Earlier, we discussed the importance of structural composition during the designing process (see this page). The designer made some calculations and limited the individual sections in the window to 12 square feet (1.2 square meters) or less and has made provisions for all other structural requirements based on the window’s design and proportion.

Lead is a very soft metal. This softness provides the flexibility that enables the fabricator to easily shape the lead came strips to fit around the curves and bends in the window’s design. Unfortunately, the lead alone does not possess sufficient tensile strength to hold any substantial weight within the frame on its own. If a stained glass window were fabricated and installed as one large section, the weight of the upper leaded section would press down upon the lower panels causing them to bow and eventually collapse. You often see this occurring in older leaded glass windows. Imagine a set of children's building blocks stacked up. As long as the blocks are stacked up in a perfectly flat vertical plane, they are stable. Once the stack begins to bow, instability increases exponentially with the increasing degree of deflection (bowing and bulging) and they all come tumbling down. The same is true of leaded glass windows.

To minimize this problem, a steel reinforcing system is used to supplement and divide the window frame (and therefore the stained glass window), into several smaller independent sections. These frame dividers are called mullions and usually add a decorative pattern to the window framing in addition to ensuring stability for the leaded glass. Any section or opening within the window’s frame that is larger than 12 square feet (1.2 square meters), should be further divided with horizontal T-bars (called muntins). These steel or aluminum T-bars are fastened securely to the window’s framework. They supplement the mullions, to transfer the weight of the upper leaded glass panels to the window frame, rather than entrusting the lower stained glass panels to support the weight of the upper panels.

In addition to the mullions and muntins breaking the window frame into smaller sections, the leaded glass windows also require supplemental reinforcing. The old-fashioned method of supplemental window reinforcing was to install the leaded window sections then affix horizontal steel bars into the sash of the window frame at 1-1/2 to 2 foot (45.7 to 61 cm) intervals. These steel bars would be on the inside of the stained glass and would be attached to the leaded panel using copper wires that were soldered to the lead came and wrapped around the steel reinforcing bar.

The more modern method of supplemental reinforcing is to use flat steel bar that is 1/8" thick x 1/2" wide (3 mm x 1.3 cm) and is cut slightly longer than the window is wide. This flat bar is set perpendicular (90°) to the glass surface and laid horizontally across the width of panel. The bar is soldered directly to the lead came at any point where they touch. If possible, the bar is centered across the panel at solder joint areas, which provide greater holding strength. To achieve maximum stability it is essential that the ends of the reinforcing bar be firmly attached to the window frame on installation.