Essential elements of a correctly designed and fabricated leaded glass window.

A professionally designed, structurally fabricated, and properly reinforced leaded glass window will require little or no maintenance for the first 60 to 70 years of its existence. In order for a stained glass window to meet this standard, the following factors must be present:

  • The framework of a large window must be subdivided with mullions to create smaller sections. These mullions usually add a decorative pattern to the window framing in addition to ensuring stability for the leaded glass.
  • Individual leaded glass panel sections should be 12 square feet (1.2 square meters) or less. 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.
  • Leaded glass designs that have a high number of smaller pieces, or designs that feature concentric geometric patterns, should be smaller than 12 square feet (1.2 square meters) per individual section.
  • Leaded glass windows must be fabricated with lead came that has sufficient tensile strength. For this reason it is crucial to use a lead came manufactured by the extrusion process using a lead alloy that contains antimony, silver, copper, or tin. Milled came made from pure lead (without alloy additives) is too soft and will not hold up for long, even under normal conditions.
  • The leaded glass windows must be properly sealed with a commercial cementing compound or putty that is pressed under the flanges of the lead came. Once the cement has set, it will make the panel more ridged and weather tight.
  • In addition to a well engineered matrix of mullions and muntins, leaded panels must have a supplemental reinforcing system. Steel re-bars must be securely soldered or wire tied to the lead came and attached to the window frame. The proper function of a reinforcing bar is to hold the stained glass in a flat plane, it is not to hold the stained glass up. Once the stained glass begins to sag or bulge out of a flat plane it becomes weak and will tend to bulge more and more until the lateral pressure on the glass causes it to break. The smaller the stained glass component parts, the closer the steel reinforcing bars need to be, on average every 18 inches (45.7cm) is sufficient spacing. Of course, artistic design requirements may also affect the placement of the reinforcing bars. Be warned, without a properly designed reinforcing system, the stained glass window will not withstand the test of time. For more information please refer to Structure and Reinforcing.
  • If an exterior protective covering is installed it must be properly vented to allow the moisture, that collects in the space between the stained glass window and protective covering, to dry out between condensation cycles. For more information please refer to Protective Exterior Covering.

Left: A window being leaded up using extruded “lead alloy” came.

Above: Nick Davis places steel reinforcing bars on this window section which will hold the stained glass panel in a flat plane.

Left: This chapel window, from Trinity Episcopal Cathedral in Davenport, Iowa, incorporates salvaged stained glass sections into a simple diamond background. Its an excellent example of how mullions are used to break a large window opening into smaller sections. The four large lower sections plus the decorative tracery mullions in the upper section add interest and intrigue to the overall design.

Left: Steel reinforcing bars need to be placed approximately every 18 inches (45.7cm), however, artistic design requirements may also affect the placement of the reinforcing bars. Here we see Mark Steele bending the rebar to follow along a gentle curve in the windows lead design.