Glass and Ceramics 171
polymer into the pores and cracks. The application of the polymer in solution
with a brush or a pipette is common, followed by the removal of excess mate-
rial with a solvent.
Glass, as a fragile material, often needs repair by edge bonding of broken
pieces together. The best visual appearance for transparent, colourless glasses
is achieved when the refractive indices of glass and adhesive match, in order
to avoid reflections from the breaks. For practical reasons, epoxy resins with
varying viscosity and curing time, commercially available under trade names
such as Epotec, Ablebond, Araldite, Fynebond or HYXTAL, are frequently
used in glass conservation. For sensitive glasses, where epoxy resins tend to
provide a strong joint, which is difficult to remove after aging, weaker bonds
created by cellulose nitrates or acrylates are preferred.
The adhesion of polymers to glass is best if the surface is clean. This
becomes a problem when setting up the conservation strategy: consolidation
before cleaning has to be favoured, for example, if the distinction between
dirt and paint is difficult or for gilded surfaces. Cleaning before the application
of a polymer is preferred, e.g. when broken pieces need strong joints.
Polymers in glass conservation may also be applied for filling gaps and
modelling missing areas in broken objects.
2.8 Restoration and Cleaning
The restoration and conservation strategy for any kind of glass object has to be
based on preliminary investigations, carried out by visual inspection and under
the optical microscope (in transmitting and reflecting light). Heavily degraded
objects or pieces with surface decoration (paint layers or engravings) will need
more careful treatments than stable, unaltered sheet glass. All initial examina-
tions should be part of the overall recording and documentation procedure.
The surface structure, and especially its roughness, is a decisive parameter
for glass weathering. Therefore, the selection of an appropriate cleaning process
has to take into account not only its effectiveness, but also the potential dam-
age for the object.
In accordance with the general conservation principle of minimum interven-
tion, the main objective is to conserve the glass, and not to recover transparency,
through removal of corrosion products and deposits. Only in exceptional cir-
cumstances, therefore, may weathering layers be removed to increase the trans-
parency of the glass or to support its interpretation. In any case, damage to the
hydrated layer must be avoided: this layer is considered to be the “skin” of
the glass, which protects it from further attack.
A variety of “dry” methods are at the disposal of the conservator to remove
dirt and other solid deposits from the surface: soft bristle brushes may be
sufficient for the removal of porous encrustations, whereas more severely