Series editors' preface - Wood Tools

340 Conservation of Furniture
methacrylate) can usually be removed in their
original solvents as a mixture of swollen gel
and sol up to the point at which the film
becomes 90% insoluble.
The third stage in the ageing of a polymer
that exclusively crosslinks occurs when the
film is almost completely insoluble but is able
to swell with solvents. With increasing
crosslink density and possible accompanying
changes in polarity progressively more polar
solvents are required to cause swelling.
Eventually a polymer may become so highly
crosslinked that it is not effectively swollen by
solvents.
The crosslinking of n-butyl and others of the
methacrylate series is a phenomenon that can
take place under ordinary gallery conditions.
Under conditions where precautions are taken
to reduce the light intensity below 150 lux and
to remove the UV component these films will
perhaps become about 50% insoluble in about
twenty-five years. Tests are available to detect
the presence of insoluble material and to
monitor the course of such changes with a fair
degree of confidence. It is possible to inhibit
the crosslinking tendencies of some methacrylates
(e.g. iso amyl methacrylate) but once the
inhibitor has been used up through preferential
reaction with light or oxygen the polymer
material itself again becomes susceptible. Copolymerization
techniques may be more effective.
The deterioration of surface coatings is
further discussed by Feller (1994; Feller et al.,
1985), Horie (1987), Koller and Mairinger
(1975) and Masschelein-Kleiner (1985).
8.9.5 Gilding
Gilded surfaces employ gold or imitations of
gold adhered to a suitable ground or support.
Supports are commonly wood but may also be
metal, ivory, stone, glass, ceramic or other
material. Gilding can be applied directly to the
support but frequently a ground is present,
especially where wooden supports are used.
This may be a simple paint or a more complex
preparation of gesso and bole. The metal leaf
can be attached with an oil-based gold size or
with a protein based adhesive such as an
animal glue or egg white. Deterioration of
gilded surfaces can occur through changes in
the support, the ground, the mordant or the
metal itself and is commonplace with furniture
and related objects. Indeed, it is rare and
pleasing to find an intact gilded surface on any
object of substantial age. Factors that may
contribute to the process of deterioration have
their origins in the materials and techniques
used in the original construction, in the pattern
of usage of the gilded object and in the nature
of the environment to which it has been
exposed. The process of gilding is not actually
difficult but requires considerable skill and
judgement by the practitioner to ensure the
longevity of the surface.
Failure in wooden supports is discussed in
Chapter 7. Unbalanced construction, poor
selection and preparation of materials predispose
gilded surfaces to failure. Gesso adheres
much better to most softwoods than to
hardwoods, though non-resinous softwoods
are preferable. Examples of gilding on birch,
maple, oak and mahogany may be found in
good condition but gilding on beech is
frequently found to have flaked off to a
marked degree. Gilded objects placed in
centrally heated environments are especially
prone to damage caused by movement in the
support. This is made worse by unbalanced
construction and by placing objects against
cold (and possibly damp) exterior walls when
a notable difference may result in the RH of
the air trapped behind the piece as compared
to general room RH. Inadequate preparation
of wooden supports to receive gesso grounds
can cause premature delamination of the
ground.
The ground, usually a gesso, can lose its
properties of adhesion and cohesion and
separate from the substrate (see section 8.9.2).
The commonly observed cracking of the
gesso/gilded structure perpendicular to the
grain of the wood is a complex phenomenon.
It may occur in part because the gesso is
under the most stress under low humidity
conditions with fractures occurring as the
gesso contracts more or less equally in all
directions while the wood does not
(Mecklenburg, 1991). The characteristics of a
good gesso and the means of applying it to
wood to create a ground are described by
Thompson (1962) and in Chapter 14. Several
things can go wrong during preparation or
application. Glue size may chill too quickly
during preliminary sizing of the wood thus
leading to the creation of a relatively weak