Series editors' preface - Wood Tools

594 Conservation of Furniture
1996). De la Rie and McGlinchey (1990a) stated
that the exclusion of UV light below 400 nm in
combination with the addition of 3% Tinuvin
292 (measured as a percentage of the weight of
the dry resin) would transform what would otherwise
be a photochemically unstable varnish
into a class A conservation material according
to the criteria proposed by Feller (1978). The
addition of Tinuvin 292 will lower the Tg of
dammar from 39.3 °C to 29.2 °C and reduce the
brittleness of the varnish film (de la Rie and
McGlinchey, 1990a).
Mastic has had centuries of use as a picture
varnish. It is glossy and saturates painted surfaces
well. As with other natural resins, mastic
has a tendency to yellow with time. It is slightly
more photochemically unstable than dammar
and thus requires the use of comparatively
more polar solvents to remove an aged varnish.
Solvents that may be substituted for the more
traditional turpentine include aromatic hydrocarbon
solvents, or aromatic/aliphatic mixtures
(approx. 1:9). Mastic is also soluble in alcohols
and ketones, though these are unsuitable solvents
for coatings applied to oil or proteinbased
paints because of their polarity. Although
many traditional recipes report 25–30% w/v
solutions, most conservators prefer a range of
about 5–10% w/v, using second or third coats
if required. A small percentage (c.3%) of resin
is usually insoluble, which will reduce the overall
percentage concentration of the final solution
(Mention, 1995). If the varnish solution
appears cloudy it may be left to stand until partially
dissolved material settles to the bottom.
The clear varnish solution can then be poured
off and the insoluble residue discarded. The
exclusion of UV light below 400 nm combined
with the addition of 4% Tinuvin 292 should provide
stabilization of mastic varnish against photochemical
degradation (de la Rie and
McGlinchey, 1990a). The addition of Tinuvin
292 will also slightly lower the Tg of mastic and
reduce the brittleness of the varnish film. A
small amount of varnish may be brushed onto
a surface then brushed out until it is completely
absorbed, or until it becomes tacky (which will
occur with second or third coats). If a week or
more is left between applying coats, successive
coats will not dissolve the previous one
(Leonard, 1990).
Sandarac resin was used in both picture varnishes
and for transparent coatings on furniture
and decorative surfaces, such as marbling or
graining, that required a colourless, hard coating
(Caley, 1990; Walch and Koller, 1997). It
could be dissolved in alcohol to create a spirit
varnish or heated with turpentine and linseed
oil to create an oil–resin varnish. Brittleness
could be reduced by the addition of materials
such as gum elemi and Venice turpentine
(Gettens and Stout, 1966; Walch and Koller,
1997). Sandarac-based spirit varnishes require
the use of polar solvents for removal and thus
sandarac is not appropriate as a coating applied
directly onto decorative surfaces. It may find
application for furniture that requires a hardwearing
coating if the surface has been isolated
with a photochemically stable hydrocarbon soluble
varnish that is not soluble in alcohols such
as Regalrez 1094.
Shellac, the staple varnish of the furniture
restorer for at least the past hundred years, is
unsuitable as a coating for varnished, painted
and decorated surfaces because of its tendency
to yellow, its polar solubility parameters and
the possibility that it may crosslink with time. If
the hardness of shellac is desired, it may be
possible to use if the vulnerable surface is isolated
with a coating that is insoluble in alcohols
such as Regalrez 1094.
Acrylics
Paraloid B72 is an ethyl methacrylate/methyl
acrylate copolymer (70:30). Proprietary formulations
include Univar varnish (production
ceased early 1990s) and Lascaux Fixativ, a fixative
spray. Paraloid B72 has been used as a
coating material because it is very chemically
stable. Its high molecular weight and low
refractive index (in comparison to natural
resins) result in moderate saturation of some oil
painted surfaces, particularly those that are
damaged or have large dark areas (e.g. earth or
carbon black pigments). The resin is soluble in
aromatic hydrocarbons as well as polar solvents
such as alcohols and ketones. The choice of
solvent will significantly affect the mechanical
and optical properties of the film (see section
12.4.3).
Paraloid B72 is roughly equivalent to
dammar in hardness (Feller et al., 1985). It can
be applied by brush (up to 20% solution, usually
c.10%) or sprayed (3–15%, usually c.10%),
usually in xylene (sometimes with the addition
of a small proportion (2%) of acetone),