Conservation Information
This document provides information about leaded glass conservation
for owners and caretakers of
leaded glass windows and panels. Please call with any conservation-related
questions.
Deflection (sagging, bulging, buckling): A
common problem with older windows. Can often
be seen by looking edgewise along the
surface of the panel, or holding a yardstick up to the panel.
As the condition worsens, glass can
crack, cames can split, and wire ties can break. This condition
although a slow process, generally
causes accelerating damage as forces increase and support decreases.
Cause: Gravity, vibration and shock in ventilator
windows, and air pressure and temperature
changes combined with inadequate support
design or quality and old hard putty that doesn't allow
expansion and contraction. Corrosion
can be a factor. Protective covers that are not adequately
ventilated can drastically accelerate
deterioration.
Solution: In the early stages, the window
can be removed and soaked to soften the putty, laid flat
to allow the panel to relax, and support
bars added. The window is cleaned and checked. When
damage has started, the window may
have to be partially disassembled and cames replaced.
The means of installing should not
interfere with the natural flat-plane expansion and contraction
of the panel. Any protective covers should be adequately
ventilated.
Corrosion: When a leaded window is subjected
to pollutants, severe acid rain, or condensation,
lead cames can oxidize too much, eating
away at the metal. Any flaws in solder joints will be
attacked. Corrosion causes weakening,
and when a solder joint is affected can contribute to sagging.
Where a protective cover is used, improper
ventilation can cause microorganisms to thrive, both
damaging wood and adding to the oxidation
of the cames.
Note: A thin film of oxidation on lead and
aluminum is actually good, acting as a barrier to further
deterioration. Long-term exposure to
moisture from improper venting of protective glazing can
cause oxidation and corrosive processes
to continue beyond the beneficial stage.
Solution: Cleaning with a pH neutral detergent,
re-soldering of affected joints, and properly
venting protective glazing.
Support Structure, Bracing Rods, Tie Wires:
The design of a window determines what
bracing and support is needed. Long
horizontal runs of cames can act just like a hinge and
without bracing can cause the whole
house of cards to collapse. See sagging to learn more about
how this occurs.
Support rods are placed to span weak
areas, stretching from strength to strength. The window is
stabilized to its support with tie
wires that are soldered onto the cames and tied to the support
rods. Rods should be run as unobtrusively
as possible, following the natural seams in the panel
where possible.
Frames and Sashes: Leaded glass windows often
make use of complicated frames with curves,
arches, and special mouldings. These
should be monitored for paint or varnish condition to prevent
costly repairs.
When repairs are necessary, they should
be made using standard restoration processes. Joint
repairs should be reversible to enable
future repair. Deteriorated wood should be stabilized and
strengthened with restoration quality
epoxies. Aluminum extruded frames are available that are
appropriate for leaded glass windows,
and can be a viable option.
Protective Glazing: Purists try to stay away
from the use of protective glazing because it
detracts from the aesthetics
of leaded glass by hiding the intricate reflections and visual interest
behind a flat reflection. A well-constructed
leaded glass window with flexible putty can be
relatively thermally efficient
on its own.
Vandalism is the most common reason
for using protective glazing. When protection is needed, it is
vitally important that it be properly
ventilated.
In historic or aesthetically important
situations, the protective panel can be constructed with
leading that follows the main geometric
elements, providing a more visually appealing
compromise.
Clear glass and tempered glass are good solutions
for most windows, and have the advantage of
being scratch proof and non-weathering.
Acrylic panels provide good protection from
accidental breakage or vandalism. Acrylic panels
should be installed under skylights
as a safety precaution, and building codes may require them.
Acrylic exposed to sunlight will become
more brittle and less transparent over time.
Lexan is a clear polycarbonate plastic that
has superior impact resistance. Although expensive, it
provides the highest level of vandalism
protection. Lexan will gradually become more brittle and
less transparent when exposed to sunlight.
Laminated Glass combines the benefits of both
glass and plastic. An ultraviolet resistant plastic
film is bonded between two sheets of
glass. Although the glass can shatter, the plastic can
maintain the integrity of the protection.
Laminated glass is stronger and offers better insulation
than regular glass.
Thermopane glass can be used where thermal
efficiency is a primary concern. It can be very
expensive.
