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Cathedral Glass Myth Shattered

The popular notion that medieval cathedral windows have thickened at the bottom--by slowly flowing like a liquid--doesn't hold water. Even after considering the specific chemical composition of stained glass windows, according to a report in the May issue of the American Journal of Physics, it would take longer than the age of the Universe for the glass to sag appreciably.

When materials scientist Edgar Zanotto of the Federal University of São Carlos, Brazil, first heard about sagging medieval windows, "I thought it was just a local [Brazilian] myth." But then he heard the same tale from colleagues in Argentina, and found echoes of it in textbooks and even in the Encyclopedia Britannica. Although glass isn't supposed to flow at room temperature, old glass has many impurities that might help it ooze. So Zanotto sat down to do the calculations.

Zanotto looked up the chemical compositions of some 350 medieval glasses and calculated a typical viscosity. The old glass should flow a little more easily than modern glass, he found, but only at temperatures above roughly 200 degrees Celsius (modern glass has to be hotter than about 250oC to flow). Below 200oC, the molecules would remain gridlocked. Zanotto also considered an extreme case: germanium oxide glass, which is thought to flow even at bitterly cold temperatures. But even a germanium oxide window would hold its shape, he concluded. Such glass would visibly sag, Zanotto says, "but only if you wait for 1033 years."

The paper goes a long way to laying the legend of flowing windows to rest, says Jonathan Katz, a materials scientist at the Washington University in St. Louis. Katz, Zanotto, and others think that cathedral glass makers centuries ago were unable to make even panes and that builders put the fat end on the bottom for stability.

If the cathedral glass really is sagging, Katz says, it could be spotted with an interferometer in the lab. "It would be a great graduate student project," he says. Still, there's good evidence the thesis would be a short one: If glass moves at room temperature, he adds, "your camera wouldn't focus right after 10 years on a shelf."