From the ground, a visitor to San Francisco’s Grace Cathedral will see beautiful, multi-colored stained glass windows. However, once those windows are removed from their stone niches, the crew at Nzilani see the windows in a much different light, literally. Take for example the following panel, (the bottom of St. Thomas’s feet, of the New Testament):
Viewers from the ground see the windows with light transmitted through the glass, while we see the windows with light reflected off the surface. This reflected light allows us to identify the many structural and cosmetic problems that can develop over many years of weathering. One such problem can be seen in the lower left corner of this panel:
Notice the amber-colored globs on the surface of the lead. In the past, fabricators used to hand mix the putty used to seal the windows. The putty is a mixture of linseed oil and calcium carbonate (chalk powder). If the mixture ratio is incorrect, (or the windows installed during a particularly hot day) the linseed oil can migrate to the surface over time, resulting in these unsightly globs. This is a good indicator that the putty is failing and that the window needs conservation treatment.
In this image, we see a number of issues. There is a white, hazy film on the surface of the glass. When lead oxidizes, it becomes white and powdery. This film could also be whiting (calcium carbonate powder) used to clean the glass surface when it was originally fabricated. Additionally, there is a noticeable white drip coming from the top of the right foot. This indicates water damage through a hole in the lead.
This panel also gives us clues as to how the panel was originally installed. At regular intervals along the surface of the lead, we can see bits of copper wire soldered the to the lead joints. These wires were twisted around steel, metal rods (called “support bars” or in stained glass parlance “rebar”) attached to the stone, providing structural support for the large panel.
Another interesting discovery was illuminated in the studio. When the panel is seen head on, something strange is revealed about the design of the feet:
Notice how the artist exaggerated the length of the toes. From the ground, the viewer sees the panel at a very steep angle. This angle makes everything above look compressed. To compensate for this, the artist intentionally painted the toes much longer than normal. Such careful thought and ingenuity is seen in nearly all the stained glass windows at Grace Cathedral.