Stained glass windows generally have long life spans―after all we still have windows that are 80 to 100 years and even older―but often suffer from environmental damage, pollution damage, vandalism or from any accidental damage. So, it is necessary to inspect the panels periodically, and carry out required maintenance repairs and restoration work. Usually, if we have a damaged glass pane in any regular window, we simply replace it with a new glass pane. With stained glass, this becomes slightly more complicated as you need to find the exact color and type of glass, and finding it depends on whether it is still being manufactured―; in cases where the glass is 100 years old or older, the likelihood of finding that same kind of glass is rare and sometimes impossible, because, maybe, nobody makes it anymore.
You will easily know your window needs some urgent repairs if there is any clearly apparent damage, like cracked or broken glass, or if the glass panels show an obvious bulging. Check if the window's reinforcing bars are broken or missing, or if the leaded framework is broken. Look out for window panes that have come loose in their leaded frames and for cracked solder joints. Press the window slightly and see if it gets pushed outwards. If it does, it needs to be repaired. All these repairs require some major and specialized work.
There is less risk of damage, and more chances of a successful restoration if you leave it to professionals. The restorers will visit your site and look over the window or windows in question and give you an idea of the scope and cost of the work. Before you finalize any deal, ask for credentials, references, and past restoration work experience. Preferably, ask where you can see samples of their previous work.
They may either repair the window in its setting if the damage is minor―they may just apply putty to fix loose glass or install protective glass covers to the window to protect it from vandalism or environmental damage―or they may decide to remove the framed window from its setting and take it to their studio for a more complete restoration. Here, they will remove the glass panes from the frame, and take rubbings of the scenes or designs on the glass panes. This is done by placing a paper over the pane and rubbing with a chalk, charcoal or crayon until the image is reproduced on the paper. A rubbing of the shape and size of the caming (the lines in the window that hold the panes together) too is made in a similar way. The restorers refer to these rubbings when repairing the window. They also take samples of the different caming sizes, which serves both as a record of how the window was originally constructed and a reference for putting in the new caming. The separated glass panes are cleaned and, if broken or cracked anywhere, taped together or treated with narrow caming. Often times the same kind of glass is not available for replacement, so the restorers try as much as possible to preserve the original glass. If the glass is beyond repair, they will try to find an approximate close replacement that will not detract from the original beauty of the design.
The restorers will also repair the window frame if this is necessary. Sometimes they may have to build an entirely new frame, but this is quite expensive and it is preferable instead to repair the existing frames. Aluminum, steel, mahogany, and epoxy are generally used to repair window frames, as these materials provide good strength and can withstand climatic variations.