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Specific-Salad3888 t1_j6e7b2o wrote

Reply to comment by Unique_username1 in ELI5- what is the difference between a liquid and a fluid? by stinkybuttttt

Doesn't glass also change shape just really slowly like over 100s of years. I was told this when on Venice years ago.

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Farnsworthson t1_j6ebhks wrote

Nope. Old fallacy based on old window glass usually being thicker at the bottom. But the way it was made at the time, it was always likely to be thicker at one end than the other* - and if your glass is like that (and expensive!) you're obviously going to put the heavier end at the bottom. Not least, there's less risk of it overtoppling and breaking while you're installing it. There's Roman glass around from over 2000 years ago; it's just the same shape as when it was made.

*(Basically, early window glass was made by spinning a blob of molten glass into a disc with centrifugal force. Which means that it gets thinner towards the edges. Then you'd cut the sheet up into panes. If you've ever seen old doors or windows with a rough, circular pattern of thickness on them - that's the cheapest bit, from the middle. Google "crown glass".)

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XenosapianRain t1_j6h61yy wrote

Career glazier... Glass used to be put through rollers,at the edges of the sheet it was imprecise in thickness.. Old glass is thicker at the bottom by installers choice. Most pressure from gravity was on bottom, so you cut the glass so the skinny side is up to reduce breakage. Taught these details restoring an old train.

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figmentPez t1_j6ec5fl wrote

Glass is not fluid enough to perceptively change shape in hundreds of years, if it's fluid at all. There may be some debate on if it would flow over the course of huge, cosmic scales of time, I'm not clear on that, but glass is a solid for all intents and purposes when it comes any earthly scale of time, even geological time.

Stained glass windows have glass panes with one side thicker than the other because of manufacturing defects, not because of fluid dynamics.

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