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@robpike The simple answer is that at room temperature, glass doesn't flow. If it did, all telescope mirrors, which need to have a surface figure accurate to about 1/10 of a wavelength of visible light (so, about +/- 50 *nanometers* across the entire surface of the glass) would be ruined within years, if not days. See, e.g. https://engineering.mit.edu/engage/ask-an-engineer/how-does-glass-change-over-time/.