section provides information about leaded glass conservation for owners and
caretakers of leaded glass windows and panels. Conservation-related questions
can be posted to:
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.
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
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
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 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 corrosive 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 dustbrush. 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!