Editing Glass (section)

=== Reputed flow ===
The observation that old windows are sometimes found to be thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a timescale of centuries, the assumption being that the glass has exhibited the liquid property of flowing from one shape to another.<ref>{{cite news|url=https://www.nytimes.com/2008/07/29/science/29glass.html?ex=1375070400&en=048ade4011756b24&ei=5124&partner=permalink&exprod=permalink|title=The Nature of Glass Remains Anything but Clear|work=The New York Times|access-date=29 July 2008|date=29 July 2008|author=Kenneth Chang|url-status=live|archive-url=https://web.archive.org/web/20090424094929/http://www.nytimes.com/2008/07/29/science/29glass.html?ex=1375070400&en=048ade4011756b24&ei=5124&partner=permalink&exprod=permalink|archive-date=24 April 2009}}</ref> This assumption is incorrect, as once solidified, glass stops flowing. The sags and ripples observed in old glass were already there the day it was made; manufacturing processes used in the past produced sheets with imperfect surfaces and non-uniform thickness (the near-perfect [[float glass]] used today only became widespread in the 1960s).<ref name=Gibbs />

A 2017 study computed the rate of flow of the medieval glass used in [[Westminster Abbey]] from the year 1268. The study found that the room temperature viscosity of this glass was roughly 10<sup>24</sup>{{nbsp}}[[Pascal (unit)|Pa]]·[[Second|s]] which is about 10<sup>16</sup> times less viscous than a previous estimate made in 1998, which focused on soda-lime silicate glass. Even with this lower viscosity, the study authors calculated that the maximum flow rate of medieval glass is 1 [[Nanometer|nm]] per billion years, making it impossible to observe in a human timescale.<ref>{{cite journal | last1=Gulbiten | first1=Ozgur | last2=Mauro | first2=John C. | last3=Guo | first3=Xiaoju | last4=Boratav | first4=Olus N. | title=Viscous flow of medieval cathedral glass | journal=Journal of the American Ceramic Society| volume=101 | issue=1 | date=3 August 2017 | issn=0002-7820 | doi=10.1111/jace.15092 | pages=5–11}}</ref><ref>{{Cite web |title=Glass viscosity calculations definitively debunk the myth of observable flow in medieval windows |last=Gocha |first=April |work=The American Ceramic Society |date=3 August 2017 |url= https://ceramics.org/ceramic-tech-today/glass-viscosity-calculations-definitively-debunk-the-myth-of-observable-flow-in-medieval-windows}}</ref>