In restoration of leaded windows, I consider three elements which are prone to damage and deterioration: the glass itself; the decorative elements (mostly applied paint); and the structural system supporting the glass.
Glass is virtually immune to natural deterioration. However, while glass does not normally deteriorate, it is susceptible to scratching or etching by abrasion or chemicals, and to breakage.
Minor sagging and bulging is to be expected in an old window and may not require immediate action. However, when bulges exceed 38mm out of place, they cross into a precarious realm; at that point, glass pieces can crack from severe sagging and pressure. Windows may have detached from the saddle bars and begun to sag, bulge, and bow extensively.
The fact of the matter is, almost all stained glass over 80 -150 years old, church or otherwise, are in need of repairs or restoration. Large panels on stained glass windows need to be strongly supported because of their incredible weight. This is most often done with steel bars and copper ties. These bars/wires are soldered directly to the lead, when they come loose or detach, bulging and then eventually failure occurs. Prevention is better than cure. Most of the windows in the two stone DR Churches in Wakkerstroom and Volksrust, were re supported and in fact extra bars for support were added.
Rebuilding or re leading a window is an expensive and involved process. The re leading process requires that a window be “unbuilt” before it can be “rebuilt.” Lead and zinc cames are the two most common assembly materials used in stained and other “leaded” glass. Came is prone to natural deterioration from weathering and from thermal expansion and contraction, which causes metal fatigue.
Leaded glass panels with a lot of breaks in the lead are removed from its frame and be repaired–sometimes in just the affected section that is re leaded. If the caming damage is considerable, it is best to have the entire panel and even the whole window re leaded to extend the life of the whole window.
Missing pieces of older stained glass need to be custom made for an exact match. The Braunschweig Church repairs, were Kiln formed to resemble the texture of the clear diamond panes. Coloured glass as replacement is usually imported. Painted glass, typically associated with pictorial scenes and figures found in church windows, often presents serious preservation challenges. If fired improperly, or if poor quality mixtures were used, painted glass is especially vulnerable to weathering and condensation.
Leaded lights can last for hundreds of years with only minimal care. Loose or leaking glass may be re-sealed within the leadwork using glazier’s putty , a mixture of linseed oil, whiting and white cement. The greatest and the most common threat to leaded glass is deterioration of the skeletal structure that holds the glass.