Series editors' preface - Wood Tools

628 Conservation of Furniture
secondary intermolecular bonds are broken.
As the solvent evaporates, these secondary
bonds reform within the varnish and between
the varnish and substrate. It may take up to
six months before the re-establishment of
secondary bonds is complete (Feller, Stolow
and Jones, 1985). It should be noted that many
transparent coatings on furniture were
polished after their application in order to
produce a high gloss finish and that treatment
with solvent may not reproduce the original
appearance. Further, swelling a natural resin
coating by the infusion of solvent may affect
its performance as a coating. Although such a
treatment may resaturate the finish it will not
reverse degradation associated with oxidation.
A traditional method of rejuvenating a
resinous coating that is in poor condition is to
apply solvent using a French polishing rubber
or pad. This technique aims to redistribute
partially solvated varnish on the surface and is
sometimes referred to as ‘amalgamation’.
Although such a treatment is not reversible, it
may be preferable to wholesale removal of a
very degraded original varnish.
13.7 Application of coatings to
varnished wood
A furniture conservator may apply a new
coating to varnished wood in order to
preserve the original coating (e.g. if it is friable
or vulnerable to damage) or for aesthetic
reasons (e.g. to resaturate a degraded surface).
In some cases the conservator may be asked
to apply a coating in order to better imitate
the original appearance of the object. In all
cases the treatment chosen should be compatible
with the preservation of original varnish
or colouring materials. The solubility region of
the new coating should be sufficiently different
from the original to allow future safe
removal. The restoration practice of stripping
and repolishing will reduce the historical value
of an object and can reduce the monetary
value of furniture by up to 80%.
When choosing a material for coating a
varnished wood surface, the furniture conservator
should consider issues of stability,
aesthetics and the time, skill and resources
available for the treatment. Historical and
physical integrity should be considered and
this may mean assessing the original layer
thickness, degree of gloss, original application
technique and the use of colourants in the
original coating.
Natural resins were historically used as the
basis for many transparent varnishes on wood.
All natural resins are prone to photochemical
degradation, and shellac in particular is known
for discolouring comparatively rapidly. Shellac
has often been used in the past because of its
excellent adhesive properties, colour and
refractive index. It is available in a range of
colours, such as button or garnet, which may
approximate the colour of an original finish
more closely than a transparent varnish. The
hard-wearing resins associated with furniture
varnishes, sandarac, and shellac, require polar
solvents for removal. Whilst this is not
problematic if they are applied to unfinished
wood, it has serious implications if remnants
of an original surface finish survive. It is
questionable whether the use of natural resins
to replicate an original finish can be successful,
given that, with a few notable exceptions,
the exact formulation of an original varnish is
unknown. The main advantages of sandarac
and shellac varnishes are that they are hardwearing
and produce an appearance that satisfies
the currently accepted aesthetic for
varnished wood.
It is essential to avoid a dogmatic approach
to the use of traditional or conservation grade
materials. In many cases a compromise
solution may be possible, for example by
using a barrier layer of stabilized varnish
soluble in non-polar solvents, followed by a
natural resin varnish if this is deemed suitable
or essential. This approach may allow an
ethical conservation treatment that combines
future reversibility/retreatability with practical
considerations of durability. It may be necessary
to degrease or dewax a surface with
white spirit or an aromatic hydrocarbon before
a new coating is applied.
13.7.1 Non-traditional materials
Synthetic resins that may be suitable for
coating a varnished surface include Paraloid
B72 or stabilized low molecular weight resins
such as dammar, MS2A, Regalrez 1094 or
Laropal A81. These materials are discussed in
section 12.4.7. The primary advantage of such