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Conservation And Preservation Of Stained Glass Nationwide
Exterior detail of new tee bar installation. A lancet window with new brass tee bars. Interior of lancer panel showing brass round bars.

Integrating Terra Cotta Window Mullion Repair
With Leaded Stained Glass Window Restoration - Part 4

St. Dominic’s Church, San Francisco

The Stained Glass Quarterly
by Lex F. Campbell; Simpson, Gumpertz, and Heger

Mullion Repair

Following the removal of the leaded glass windows for restoration, the terra cotta mullions were repaired. The terra cotta mullions are formed by stacked indi-vidual hollow terra cotta blocks filled with grout and supported by steel “tees.” To repair the mullions, the terra cotta mul-lion blocks were disassembled, and cracked mullions were replaced. Where possible, intact mullion blocks were sal-vaged and reinstalled. The corroded sup-porting steel tees were removed and replaced with new stainless steel support tees and anchoring.

Generally shaped in paired cavetto or cyma reversa profiles, the mullions varied in size from window type to window type. An inventory was developed identifying the different types of blocks and quanti-ties to be replaced. The final replacement inventory was subsequently used for pric-ing and placing orders with the terra cotta manufacturer for reproduction. New blocks were manufactured to be identical in size, profile, texture, and color to the existing blocks.

The design of new steel tees and anchoring had to contend with the size of the terra cotta mullion blocks, overall size of the window openings, and the existing conditions of the mullion connections to the concrete structure. Unique sets of details were developed for each window type; however, various aspects of the designs were conceptually similar. In gen-eral, to compensate for wind loads, the new stainless steel tees increased slightly in thickness from the original tees. Where possible, stainless steel anchor plates were used to connect the ends of the steel mullions to the concrete wall structure. In other locations, the ends were embedded in grout-filled notches cut into the con-crete wall opening at the sill or lintel. Stainless steel wires wound through the webbing connected the terra cotta mullion blocks to the steel tees. The entire mullion cavity finally was filled with masonry grout to provide additional structural stability.

In some cases, such as on some larg-er windows, the steel mullion was not suf-ficient to meet code requirements in order to withstand wind loads. At these win-dows, an H-shaped steel cross bar was used to connect the mullion to the win-dow jambs, providing additional lateral support. Because the cross bar was exposed, there was a visual impact on the window with the introduction of a hori-zontal element. To minimize the impact, the horizontal bar was located between the leaded glass panels in individual lancets, replacing the bronze tee that would normally provide horizontal sup-port. Because the cross bar was thicker than a typical tee bar, adjustments were necessary in the overall vertical alignment of the lancet panels. The panel located above the horizontal bar was raised slight-ly to maximize the visibility of the glass design.

The terra cotta work was completed with the preparation of the groove between the terra cotta cladding and inside stone to accommodate the leaded glass setting. Much of the work required leveling out existing bumps in the grout fill and filling in gaps to create a consis-tent one-inch deep groove or reglet. With the completion of this work, the window openings would be ready for the leaded glass to be installed.

Leaded Glass Setting and Support

In the new design for the window set-ting, two primary issues needed to be addressed: waterproofing and adequate vertical support for the restored leaded glass panels. Both of these problems pre-viously contributed to the deterioration of the windows as well as the mullions.

Originally, the leaded glass windows were set in a mortar-filled reglet. At some locations, sealant (on the outside) and glazing (on the inside) fillet joints were installed to provide added weather protec-tion. The failure of these joints allowed water into the morter and terra cotta cavity, ultimately contributing to the deterioration of the steel and terra cotta.

The Stained Glass Quarterly

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