Is glass a slow-flowing liquid? Why old windows are thicker at the bottom

Glass does not flow at room temperature in any way a person could ever observe. The idea that it is a very slow liquid is a long-running myth. Materials scientists describe glass as a highly resilient elastic solid, and say that glass left undisturbed does not change, however old it is. [1] [2]

The story usually points to old stained glass windows, which are often thicker at the bottom. Archaeologist Nadine Schibille adds that the atomic structure of glass is a disordered jumble rather than a regular crystal, resembling the arrangement seen in liquids, and that this is part of why people assume it flows. She also notes that medieval window glass is often badly preserved and can corrode from pollutants, which adds to the impression that glass is unstable. [3]

The unevenness comes from manufacturing. Medieval windows were typically made by the crown process, in which glass was blown into a hollow globe, flattened and spun out into a disk. Panes cut from the disk were thicker toward the center and thinner at the edges. Glass scientist John Mauro notes that with uneven glass, it is natural to place the thicker part at the bottom because it looks more stable. MIT's Michael Cima makes the same point: the ripples, warps and bull's eye marks in old glass were created when it was made. [4] [2]

Corning's museum also points out that glass objects thousands of years older than cathedral windows show no sign of flow. [1]

A 2017 study by Corning researchers, published in the Journal of the American Ceramic Society, tested a glass composition typical of Westminster Abbey's windows from 1268. They found its room temperature viscosity was on the order of 10^24 to 10^25 pascal-seconds, about 16 orders of magnitude lower than an earlier estimate based on soda-lime glass. Even so, it was far too high for flow to be measurable over a human lifetime. Their calculation gave a maximum flow of about 1 nanometer over a billion years. Mauro said the glass composition would have to change dramatically to give a qualitatively different result. [5] [4]

Not flowing does not mean glass is indestructible. Cima explains that water can attack tiny flaws in common lime glass and greatly weaken it over time, and that abrasion, stress, caustic chemicals and temperature extremes can also damage it. Some glasses, such as the silicate glass used for Pyrex, resist these better. [2]