficial effects are now well
known. The new dyestuffs were either used alone or together with mordants,
such as potassium bichromate, iron sulfate or tin chloride which, unlike alum,
actually have a damaging effect. Second, it was found that in order to “fix”
the dyestuff to the leather and “clear” the dyebath it was necessary to increase
its acidity. Sulfuric acid was generally used with extremely deleterious results.
It is now known that sulfuric acid either derived from atmospheric pollution
or added during processing has been the major cause of the deterioration of
bookbinding leathers and that the presence of strong acids, whatever the source,
is particularly damaging.