Glass Isn’t a Fluid

Mark R writes:

Glass is a Fluid, Too
Post complex equations regarding how long it would take a certain window to flow, and post pictures of sunken glass. This would be educational.

This is a pretty widespread myth. Actually, glass is not a fluid and does not behave like one as long as it is below the glass transition temperature. It’s a bit difficult to classify glass under the traditional categories for a solid due to its phase transition behavior and its lack of crystallization, but it is usually classed as an amorphous solid.

The observation that old panes of glass tend to be thicker at the bottom is usually used as evidence that glass flows over the centuries, but this assumes that the glass was flat to begin with. However, glassblowers at the time usually made panes by spinning molten glass to create a round, mostly even flat, which was then cut to fit. Although spinning made the glass mostly flat, the edges of the disc tended to be thinner. When installed, the glass was typically placed thicker side down for stability purposes. One researcher even calculated the time period necessary for glass to flow and deform at ordinary temperatures as 10^32 years–longer than the age of the universe.

If that is not convincing, consider this: if glass flows at a rate that’s discernible to the naked eye after a couple of centuries, then the effect of this deformation should be extremely noticeable in antique telescopes since a slight change in the lens’ optical properties should dramatically affect performance. But no such degradation occurs. (Photo credit: Vincent van der Pas)

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