Rebuilt Transom Design
Like most prismatic transoms, the zinc came and the tiles themselves provided the only structural strength to the assembly. When the transoms were originally designed the manufacturers figured that zinc was rigid enough to not require additional support.
In some cases, such as with the Hamm Building panels, time and weather have proven otherwise. Periodic high winds in downtown St. Paul tended to push the large transoms inward, while unequal atmospheric pressure between the building interior and exterior exerted an outward force. The resulting distortion cracked solder joints and separated came and glass from the waterproofing grout, causing or exacerbating leaks.
To resist these tendencies, the architect and glass contractor attached four vertical bars to the interior face of each panel to provide additional structural reinforcement. Flat steel bars 1/2" deep were spaced three or five tiles apart and soldered on edge at points where the zinc came intersects. Applying continuous solder along the length of the reinforcing bar is not recommended as it may impede future repair efforts. The ends of the bars were then secured to the edge of the frame so that each bar supported sections of less than eight perimeter feet. Because of their thin, 1/8" profile and placement on the inside of the panel, the braces are invisible from the exterior. A similar approach is often used with leaded glass, where horizontal or vertical saddlebars are tied to the came in order to assume the structural load of the windows.
Repair Work
After the transom contractor established a temporary workshop in one of the Hamm Building's vacant storefronts, each panel was removed and disassembled. Because the shop did not include cleaning facilities, the tiles were inventoried, packed, and sent to a furniture stripping company, where they were immersed in a solvent bath to loosen paint and dirt. The solvent used by the stripping company had been previously tested to ensure that it did not etch, cloud or discolor the glass. In the meantime, the transom contractor built plywood jigs and workbenches in the shop that would facilitate precutting came and assembling the panels. When the cleaned tiles were returned to the site, reglazing began.
The majority of the rebuilt panels have prism glass tiles that were original to the building. Both ventilator hardware and the historic trademark prism border were included in the rebuilt panels. The assembly process was similar to that used for stained glass windows. Individual prisms were set in a zinc H-came matrix that was gradually soldered together as more tiles were added. Reinforcement bars were then soldered to the panel. The last major step before reinstallation was to waterproof the assembled panel. A waterproof grout made from a traditional mix of putty, boiled linseed oil and lamp black was forced into the spaces between the came flange and the glass.
The four reproduction panels containing new glass were reglazed in the same manner. A clear, patterned glass called "moss glass" that resembled the tone and texture of the historic prism tiles was used as a substitute material. Where historic ventilators were beyond repair or missing, the transom contractor fabricated a simulated vent with strips of capping zinc (a material normally used to cover the edge of the transom frame). Reproduction panels also differ from the historic assemblies in that they did not have a border of trademark tiles along the perimeter. The resulting panels were distinguishable from the historic units up close, but when viewed from the street or the opposite sidewalk, seemed only slightly different from the original panels.
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