Series editors' preface - Wood Tools

564 Conservation of Furniture
extent of damage may not be revealed until
everyday stresses or use result in structural failure.
This may involve complete breakage of a
rail or other component, or the disintegration of
edges as a result of handling. The surrounding
wood may be too weak to support the cutting
edge of tools and/or the pressure of cramps or
other apparatus needed to effect repair. Insect
infestation of a wooden substrate may disrupt
decorative layers as a result of areas of loss, a
large number of exit holes or the collapse of
larval tunnels. In such cases a consolidant
should be compatible with the wood, surface
decoration (e.g. not cause darkening) and subsequent
treatments such as filling and retouching.
If Anobium infestation has caused severe
structural damage it may be necessary to impart
additional strength to structural components.
This can be done by consolidation, reinforcing
the structure by introducing dowels or loose
tenons, or in severe cases, replacing the entire
component or, if the surface is valued, its core.
The extent of reinforcement or replacement
may be determined by balancing the loss of
original material against the risk of further damage
to the object or injury to people. An independent
support for the structure of the object,
such as supports extended from a wall, may
provide greater longevity for the object than
impregnation with resins.
The purpose of consolidation is to impart
strength to the structure of the object so that it
can bear its own weight, permit essential conservation
processes and ensure it is fit for its
purpose, whether that is display or use.
Although some properties for consolidants will
overlap with those discussed above, such as the
need for chemical and mechanical stability, a
consolidant for worm-damaged structural components
must have good adhesive properties,
impart strength, should penetrate the wood
fibres well, combine a degree of flexibility with
hardness, should not creep, and ideally combine
high concentration with low viscosity.
Ideally conversion from liquid to solid state
should occur at room temperature, at a controlled
rate and without shrinkage. Other considerations
include the retention and toxicity of
solvent, lack of reactivity with non-wood components
such as metals, extant adhesive, leather
or glass, and compatibility with other adhesives
that may be used as part of the conservation
treatment. Some solvents used to deliver consolidant
solutions are incompatible with many
varnished and decorated surfaces.
The consolidation of wood by impregnation
with thermoplastic resins cannot be considered
completely reversible. Attempts to remove consolidant,
where this is possible, may result in
further degradation of the wood. Schniewind
(1987, 1988) demonstrated the retention of up
to 6% of residual thermoplastic resin used to
consolidate small pieces of wood after they had
been soaked in solvent to remove the consolidant.
The practicality of immersing large components
in a suitable solvent may render this
finding moot, particularly when varnished or
decorated surfaces are present. However, even
such partial reversibility is more desirable than
a single treatment with a thermoset resin,
where the lifetime of the treated wood is
extended only by the working life of the resin.
Improvement in strength after consolidation is
strongly influenced by the degree of degradation
of the wood. More degraded wood allows better
penetration of consolidant, increased resin
loading, and thus a greater improvement in
strength after treatment (Schniewind and
Kronkright, 1984). The tensile and compressive
strength of the consolidant itself must exceed
that of the wood to have a noticeable effect.
The method of application will affect the
amount of consolidant absorbed into the wood.
Application techniques include brushing or
spraying the surface, injecting wormholes,
vapour phase (Schniewind and Kronkright,
1984), vacuum impregnation (Schaffer and
Brisebois, 1974) and total immersion, though
the last two are only feasible for smaller objects
or components uncomplicated by the presence
of coatings. Barclay (1981) reported that, in
comparison to brushing, the use of a portable
vacuum chamber increased the amount of consolidant
introduced into the wood by a factor
of ten. Carlson and Schniewind (1990) considered
the effects of solvent retention on the glass
transition temperature of thermoplastic resin
used for consolidating wood, whilst
Schniewind and Eastman (1994) examined consolidant
distribution in deteriorated wood.
Materials used to consolidate wood
A wide variety of materials have been used to
consolidate wood. These materials have been
reviewed by Schniewind (1998).