118 Chapter 5
As a method of avoiding the damage caused by aqueous solutions, an
attempt was made to find an aluminium compound which was both soluble
in a non-polar solvent and had significant tanning powers. A wide range of
compounds was screened and one, an aluminium isopropoxyde derivative
chelated with ethyl acetoacetate called, for simplicity, aluminium alkoxide,
was found to have the optimum combination of properties. This is used as
follows.
A solution of aluminium alkoxide in white spirits is applied to the acid
damaged leather and the solvent is allowed to evaporate. The reaction takes
place over an extended period of time depending on temperature and humid-
ity and is thought to proceed thus: the organic complex does not react with the
leather and can therefore penetrate fully into the fibre structure; the solvent
then evaporates and water, in the form of moisture present in the leather, reacts
with the compound, changing it from an organic to inorganic co-ordination
complex. It is this complex which reacts with both the degraded vegetable
tannins and the partially denatured proteins, forming new chemical links
between the tannins and the collagen, in effect retanning the leather. This
is indicated by an increase in the shrinkage temperature. The inorganic
aluminium compound also reacts with free acid, buffering the leather at about
pH 4.5.
All conservation treatments should ideally be reversible. The use of alu-
minium alkoxide is most definitely not. It should be noted also that while
treatment with aluminium alkoxide improves chemical stability, there is no
improvement in physical characteristics and the use of suitable consolidation
treatments should be considered.
6 Conclusions
Leather can be defined as a material made from the outer skin of any ver-
tebrate (fish, reptile, bird, mammal, etc.) by any process which renders it
non-putrescible under warm, moist conditions. A true leather will retain this
fundamentalproperty even after repeated wetting and drying. These proper-
ties are imparted to the main constituent of the skin, the protein collagen, by
introducing additional microbiologically-stable cross-links into the polypep-
tide structure.
It will be seen that the successful treatment of any object requires the
conservator to have a wide range of skills. These include an understanding of
the scientific principles behind any treatment process, a knowledge of the
technology of the material from which the object was made, an appreciation
of the techniques employed in its manufacture and the craft skills to undertake
the conservation procedure. This applies just as much to the treatment of
leather artefacts as to those which are sometimes more conventionally valued.