Series editors' preface - Wood Tools

ensure that any water residues are removed.
Any other treatment should be undertaken by
a specialist.
15.5 Flat glass, mirrors, reverse
painted and gilded glass
Glass may be found association with many
different forms of furniture including glazed
doors and door lights, wall mounted framed
mirrors and reverse paintings on glass, mirrors
within dressing tables and boxes or as small
decorative elements such as painted or gilded
glass on frames and drawer fronts. These
glasses cannot be considered in isolation from
the furniture of which they form a part. As
with a framed painting, the glass is the
functional part of a framed mirror. The mirror
frame, like the picture frame, is a support for,
and an enhancement of the looking glass.
Both glass and frame make up the mirror and
it would be unethical to separate them. Where
both furniture and glass are to be conserved,
it is necessary to consider the level of conservation
treatment in order that visual harmony
is maintained. All glass can be conserved if the
budget allows and the owner really wants to
keep the original. Replacing original glass with
a reproduction can significantly reduce the
value of the whole object.
15.5.1 Flat glass
Flat glass can be made by various methods
(see section 5.6) that can be determined by
visual examination and analysis (Frank, 1982).
Glass is a non-porous, brittle and may be
considered a comparatively ‘unforgiving’
material. Mechanical damage is the most
common cause of deterioration because of the
inherent brittleness of the material. Scratches
can be ground from the glass, but will leave
a dip that may prove obtrusive, particularly on
mirrored glass. Handling glass can be
problematic. Not only can it be very heavy
and unwieldy, but also there is a possibility
that cracks may propagate rapidly and lead to
breakage. Such cracks may be small or invisible
to the naked eye but may extend when
the glass is handled. Changes in temperature,
especially sudden cooling known as thermal
shock, may cause glass to crack.
Conserving other materials I 705
Broken glass is difficult to re-assemble and
bond, both because of the nature of the glass
and the relatively few materials with a refractive
index suitable for glass repair (Tennent
and Townsend, 1984). Although generally
found in vessel glass, ‘glass disease’ may also
be encountered in furniture, particularly where
the glass is enclosed, for example in toilet
boxes, clocks or barometers. Much of the
information on the conservation of glass deals
with vessel glass rather than the flat glass that
is likely to be encountered in furniture (e.g.
Tennent, 1999).
15.5.2 Mirrored glass
Glass may be decorated in many different
ways. The most common forms are mirrors,
reverse painted glass (hinterglasmalerei) and
verre eglomisé. These decorative techniques
may be found together on the same object or
even on the same plate of glass.
Mirrors have been produced since early in
the fourteenth century by applying a metallic
layer to the reverse side of a flat sheet of glass
(Child, 1990; Wills, 1965). Two distinct types
are mercury/tin amalgam and silvered mirrors
(see section 5.6). Tin/mercury mirrors were
produced up to the beginning of the twentieth
century, whilst silver-backed mirrors were
produced from the middle of the nineteenth
century. The two differ in both colour and
reflectivity. Tin/mercury mirrors reflect darkly
and have a grey tint, whilst silvered mirrors
are brighter and have a slight yellow reflection.
Whilst the back of silvered mirrors were
always given a protective coat of paint,
tin/mercury mirrors were rarely painted,
except, for example, those used at sea.
Mercury mirrors can be recognized by the
rough matt grey (verso) and by the distinctive
characteristics of the deterioration seen from
the front.
The tin/mercury surface is a two-phase
system in which crystals of a tin–mercury
compound are surrounded by a mercury-rich
liquid phase. This two-phase amalgam layer is
unstable. The crystals change shape and grow
larger whilst the amalgam layer oxidizes and
releases liquid mercury that slowly evaporates.
The liquid phase moves slowly to the bottom
edge of the mirror, where it is not uncommon
to find droplets of mercury. Whilst the upper