Just as the demand for leather outstripped the supply of vegetable-tanning
materials, there was an increasingly serious shortage of skins for the tanners
to process. This problem was overcome by the importation of crust leathers
which had been roughly tanned in their country of origin either using local,
traditional methods or local variations of “European” systems. Indigenous tan-
ning materials were employed, many of them of the condensed type. These
skins were usually tanned locally on a very small scale and gathered together
from over a wide area to a centralised trading depot. Here they were sorted,
mainly into size and quality, and exported. Once in Europe, they were often
sorted again before selling on to a tanner or leather dresser. Crust skins were
sold by weight and often contained adulterants, such as excess tannins, oils,
earthy materials and soluble salts. In addition, the dresser was purchasing a
very mixed lot of skins from a wide range of sources having very different
processing histories. It was his job to minimise these differences and produce
as uniform a batch of leather as possible.
In order to achieve this, he first washed the skins thoroughly to remove as
much of the unwanted weighting material as possible. He often added alkalis to
the wash water in what is known as the stripping process. This was designed
to remove excess tannins and to unify the dyeing properties of the mixed
batch of leather but, unfortunately, it also removed non-tans. These include
various organic salts which are present in the leather but do not actually con-
tribute to the tanning reaction. They are, however, effective buffers and have
a protective action against the action of acids.
Next, the skins were shaved to the required thickness. By the last quarter of
the nineteenth century various shaving machines had been developed which
replaced the laborious, skilled, hand operations. With these machines it was
possible to cut skins down to the required thickness, cheaply and accurately.
One feature of these machines, though, was that in order to ensure a clean cut,
their blades had to be sharpened continuously. This resulted in a shower of
sparks and small specks of iron dropping onto the leather. These caused a pattern
of blue-black iron stains, which had to be removed in the clearing process. In
this, the skin was immersed in a solution of sulfuric acid, which not only
dissolved the iron salts but changed the colour of the leather from a reddish
brown to a pale yellowish buff. Once again, dangerous sulfuric acid was
introduced into the leather. In order to produce a more uniform substrate for
the dyeing process, the leathers were then retanned, possibly using tanning
materials of the condensed type. They were then dyed, often with synthetic
colours, acidified, again with sulfuric acid, dried out and finished.
By the last decade of the century, the effects of these changes had become
only too apparent, and efforts were made to determine the causes of the prob-
lems and eliminate them. Conservators are, however, still working today on
108 Chapter 5