Series editors' preface - Wood Tools

326 Conservation of Furniture
verditer or blue bice has been regarded as an
unstable pigment due to its conversion to
green, or its darkening in the presence of
hydrogen sulphide.
Lakes, which are pigments made from dyes
by precipitation, are particularly prone to light
fading. Commonly encountered lakes are the
bright reds produced from various scale
insects: including carmine from cochineal
(new world) and kermes (old world) and lac
lake from raw shellac resin. Alizarin red was
derived from madder root and indigo blue
from plants of the genus Indigotifera. Of the
naturally derived lakes, madder and indigo are
the most permanent, but modern synthetic
organic pigments are now available that far
surpass these in stability.
It should be emphasized that the behaviour
of pigments may differ significantly in different
media. Some illustrations of this have been
given above with respect to oil and water
colour media. Further differences can be
expected with other media such as shellac,
animal glue, casein, urushi and japanning.
8.8 Plastics and polymers
The use of polymers in furniture, including
upholstery materials and composites and
indeed as materials for conservation, continues
to grow. Recent collection condition audits
have shown that the problems of modern
materials are becoming increasingly serious
and demanding of urgent attention (Then and
Oakley, 1993; Griffith, 1997). From the point
of view of degradation, polymers will behave
similarly whether they are used as bulk materials
(plastics) or function as adhesives, paint
media or coatings. Obviously, however, the
life expectancy of a polymer material is influenced
by the nature of the application for
which it is used and the type of object in
which it is incorporated and is therefore not
precisely defined. The form of degradation
also depends on the material, its history,
including manufacturing process and the
environment to which it has been exposed.
There is considerable variation between
polymers. Polymers encompass such a chemically
diverse range of materials that it can be
said that as a category they may be degraded
by every known agent of deterioration. Most
polymers are far from inert and are prone to
a host of ills.
Physical causes may be responsible for
distortion or dimensional change, crazing or
cracking, surface deposits, which are often
tacky and changes in flexibility. Migration of
plasticizers in materials such as polyurethanes
(PU) and poly(vinyl chloride) (PVC) results in
these materials becoming rigid which in turn
may lead to distortion and cracking. Chemical
effects are more serious than physical effects
and are nearly always progressive and
irreversible. Degradation resulting from
reaction with oxygen is the most important
reaction leading to polymer failure. Evidence
for chemical effects includes: colour change,
chalkiness or surface bloom, crazing, embrittlement
with loss of strength and evolution of
degradation products (often acidic). Major
factors involved in bringing about chemical
change include light and UV radiation, heat,
stress, oxygen and ozone, contact with other
substances such as moisture and pollutants
and some forms of biological attack. Chemical
changes in polymers may also be produced by
ionizing radiations.
Almost all polymers age at a significant rate
unless steps are taken to stabilize them.
Degradation is a major factor limiting the application
of these otherwise remarkable and
versatile materials. Even the mildest of human
environments may be extremely aggressive to
synthetic materials and in some cases the rate
of deterioration of commercial formulations
may set surprisingly short limits to the
expected lifetime. Unacceptable aesthetic
changes frequently occur long before the
component becomes functionally unserviceable
due to loss of mechanical strength. Weathering
is the result of exposure of polymers to conditions
under which thermal oxidation and photo
oxidation may occur together with the effects
of water, abrasion and atmospheric pollution.
The extent of weathering varies with surface to
volume ratio and is sensitive to small variations
in conditions that may quite easily become
established even over small distances on the
surface of an object due to the aspect of the
object in relation to light exposure, dirt
accumulation or mechanical stress etc. The
chemistry of polymers is highly complex and
these materials contain more than a few
surprises in comparison to some of the more