Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 143
based gilding ground such as those commonly
used in southern Europe. In the general context
of painted, japanned or gilded furniture,
the term is extended to include grounds based
on calcium carbonate. This may not be the
case in other conservation disciplines, where
the use of the term ‘gesso’ may be specific to
calcium sulphate grounds and where other
grounds, such as those based on calcium carbonate,
may be simply described as ‘white
ground’. Cennini’s fifteenth century commentary
on artists’ materials recommends that the
best size for gessoing panels is made from
parchment prepared from the necks of goats
and sheep by trimming, washing, soaking and
then boiling it (Thompson, 1960). Other historic
treatises such as those by Theophilus and
Watin also advise on the preferred animal glue
for making gesso, although interestingly, none
mentions rabbit-skin glue (Souza and Derrick,
1995).
The filler is typically calcium carbonate or
calcium sulphate but other materials can be
used. Historically, calcium sulphate has been
used in the Mediterranean basin and calcium
carbonate in Northern Europe, where chalk
was plentiful, but this is not invariable. Kaolin
(A 12O 3.2SiO 2.2H 2O) and sometimes dolomite
(CaMg(CO 3)2) were also used (Cession, 1990).
Large deposits of gypsum are found near
Bologna and Volterra and also occur in varying
grades throughout the world. Chemically, gypsum
is calcium sulphate dihydrate formed by
the deposition of salts in inland lakes. The raw,
unburned gypsum has no function in the making
of gesso and is only used once it is calcined
and ground to make plaster of Paris
(CaSO 4. 1 ⁄2H 2O). The powder, known as gesso in
Italian, is used in two ways. It is soaked in
excess water (so that it does not set) for four or
five weeks with frequent stirring and changes
of water. The residue, after the water is poured
off, is made into small loaves and dried. This
very fine powder, known as gesso di Bologna,
can then be crushed, sieved and mixed with
size to make gesso sottile. To make gesso grosso,
unadulterated plaster of Paris is sifted and
mixed into a paste with either parchment or
rabbit-skin size. Gesso di Bologna is slightly
greyer than chalk whiting (see below), it feels
cooler and softer, and compacts more easily. It
is lumpy and therefore it is always necessary to
sieve it before mixing it into the size.
In Northern European regions, particularly
England and France, where the chalk deposits
are plentiful and of a high quality, a very fine
variety of calcium carbonate (chalk whiting –
CaCO 3) known as gilders’ whiting is used to
make gesso. Natural chalk is a soft, white rock
which is largely composed of the remains of
minute sea organisms. It is extracted by opensurface
mining of deposits with minimal impurities
and a high degree of whiteness. Once the
material is extracted it is dried to lower its
water content from 20% to 0.2%. For gilders’
whiting the rocks are ground to a particle size
of 3.3 or 4 μm. The fine, soft, white powder is
added to glue size, made from parchment cuttings
or rabbit skins, to make gesso.
Southern European grounds prepared with
gypsum (natural gypsum is calcium sulphate –
CaSO 4.2H 2O), usually consist of a coarse preliminary
preparation followed by a smoother
one. Cennini describes the two stages, gesso
grosso (coarse) and gesso sottile (fine). Typically,
gesso grosso refers to anhydrite (CaSO 4)
or plaster of Paris (CaSO 4. 1 ⁄2H 2O), which is then
coated with a gesso sottile made of calcium sulphate
(CaSO 4.2H 2O), the difference being the
degree of hydration and the consequent chemical
link. Anhydrite and plaster of Paris set by
hydration, regaining the water lost during heating,
but when mixed with glue, they become
much harder.
Von Endt and Baker (1991) explain that in a
filled animal glue system, the hydrogen bonds
of the glue are pulled apart to allow the filler
into the matrix, which reduces the flexibility of
the chains. This makes the matrix stiffer, but
also weaker as less force is required to pull it
apart. Fillers such as calcium carbonate and
sulphate do not interact chemically with the
matrix and consequently form a system that is
weaker than those in which there is chemical
linking between the filler and the polymer.
Although filled glue system are less permeable
to water than unfilled ones, gesso can be permanently
deformed by the introduction of
humidity because the molecules do not always
go back to their original positions and less
stress is required to provoke subsequent deformation.
Bole
A type of clay also called bole is used to facilitate
the burnishing of water gilding. The name