The glass itself is quite permanent and the colors remain fairly true because the pigments, even in painted glass, are actually melted into the glass . Glass several hundred years old can be restored in many cases with careful and thorough cleaning. However, when the glass is assembled with lead strips or cames to make a window, especially a large window, some problems become inevitable. These potential problems are, for the most part, the direct result of the window's nature and use . Deterioration of a leaded glass window is progressive in nature.
Weight is one of the primary causes of the structural deterioration of stained glass windows.
Glass is quite heavy. Over a period of time the very weight of the glass will stretch the lead and the glass will become progressively more loosely seated in the leading. As the glass loosens in the lead the windows becomes more susecptible to wind loading and further stretching of the leading. Loosening of the lead and glass along with weathering can result in the deterioration and loss of waterproofing putty, resulting in further loosening of the window as well as water leakage. Attack by water and atmosphere borne contaminants will continue the deterioration process.