Page 124 - The Building Conservation Directory 2013

1 2 4 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 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
3.3
Structure & Fabric :
Metal,
Wood & Glass
Cleaning Historic
Stained Glass Windows
Léonie Seliger
Stained glass and leaded windows
in churches do not normally require
regular cleaning. There are, however,
reasons why cleaning may become necessary.
This article provides an introduction to
the types of soiling found on the internal
and external faces of stained glass,
when and how to clean them safely and
sympathetically, and the kinds of damage
that can result from inappropriate cleaning.
Surface deposits
Surface deposits and accretions on windows
come in a great variety of forms, on both
internal and external surfaces.
On the external surface
Rainwater running down the outside of the
building and onto the windows slowly deposits
particles of the surrounding materials onto the
glass surfaces. These deposits include limescale
from render, mortar and limestone; and rust
from ironwork. Over time they form a thin but
very tough patina. On stained glass windows
this usually just mellows the intensity of the
sunlight passing through the glass but on clear
windows a dense patina can be quite intrusive.
Airborne particles can attach themselves
to the glass surface and to the leads. Heavy
traffic or industrial pollution can deposit thick
crusts, which are most visible in those areas
that are protected from direct rain, such as
at the top of a lancet, in small tracery panels
or under a horizontal bar. These crusts can
be quite loose and flaky, but they can also be
extremely hard. Tree sap may regularly coat a
window in sticky droplets, which then allow
dust to adhere to the glass. Over time, this can
result in similar crusts.
Bird guano is another frequent nuisance.
Because there are serious health risks
associated with bird guano, it should only be
removed by trained people who are aware of the
risks and use the appropriate safety equipment.
Organic growth such as algae and lichens
(top right) can also be found on the outside
of windows.
Mould growth
Lichens on the external surface of medieval stained glass
Limescale deposits