10
BCD SPECIAL REPORT ON
HISTORIC CHURCHES
21ST ANNUAL EDITION
PROTECTIVE GLAZING
Tobit Curteis and Naomi Luxford
D
ESPITE ITS
fragile and brittle nature,
stained glass has survived in many
churches and cathedrals from as early
as the 12th century. Often it is the last vestige
of the rich colourful decoration that covered
walls, fittings and furnishings in the medieval
church interior, and early examples are of
huge significance. Unlike other works of art,
stained glass also forms an integral part of
the structure and envelope of the building.
As such, it is of particular importance that
stained glass of all periods should be conserved
in situ wherever possible, an approach which
often requires the use of protective glazing.
As part of an initiative to improve the
understanding of protective glazing systems,
a detailed research project commissioned by
the Building Conservation and Research Team
at English Heritage is currently under way.
DETERIORATION
Seen from a distance inside a church, historic
stained glass can look remarkably well preserved.
However, distance and transmitted light often
disguise problems which are only apparent
on much closer inspection. Medieval stained
glass, and some later painted decoration, are
often chemically unstable as a result of the
materials and techniques used to create them. In
particular, the leaching of potassium and sodium
ions from the glass in the presence of water can
cause severe physical deterioration of the glass
body. Stained glass windows are also subjected
to wind-loading which can damage the entire
structure. The risk of vandalism and deliberate
damage add further weight to the arguments
for protecting the exterior face in particular.
In addition, the internal face can
be damaged by repeated high levels of
condensation, which facilitate the dissolution
of soluble compounds within the glass paint
and body causing pitting. Differential thermal
stresses can lead to delamination of paint and
enamel layers. High levels of condensation
can also lead to substantial microbiological
growth on the surface of the glass. Not only
can this growth be disfiguring but it can
also retain moisture and the physical and
chemical side-effects of the lifecycle of the
organisms can cause further damage.
OPTIONS FOR PROTECTION
Although there are a number of ways of
protecting stained glass from physical
damage and vandalism, for example the
use of wire mesh, the only system which
can control chemical and environmental
deterioration in situ is protective glazing.
Protective glazing has a long history:
it was used at York Minster from 1856 and
Lindena church in Germany from as early as
1796. However, the benefits of this approach
were first recorded during the 1960s in Bern
Minster in Switzerland, where some of the
stained glass had been reinstalled in frames
behind new glazing after the war. These
were in noticeably better condition than
the panels that had been reinstalled in their
original positions, which were unprotected.
Protective glazing is now widely used to
conserve important and vulnerable stained
glass. However, just as no two windows or
churches are the same, no two protective
glazing systems are the same. As a result
it has been difficult to determine the best
design features for specific installations.
CURRENT PRACTICE
Protective glazing involves the installation of a
layer of new glass on the exterior of the window.
In some cases the historic glass is left in situ,
while in others it is moved forward (towards the
interior) on a metal support with the new glazing
installed in the original grooves in the tracery.
In most cases the gap between the original
glass and the protective glazing is ventilated at
the top and bottom, which allows air to pass
through the interspace between the two. Some
systems are vented to the outside (externally
ventilated), some to the inside (internally
ventilated) and some combine the two.
In some cases sealed units that are more
similar to double glazing have been used.
However, due to the difficulty in creating
an effective seal, significant condensation
can occur and this approach is now rare.
Most current protective glazing is internally
ventilated, allowing the historic glass to
be surrounded by air from within the
building, which also, generally, has the
advantage of lower levels of pollution.
AESTHETICS
One of the main concerns with the use of
protective glazing is its impact on the appearance
of the building. A wide variety of approaches
can be considered to modify the external
appearance of the protective glazing, including
glass type, glass surface treatment, and whether
the glazing is constructed in panels or is leaded
to imitate the original glass and mounting.
However, change in appearance is nothing
new. Corroding 20th-century metal mesh is
often tolerated because it appears always to have
been there. Increasing opacity and distortion
of the original glass is tolerated because it
occurs gradually. However, both cause at least
the same level of visual impact as protective
glazing when viewed externally. Internally,
the shadows cast by grilles are particularly
damaging to the appreciation of works of art.
PREVIOUS RESEARCH
Over the past 40 years research has been
carried out, mostly in France and Germany,
into the efficacy of different protective glazing
systems. Between 2002 and 2005 an EU-funded
project (VIDRIO) studied protective glazing
in a number of churches and cathedrals across
Europe. This project looked at how windows
were ventilated, the amount and duration of
condensation periods, pollution levels within
The effects of moisture: left, pitting caused by the dissolution of soluble components (Photo: Tobit Curteis Associates) and, right, microbiological growth (Photo: Holy Well Glass)
