Series editors' preface - Wood Tools

648 Conservation of Furniture
and provides the base for the gesso. Rabbit skin
size is now most commonly used throughout
the gilding process, but traditionally parchment
cuttings were used. Parchment size is finer and
less viscous than rabbit skin size and is still used
by some modern gilders, even though it is
inconvenient and more time-consuming to
prepare in the quantities necessary for the size
coats and gesso layers. Rabbit skin granules and
sheets are readily available, easy to prepare in
quantity, and are more consistent in strength.
They are therefore ideal to use for preparing the
size coat and binder in the gesso.
14.2.3 Preparation of glue size
Glue size and gesso should always be
prepared in a double boiler or ‘bain marie’.
The temperature of the mixture should be no
higher than 55 °C in the preparation and
subsequent reheating of the rabbit skin size.
Higher temperatures will denature the protein,
impairing the elasticity and adhesive properties
of the glue.
The strength of the prepared size is determined
by the ratio of dry glue to water. Some
gilders prefer a very strong solution of four
parts of water to one part of dry glue whilst
others use a medium strength solution of ten
parts water to one part dry glue. There are
dangers in making the glue too strong or too
weak: if it is very strong it will be difficult to
smooth once dry and will burnish poorly,
whilst if it is too weak the gesso will have
little strength and may crumble.
A medium strength solution of 1:10 v/v
rabbit skin granules to water is recommended
here. After pre-soaking in water, preferably
overnight or for at least four hours, the rabbit
skin granules should be gently warmed
(40–50 °C) in a double boiler and occasionally
stirred until the glue has dissolved. If a skin
forms it indicates that either the glue is too
hot or the room is too cold. Once evenly
melted, the size should be set aside, allowed
to cool and the gel assessed for strength.
Parchment size may be prepared by cutting
the parchment into small pieces about an inch
square and soaking them in cold water for at
least an hour. The water is discarded and the
clippings rinsed in clean running water. The
cuttings are placed in a double boiler and the
outer water simmered for two to four hours.
One pound (454 g, dry weight) of cuttings
may be simmered in four pints (approx. two
litres) of water and will result in a fairly strong
size roughly equivalent to a 1:6 or 1:7 (v/v
water:glue granules) mix prepared with rabbit
skin glue. Some gilders prefer a slightly
weaker size prepared with 1 ⁄4 pound parchment
clippings (113 g) in two pints (1.137 litres) of
water (Green, 1979; Sheraton, 1803). In either
case the clippings are discarded before the
parchment size is strained through two thicknesses
of fine tights or silk, allowed to set and
assessed for gel strength. The liquid should be
clear and when it has set should appear transparent
with a yellowish tinge. Although
believed to be necessary for the best quality
gesso, parchment may be difficult to acquire
in large quantities. Parchment size may be
reserved for mixing with the bole or for
matting and protecting the gold.
14.2.4 Assessing gel strength
Rabbit skin glue and parchment size may vary
in the gel strength produced from batch to
batch. Since the gel strength is critical to the
success of the gessoing, it is essential that each
mix of size is allowed to cool and set to allow
gel strength to be assessed. The size should
be left overnight for this purpose. Gel strength
is a matter of feel. Run a fingertip over the
surface of the size – it should have the feel of
a medium to stiff table jelly. If the gel is
broken easily and cleanly it is the right
strength. If a fingertip can be pushed into the
size without the glue breaking apart, it is too
strong and must be warmed and diluted
before testing gel strength again. If the gesso
cannot be cleanly parted it is too weak and a
fresh batch should be made.
14.2.5 Sizing the wood
Once the surface has been cleaned and dried
a thin layer of glue size is applied to the
surface. It is important for this coat of size to
penetrate the wooden support as much as
possible in order to provide adhesion and
stability to the subsequent gesso layers.
The wood may be coated with pure size, or
size with a little whiting added, this last
known as clair colle or clearcole. If the wood
is very absorbent and open grained it may be