Series editors' preface - Wood Tools

necessary to use a size coat as strong as six
parts water to one part glue and to apply a
second coat. The strength of the size coat may
vary from a glue strength of 4:1 to 10:1 v/v
water to dry glue – the fundamental principle
of gilding is that size and gesso should
progress from strong to weak as the layers are
built up – subsequent gesso layers should
never be based on a stronger mix than was
used for the size coat. Care should be taken
to avoid applying too much size or allowing
it to puddle. The wood should not have a
shiny appearance after the size has dried.
Sufficient size to cover the job should be
warmed in a double boiler. Some gilders apply
the hot glue to the surface using a stiff gesso
brush to work the size into the grain, using a
tamping motion to reach recesses and undercut
carving. Some gilders stiffen a hog’s-hair
brush by wrapping the hairs to within a
couple of inches of the end with butcher’s
twine and coating the wrapping with epoxy.
Some gilders prefer to use a second size coat
of clair colle (thin white), made by adding a
small quantity of whiting (calcium carbonate)
or calcium sulphate to the size. This allows the
gilder to see where the size has been applied
and may provide a mechanical key for the
subsequent gesso layer (Watin, 1755). The
whiting should be added when the glue has
been warmed to around 37–40 °C (blood
temperature). The amount of whiting is not
critical but should be enough to give the size
a milky appearance – a rounded tablespoon
of whiting added to 300 ml (10 fl oz) will be
sufficient. The whiting should be sieved and
then sprinkled onto the surface of the size,
allowed to settle and then stirred very gently
with a hog’s hair gesso brush gently
backwards and forwards on the bottom of the
container until the whiting is mixed into the
size. A wetting agent, such as few drops of
methylated spirits (denatured ethanol) or
Photoflo, may be added to reduce the surface
tension of the size coat. Whether size or clair
colle is used, the surface should be allowed
to dry thoroughly before subsequent coats of
gesso are applied.
14.2.6 Gesso
The function of the gesso layer is to provide
a surface that can be smoothed to a fine finish
Introduction to traditional gilding 649
onto which gold can be laid. The gesso layers
obscure both the grain and any imperfections
in the wooden substrate and provide a
cushioning layer that allows the gold to be
burnished. Gesso is an adaptable material that
can be textured, recut, incised or engraved and
can also be used to make raised designs. The
polymeric coiled structure of collagen glue
produces a material that is slightly elastic, even
when dried to equilibrium with the ambient
environment. This allows the gesso to tolerate
a certain amount of substrate movement
without flaking. Gesso provides a fine, white,
uniform surface and has traditionally been used
as a ground for painting, water gilding and, on
occasion, oil gilding. Additional information on
gesso can be found in Chapter 4.
The term gesso originally referred to a
gypsum-based ground such as those
commonly used in southern Europe, and in
some conservation disciplines this specific
definition remains. In the context of gilded
furniture, the term gesso now commonly
includes gesso based on calcium carbonate.
Thus ‘gesso’ describes a mixture of either
rabbit skin or parchment size combined with
an inert filler. In Northern European regions,
particularly England and France, a very fine
variety of calcium carbonate (chalk whiting –
CaCO 3), known as gilder’s whiting, is used to
make gesso. This whiting produces a gesso
that is fine and hard, suitable for recutting and
crisp outlines. In Italy and other regions of
Southern Europe, calcined gypsum (plaster of
Paris) is used as a filler to make gesso.
The use of different fillers produces gessoes
with distinct characteristics. Each type will
produce a different appearance on the final
surface. It is not possible, for example, to recut
Italian gesso sottile as finely as is necessary to
reproduce the sharp and intricate designs on
many pieces of French furniture. Similarly, the
soft look and fluid feel of Italian and Spanish
gilded works, which is partly achieved by the
malleable nature of gesso sottile, cannot be
reproduced using calcium carbonate as a filler.
Kaolin (China clay) has also been used
occasionally as a filler, for example on highly
detailed French carving in the nineteenth
century and on oriental objects (see, for
example, Larson and Kerr, 1985). The detail in
the carving is not lost because only one or
two coats are applied and a relatively good