FREQUENTLY ASKED QUESTIONS:
STAINED GLASS RESTORATION
Much of our country’s stained and leaded glass is now over 100 years old—yet much of this glass has not been professionally cleaned or repaired since it was first installed.
With proper care, the stained glass windows that add character and value to our historic buildings can easily last another century. The following information will help guide you in understanding and approaching the restoration process. Also see our stained glass FAQ, or contact us with any questions.
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- How does stained glass deteriorate over time?
Windows deteriorate in many ways, often as their supporting structure weakens (see sidebar) allowing air to come through and moisture to invade the frame and lead. Water expands and contracts with heat and cold—but the lead, once expanded, stays stretched out. So the lead oxidizes and fatigues, and, over time, becomes brittle and thin to where it's incapable of holding the glass. When the lead cracks and pulls away, windows sag and bulge, and you’ll need repair and releading. top↑
- What specific techniques or skills does restoration require?
When working with fragile, aging glass, it’s essential the job’s done right. First, the restorer must have expertise in evaluating the damage. For instance, testing stability of the glass paint can prevent disaster during cleaning. Being able to recognize corrosion of the frame, to evaluate torque and stress on the window, and to know what should and should not be done on-site is essential. Fixing a rattling window with acidic silicone caulk can save time but do serious long-term damage. And forcibly flattening a window in place with added bracing can cause stress cracks in the glass and broken solder joints.
Dissembling antique windows also requires experience in evaluating patterns of cracks and fractures. Windows should be securely taped—or a 75–year–old, solid-looking window could easily start crumbling as it’s removed.
With multi-layered glass (such as Tiffany’s) that lines up precisely to create a certain effect, you’ll need a skilled restorer to recreate that effect. Windows must be unbuilt before they can be rebuilt. Each layer must be photographed as it's dissembled and rubbings made, which is time and labor intensive—and some restorers don't do it, not realizing what's involved.
Recreation of antique glass and reassembly are also complex tasks. Thick or layered glass may require high-hearted, specially-alloyed lead came, requiring a restorer familiar with custom extrusion dies. And, once releaded, windows must be properly cemented to assure they remain stable and waterproof. top↑
- What about painted glass that’s damaged or missing?
Decay or vandalism to valuable painted glass can seem tragic to a congregation. The best protection is a file of detailed photos that can provide the restorer an idea of what’s been lost. In cases where parts of the pattern remain—for example, if a robe is partially missing—glass can be replaced and repainted to match. Otherwise, restoration depends on the historical knowledge and skills of the restorer. Our owner Jeffrey Mueller, a painter and expert in 1800s European glass, is often able to faithfully reproduce historically accurate work, even matching the faded effect of aged paint on other sections of the design. top↑
- Can a window be repaired without removing it?
If a window’s deterioration is caught early enough, work can sometimes be done on-site. For instance, if the problem is minor bulging, we can recement or flatten the window in place. Broken glass and small releading work is also possible. However, conservationists don’t recommend stop-gap measures that address only part of a window. And full recementing, correctly done, requires that a window be laid flat. For a window that shows significant weather-related wear, a proper restoration assures that your stained glass will last another 75 to 100 years. top↑
- How can I keep my stained glass in top condition?
First, if your window is sealed with a protective covering, make sure there’s no heat buildup between the stained glass and protective glass, which can cause a window to deteriorate much faster. Always vent the window so hot air can escape.
Also, make sure windows are supported properly and that the supports are securely attached. Keeping the frame in good condition is also critical—so make sure the glazing compound or caulk around the window where it meets the frame is intact and that the frame is painted so it doesn’t rot. If the frame gets weak, the window may pull away from the frame.
In a home, you’ll want to reglaze the putty around old windows where they are attached to the house. If they rattle and cold air comes in, they may be to the point where they need recementing. A window in good shape can be taken out and recemented to keep it waterproof and stable. Recementing is the easiest way to protect old stained glass.
Also, the perimeter lead or zinc came can separate over time, and the glass will drop out. This is due to moisture from weather or humidity dripping down to the bottom of the sill and freezing, spreading the lead open. Once this occurs, you may need to replace the perimeter lead came, which requires a professional restorer. top↑
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issues: cracked and chipped stained glass
CRACKS AND CHIPS
Stained glass is extremely durable, yet fragile and easily broken due to weather damage, accident or structural stress. Broken glass can usually be replaced with antique or newly fabricated glass fused or painted to closely match the original.
issues: bulging stained glass panels
Older stained glass is vulnerable to sagging and bulging due to lead fatigue, inadequate support, or deteriorated cement. After restoration, steel braces may be soldered to the window to prevent the bulge from recurring.
issues: broken leading in deteriorating stained glass
Lead does not rot or rust, but will lose its elasticity over time. Brittle lead will break, often developing hairline cracks on the face of the came as well as between the solder joints.
issues: oxidation as a cause of deterioration in leaded windows
Trapped moisture can easily corrode lead or zinc came, tie wires and support bars. A white "lead oxide" powder (the equivalent of rust) on the lead surface can indicate a moisture build-up between the protective covering and glass. Trapped moisture can also rot wooden frames and leach mortar from masonry, leading to severe structural damage.
issues: leaks and rattling in stained glass requires recementing
LEAKS AND RATTLING
When glass becomes loose, due to worn or missing cement, the window becomes more vulnerable to environmental damage. Non-professional sealing of stained glass with paint or caulk can do irreversible harm. Recementing is often the best way to extend the life of your windows.
issues: decaying frames are a major cause of window damage