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