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Like many other students, I remember being taught during my early education that glass—because the material is somewhere in between liquid and glass states—is actually flowing, ever so slowly. Old windows, my teacher explained, are measurably thicker at the bottom than at the top.

I remember being astonished by this fact as a child—how could something that seems so physically solid actually be more like a liquid? It was an important early lesson to a curious mind that some things are not always as they seem.

Although today, that lesson applies itself to that very legend that taught it.

Glass scientists, including ACerS Fellow Edgar Zanotto, have previously shattered the flowing glass window legend.

But recent advances have now allowed glass scientists at Corning to take another closer look at this urban legend by calculating the rate of glass flow in medieval windows.

The team combined glass transition theory and experimental characterization techniques, which, the scientists report, had astounding agreement. Their results indicate the highest ever direct measurement of glass viscosity at low temperatures.

The scientists—including ACerS members Ozgur Gulbiten and John Mauro, now at Penn State University—used medieval glass windows in Westminster Abbey from 1268 AD as the basis for their calculations.

Their measurements reveal that medieval glass has a much lower viscosity than expected at room temperature—16 orders of magnitude less than previous estimates, which were based on soda-lime silicate glass.

However, despite the low values, the glass’s viscosity is still “much too high to observe measurable viscous flow on human time scale,” the authors write in a paper describing their findings, published in the Journal of the American Ceramic Society.

New calculations show that medieval glass windows, like these at Sainte-Chapelle in Paris, France, are not thicker at the bottom because of glass flow. Credit: John Mauro

Just how slow is too slow?

The team’s calculations show that the medieval glass maximally flows just ~1 nm over the course of one billion years.

That’s just 0.000000001 nm per year—which, although is theoretically measurable, would be practically impossible to achieve.

“This result confirms that the long-lasting myth about the flow of glasses at room temperature is still just that: a myth,” the authors conclude in the paper.

While the results are based on calculations and experiments for those specific Westminster Abbey windows, however, the results extend beyond those examples.

“The flow rate is specific to the particular viscosity curve, which is typical for medieval cathedral glass compositions,” Mauro explains in an email. Those compositions usually included higher K2O and MgO concentrations and lower SiO2 and Na2O concentrations than modern window glasses. But “the glass composition would have to be changed rather dramatically to get a qualitatively different result.”

In other words, although different glass compositions will have different flow rates, the rate is still going to be too slow to account for any measurable changes. So my teacher, and many others, were definitively wrong in stating that those old glass windows were slowly seeping down toward the earth.

However, my teacher wasn’t wrong that many old glass windows are actually measurably thicker at the bottom—but that difference can be traced to manufacturing inconsistencies.

Medieval windows were typically manufactured using the crown process, in which glass was blown into a hollow globe, flattened, and spun out into a flat disk. Glass window panes were cut from the non-uniform disks, which were thicker in the center and thinner at the edges.

“Given non-uniform glass, it is only natural to orient the thicker part of the glass at the bottom, since it gives the appearance of being more stable,” Mauro explains.

In addition to definitively debunking the flowing glass window myth, the new JACerS paper represent an important development in available methods for studying low-temperature dynamics of glasses, especially commercially relevant glasses, such as glasses used in flat panel display and chemically strengthened cover glass—like Gorilla Glass.

“This work represents significant advances in both theoretical and experimental characterization of low temperature viscosity, which is also extremely valuable for modern industrial glasses,” Mauro says.

The paper, published in the Journal of the American Ceramic Society, is “Viscous flow of medieval cathedral glass” (DOI: 10.1111/jace.15092).