Series editors' preface - Wood Tools

518 Conservation of Furniture
a decorative substrate will depend on several
factors such as chemical compatibility and the
surface area of the coating that is exposed to
solvent. The presence of cracks, pores or
cleavage between layers can significantly
increase surface area, with a concomitant
increase in solvent penetration and retention.
The amount of swelling that occurs in the
substrate depends on the relative rates of
solvent diffusion into the substrate and the
dissolution of the varnish (Figure 11.9).
Organic solvents that are able to remove
varnish layers are also able to affect paint
layers. The degree of effect depends on the
chemical compatibility of the solvent with the
paint medium. A slow diffusing solvent will
swell a chemically compatible substrate layer if
the solvent is also slow in penetrating and
dissolving the overlying varnish. In contrast, a
slow diffusing solvent that is chemically incompatible
with the substrate will have little or no
swelling effect during the short exposure of a
varnish removal treatment. This is one reason
for the development of aqueous solutions such
as resin and bile soaps to remove degraded
varnish from oil paint substrates.
Solubility parameters
The principles of solubility are understood
intuitively by most practitioners as ‘like
dissolves like’. This is an expression of the
principle that the intermolecular bonding of
the solvent should be similar to the intermolecular
bonding within the material that is to
be removed. Water, for example, characterized
primarily by hydrogen bonding, is a good
solvent for traditional furniture-maker’s hide
glue, the adhesive properties of which are also
characterized by hydrogen bonding. Waxes,
characterized by van der Waals forces, are
insoluble in water but can be brought into
solution by hydrocarbons such as white spirit
or xylene. Thus understanding the composition
of the materials to be removed will help
in selecting a solvent with a similar chemistry.
Conversely, knowing the nature of what is to
be retained will aid the selection of a dissimilar
solvent. The ‘strength’ of a solvent is not
an intrinsic property but is relative to the
chemical similarity of the solute and the
solvent, the molecular weight of the material
that is to be dissolved, duration of exposure
and temperature.
Predicting solubility
Several models that assign numbers to the
intermolecular forces in liquids have been
developed, primarily in response to the need
of the coatings industry to match solvents with
polymers. Three models are discussed here.
Hildebrand solubility parameters ( s) were the
first to relate the solubility characteristics of a
solvent to the intermolecular forces within a
liquid:
s = ℘(cohesive energy dispersion, or CED)
The cohesive energy dispersion was a measure
of the energy required to vaporize one cubic
centimetre of liquid solvent. The stronger the
intermolecular forces, the higher the CED and
s. Hildebrand and Scott (1948) predicted that
solvents with a similar s would be miscible.
Hildebrand solubility parameters are not
particularly effective for predicting the solubility
of solids, though they have been used to
predict the swelling effect of solvents on oil
paint films (Feller et al., 1985). They are still
widely used in the industrial solvent and paint
industries.
Hansen (1967) proposed a refinement of
Hildebrand parameters by suggesting that the
single solubility parameter s could be divided
according to the contribution of dispersion
forces ( D), polar forces ( P) and hydrogen
bonding ( H):
s = ℘( D 2 + P 2 + H 2 )
Teas (1968) proposed three parameters that
were based on Hansen parameters but relative
to each other as a proportion of 100:
dispersion forces fd = 100 D/(D + P+ H) polar forces fp = 100 P/(D + P+ H) hydrogen bonding fh = 100 H/(D + P+ H) Solvents that are predominantly characterized
by dispersion forces are non-polar (e.g. mineral
spirits). Polar solvents can be categorized
according to whether hydrogen bonding
predominates, in the past referred to as ‘wet’
polarity (e.g. water), and those in which dipole
bonding predominates, previously known as
‘dry’ polarity (e.g. dichloromethane). Within
these broad categories, subtle variations exist