Series editors' preface - Wood Tools

(a)
(b)
Figure 13.11 Use of oils
(a) Detail of carving on a Philadelphia Chippendale
style side chair. Dust and lint accumulated in the
recesses of the carving were adhered to an oiled finish
(b) Accretions of old linseed oil on a varnished
mahogany surface. Patches of darkened oil disfigure the
surface
their showrooms for this purpose. Proprietary
preparations such as Danish oil and tung oil
are often used by the amateur furniture
finisher because they are easy to apply. Oils
are not usually employed in furniture conservation
because they can remain sticky for an
extended period of time and trap dirt that
becomes incorporated into the surface coating
as the oil slowly dries (Figure 13.11a). In
addition, drying oils darken as they age,
crosslinking to produce large insoluble
molecules that are difficult to remove without
the use of aggressive cleaning methods (Figure
13.11b). Oil should never be applied to
saturate the colour of a degraded varnished
Conserving transparent coatings on wood 631
surface. It should be noted that rags used to
apply linseed oil may spontaneously combust,
thus used or discarded rags must be stored in
a flameproof container.
Non-drying essential oils such as lemon or
almond oil are used in numerous proprietary
polishes and have been used in the past to
saturate polished and decorative surfaces.
While they do not polymerize as a drying oil
would, they remain sticky and serve to bind
dust to the surface. These materials may be
removed with white spirit.
Natural resins
Natural resin varnishes were applied to wood
surfaces to saturate the colour and protect the
surface from dirt. Some eighteenth century
sources refer specifically to the desire that
varnishes should impart durability and gloss to
a wood surface. Despite a comparatively
limited range of natural materials and solvents,
historical treatises attest to the many possible
permutations for their preparation and combination
in varnish formulations. Information on
the ingredients and preparation of varnishes
can be found in many historical treatises.
Walch (1997) has suggested three main classes
of varnish were applied to wood in the
eighteenth century: oil–resin varnishes, usually
based on amber or copal, spirit varnishes and,
less commonly, essential oil varnishes.
Oil–resin varnish generally contained a high
proportion of a drying oil such as linseed oil.
Fossil-type resins, such as amber and copal,
could be heated with linseed oil at around
300 °C to produce hard-wearing oil–resin
varnishes. Such oleo-resin varnishes could be
thinned with turpentine and applied in a few
thick layers. Although such varnishes could be
diluted with alcohol when fresh, aged films
are only sparingly soluble in this solvent.
Natural resins, such as sandarac, larch resins
and mastic, could be heated with linseed oil
to produce an oil–resin type varnish. Natural
resins could also be heated and combined
with essential oils such as oil of spike, lavender,
or rosemary, and diluted to a workable
consistency with turpentine. Walch (1997)
refers to such formulations as essential oil
soluble varnishes.
Whilst a range of natural resins have historically
been applied to paintings, only sandarac
and later shellac were hard-wearing enough to