Series editors' preface - Wood Tools

although slower evaporating solvents, such as
diethylbenzene, diacetone alcohol or Cyclo Sol
100 (Shell) may be used to adjust working
properties. If the varnish is applied to a painted
surface, consideration should be given to the
effect of the solvent on the paint film. Even a
small amount of benzyl alcohol or diacetone
alcohol, for example, can have a significant
swelling effect on oil paint and this may cause
the varnish to penetrate into an oil paint layer
below and complicate future removal. Paraloid
B72 exhibits poor wetting on unevenly cleaned
surfaces where residues of old varnish remain.
It is less ‘forgiving’ than natural resin varnishes
such as dammar and mastic and must be
worked very quickly. Once the varnish begins
to dry a softer brush must be substituted for the
stiff natural bristle brush if the surface is to be
worked further (Buckley and Houp, 1996).
Spraying may produce a surface that is electrostatic
and will attract dirt. This effect may be
exacerbated by a matte finish (Hackney, 1990).
Paraloid B72 has been used in conjunction with
other varnishes, for example more saturating
varnishes (e.g. dammar, MS2A) have been
applied over B72 to improve the optical properties
of the surface. It should be noted that, in
a multi-layer system, the final varnish layer has
the most marked effect on gloss.
Paraloid B67, poly(isobutyl methacrylate), is
soluble in hydrocarbon solvents with a K-B
value above c.35, such as VM&P naptha, white
spirit and polar solvents such as alcohols and
ketones. A small proportion of aromatic hydrocarbon
solvent (10–15%) may be added if difficulties
are encountered dissolving the resin in
aliphatic solvents. Paraloid B67 saturates somewhat
better than B72 but exposure to UV
causes crosslinking and the aged resin requires
the use of comparatively polar solvent combinations
for removal (Feller et al., 1985;
Thomson, 1957). In some experiments, aged
poly(isobutyl methacrylate) films have become
completely insoluble. Horie (1992) has suggested
that the addition of an unspecified additive
to B67 renders it more stable than might
otherwise be expected, but concerns about
photochemical stability have resulted in most
painting conservators discontinuing use of this
resin (Vagts, 1996). B67 can be brushed or
sprayed in solutions of 5–20% (usually c.10%)
w/v in a solvent mix of aliphatic and aromatic
hydrocarbon solvents. It produces a glossy fin-
Principles of consolidation, aesthetic reintegration and coatings 595
ish, though this may diminish with time, which
may be prone to attract dust, particularly as a
result of spray application. B67’s comparatively
high Tg makes the varnish film very brittle,
prone to fracture and scratch. Other products
based on poly(isobutyl methacrylate) include
Elvacite 2045 (previously known as Lucite 45).
Paraloid F10, Elvacite 2044 and Plexisol 550,
all poly(n-butyl methacrylate)s, are soluble in
hydrocarbon solvents with a K-B value of 35–55
or more, as well as polar solvents such as alcohols
and ketones. These resins, with a Tg
around 20 °C, will retain dirt and dust and are
therefore unsuitable for use as coatings, particularly
for horizontal surfaces on furniture,
though they may be added to other formulations
to reduce overall Tg and brittleness. After
a comparatively short period of artificial ageing,
de la Rie (1993) found that a significant amount
of material insoluble in either toluene or acetone
had formed in test films of Elvacite 2044
(poly n-butyl methacrylate) and 2045 (poly
isobutyl methacrylate). The addition of Tinuvin
292 had little effect on the formation of
crosslinked material in these two resins.
Synthetic low molecular weight varnishes
Ketone resins Ketone resins, also known as
polycyclohexanones, are a class of synthetic
low molecular weight varnishes developed in
the second quarter of the twentieth century.
They have been used as retouching media and
as varnishes. Proprietary products available
since 1970 include MS2A (Linden Nazareth),
Ketone Resin N and Laropal K80 (BASF). De la
Rie and Shedrinsky (1989) found Ketone Resin
N and Laropal K80 to be chemically indistinguishable.
As a class, ketone resin films are
comparatively weak, hard and brittle (Horie,
1992). They are prone to yellow in the absence
of light, have a tendency to bloom, and
although they are initially more stable than the
natural resins, in the long term oxidation results
in the need for polar solvent mixtures for their
removal. Additives tested to date have had no
effect on the photo-oxidation of Laropal K80
(Lafontaine, 1978; de la Rie and McGlinchey,
1990a), though it has been suggested that the
addition of bleached beeswax reduces the
effect of photo-oxidation and improves resolubility
(Raft, 1985). Ketone Resin N/Laropal K80
are the primary ingredients in many proprietary
varnishes, including Artists’ Gloss Varnish