Series editors' preface - Wood Tools

may not be representative of the kinds of surfaces
encountered in decorative paints on furniture.
It is therefore safer to assume that
leaching will be a problem and to choose a
cleaning strategy accordingly, rather than
assuming it is not and potentially causing significant
damage.
Alkaline reagents
Natural resin varnishes become increasingly
acidic in character as they oxidize and age.
Alkaline liquids, such as ammonium hydroxide,
will have some effect on those materials and, in
some cases, will dissolve them. Unfortunately,
many paint binding materials also become
more acidic as they age and become increasingly
susceptible to damage from alkaline
cleaning solutions. The traditional approach
with ammonium hydroxide has been to rely on
its comparatively fast evaporation to limit penetration
and potential damage to an underlying
surface. The most dramatic effects are usually
observed at significantly elevated pH (i.e.
above 8.5), but there is a concomitantly high
risk of damaging the substrate, e.g. by the
saponification of oil paint films or the dissolution
of proteinaceous films. In some cases,
where the pH is buffered, e.g. to 8.5 or less for
oil bound paint, an alkaline solution such as
ammonium hydroxide may cause less swelling
than a solvent-based system.
Aqueous methods
Water has been suggested as a method for
removing varnish from oil paintings (Wolbers
et al., 1990; Wolbers, 2000). The primary
advantage of aqueous varnish removal solutions
is that water is a poor solvent for oilbased
paints. Where varnish is very degraded
(and therefore more polar), the use of an aqueous
solution may have less potential to damage
the paint layer than the use of a very polar
solvent blend. Aqueous cleaning solutions
offer the conservator the potential to manipulate,
monitor and control the parameters of pH,
conductivity, the addition of surfactants or
detergents, the addition of chelators, and viscosity.
It is important to use measured amounts
of these materials and to define safe limits for
their use (Wolbers, 1992). The use of nonvolatile
materials will, however, require a wellformulated
clearance protocol. The disadvantages
of aqueous cleaning include the
Conserving other materials II 749
fact that the formulation of aqueous cleaning
solutions is often more complex than solvent
blends and is not easily undertaken without a
good grasp of the theory and chemistry
involved. The aspect of aqueous cleaning solutions
that has caused most concern is the use
of non-volatile materials, the effectiveness of
clearance procedures, and the ageing characteristics
of any residues that may be left on the
surface. The research that has been undertaken
on the subject of residues varies in quality
(Lang, 1998). It is important that conservators
keep abreast of current research in this specialized
field.
Resin and bile soaps Resin and bile soaps are
aqueous systems developed for the removal of
aged and degraded natural resin varnishes,
usually dammar or mastic, from painted surfaces
(usually nineteenth-century paintings
with an oil–resin binding media) (Wolbers et
al., 1990). Varnish removal relies on the differential
solubility of varnish and paint. As natural
resin varnishes age they become more polar,
and require polar solvents in order to dissolve
them. These polar solvents are more likely to
swell or leach an aged oil paint below, particularly
where an oil–resin binder has been used
for the paint. In contrast, within the time an oil
paint film may be exposed to water during
cleaning, it is unlikely to swell oil-based paint
(Browne, 1954). Water can be used to provide
the necessary polarity for removing degraded
varnish, whilst the addition of a detergent or a
small amount of an aromatic solvent such as
benzyl alcohol can be used to bring non-polar
components of degraded natural resin varnishes
into solution.
Dammar and mastic do not polymerize as
they age, though they do oxidize and become
more polar. This lack of polymerization implies
a low molecular weight and the potential for
detergent micelles to bring moieties into solution.
Oxidized varnish may contain relatively
polar material that may be sparingly soluble in
water, but the bulk of natural resins such as
dammar and mastic is made up of non-polar
molecular groups. Any detergent would aid in
solubilizing such material, but Wolbers based
his selection of certain soaps for solubilizing
aged resin varnishes on the principle of ‘like
dissolves like’. He argued that, at any given
HLB, a close structural similarity of soap and