Series editors' preface - Wood Tools

ond with the support. Uneven drying of
gesso, which may occur for example through
direct heating by the sun or by too rapid
drying, may lead to the formation of a hard
dry layer over a softer wetter layer during the
build up of the ground and cause premature
cracking. Overheating the gesso will denature
the protein in the animal glue binder and
cause loss of strength in the ground and is a
frequent cause of air bubbles in gesso.
Changes in strength of gesso in the ground
layers should be gradual with the strongest
layers next to the support. Slightly weaker
layers may be applied satisfactorily over the
first layer but application of stronger layers
over weaker layers may cause the top coat to
curl up and pull off the layers beneath. This
can happen if too much water is allowed to
evaporate from the gesso mix during the
application of the ground. Size that is too
strong will lead to the ground flaking and
cracking off while gesso grounds that are too
weak will tend to chalk and burnish poorly.
With the exception of the application of the
initial size layer, it is important that the gesso
ground should be completed from a single
batch of gesso and that the ground is not
allowed to dry out completely once the
process of applying gesso has started. Faults
can also occur in the application of both gesso
and bole layers. Bridges, which may form over
recessed parts, may later break. Attempts to
rectify imperfections in wet bole during its
application may lead to later failure where the
ground has been unduly disturbed. Gilders
composition, frequently used for imitating
carved ornament, is prone to shrinkage, cracking
and embrittlement on drying. Dirt and
cleaning materials penetrate the cracks and
weaken the bond with the substrate.
Oil gilding, though easier to do than water
gilding, still requires skill and judgement. Dust
allowed to settle over a slow drying oil size
will spoil the surface and render it more liable
to subsequent tearing and abrasion (e.g.
during cleaning). If oil size is applied over too
large an area at one time it will not be possible
to complete the gilding while the mordant
is in the correct state to receive the gold and
this will lead to uneven brightness. Gold that
is applied while the oil is still too wet will not
give a good lustre, whilst that applied when
the size is too dry will not adhere properly. If
Deterioration of other materials and structures 341
the oil size layer is too thick or if too much
oil is added to it a soft film will remain under
the gold which will cause it to mark easily and
wrinkle. Oil gilding is easily damaged by polar
organic solvents but is resistant to water.
Water gilding is very easily damaged by water
but is resistant to non-polar organic solvents.
The gilding itself can corrode, particularly
when it has been alloyed with other metals or
when its primary component is a metal other
than gold. Gold leaf alloys below 22 ct may
be subject to sulphide tarnishing (Linns, 1991).
High purity gold is resistant to corrosion but
leaf alloyed with silver and copper is susceptible
to tarnish from pollution in various forms.
Skin salts and acids will frequently cause
finger prints to appear on the gilded surface
shortly after handling and these will be permanently
etched into the surface. Silver leaf
Figure 8.9 Spirit gilding that has taken on the
craquelure of aged varnish can be seen in this detail of
a dressing bureau, rosewood (Dalbergia latifolia),
Philadelphia, Pennsylvania, c.1830. Part of the ‘Empire
Bedroom Suite, Winterthur Museum