The most overlooked and among the largest topics of debate among conservators worldwide is leadlight cement. Loss of cement is the foremost common cause of lead failure. The lead matrix of a stained glass window is extremely soft and malleable. When exposed to high temperatures along with expansion and contraction of glass and other building materials, it is subjected to a great deal of movement. This matrix is not capable of supporting its own weight, not to mention the glass, and needs additional support to combat gravity and lateral pressure. This is where the leadlight cement takes over. When assembled, panels are grouted similar to ceramic tile with a cement mixture. This material is not easily visible; however it is the backbone of a leaded glass window. It is such an important element in leaded glass; one might argue the lead matrix is merely a vehicle the cement uses to cast the glass pieces in place.
There have been many formulas over the years for different types of cements and putties found in leaded glass. A desirable cement should provide a weather tight seal between the lead flanges and the glass and remain flexible enough to absorb physical impact and building movement as well as allow for future repair. We mix our own cement in house with a time honored recipe that contains no solvents or driers that would cause premature failure.
This organic mixture offers superior longevity when paired with a periodic maintenance program. Plaster of Paris is gypsum that has been kiln dried to remove all H2o. When mixed with boiled linseed oil it becomes protected in a barrier of water resistant oil which will remain flexible and forgiving enough to absorb most structural movement, future repair and even physical impact, without fracturing a piece of glass. Since the oil repels water, the plaster never absorbs the moisture which would initiate the process of hardening and returning to its porous form gypsum.
Over time the organic linseed oil, which cures through oxidation rather than evaporation, will dry out and begin to decompose. In turn, this will expose unprotected plaster which will begin to absorb water. Rain and other moisture buildup will siphon into the cement and interior side of the windows leaving calcium deposits in its path and to an extent actually fill small voids and self repair. It is at this stage the panels are brittle and "thirsty" for fresh oil. If simply supplied with fresh oil, the cement mix will absorb it through capillary action and become fresh again. The lead can actually absorb and benefit from the oil treatment as well.
Panels that experience widespread cement loss will require complete re-grouting. This can be fulfilled in place, however many other structural problems and neglect observed with historic glass in our region warrant the removal to address other issues. A panel with a structurally sound lead matrix can be re-cemented and returned to as new quality. If neglected the cement will continue to crack and dislodge. At this stage the lead matrix is bearing more weight than it can support, leading to advanced problems which will become more costly and difficult to rectify as time passes.
Leaded glass that has been cemented with unacceptable alternatives can be rendered nearly disposable. Many commercially manufactured cement systems contain binders (including Portland cement or epoxies) that will not release the glass panes when subjected to movement, impact, or even repair! This bond can and will result in widespread glass fracture.