Series editors' preface - Wood Tools

If the surface is unaffected by spot tests,
there are five parameters that may be manipulated
when working with aqueous solutions.
These are pH, conductivity, the addition of surfactants
or detergents, the addition of chelators
and raising the viscosity of the cleaning solution.
The principles behind the manipulation of
these parameters are discussed in detail in sections
11.5 and 11.6. As a general rule the following
may be useful guidelines for aqueous
cleaning solutions applied to painted surfaces:
• pH 5.5–8.5 for oil paint films. The older the
oil paint film, the greater the risk that it will
be damaged by elevated pH, so the upper
limit of 8.5 should be adjusted downwards
according to the age of the paint film.
• pH 5.5–7.0 for casein based paints,
although in some cases the use of water to
clean casein-based paints may be inappropriate.
At a pH around 4.6 (the isoelectric
point) casein is at its least soluble in water.
• Natural resins are weakly acidic materials
and ageing will also contribute to this acidity.
As pH rises above 7, alkaline solutions
will increasingly interact with such surfaces,
and there is a concomitant risk that blooming
will occur in the varnish.
• The presence of distemper (collagen gluebased
paint) or gums may preclude the use
of water-based cleaning solutions.
• As a general rule, the conductivity of aqueous
solutions used to clean painted surfaces
should not exceed 5000 μS. Conductivity
may be excessive if ionic materials such as
buffers or chelators are incorporated into a
cleaning solution (see section 11.5.3).
• The HLB of surfactants should, as a general
rule, fall between 13 and 20. Low CMC surfactants
may be difficult to clear from
lipophilic surfaces such as unaged oil paint
films. Surfactants with an HLB above 20
have an increasing ability to emulsify water
into oil, and thus blanch an original paint
layer.
• Weak chelators (such as citric acid), should
be used in preference to stronger chelators
(such as EDTA) (see section 11.5.5).
• Viscosity may be raised, for example with a
cellulose ether or polyacrylic acid (e.g.
Carbopol), to give greater application control
(see section 11.6). The thicker the gel,
the less the possibility of liquid water (or
Conserving other materials II 745
solvent) penetrating a complex surface and
disrupting sensitive layers below.
Unvarnished paint is more susceptible to damage
during cleaning. In some cases, for example
if the surface was originally varnished, such
areas may be sealed before cleaning commences.
Hackney et al. (1990) and Green
(1990) set out strategies for cleaning unvarnished
painted surfaces including the use of
erasers, aqueous cleaning solutions and solvent/detergent
emulsions.
Removal of surface dirt from painted furniture
may be complicated by the presence of
grease, wax or oil. In some cases varnish or oil
layers may have been applied over disfiguring
soiling material. Aliphatic hydrocarbon solvents
may be used to remove wax and grease from
natural resin varnish coatings, but will remove
many synthetic coatings such as dammar,
MS2A and hydrogenated hydrocarbon varnishes,
particularly where these varnishes are
unaged or have been stabilized with Tinuvin
292 (see section 12.4). Aromatic solvents will
remove Paraloid B72, moderately aged unstabilized
dammar and MS2A. Small spot tests
will establish whether a surface coating is
sensitive to a particular hydrocarbon solvent.
Traditional recipes for varnish removal or
degreasers often relied on the addition of a
small amount of a reagent such as ammonium
hydroxide to water. If this approach is used,
the pH of the solution should not exceed 8.5
(Wolbers et al., 1990). Proprietary cleaning
solutions should be avoided because they are
often formulated at alkaline pHs, often incorporate
an excess of strong chelating materials
and may contain high HLB detergents. As a
general rule such conditions are not appropriate
for historic finishes or surfaces.
16.6.3 Removal of varnish
Discoloured varnish may be considered more
of a problem on painted surfaces than on varnished
wood. Discoloured varnish may disrupt
the intended aesthetic of the decorative
scheme. As varnish yellows or darkens it may
obscure the colours below and can alter colour
relationships and balance within a decorative
scheme (Figure 16.15). The goal of treatment
in such cases may be to remove a layer that
has significantly disturbed the original intended