The sections of glass that make up a stained glass or lead light windows are held together with an H section lead calm. The lead construction (matrix) that is formed when constructing the window is made up of many small sections of lead and in the some windows many thousands of sections, all of which are soldered together at each individual joint to form a structure that forms the completed window.
Before installation this structure of lead is required to undergo two further processes, firstly that of being what is termed in the trade as ’Cemented’, this will waterproof the glazing for the life of the lead structure. Secondly, copper wire ties are soldered to the lead work joints to allow fixing to the support bars that will form part of the fixing mechanism when the window is installed in its final position.
The lead work of windows, whether it be stained glass or lead lights has a useful life and depending on various elements, elevation on the building, heat, wind, support, fixing method, quality of lead, has a life of 100-150 years as a general rule. At the end of that useful life the window needs to be removed from its fixings, the glass stripped from the old lead work and the original glass re-leaded in a new lead calm of the same profile as the original, ensuring as much as is possible to retain the original visual appearance.
There are many factors to take into consideration that govern when it is appropriate to remove glazing for re-leading. These may include; generally very poor condition where the glass is coming out of a weak lead structure, bowing of the glazing, failure of copper ties resulting in lack of support, cracking of solder joints, ingress of water, damage to glazing caused by rusting of associated ferrous metalwork, failure of the stonework/timber or metal frame that contains the glazing, or a combination of the above.
In every case where there is an issue with the structure of stained glass and lead light glazing it is the duty of the glazier/conservator to give best advice and that may or may not be to re-lead the glazing, but it should be remembered that replacement of lead work is part of the life of a leaded window. They have been designed with that in mind and it is required to keep the glazing in a condition fit for purpose and is a necessary part of apprentice training to ensure that good basic craft skills are kept alive in our trade for future generations.
View the process of re-leading an ancient stained glass panel from rubbing to completed panel here:
A stained glass window in the making >>