Series editors' preface - Wood Tools

cation and characterization of the stratigraphy of
the decorative surface. Wolbers and Landrey
(1987) described the use of reactive fluorescent
dyes for the characterization of binding media in
decorative surfaces. Sigel (1993) tabulated many
reactive fluorescent dyes and the materials they
tag. If access to a UV microscope can be gained,
this can be an effective tool that allows the conservator
to formulate cleaning, consolidation and
recoating strategies specifically designed for a
given painted surface. The microscopic examination
of paint layers has been considered by
Plesters (1956) and reviewed by Martin (1998).
Such microscopic analysis is usually conducted
in conjunction with careful solvent testing. If
microscopy or other analytical techniques are
not available, examining the surface under handheld
UV, before and after surface cleaning, will
identify the presence of an auto-fluorescent natural
resin varnish. Removal of discoloured varnish
may be complicated by the presence of
‘maintenance’ coatings such as wax, linseed oil,
or multiple layers that may comprise natural
resins or oil-resin varnish/es.
A variety of techniques have been used to
remove discoloured varnish from painted surfaces.
These techniques include mechanical
removal, the use of solvents and solvent gels,
alkaline reagents and, more recently, aqueous
methods (Wolbers, 2000), each of which has
advantages and disadvantages.
Mechanical removal
Mechanical reduction of varnish may be an
option when other methods are precluded. A
traditional technique for the removal of varnish
from easel paintings was to grind a natural
resin such as dammar, mastic or colophony,
which had originally been used to varnish the
painting, and to rub it over the surface with a
finger. This method has the advantage of not
requiring the use of solvents or water, and may
be a controllable technique in skilled hands. It
is not usually used in conservation treatments
because there is a significant danger of abrading
the paint below. In some cases, however,
where the varnish is very oxidized and brittle,
this may be considered less damaging than
other cleaning options.
Solvents
The most common method of removing a varnish
from a painted surface is the use of sol-
Conserving other materials II 747
vents. Varnishes are prone to oxidize, discolour
and become soluble in more polar solvents
over time. The strategy for solvent removal of
varnish is to find a solvent or solvent mixture
that is polar enough to rapidly swell or dissolve
the bulk of the varnish film without affecting
underlying materials. Thus a typical blend may
be based on acetone, isopropanol or IMS, often
mixed with a non-polar solvent such as white
spirits to adjust the degree of polarity and position
of the mix on a TEAS diagram (see discussion
of solubility parameters under section
11.3.3). Ruhemann (1968) and Feller et al.
(1985) describe this cleaning strategy.
The advantages of solvent cleaning are easy
formulation if the conservator can find a blend
with suitable solubility parameters, rapid dissolution
and removal of a coating. The materials
are volatile, leave no residues and the treatment
is therefore uncomplicated by problems
of clearance. The disadvantages of solvent
cleaning are that it is often difficult to get the
solubility parameters exactly right. Varnishes
used on furniture are often complicated by the
presence of oil, fossil resins or other polymer
containing or forming materials. Simple solvent
mixtures may not work well with such complex
films. As both the varnish and decorative
layers age and oxidize, solubility parameters
may increasingly overlap. In some cases, the
original materials and coating may have originally
had similar solubility parameters, e.g.
Victorian reds and browns often contain an
admixture of natural resin. One strategy for
dealing with this problem is to limit the diffusion
of solvent into the surface by using a viscous
solvent gel (see section 11.6).
Leaching and swelling A serious problem associated
with solvent cleaning, particularly of oil
painted surfaces, is the removal of unpolymerized
components of the decorative surface, an
irreversible process known as leaching. The
leaching of unaged oil paint films was quantified
by Feller et al. (1985). The loss of soluble
components occurs during cleaning as the solvent
diffuses into the paint film. Small soluble
molecules will be lost independently of
swelling, but leaching will markedly increase
when accompanied by swelling (Figure 16.16).
Thus the relationship between swelling and
leaching is not directly proportional. The overlap
of solubility parameters of the solvent and