Series editors' preface - Wood Tools

566 Conservation of Furniture
The effect of synthetic materials on the
hygroscopicity of consolidated wood is
dependent on resin loading (Schniewind,
1998). The extent of such changes is proportional
to polymer loading, though Schniewind
(1990) noted no effect at 24% resin loading.
Solvent selection for thermoplastic resins The
choice of solvent used with a thermoplastic
resin can influence a range of physical properties.
Though largely a function of resin concentration,
penetration of consolidant may be
influenced by solvent polarity or the solvent
effect on solution viscosity. Although non-polar
solvents penetrate wood more than polar solvents,
Paraloid B72 and AYAT resin delivered in
polar solvents imparted greater strength to
treated wood (Wang and Schniewind, 1985).
Although solvents with a higher boiling point
are retained for longer, their slower evaporation
rate may allow deeper penetration of the resin
and prevent reverse migration of consolidant to
the surface. Reverse migration will result in a
concentration of solvent at, or close to, the surface
of the wood, which will in turn result in an
increase in bending strength. Brewer (1994)
suggested the use of consolidant reservoir under
Melinex/Mylar in order to facilitate penetration
of consolidant and address concerns of reverse
migration. The presence of a decorative surface
or varnish may complicate solvent selection.
12.2.4 Consolidation of painted and
decorated surfaces
All decorated (e.g. painted, japanned, gilded or
urushi) surfaces have a laminate structure that
is comprised of materials with different physical
and chemical properties. The laminate structure
may be very complex and inevitably has
inherent flaws and incompatibilities. These may
lead to adhesive failure between, or cohesive
failure within, any of the layers. In many cases
some sort of consolidation treatment may be
necessary if the decorative surface is to survive
in a coherent and ‘readable’ state. It is often
helpful to examine a surface that requires consolidation
with a binocular microscope before,
during and after treatment to assist in understanding
the object and the locus of failure,
monitor treatment and evaluate its success.
Damage to decorative surfaces is overwhelmingly
caused by three main agents of
deterioration. First, the materials used in the
original manufacture of the object and the techniques
used to create it can cause premature
deterioration of a decorative surface. This category
includes the deliberate or accidental use of
adulterated materials, the construction methods
used for the substrate such as cleated boards,
or short cuts taken by the manufacturer to
reduce the time or cost of production.
Secondly, unsuitable environmental conditions,
particularly high light levels and excessively
low, high or widely fluctuating relative humidity
(RH), will radically shorten the life of most
decorated surfaces. RH levels play an important
role in the adhesive failure between gesso-type
layers and wood substrates. Long exposure to
high humidity will result in a friable, crumbling
gesso layer, whilst long exposure to low
humidity may lead to gesso cleaving away from
the substrate. The third category includes wear
and tear, poor handling and unsuitable past
restorations. Unsuitable consolidation treatments
can be included in this last category,
either as a result of the use of inappropriate
materials or by stress between consolidated and
unconsolidated areas. Unsuitable consolidation
treatments may have been the result of inexpert
treatment, or the use of materials that have
since been proven unsuitable.
There is no one definitive answer to the
question of consolidation. Each surface or
object must be considered carefully, its problems
identified and addressed. In some cases
the priority may be a minimal change of
appearance (e.g. avoiding excessive saturation
of matte paint) whilst in others a strong (or
weak) adhesive bond may be the primary consideration.
In every case, however, compatibility
with existing materials, previous and
subsequent treatment stages (e.g. infilling,
retouching or the application of a coating) is
essential. The potential for future treatments
(retreatability) should be clear, and the
mechanism and future consequences of deterioration
understood as far as possible.
In some cases, for example where the surface
is extremely friable, it may be necessary to consolidate
before cleaning. In the past such treatment
has often involved applying a consolidant
and, once dry, removing excess consolidant
and some surface dirt. This inevitably consolidates
dirt onto the object, but for underbound or
very fragile surfaces there may be no alterna-