Leather 117
and hydrolytic degradation of vegetable-tanned skins, leading to what is
known as red rot. This form of decay is associated with the presence of strong
acids, particularly sulfuric acid, in the leather. It has been found that a
vegetable-tanned leather with a pH of over 3.2 is unlikely to be suffering
actively from this form of deterioration. Those with a pH of 2.8 or less are
either exhibiting the symptoms of acid deterioration or will probably develop
them in the near future. This finding led to a number of modifications being
incorporated into the manufacturing process employed to make furniture,
bookbinding and other leathers which might be expected to have an extended
life. These were aimed at neutralising any acids present or making the leather
resistant to acidic atmospheric pollutants. The use of strong alkalis had to be
avoided as increasing the pH of a vegetable-tanned leather above 5.5 can
cause darkening, detannage and embrittlement. Salts such as lactates, citrates,
phthalates and tartrates, which buffer leather between 4.0 and 5.0, were there-
fore employed.
A number of techniques using similar concepts were developed over the
years to treat leathers that had already deteriorated. Aqueous solutions of
potassium lactate were routinely applied to the surface of bookbindings and
other leather objects. Unfortunately, in many cases, the deleterious effects of
the liquid water solvent on acidic leathers took place before the buffer salt neu-
tralised the acid, resulting in dark, seriously embrittled leather. In order to
avoid the effects of aqueous solutions, methods were tried employing ammo-
nia vapour. The object was placed in an enclosed chamber over an open dish
of ammonium hydroxide solution. However, ammonia vapour is a strong
alkali and there is a danger that even with the use of dilute solutions, leathers
that were too acidic would be transformed into ones which were too alkaline.
Another alternative was to use an organic base such as imidazole in a non-
aqueous solvent. However, this was also liable to result in leathers that were
too alkaline.
It has been shown that by retanning vegetable-tanned skins with aluminium
salts, leathers could be produced which resisted the effects of acidic atmos-
pheric pollution. It was thought, therefore, that treating acid-deteriorated
leathers with solutions of aluminium compounds such as sulfate, chloride or
formate could have a beneficial effect. Trials using artificially-aged leathers
showed that the acid was neutralised and the pH was raised. Moreover, the
shrinkage temperature of the damaged leather was increased showing that the
aluminium salts were reacting with the partially-degraded collagen, introdu-
cing new chemical stabilising bonds. However, when the system was applied
to naturally damaged leather, it was found that, as with the use of aqueous
buffer solutions, in a significant proportion of cases the damaging effects of
water occurred before the beneficial protective reactions of the aluminium
compounds could take place.