Series editors' preface - Wood Tools

330 Conservation of Furniture
acid from degrading cellulose nitrate, or
hydrochloric acid from poly(vinyl chloride).
Both natural rubbers and synthetic elastomers
are attacked by ozone (O 3). The ozone content
of air is usually in the range 0.01–1.0 mg/kg
(1 part in 10 8 – 1:10 6 ). Even at such low ozone
levels as these rubbers require stabilization
against ozone attack to achieve acceptable
service lives. The O 3 molecule attacks CC
bonds (not a free radical reaction) and can
cause scission of the main chain. The effects
of solvents on polymer molecules are usually
physical rather than chemical. The primary
chains remain intact and the polymer can
usually be recovered chemically unchanged
though the microstructure (morphology) may
be very different from that of the original.
Biological damage to polymers
Synthetic polymers are not attractive to most
biological pests but may sustain damage due
to association with other target food stuffs
such as wood, glue or coatings. Oil, that universal
folk panacea for all observed problems,
will provide a nutritious medium for mould
growth. Mould has even been observed on
acrylic sheet, probably due to oil residues
present from handling. In some environmental
conditions polymers based on cellulose,
protein and latex may support bacterial and
fungal life resulting in loss of strength.
Prevention and care
In the conservation of polymeric materials
good handling and environmental control are
of primary importance. Many physical effects
can be controlled by maintaining stable conditions
of temperature and relative humidity, by
avoiding mechanical stress, by avoiding
contact with liquids and vapours that might be
absorbed by the polymer material. Low light
levels or dark storage, cool temperatures and
average (40–55%) relative humidity are useful
general guidelines. Conditions should be kept
constant to avoid the imposition of thermal
stress. Objects should be protected from dust
by Tyvek ® dust covers or other means. Storage
in low oxygen environments may be appropriate
for some materials. Work on the use of
low oxygen environments has been published
by Shashoua (Shashoua and Thomsen, 1993;
Shashoua and Ward, 1993) and Gilberg and
Grattan (1994). For materials such as PVC and
cellulose nitrate, which produce auto-catalytic
degradation products, good ventilation is
essential unless degradation can be otherwise
prevented.
The fact that objects made of plastic look
modern devalues them in the eyes of many.
There is a misplaced perception that they are
new, cheap and strong. A careful educational
campaign (even in museums) is required to
impress upon those handling these objects that
they are both important documents of technological
history and fragile. When objects of this
type are moved every effort should be made
to avoid touching the material, which may be
tacky or very brittle. Trays placed under pieces
of furniture in storage may be used to help lift
the object onto wheeled transport to display
or treatment. Display should be short term if
possible as the materials are highly light-sensitive.
Early detection of degradation and its
likely causes is most important but can only
be achieved if objects are examined regularly
Frequent condition checks are advocated so
that adequate treatment responses are made.
The first step in identifying a particular
cause of damage must be identification of the
polymer and investigation of the specific literature
to provide appropriate conditions for
display or storage. Polymer materials and their
degradation products can be identified using
various techniques (see Chapter 4). Among
these FTIR and various forms of GC MS are
the most useful (Learner, 1995; Shearer and
Doyal, 1991). FTIR is suitable for identification
of base polymers but not suitable for identification
of additives and plasticizers.
Ultimately the best means of control of some
of the problems of plastic materials outlined
above is to develop more durable materials.
However, a serious current problem is the
prediction of the ageing and weathering
properties of a polymer over a long period of
time. Accelerated tests are potentially an
invaluable way to get this information while
there is still time to act on it, but there are
several problems with such tests that complicate
their interpretation. Several agents of
deterioration acting together may produce an
overall effect that is different from the sum of
their individual effects. Higher temperatures
and higher light levels may lead to fundamentally
different kinds of reactions taking place
than would normally occur. Rates of reactions