Series editors' preface - Wood Tools

ecoming progressively harder and darker (see
Mills and White (1994) for a description of the
oxidation of drying oils). Dirt may become
incorporated into the film structure as it
crosslinks. Coatings with a significant oil
content are sensitive to, and may be damaged
by, aqueous solutions with a pH higher than
8.5, bases, polar organic and chlorinated
solvents. Water may be used to clean an oil
finish if it is applied sparingly on a damp swab
or cloth and the surfaced is wiped immediately
with a dry cloth. Although original intent
should be a consideration in selecting a treatment,
maintaining an oil finish with wax may
be preferable to an irreversible re-oiling treatment.
Resinous coatings, e.g. sandarac or shellac,
may appear matte, semi-matte or highly glossy
depending on age and the use of matting
agents or others dulling treatments. They are
auto-fluorescent under UV, though the colour
of the fluorescence varies according to the
resin type/s and the age of the coating. Aged
resinous coatings are usually insoluble in
hydrocarbon solvents but may be dissolved by
alcohols and ketones. In addition, resinous
coatings are generally sensitive to elevated pH
and polar organic solvents. The removal of dirt
from natural resin varnishes that are in good
condition is usually straightforward (Figure
13.2). Wax or greasy accretions may be
removed with a swab dampened with an
aliphatic or aromatic hydrocarbon solvent. It
should be noted that these solvents will
remove some synthetic resins. Cleaning may
utilize a swab or cloth dampened with water
followed immediately by wiping over with a
soft dry cloth. Aqueous cleaning of resinous
transparent coatings requires some care as the
water molecules may diffuse into the coating
and cause blooming. The risk of bloom
increases with the degree of ageing of the
varnish, exposure time and as the pH of the
cleaning solution rises. Some very degraded
natural resin surfaces may be partially soluble
in water with a pH of only 8–8.5, a fact that
has been exploited in varnish removal treatments
(Wolbers et al., 1990). The principles of
aqueous cleaning are discussed in section
11.5. Blooming of resinous coatings is also
often associated with long exposure to
moisture or a combination of heat and
moisture, for example if excess warmed
Conserving transparent coatings on wood 609
(a) (b)
Figure 13.2 Scanning electron microscopy (SEM)
photomicrographs of an aged natural resin varnish
(a) Before cleaning: accretions darken and obscure the
mahogany substrate. (b) After cleaning: accretions have
been removed and the original varnish remains intact
animal glue used to repair the wood substrate
is left in contact with the varnished surface
when the repair is cramped up.
Oleo-resinous varnishes were often
employed where a hard-wearing surface was
required and are particularly associated with
the nineteenth century. Drying oils such as
linseed or tung oil were used in combination
with natural resins such as sandarac, mastic or
gum elemi, with fossil resins such as copal or
amber, or in combinations of these materials
with the addition of colophony.
Limed finishes were created by scrubbing the
surface of an open-grained timber such as oak
or ash to accentuate and open the large pores
associated with early seasonal growth.
Traditionally, quick lime (calcium oxide) was
dissolved in an equivalent quantity of water to
produce highly alkaline slaked lime (calcium
hydroxide). A thick mixture was rubbed into
the surface of the wood and lodged in the large
opened pores of the early wood. Excess was
removed from the surface and the lime that
remained in the pores created a striking decorative
effect. The alkaline liming treatment may
have had the additional purpose of disinfecting