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Comment by helltone | Hacker News Reader

I thought glass was a solid?

I lived in a house built around 1605 in the Netherlands, with some of the windows being hundreds of years old, based on expert opinion. The glass had started to droop downward in certain places, almost like the resin you see in trees. It made me realize that glass is not as rigid as I thought.

I think that glass flowing thing is a myth. The manufacturing method back then resulted in uneven panes and they were installed with the thicker side at the bottom.

I'm not entirely convinced it is a myth, but the practical effect surely is.

I do suspect that over insanely long periods an otherwise undisturbed physical object would flow-sag at some rate under the influence of a constant force in a given direction.

No idea how long that would take, but erosion and other destructive forces appear to dominate so for practical measures any such effect is likely against the noise floor.

It's an amorphous solid last time I dove into this.

The "well it's technically a liquid!" because it "flows" is really not telling the whole story. Like most science, it's just more complex than can be quickly summarized with one sentence, and doesn't quite map to just high school simplifications.

Nope, lots of fluids that just flow over such a long period they appear solid.

That’s a misconception. Glass does not flow over time; it is a rigid, amorphous solid. The uneven thickness seen in old cathedral windows is a result of historical manufacturing. Glass was spun into discs that naturally became thicker at the edges, and builders installed the heavier side at the bottom for stability. Physicists calculate that it would take longer than the age of the universe for room-temperature window glass to visibly deform under gravity. Reference: Zanotto, E. D. (1998). "Do cathedral glasses flow?" American Journal of Physics, https://doi.org/10.1119/1.19026

> That’s a misconception. Glass does not flow over time;...Physicists calculate that it would take longer than the age of the universe for room-temperature window glass to visibly deform under gravity.

so then it does flow over time, just not on a scale where it matters to us.

But this is true for lots of solids, not just glass. Consider creep deformation [0] which is deformation mediated by diffusion of defects in solid materials. It’s a big problem in metal turbine blades, it limits the maximum usable temperature to well below the melting point.

The physical mechanism would be similar in glass flowing in this way, so I don’t think evidence of glass flowing like this should make us think of it as a liquid instead of an amorphous solid

0: https://en.wikipedia.org/wiki/Creep_(deformation)

How do you go from “calculate that it would take longer than the age of the universe for room-temperature window glass to visibly deform” to “so the. It does flow”?

Glass flows a higher temperatures where it does act like a liquid, you can study glass blowing for examples, however no, room temperature glass is not a liquid and does not flow, for any practical definition of what flowing is.

That ‘long period’ can be many billions of years, glass is an amorphous solid.

Not sure why klustregrif got flagged - the answer was informative and correct.

What I said is true but not for glass. Pitch is a liquid even though it feels like a solid and shatters when smashed.

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