Series editors' preface - Wood Tools

650 Conservation of Furniture
burnish can be achieved using this method.
Thin coats of gesso, however, have a tendency
to flake and gesso made with kaolin cannot
be built up as successfully as calcium carbonate
into a thick gesso layer.
The strength of glue size used in gesso
recipes varies widely, but in general varies
between 15:1 and 10:1 v/v water to rabbit skin
glue granules. It is essential to prepare enough
gesso to complete the whole job because if
more than one preparation of gesso is used
on the same piece of work the glue strength
may vary between the batches. This can lead
to delamination if later layers have utilized a
stronger size. The gesso should be made using
size that has been pre-prepared and tested for
gel strength. The size should be warmed in a
double boiler and allowed to cool slightly
before adding the whiting. The whiting may
be ground in a mortar and pestle and/or
sieved to remove lumps and large particles.
The whiting should be sprinkled slowly onto
the surface of the glue. The amount of whiting
added will depend upon the thickness of
gesso required for the job. Some sources
recommend a thinner gesso mix for softwoods
(Wetherall, 1992). Some types of carving may
require only two or three thin coats of gesso
in order to fill the grain and smooth out any
tool marks. Work that requires extensive recutting
should receive more coats of gesso.
A medium viscosity gesso may be produced
by sprinkling the whiting into the glue until a
cone covers about half the surface of the size
(approximately a 2:1 w/w ratio of whiting to
glue) or until the whiting is level just below
the surface of the size. Some gilders allow the
whiting to settle and absorb the size and then
stir gently with a wooden stick, whilst others
stir the mixture as the whiting is added.
Stirring should always be gentle – excessive or
vigorous stirring will cause air bubbles in the
gesso. The mixture should be about the
consistency of thick cream and may be
brushed out on a test piece of wood to check
that it produces a smooth even layer. Some
gilders add a few drops of linseed oil, a pea
size lump of tallow wax, a drop of alcohol or
other surfactant to the gesso to improve the
working properties of the gesso, prevent a
skin forming on the top and reduce the
number of pin holes. It is critical to avoid or
minimize the formation of bubbles in the
gesso. The likelihood of bubbles increases as
temperature rises, or if excessive agitation is
used when stirring the whiting into the glue.
The gesso may be strained through a double
layer of fine nylon tights or silk. Once the
gesso is made it may be left to sit. This allows
for better absorption of glue in the whiting
and gives any air bubbles time to rise to the
surface where they can be removed once the
gesso has cooled. If the gesso has gelled it
should be slowly reheated to around 37–40 °C.
14.2.7 Application of gesso
Some gilders prefer to work by slicing enough
gelled gesso from the stock batch to apply a
coat to the work whilst others simply use one
large batch. In either case, warm the preprepared
gesso in a double boiler and ensure
that the temperature of the gesso does not
exceed 45 °C. Once warmed, the gesso should
be applied to the work. Some gilders take
gesso onto the brush from the centre to
bottom of the gesso container, swirling the
gesso gently each time to avoid the formation
of a skin on the surface, whilst others take
slowly from the side of the container. In either
case excess gesso should be removed from the
brush before it is applied to the work. Always
try to brush on each gesso layer in one application
without breaking off, which will avoid
the formation of ridges or an uneven build-up
in the gesso.
All the necessary coats should ideally be
applied on the same day, using the same
batch of gesso, to avoid the possibility of the
layers delaminating. If this is not achievable it
is possible to go over the dry gesso with a
damp brush. This will reactivate the glue in
the dry gesso reducing both the possibility of
‘shelling’ and the occurrence of pinholes in the
next layer. Each subsequent layer should be
applied once the preceding layer has become
dull or matte, which indicates that the glue has
gelled but is not yet completely dry. The gesso
will tend to thicken during the extended
process of applying multiple gesso coats.
Some gilders add a little water to reduce the
viscosity of the gesso and to slightly reduce
the strength of the upper layers, reducing the
risk of delamination and allowing for a better
burnish to the gold. Adding water in this way,