damaged windows

Repairing Damaged Windows

Repair of damaged stained glass windows varies from a single shattered piece of glass to replication of several missing pieces.

Replacing a single broken piece of stained glass: It is possible to replace a single piece of glass without removing the window panel.  A rubbing is made of the broken piece to use as a pattern to cut and, if necessary, painted to match the existing window design.  The new piece of glass is then carefully installed into the damaged window.

Replacing several broken pieces of adjacent stained glass:  This type of repair is more difficult to accomplish in place.  Often the lead is mangled and/or stretched.  Typically a repair of this nature requires the removal of the panel.  New pieces of stained glass glass are cut and, if necessary, painted to match the existing window design.  The lead in the damaged area is replaced with new lead came.  Depending on the age and condition of the window,  the recommendation may be to completely re-lead the damaged panel.

solder joint

Broken Solder Joints

Broken solder joints are a sure sign that a stained glass window is under duress.   As a window ages, the lead structure within the window gets brittle.  Brittle lead is more apt to crack under stress.  Stress can come from multiple of sources:

  • The weight of the stained glass:  The panel weight us supported by the lead structure.  As joints break, the weight redistributes thus adding more stress to the remaining joints.  Therefore lead breakage is a progressive failure increasing in severity if left unchecked.
  • Expansion and contraction due to temperature fluctuations:  Although not visible to the human eye, window materials expand and contract daily with fluctuations of solar temperature.  These tiny movements stress the lead, sometimes even pushing the window out into a bowed condition. Repeated expansion and contraction introduces material fatigue which weakens the lead. A good example of this is a simple paper clip.  Bending a paper clip back and forth weakens the material and intimately results in breakage.  Similar is the break olead due to expansion and contraction.

Broken lead often occurs at the solders joints.  This is because of the difference in material properties between the lead and solder.  The solder joint is harder than the lead thus the lead usually breaks at the edge of the solder joint instead of through the solder joint.

Depending on the age and condition of the lead, repairing these broken joints can be difficult to accomplish.  Often this condition is a sign that the window is in need of re-lead restoration.  Re-lead restoration is the process of removing, dismantling, and reassembling the window with new lead.

Faded Paint on Antique Stained Glass

The traditional art of crafting stained glass often includes painted detail on the glass. The paint is fired at a high temperature to permanently set the paint into the glass.  Unfortunately, history tell us that this process was not always executed successfully.  A window which has not been properly fired can result in unstable paint; evident where painted detail has faded or is completely missing.

Unstable or faded paint is the result of under-fired glass paint. Glass paint is permanently set into the glass at approximately 1200 degrees F.  At lower temperatures the paint will not set and therefore is not permanent.  Under-fired glass paint has a dull matte finish and will fade over time.

Treatment of Unstable or Faded Painted Glass

Painting on stained glass requires that the painted piece be fired at a high temperature to permanently set the paint into the surface of the glass: approximately 1200 degrees F.  If fired too hot the paint merely burns away.  If fired too low the paint does not set properly and will fade over years of service.

We do not recommend adding paint to the existing painted glass in an effort to replace what is missing.  The firing process which sets the new paint can burn off the remaining unstable paint and may not accomplish the desired result.

The recommended repair may vary depending on the type of window and wishes of the church.

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bowing stained glass

Bowing in Antique Stained Glass Windows

One of the most common problems found in antique stained glass windows is out-of-plane bowing. Window bowing is a progressive failure and can take years to become a serious problem. However, other factors can accelerate the condition.

  • Oxidation of the lead:  Oxidation is the white chalky substance that appears on the exterior of the lead. Extreme oxidation can weaken the lead and thus the window.
  • Brittle lead:  Window leads get brittle with age and become prone to breakage at the solder joints.  Broken joints weaken the overall window.
  • Breakage of wire ties:  Twisted copper wires bond the stained glass window to bracing rods extending across the window.  This is designed to support the weight of the window and keep it flat. These wire ties can break over time and lessen their support to the window.
  • Building settlement:  Settlement can induce pressure on the perimeter on the window. This pressure is relieved as the panel bows.
  • Fluctuations in temperature:  Stained glass is exposed to a range of daily solar temperature change. As the window heats it expands.  As the window cools it shrinks. This daily heating/cooling cycle creates stresses in the stained glass which are relieved by out-of-plane bowing.

Bowing ultimately allows stained glass to work free of its confining lead, in some cases actually dropping glass.  Bowing can result in broken glass.  At the very least, windows may become less weather resistant.

Inward bowing is partly confined by steel brace bars; with bowing generally occurring between the brace bars.  Outward bowing can be more pronounced, in some cases breaking the wire ties designed to secure the window to the brace bars.

Window bowing cannot typically be repaired in place.  The window will require removal to flatten.

If the window is generally in good condition (other than the bowing) it is possible that the panel can be flattened and reinstalled.  However, more often than not, the window is old enough that is has many of the degrading symptoms described above. If this is the case, it is possible that the window will require re-lead restoration.

Re-lead restoration is the process of dismantling, cleaning, and reassembling the window, replacing the old lead with new restoration grade lead.  Structurally speaking, the result is a “like-new” stained glass window. The recommendation for re-lead comes after careful consideration of the project as a whole.

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