Is protective glazing slowly destroying your stained
glass windows?
This is very possible, and unfortunately
probable,
if the protective glazing system was
installed without ventilation. In the
1960's and 70's the issues of energy
conservation and protection from vandalism were common
concerns. Well-intentioned installers
of protective glazing often did not provide ventilation, in an
effort to conserve energy, during the
heyday of protective glazing installations.
We as an industry have learned from
on-going studies here and in Europe that stained glass
windows do not need protection from
the elements and pollution -- only from insensitive people!
Also, any protective glazing must be
ventilated to prevent heat build-up, to avoid condensation,
and to prevent air pressure build-up.
How inadequately vented protective glazing
works to destroy your stained glass windows:
A) Heat gain from the sun
The air column that is trapped between
the protective glazing and the stained glass is being super-
heated by the sun, just as the air
in your car is heated when it is parked with the windows closed.
Temperatures can reach 140-150 F, causing:
1) A panel of leaded stained glass
experiences a daily expansion/contraction cycle. Flexing to the
lead came matrix over a long period
of time results in failure due to metal fatigue. With
inadequately ventilated protective
glazing, sun heating increases the amount of daily temperature
change, thus dramatically accelerating
the negative effects of the expansion/contraction cycle.
2) As the air column inside inadequately
ventilated protective glazing heats up, it expands, forcing
the window to bow toward the interior.
Imagine what would happen if you inflated a balloon
(in the air space between the protective
glazing) in the middle of your window. Would you even
want to try it? Every day?
3) The high temperatures reached with
inadequately ventilated protective glazing makes the lead
cames somewhat softer and easier to
bend, maximizing the destructive effects of 1) and 2).
B) Moisture and condensation
Moisture trapped between the protective
glazing and the stained glass is conducive to
microorganisms that directly attack
wood frames, and whose by-products attack the lead cames
and the glass. Daily cycles of condensation/evaporation
accelerate rusting of any iron and steel
(like support bars, ventilators, or
frames), and the oxidation of the lead cames.
What to look for:
1) Dust or stain trails where condensation
forms and runs down the glass, either
on the protective glazing or the stained
glass.
2) Any signs of mildew, particularly
on the frames,
usually seen as a dark patch or splotches
or speckles.
3) Any sign of bowing of the leaded
glass.
4) White powder (excessive oxidation)
on cames.
What to do: In almost all cases, protective
glazing should serve only one purpose -- to protect your
window from accidental damage (tree
branches, etc.) and/or vandalism. Abundant ventilation will
keep the protective glazing from doing
any damage itself.
If you suspect that your protective
glazing may have problems, or even if you are simply
concerned, have an experienced stained
glass professional come to evaluate your windows and
prepare a condition report. The nominal
expense of an inspection is a good investment towards
the long-term health of your beautiful
stained glass windows.
Venting: Ventilation is an absolute necessity
between leaded glass and its protective cover. With
no ventilation, moisture condensation
can build up allowing microorganism growth and corrosiv
acids that damage the cames, glass,
and frames. Also, daily air pressure changes will be equalized
through bending of the leaded glass,
weakening the panel.
Cleaning leaded glass: Cleaning needs to be
done very gently with just water, or dusted with a
cloth or soft dust brush. No pressure
should be applied to the window. Absolutely no scrub
brushes, steel wool, abrasive cleaners,
etc. should ever be used.
Important: If your windows have painted decoration
on the glass surface and have not been
cleaned in many years, they should
be inspected by a professional to insure that the painted glass
is stable and will not be removed or
damaged by cleaning. Windows with evidence of paint
deterioration (flaking or faded paint)
should not be cleaned.
Unfortunately, many windows made in
the nineteenth and early twentieth centuries are plagued
with the loss of painted decoration.
The great demand for stained glass during this period meant
that numerous shortcuts were used,
often compromising the proper firing times and temperatures
necessary to fuse the painted decoration
to the glass permanently.
Important: Painted glass decoration is not
always obvious. Techniques are often used that to a
casual observer would appear to be
the color of the glass, but is in fact painted and fired
decoration. Be careful, be gentle,
and be a little bit paranoid when (or if) you clean your
windows!