The following is a brief list of things to look for.
- Broken Glass-
Cracks or even shattered glass pieces within the window are the most common problem. They can be caused by many things including something striking the window, deflections (bowing) of the window, even settling of the building itself can add enough stress to break pieces. Broken glass causes energy loss and can develop rapidly into major problems for your windows.
- Lead Problems-
Another common problem in older windows is the deterioration of the lead came which makes up the skeleton of the window. Look for cracks in the lead especially around the joints. Oxidation is another common problem; look for a white powdery looking substance on the face of the lead. This is basically the equivalent of rust to lead. Delaminating is something else to keep an eye out for, areas where the face has come apart from the heart of the lead.
- Structural Problems-
As stained glass ages the cement or putty that is under the lead dries out and begins to come loose. This can be a major cause of energy loss if the windows do not have proper protective glazing. It can also weaken the window, especially if there is inadequate or improperly attached reinforcement. Look for powdery residue at the base of the window. Also watch for deflections or bowing / sagging areas of the windows; this may start slowly but can accelerate over time.
As with most things in life stained glass breaks down naturally. Every thing eventually returns to the earth from whence it came. However, with a pro-active approach to maintenance stained glass windows can be kept looking beautiful for centuries.