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A vertical crack in a terra cotta mullion. A window reglet or groove in poor condition. New stainless steel “T” mullion and terra cotta blocks.

Integrating Terra Cotta Window Mullion Repair
With Leaded Stained Glass Window Restoration - Part 2

St. Dominic’s Church, San Francisco

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

St. Dominic’s Tracery Windows

Designed by Arnold S. Constable in conjunction with the Beezer Brothers, the current St. Dominic’s Church (there were two other buildings) was built in 1928. The Gothic Revival building is constructed with a reinforced cast-in-place concrete frame and walls and architectural terra cotta cladding on the exterior. The interior walls are finished with cast stone. Aseismic retrofit in the early 1990s added a concrete ring beam around the roofline and a series of flying buttresses along its perimeter. The main building forms a cruciform plan characterized by a long nave crossed by a transept.

The Gothic Revival style of the church is primarily characterized by a series of large arched tracery window openings with multiple lancets in the nave clerestory, transepts, and apse. A series of multi-paneled and single arched foil and simple lancet windows are found on other walls of the building, including in the Sacristy and Confessionals.

In multi-lancet windows, terra cotta window mullions separate the lancets and support the corresponding tracery or quatrefoils. The window mullions range from seven to 14 inches wide and rise three to 20 feet high, where they meet the tracery spring stone. There are a total of 76 window openings, 41 with at least one or more mullions. Installed within the window openings are stained glass windows by the Connick, Ingrand, and Cummings studios and some amber glass windows of unknown origin.

Cracked Window Mullions

In 2001, the engineers Simpson, Gumpertz & Heger were hired by the church to investigate the conditions of the walls and windows, with a particular emphasis on the window mullions. Open vertical cracks were visible on select window mullions. Deterioration was also noted in the leaded glass windows, which, according to church records, had never been serviced since their installation.

SGH conducted a ground-level binocular survey of the terra cotta walls, window mullions, and window openings. In their initial findings, SGH observed that a substantial number of terra cotta mullions were cracked, with varying levels of severity. In some windows, an open vertical crack extended the height of all the mullion blocks. Some of the more severe cracks were open with rust staining evident on the surface of the terra cotta. A select group of mullions on the north elevation were less damaged, with single blocks being cracked if at all. SGH suspected that corroded structural steel embedded in the mullions was the cause of cracking. Severely cracked terra cotta potentially can lead to loose spalls, pieces which can fall from the building. The poor condition of the window mullions was also an obvious threat to the condition of the leaded glass windows and tracery matrix.

Physical investigations of cracked mullions and structural reinforcement by removing terra cotta blocks confirmed that corroded steel mullions were the problem. Pressure from flaking and expanding corroded steel caused the mullions to expand and crack. A likely cause of the corrosion was water leaking into the mullions from deteriorated window perimeter mortar and sealant/glazing putty joints. The steel mullions also lacked sufficient anchorage back to the structure and were typically embedded into the surrounding concrete or brick masonry substrate. Many of the embedded connections were now cracked around the masonry substrate due to the corrosion. In some windows, the tracery framework did not permit the extension of a mullion up to the lintel, creating a cantilevered mullion potentially with inadequate lateral support to resist wind and seismic loads.

Concurrent with the terra cotta survey,the leaded stained glass windows were surveyed by a conservator. Common problems included weakened, broken, and deflected lead cames, lead corrosion, insufficient and poorly applied support bars, hard-setting putty inhibiting movement of the panel within the frame, deteriorated glazing compound, broken glass, and built-up dirt. The setting detail in the frame or reglet was in poor condition. The hard cement and glazing putty used to set the windows were cracked and deteriorated, allowing water into the window frame and wall. Finally, all the setting materials, including some sealants, tested positive for the presence of asbestos fragments.

A complete body of schematic design information included a comprehensive visual survey and evaluation, invasive investigation of the terra cotta substrate, a conservator’s report, and hazardous material report. SGH engineers and architects effectively used this information to design and plan a phased restoration.

The Stained Glass Quarterly

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