[PA-NJ Glassblowers] Glass viscosity calculations definitively debunk the myth of observable flow in medieval windows

Tony Patti gaffer at glassblower.info
Fri Aug 4 21:26:36 EDT 2017



Published on August 3rd, 2017 | By: April Gocha, PhD


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  <http://www.cmog.org/article/does-glass-flow> 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


Glass scientists, including ACerS Fellow
<http://ceramics.org/?award_winners=edgar-d-zanotto> Edgar Zanotto, have
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
<http://ceramics.org/?award_winners=john-c-mauro> 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
<http://onlinelibrary.wiley.com/doi/10.1111/jace.15092/full> 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
-length-announced/> 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


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


"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


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
<https://en.m.wikipedia.org/wiki/Crown_glass_(window)> 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
ghts-with-gorilla-glass-5> 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 "
<http://onlinelibrary.wiley.com/doi/10.1111/jace.15092/full> Viscous flow of
medieval cathedral glass" (DOI: 10.1111/jace.15092).



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