Series editors' preface - Wood Tools

chemical stability, flexibility and solubility parameters
or application technique that ensures
that future removal will not damage original
material. Retouching media with proven long
term photo-chemical stability include Paraloid
B72, Laropal A81 and PVAC (de la Rie et al.,
2000). Retouching media need to be compatible
with both the surface to which they are
applied and any subsequent coating that may
be applied. For example, retouching a resinbased
japanned surface requires a retouching
medium that will not affect the original surface
(e.g. one soluble in hydrocarbon solvents), and
that in turn will not be affected if a varnish is
to be applied. Other important properties of
retouching media include refractive index and
molecular weight.
The refractive index of many synthetic materials
is slightly lower than that of many traditional
binding media (Table 12.2). This lowered
refractive index can reduce the chroma of the
paint because it is perceived as greyness (see
section 12.4.2). This effect can be reduced or
overcome by ensuring good wetting of pigment
particles, by careful selection of pigments used
for retouching, or by glazing with a higher
refractive index material.
The handling properties of a binding
medium are a result of both the molecular
weight of the medium and the compatibility
and evaporation rate of the diluent. The higher
the molecular weight and the faster the diluent
evaporates, the more rapid the onset of the formation
of a gel phase and the more likely the
paint will begin to feel ‘rubbery’. Low molecular
weight resins stay mobile longer because of
the later onset of the gel phase, and often have
better wetting of both the pigment particles in
the paint and the surface being retouched. The
compatibility of solvent and resin will affect the
appearance and ‘feel’ of the paint, particularly
for high molecular weight polymers such as
Paraloid B72. A good solvent will allow the
polymer to ‘uncoil’ to a greater degree, increasing
viscosity (Hansen, 1994). The more
uncoiled the polymer, the better it can wet individual
pigment particles and the more saturated
the paint will appear.
Retouching requires that the application of
successive thin layers does not disturb those
lying underneath. Some binding media need
only adequate drying time between coats, whilst
others may require the use of isolating layers
Principles of consolidation, aesthetic reintegration and coatings 583
Table 12.2 Comparative glass transition
temperature (Tg) and refractive indices (RI) of
some natural and synthetic resins
Resin Tg °C RI
Dammar 39.3 1.539
Mastic 34.7 1.536
Sandarac a 1.545
Shellac b 1.52
Laropal K80 50.8 1.529
MS2A 54.1 1.518
Paraloid B67 c 50 1.486
Paraloid B72 c 40 1.487
Paraloid F10 20 d 1.476 e
Arkon P90 35.6 1.522
Regalrez 1094 33 f /43.8 1.519
Source: de la Rie and McGlinchey (1990b) except:
aGettens and Stout (1966)
bAngelo Bros (1965)
cManufacturer’s literature and Samet (1998)
dManufacturer’s literature
eTennant and Townsend (1984)
fManufacturer’s literature – differences may be due to
experimental conditions
with a different solubility parameter. Although
the substrate and any adjacent area are usually
isolated from retouching materials, reversibility
is a property of an integrated system rather than
a material. Thus the use of a binding medium
that is only re-soluble in acetone or alcohol may
be unsuitable for resin-based japanned surfaces.
In other cases it may be possible to select a
retouching medium that is soluble in acetone or
alcohol, which will allow the application and
future removal of a hydrocarbon soluble varnish
without disturbing the retouch.
In an ideal world the appearance of the paint
used for retouching would remain the same on
the palette, on the retouching, and before and
after varnishing. It may be difficult for an inexperienced
retoucher to compensate for colour
or saturation changes that may occur at each of
these stages. One way of minimizing such
change in appearance is to aim to use a paint
that is sufficiently medium rich to match the
gloss of the surface being retouched. Some
conservators wet out with white spirit to replicate
the appearance after varnishing, but it may
be difficult to remember detail accurately after
the solvent has evaporated. Other conservators
prefer to saturate and isolate progressive
retouching layers with a thin coat of varnish. If
this approach is to be successful, the isolating/