Stained-Glass Out-of-Plane Bowing
Out-of-plane bowing forms one of the most common problems found in antique stained-glass windows. As a progressive-failure issue, window bowing can take years to become a serious and noticeable. However, other factors can accelerate the condition.
Bowing ultimately allows stained glass to work free of its confining lead; in some cases, glass actually drops out. Therefore, bowing can result in broken glass. At the very least, windows may become less weather-resistant.
Steel brace bars partly prevent inward bowing; however, the issue can still occur between brace bars. Outward bowing can be more pronounced, in some cases breaking the wire ties designed to secure the window to the brace bars.
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, old windows have many of the degrading symptoms described above. If this is the case, windows will probably require re-lead restoration.
In the process of re-lead restoration, we dismantle, clean, and reassemble the window, replacing old lead with new, restoration-grade lead. Structurally speaking, the result is a “like-new” stained-glass window. A recommendation for re-leading comes after careful consideration of the project as a whole.
To prevent bowing, it is important to ensure that stained glass windows are properly supported and that the surrounding framework is designed to accommodate changes in temperature and humidity. Additionally, it may be necessary to reinforce the glass with additional support structures.
Factors Causing Accelerated Stained-Glass Bowing
- 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: Building settlement can induce pressure on the perimeter of the stained glass window. This pressure is relieved as the panel bows.
- Fluctuations in temperature: Temperature and solar exposure on stained-glass windows and surrounding framework vary greatly throughout the day. Windows expand as they heat and shrink as they cool. This daily heating/cooling cycle creates stresses in the stained glass, and out-of-plane bowing relieves that stress buildup.