Series editors' preface - Wood Tools

as odourless mineral spirits or white spirits,
xylene and IMS or acetone (Table 11.4).
The primary problem with using solvents to
remove varnish is that both coatings and
decorative layers are organic polymeric materials
whose solubility parameters often overlap.
This problem is often found where oil-resin
paint media have been coated with a natural
resin varnish. Underbound pigments are
vulnerable during solvent cleaning, both from
dissolution of the minimal binding medium
and from the mechanical action of swabs. The
interaction between pigment and binder should
be understood before cleaning is attempted
because some pigments may be removed by
careless cleaning (e.g. carbon black and vermilion
are poor driers in oil). Pigment–solvent
interactions should also be taken into consideration,
for example the affinity of zinc derived
pigments for water and alcohol. Glaze layers
are vulnerable to damage or inadvertent
removal because they are usually the uppermost
decorative layers, are very thin and the
pigment is sparsely dispersed in binding
medium. The belief that glaze layers have been
lost as a result of varnish removal treatments
has provoked significant criticism of painting
conservators in the past (Keck, 1984).
A potential problem associated with solvent
removal of varnish from painted surfaces is the
removal of unpolymerized components of the
decorative surface, an irreversible process
known as leaching. Leaching of oil-painted
surfaces as a result of varnish removal is
discussed in the section on solvents under
section 16.6.3).
Swelling in the decorative surface layer as a
result of exposure to solvent during varnish
removal will result in damage. The decorative
surface, after exposure to solvent, may attempt
to swell in all directions but can only freely
swell upwards, resulting in plastic compression
(Figure 11.18). Whilst leaching of soluble
low molecular weight components from within
the decorative surface may cause shrinkage of
around 15% for oil paint, plastic compression
may result in up to 80% shrinkage of the paint
film on drying. In addition, micro-voids may
be created within the paint layer (Michalski,
1990).
Both polar and non-polar solvents can cause
irreversible changes in solubility and other
properties of protein-based layers. In particu-
(a)
(b)
(c)
Varnish
Decorated layer
Ground
Wood
Principles of cleaning 525
Figure 11.18 Plastic compression
(a) Diagrammatic representation of a decorated surface
made up of varnish, decorated and ground layers on a
wood substrate
(b) Solvent is introduced to remove the varnish layer.
As varnish is removed, solvent enters the susceptible
lower layer, which attempts to swell in all directions,
but can only freely swell upwards. In other directions,
the susceptible layer/s are compressed (arrows indicate
direction of force)
(c) Solvent evaporates and swollen layers contract.
Where compressive forces have exceeded the elastic
properties of the binding material, cracks will develop.
Such plastic compression is irreversible
lar, low molecular weight alcohols will tend to
remove bound water from the protein structure,
altering chemical and physical properties.
Some organic solvents may react chemically
with specific peptide chains, for example
trichloroethylene and cysteine (Karpowicz,
1981).
Mixing solvents
Miscibility is the capacity of one liquid to mix
with another to form a stable solution. When