Series editors' preface - Wood Tools

332 Conservation of Furniture
It is easy to overlook the most obvious and
mundane types of damage in favour of the
more esoteric chemical deterioration but
surface coating materials are also subject to
deterioration through wear and tear, physical
abrasion and biological causes. Chemical and
mechanical changes may be seen as two
aspects of deterioration that are closely
connected. Changes that occur through
exposure to light and oxygen, for example,
cause changes in the physical properties of
coatings (e.g. modulus of elasticity) that make
them more sensitive to abrasion, contusion
and other forms of mechanical damage. The
dirt and grime that are a part of use may also
contribute to the process of chemical degradation.
Most furniture items were intended to be
used and this has an impact on the condition
and the treatment of the object. Surface
coatings will naturally deteriorate through the
wear of regular use. The finish that has been
rubbed away from the arms of chairs is a
common example. This mechanical deterioration
of the surface coating may or may not be
deemed a desirable aspect of the object when
treatment choices are made. It is useful to
distinguish between damage and ‘fair wear
and tear’. The condition of a surface coating
and its substrate are closely interlinked. This
is emphasized as problems with the substrate
will inevitably transfer to the surface.
Expansion and contraction of wood in an
unequal manner, due to the anisotropic nature
of the material, causes surface coatings to
deform or even cleave from the surface.
Another obvious change at surfaces is the
accumulation of atmospheric dirt and grime.
Specks of dust once adhered to the surface
can become the focal point for condensation
of moisture thus becoming more tightly
adhered. The hygroscopic nature of dirt allows
it to serve as a reservoir for water furthering
its negative impact on coatings, particularly in
instances of high humidity. Dirt can also be
acidic, presenting free H+ ions which will
accelerate degradation. Nitrous oxides, ozone,
sulphur and other common gaseous pollutants
have long been known for their negative
impact on art objects in general.
There are many steps that can be taken to
prevent damage and deterioration and mostly
they are of the common sense variety.
However the complete elimination between
such problems is not easy. Deterioration of
coatings affects the data produced in the
course of characterization and analysis
(Bentsen et al., 1990; Birstein and Tul’Chinsky,
1981). The deterioration of coatings and
adhesives is discussed in two parts; the first
part discusses the interaction of the components
of commonly encountered systems of
surface decoration under the influence of
different environmental factors; the second
discusses the materials in more detail as
members of their chemical class.
8.9.1 The support
The nature and behaviour of supports can
profoundly affect the performance of coatings
and it is important to consider the support
structure before carrying out treatment.
Supports used for decorative surfaces on furniture
include leather, textile, glass and metal
but wood is the most common. Wood as a
physical surface for paint application is
discussed by Hoadley (1998). Differences in
the dimensional response of wooden supports
and of grounds applied to them can have
profound deleterious effects on the ground
and on decorative layers applied to it. As an
example, a gesso and gilt surface on a turned
chair leg may de-laminate due to the shrinkage
of the wood. The volume of the leg has
decreased while the surface area of the gesso
and gilt has not. The majority of the factors
discussed in Chapter 7 are relevant to the
interaction between wooden supports and
coatings applied to them. These include
natural and artificially induced defects in the
material, differential shrinkage, differences in
hygroscopic sensitivity and rate of response of
different structural components, the type of
construction employed, poor quality materials,
poor workmanship and biodeterioration.
Any distortion in the support will be
reflected in stress applied to the decorative
layers of ground and paint applied to it.
Differential responses to moisture content
changes commonly cause some distortion of
design and may cause splits. Compression set
in wooden supports leads to slight reduction
in width of wooden panels over time. Various
construction techniques were used to minimize
distortion or weaknesses in the substrate.