Page 125 - The Building Conservation Directory 2013

t w e n t i e t h a n n i v e r s a r y e d i t i o n t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 1 2 5
3.3
Structure & Fabric :
Metal,
Wood & Glass
can trigger corrosion damage even on post-
medieval glass and paint.
Painted decoration is particularly at risk.
Glass paint is similar to a pottery glaze and
is fused to the glass surface in a kiln before
the window is assembled. Windows of all
ages can have problems with poorly fired,
damaged or decaying glass paint (above and
right), and it takes an expert to tell whether
or not the paint is stable.
How to clean stained glass
Assessing the condition of glass and painted
decoration and advising on a suitable
cleaning method should always be carried
out by an accredited conservator, even if
the actual cleaning can in some cases be
carried out by non-professionals. Even plain
unpainted glass may be very old and can be
damaged by the wrong choice of cleaning
method or by unskilled hands.
Church windows consist of a mosaic of
small glass pieces which are held together
with lead profiles and weather-proofed with a
grout. They are typically inserted into stone
surrounds and strengthened against wind
pressure with horizontal metal bars that are
inserted into the surrounding stonework and
attached to the windows with wire.
The complicated construction of church
windows means that they have to be cleaned
by hand, piece by piece. Depending on the
nature of the deposits, cleaning can be
time consuming and potentially damaging,
and may need to be carried out by skilled
professionals. In some cases, gentle and
comparatively quick cleaning with cotton
wool and distilled water can remove simple
loose dirt and bring a window back to its
original splendour. In other cases, cleaning
may be impossible, or can only be done under
a microscope in a conservation studio.
Deposits on windows can include bits of
grit from surrounding stonework or rust from
iron bars, which could be trapped in the other
dust deposits. These can easily act like an
abrasive powder and scratch the glass surface.
Limescale is also often present and this can
leave very unsightly smear marks that may
On the internal surface
Even if a window is not leaking, water in
the form of condensation will regularly run
down the inside surface and can create thick
limescale deposits (facing page). Soot from
decades of burning candles can gradually
cause window glass to darken.
Algae, fungi and moulds (facing page)
are more often found on the inside of a
window, as there they have a regular supply
of condensation water but are not washed
away by rain.
Corrosion of medieval glass
Medieval glass will often show corrosion
damage and weathering crusts on the inside
as well as the outside.
On medieval glass, corrosion processes
can result in ‘weathering crusts’. These can
cover entire pieces evenly with brownish or
whitish crusts but they also often erupt from
distinct pits, covering the surface in white
spots (above). These crusts are the result of
a chemical interaction between the glass
and the atmosphere under the influence of
water. Underneath the weathering crusts
the medieval glass surface is always severely
damaged and very fragile.
Damage caused by deposits
Some glass types are more prone to
damage than others and medieval glass
is particularly vulnerable. Some deposits
can be harmful to the glass while others
are not. Patina and some hard crusts,
although they can be unsightly, are unlikely
to cause damage to the underlying glass.
Soft deposits that attract and hold water
on the surface, and particularly organic
growth, can actively damage the glass by
keeping it damp. Organic growth often has
acidic metabolic by-products and, over time,
Typical corrosion damage to the exterior face of a medieval stained glass window Loose flaking paint on medieval glass
Paint loss on 19th-century glass
Rust staining before and after cleaning