What are bulging or sagging stained glass windows and how are they repaired?
Lead is a soft metal which is very pliable and flexible. This allows it to adhere to whatever shape the glass is cut. However, a leaded stained glass window needs extra support to stand up vertically. Stained glass designers use varies techniques to strengthen the window like staggering borders, using larger lead, or using large pieces of glass in certain areas. But these design techniques are not enough to strengthen the window alone. Another metal is needed to strengthen it. So steel braces are soldered to the inside of the window to help keep it straight.
The leaded window is continually fighting gravity. A window may bulge simply because the panel is too big or too wide. A panel over 15 or so square feet is too big. A panel over 48 to 52" is too wide. Sometimes, extra steel braces will be enough to straighten the windows and keep them straight. Windows or panels larger than these sizes usually need extra framing support, not just steel braces. All windows are different and depend on the design, the shape, the size, the leading, the location, and the existing braces.
Expansion and contraction of the leaded glass windows caused by temperature changes can cause the solder joints to loosen. Extended heat can cause the solder joints to fail. Sometimes, the joints loosen so much that the steel braces fall off. Without the braces, the window slowly sags and buckles due to the pull of gravity.
Too many panels stacked on top of one another can cause the bottom portion of a window to bulge. Leaded glass windows are heavy - about 3-7 lbs. per square foot, depending on the design and amount of lead used. The weight of panels stacked upon each other can really add up. If T-bars or H-bars are not anchored to the sides of the frame to alleviate some of the weight, then the window will fail and begin to bulge.
When a bulge becomes really bad, the glass may begin to crack and break. In worse case scenarios, the window has to be disassembled and re-leaded. But many times, the bulges can be fixed on site. Some minor bulges can be straightens in place without even removing the panel. New or additional braces are added. Other times, the panels are laid down flat and straighten out while glass is replaced and additional braces are added.
Here is a window bulging toward the outside. Note that it is not bulging where the two steel braces are.
This is a 53" wide window that is quite tall. Although it has t-bars and some steel braces, there is a bulge in the ventilator section caused by the weight or the top sections and maybe excessive movement of the vent.
This is a close-up showing the bulge in the ventilator section. Note that the lead has broken away from the glass.
Here's a window with pretty colors and design, but the lead is small and the borders are not staggered. Note the bulging in the second section up from the bottom.
This is a close-up of the same window from a different angle showing the extent of the bulging. Staggering the borders and adding a steel brace in this area would have prevented the bulge in the beginning.
Note how both of these windows bulge at the bottom. The window on the left has bulged so much that glass has broken and came out.
This is a close-up view of the window on the left in the above photo. Note how glass has broken and fell out from the severe bulging.
This is an outside picture of the window on the right in the previous photo. Note how much it bulges to the inside.