Series editors' preface - Wood Tools

The materials used in the decoration of glass
are fragile and easily damaged by abrasion,
moisture, organic solvents and corrosive
vapours (e.g. carbonyl pollutants). The glass
was intended to be a protective face and the
reverse was almost always enclosed by a
backboard or mounted onto the furniture and
thus protected by it.
Glass exhibits a degree of shrinkage and
expansion in response to changes in temperature.
Unlike a painting on canvas, the binding
media of the paint cannot physically soak into
the glass support, thus the two layers are not
mechanically bound together. Over time, the
painted layer will become brittle and shrink
due to loss of solvent. Thus the main problem
associated with decorated glass is the detachment
of the painted or gilded layer from the
glass and its subsequent loss.
Cupping and shrinkage of the paint result in
losses of the metal leaf. The painted glass will
appear grey where leaf has detached. Corrosion
of silver leaf will occur wherever it is exposed
to the environment, mainly along the edges or
where there are splits in the backboard. Animal
glue may have been used to attach backing
papers and can also cause deterioration as it
ages and becomes brittle. The pressure exerted
by the glue can pull the painted layer and metal
leaf away from the glass. Oxidation of the paint
may also cause colour change. Although the
painted layer is protected from the front where
it is securely bound to the glass, the reverse
may be discoloured. UV light can penetrate the
glass and cause bleaching, particularly of dark
colours, but even in the same media, various
pigments are affected differently. Humidity can
also cause a problem as animal glue or gelatin
may have been used as a coating on the glass.
Mould growth can develop in damp conditions.
15.5.4 Repairs to adjacent wood
Potential damage to the glass as a result of the
handling or transport of the object must be
considered before repairs to adjacent wood or
a frame are undertaken. As a general rule, if
the glass is well supported, then it is safer for
it to travel with the object (see section 10.2.5
for guidelines for handling mirrors). Where the
glass and furniture are to be treated separately
the object should be first delivered to the glass
conservator where it can be taken apart.
Conserving other materials I 707
Figure 15.17 Missing sections of a mirror backboard
replaced with seasoned obeche wood
Depending on the work to be carried out on
the furniture, it may be possible for the glass
to remain in situ. However, the risk of breakage
or scratching from the use of hand tools
or abrasive papers near glass cannot be overemphasized.
The potential for chemical contamination
must also be considered. Off-gassing from
adhesives or wood used for the repair of
adjacent wood can corrode metal, particularly
if a microclimate has been created. PVAC
adhesives, for example, off-gas acetic acid
(Down et al., 1996), whilst polyester resins offgas
styrene that will blacken silver and tin.
Wood preservatives, varnishes and paints that
off-gas carbonyl pollutants should be avoided.
One of the criteria in the selection of wood
for repairs or board materials for backing
should be minimal off-gassing of acetic acid
(see Table 10.1) (Figure 15.17). Repair wood
or backing board may be isolated from the
decorated or mirrored glass by a strip of
Melinex/Mylar placed between the glass and
the wood. This reduces the rate of, but does
not completely prevent the, emission of
carbonyl pollutants. It is extremely important
to allow for some air movement, whilst also
minimizing the ingress of dust and dirt, to
prevent the formation of a destructive microclimate
(Thickett et al., 1998).
15.5.5 Removing glass
As a general rule the glass should be treated
in situ wherever possible. However in some
cases it may be necessary to remove glass to