Shattering the myth of fast-flowing medieval glass
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Shattering the myth of fastflowing medieval glass
P
eople often reference cathedral windows constructed during medieval times, found in places like Notre Dame in Paris and Westminster Abbey in London, when they want to provide an example of glass flowing at room temperature. Glasses in medieval cathedral windows are 1015 times less viscous than initially calculated, but glass still only flows at a rate of 1 nm over 1 billion years. These results, published recently in the Journal of the American Ceramic Society (doi:10.1111/ jace.15092) by John Mauro of The Pennsylvania State University, will allow manufacturers to improve glass substrates used in high-resolution displays. A glass is a nonequilibrium material. Besides being dependent on its current physical state, the properties of a glass are heavily influenced by its thermal history, which describes how quickly or slowly a glass was annealed. “If you have a glass that’s cooled more quickly, you can end up with a glass with very different properties compared to a glass that’s cooled more slowly,” Mauro says. The atoms in a glass are also constantly trying to reorganize by relaxing toward a more thermodynamically favorable arrangement, so over time the glass will change shape. At room temperature, these miniscule changes are beyond detection in even the most advanced experimental measurements currently available. These medieval cathedral stained-glass windows are noticeably thicker at the bottom than the top. Research performed on modern glass compositions almost 20 years ago showed (see American Journal of Physics doi:10.1119/1.19026) that the manufacturing methods used in medieval times are actually responsible for this thickness gradient, countering the belief that glass flows noticeably over centuries. This older study, however, did not explore glasses with the same composition as those used in medieval buildings, or consider how different thermal histories affected the roomtemperature viscosity. Redoing calculations using a model that includes these physical properties could significantly change
the theoretical roomtemperature viscosity. Having developed a model of glass viscosity that took both the thermal history and composition of the glass into account (see Physical Review B doi:10.1103/Phys RevB.80.094204), Mauro and co-author Xiaoju Guo at Corning Incorporated calculated the viscosity for the specific glass mixture used in Westminster Abbey. Their results revealed that the viscosity of such glass was actually 16 orders of magnitude smaller than previously thought. To confirm the model’s validity, Ozgur Gulbiten, a researcher Example of medieval glass seen in the south rose window of Notre at Corning, fabricat- Dame, Paris. Built in 1260 AD. ed a sample with the same composition of the glass in cathedral windows. Guilbiten of glass are fused together to create modmeasured the sample’s viscosity up to a valern displays for computers or televisions. ue of 1018 Pa s—a heroic meas-urement Manufacturers must heat the glass panthat took tremendous patience, ac
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