Series editors' preface - Wood Tools

316 Conservation of Furniture
characteristics, which causes them to check or
crack as they dry. In bulk specimens or large
objects, the cracking pattern can be material
specific. Ivory, for example, cracks in both a
circumferential cone-within-a-cone pattern as
well as developing longitudinal splits in the
radial plane (Figure 8.1). In veneers both
types of cracking will show up as small parallel
cracks. Horn exhibits a fine parallel surface
cracking or, depending on its orientation, may
delaminate in fine layers much like the mineral
mica, or like one’s own fingernails. The most
common moisture-related conservation problems
involving ivory, horn and turtleshell
veneers are due to differential movement of
the wood, glue and covering material and
must be seen as systems rather than isolated
reactions.
Proteinaceous materials are attractive to a
variety of animal pests including rats and mice
and protein-loving insects such as cockroaches,
silver fish, dermestid beetle larvae
and clothes moths. In some cases, veneers of
ivory and horn are perforated by adult powder
post beetles or Anobium exiting from the
wood substrate.
These materials do not normally support
fungal activity unless they are very wet.
However, the application of oils will often
furnish a more attractive culture medium that
will support mould at lower RH levels. While
ivory has often been described as ‘oily’ this is
usually due to its having been oiled. The
natural lipid content of ivory is quite low,
certainly less than 2% and normally, much
less. A figure of 0.24–0.34% is given by Thorpe
and Whiteley (1946). However, people have
often oiled ivory (as they have done with
wood) in an attempt to prevent drying cracks.
Although no organic coating will prevent
drying and cracking, oils are particularly
undesirable because they remain soft and
entrap atmospheric particles and because they
are nourishing to fungi.
All of the animal-derived organics are fairly
tough when fresh and hydrated, but will
become progressively more brittle with age
due to degradation of the constituent proteins
and loss of water. Drying out is one form of
degradation but there appear to be many
others (Mills and White, 1987). Scission of
polymer chains can occur, usually due to
extreme low or high pH. Stability is also
affected by electromagnetic radiation,
especially in the UV range. Ivory yellows in
the dark but stays white in the light. All of
these materials are relatively soft and susceptible
to mechanical damage (e.g. scratching),
especially when aged. Horn and turtleshell
may also lose their translucency as a result of
internal degradation leading to microscopic
cracks.
Ivory objects should be wrapped in acidfree
tissue paper for storage. Cotton wool, soft
paper and similar materials should be avoided
as they are more hygroscopic and could hold
moisture that would cause warping of the
ivory. The hydroxy apatite mineral component
of ivory readily absorbs a variety of ions,
particularly metal ions, on to its surface that
may lead to discoloration and staining. Ivory
must therefore not be left in contact with iron,
copper, brass, or any coloured material.
Rubber containing materials should also not be
used as sulphur from the rubber can cause
ivories to become stained a disagreeable
yellow colour. Ivory should not be allowed to
get damp and whenever possible, air circulation
should be encouraged to inhibit the
growth of mould on the surface of the ivory
or the packing material, as this can cause
pitting and staining of the surface. The results
of studies by Matienzo and Snow (1986)
indicate that solvents commonly used in
conservation treatments may affect the surface
composition and morphology of ivory.
8.2 Mollusc shell – mother-of-pearl
and related materials
Shell materials are prone to deterioration by
acid fumes. Volatile organic acids, such as
acetic acid and formic acid, given off by wood
(particularly oak) and other materials can react
with calcium carbonate to form hydrated
calcium acetate and formate salts. Acetic acid
comes from hydrolysis of acetylated hemicelluloses
in wood and formic acid can be
formed by the decomposition of formaldehyde
to formic acid and methanol (a thermodynamically
easy reaction). White powdery
water-soluble salt efflorescence appear on the
surfaces which are etched as a result. This
condition, called Byne’s disease, has been
observed on mollusc and egg shell collections