Series editors' preface - Wood Tools

Thus conservation grade materials may be
used for retouching or for isolating layers,
whilst traditional materials may be used to
provide harder wearing, or more aesthetically
acceptable surfaces. Whilst the need for
accurate documentation is widely accepted in
the conservation profession, it is particularly
important if different materials are used for
isolating, retouching and coating layers.
Careful selection of materials can allow future
conservators to remove a discoloured upper
coating without disturbing retouching below.
Although easel painting conservators avoid
the use of shellac as a retouch medium because
of its tendency to yellow, shellac has been the
retouching binding media of choice for many
generations of furniture restorers. Thin coats of
shellac dry rapidly and each successive layer
partially dissolves and fuses with the preceding
layer, a property that is exploited in French
polishing. In spite of this, a touch-dry layer of
shellac is not immediately dissolved when a
subsequent coat is applied. As a result, successive
coats do not cause reticulation in lower
layers. Shellac-bound colourant, sealer coats
and varnish may be applied with a brush,
French polishing rubber or sprayed – the latter
two methods tend to produce a thinner and
flatter surface. It is possible to work in very thin
layers, for both coloured glazes or sealer coats,
using a French polishing rubber. Although
shellac discolours more rapidly than dammar or
mastic, it forms a hard wearing durable film.
The apparent lack of concern from furniture
restorers about the substantial and relatively
quick yellowing of shellac may be a result of
its durability in use combined with the fact
that the wood itself is subject to colour change
as a result of photochemical oxidation. Thus
by the time the shellac retouch had yellowed,
the background to which it was originally
matched had itself altered. Since the aim of
modern conservation is to limit such photochemical
change of the wood by excluding or
limiting exposure to UV, retouching often
requires a longer stable lifespan than was
previously the case. The use of shellac as a
retouching medium may cause problems for
the survival of adjacent original or early transparent
coatings. Shellac is chemically complex
and may partially crosslink, making the whole
more resistant to organic solvents after around
150 years (Horie, 1992).
Conserving transparent coatings on wood 625
Applying pigments
Powdered pigments may be applied in an
opaque layer to obscure the wood or other
filler below or may be thinly dispersed in a
binder and applied as wash. Pigments with
extremely small particle size, such as microlith
pigments (Ciba–Geigy), have made it easier to
achieve a translucent wash. A comparatively
small palette is required for matching most
wood. Such a palette might include flake or
Chinese white, yellow ochre, raw sienna,
chrome yellow (or its equivalent), burnt
sienna, Venetian red, raw umber, light umber,
burnt umber and lamp black. Many furniture
conservators also use proprietary artist’s water
colours. Whilst some furniture conservators
use the conservation grade materials described
in section 12.3.2, others may choose to isolate
the adjacent surface and use a shellac binder.
Furniture restorers often used powder
pigments in a shellac binder. The process
involved using a scrap of used abrasive paper
folded into a cone into which a small amount
of shellac was poured, diluted with a small
amount of methylated spirits. Dry pigment
powder was mixed into the resin at the tide
edge of the shellac. Whilst there is no doubt
this method is convenient, the pigments are not
well ground and may not be well integrated into
the varnish (i.e. particles may not be thoroughly
wetted). The pigment:binder ratio will inevitably
vary, sometimes from stroke to stroke. Though
more consistent results may be achieved by
mixing pigment and resin on an artist’s palette,
the rapid evaporation of the alcohol binder
made this unsuitable for shellac based paint.
If pigment has been as a base coat, colour
and tone may be adjusted using transparent
glaze layers. Pigmented or transparent glazes
may be used to simulate bands of colour. The
last step may be simulating the pores or grain
of the wood. This may be difficult using
pigments. A thick Vandyke brown, made up
to the consistency of a paste, is particularly
effective for this purpose and can be applied
in extremely fine lines. The technique
described below for sealing water stain
between sealer coats should be used to ensure
the fine detail is not removed or disturbed.
Applying stains
Stains, by definition, are translucent materials.
They may be used to adjust the colour and