Series editors' preface - Wood Tools

erty of the adhesive polymer rather than the
result of the presence of retained solvent or
plasticizers. The effect of the fill on adjacent
material should also be considered; for example,
water-based fills with a high water content
may swell unsealed adjacent original gesso,
whilst some solvents may swell adjacent pigmented
layers.
Bulking agents Calcium carbonate (chalk,
whiting) is the traditional material used to bulk
out a fill. It does not chemically interact with
many adhesives and results in a comparatively
weak fill, which may be an advantage in conservation.
The amount of chalk to be added is
gauged in terms of the viscosity of the mixture
– a dough-like material for deep fills or a
cream-like material that can be applied in
stages. Glass (3M) and phenolic (Union
Carbide) microballoons have been used as
fillers for conservation. Cellulose or paper pulp
may be used for low density fills.
Wax Wax has been used as a filling material
in the past but is no longer generally used as a
filler in decorated surfaces, though it may find
application for shallow losses. Residues of wax
may be difficult to remove completely and are
likely to interfere with the adhesion of non-wax
fills in the future. If wax is used as a filler, it
should be isolated from the original surface,
particularly where the fill would otherwise be
in contact with a porous ground or decorative
layer. Although wax fills are easily levelled,
many coatings will not adhere to them due to
their low surface energy and this may make
sealing and retouching difficult. Further, many
conservation inpainting media have a hydrocarbon
solvent component, which may disturb
the surface of a wax fill. Shellac, often used for
sealing and retouching wax fills in wood, is not
generally used in the conservation of decorative
surfaces due to the polarity of the solvents
needed to reverse it and a reported tendency to
crosslink over time (Horie, 1992).
Gesso Gesso is widely used in the West as a
ground for many decorated surfaces and, as the
interface between the substrate and the surface
layers, is often the locus for adhesive failure.
The term gesso originally referred to a gypsumbased
ground such as those commonly used in
southern Europe, and in some conservation dis-
Principles of consolidation, aesthetic reintegration and coatings 577
ciplines this specific definition remains. In the
context of furniture conservation, the term
gesso commonly includes ground layers based
on collagen glue and calcium carbonate
(chalk). Mecklenburg (1991) has demonstrated
that the proportion of chalk in the gesso will
affect both its strength and stiffness.
Mecklenburg’s experiments demonstrated that
the tensile strength of gesso is inversely proportional
to the chalk-to-glue ratio. A higher
proportion of chalk will reduce shrinkage and
produce a fill that is softer and more easily
reversed. Fills with a high proportion of chalk
can be very porous and may require sealing
(e.g. with varnish) before they are retouched, in
order to prevent the binding medium being
absorbed into the filler and leaving an underbound
retouch.
PVAL PVALs will not be exposed to UV when
used as fill materials and therefore crosslinking
should not be an issue. PVAL and whiting can
be used instead of the traditional rabbit skin
glue gesso. Such fills are water soluble and
readily reversible using only slightly dampened
swabs. The surface of the fill can be dampened
to allow easier removal of excess or light
moulding of the surface. For use as a fill, PVAL
is dissolved in water to make a solution
between 6% and 8%. Calcium carbonate is
added to the solution until the mixture reaches
the desired consistency, a dough for deep fills
or much less viscous for shallow fills. The
resulting mixture is thixotropic, which makes it
difficult to apply in the same way as a traditional
gesso is applied. It is best to spread it
with a rubber spatula in order to obtain a
smooth finish. It can be wetted again later for
further smoothing, but can be dry sanded for
the final finish. As with any fill material, care
should be taken not to expose the adjacent surface
to the fill material. PVAL has good working
properties, porosity comparable to gesso
and is reversible in organic solvents such as
alcohol. The use of PVAL as a filling material is
discussed by Hebrard and Small (1991) and
Webb (1998).
Commercial preparations Commercial preparations
such as Polyfix (calcium carbonate and
other fillers in a vinyl acetate emulsion),
Polyfilla (Canada) (cellulose ether and plaster)
and fine surface Polyfilla (UK) (vinyl acetate