While I'm a little reluctant to contradict the word of a 30 year leadlighter, I can't let the "glass flowing under gravity" statement go un-challenged. Sorry John
The story of old window panes flowing over time is a myth. Any sagging or thickening at the bottom of the glass was there the day it was fitted. Once glass solidifies from the molten state, that's the shape it stays. The reason that many old window panes are thicker at the bottom is that it probably just made sense for the glass fitters to install it that way. There may be rare examples to be found where it's thicker at the top, but nobody mentions those.
I suppose the idea that glass flows may have been given some credence because glass doesn't have an obvious melting point. It just goes from hard to soft, to a syrupy liquid as it's heated. So there's a theory that glass is a supercooled liquid. But it's not. It's an amorphous solid (has no crystal structure) but that doesn't mean it can't be rigid. That melting behaviour is typical of amorphous solids (like wax for example) because there's no change of crystal structure and no latent heat of fusion.
Anyway, a simple thought experiment will reveal the truth. If glass did flow, it may indeed get thicker at the bottom, but it won't get thinner at the top. It'll just slump down and get shorter. Anyone who has taken jelly out of a mould or done a concrete slump test knows that. So why isn't there a gap at the top of all those old windows?
There are solid(ish) substances that slowly deform and indeed flow under gravity. Pitch is a good example. Refer to the pitch drop experiment
Pretty much anything will deform if the conditions are right. Rocks like granite can flow, but not under gravity and not in human time scales. It takes geological time scales under extreme pressure and high temperature. Such rocks called "rheids" by geologists. The capacity of a such materials to flow is called "rheidity" (similar concept to viscosity in fluids).
en.wikipedia.org
Perhaps glass would flow under similar geological conditions. It's a difficult question for geologists to answer because naturally occurring glass (obsidian) has no crystal structure that would give any clues as to its deformation since it cooled from the molten state. And naturally occurring window panes are pretty rare.
I guess the distinctions between viscosity, rheidity, creep, plastic deformation and change of state can be somewhat blurred. The subject seems to cross over geology, metallurgy, fluid dynamics, physics. chemistry* and maybe even quantum mechanics. I don't know the answers. I'm not an expert in any of this shit. I'm just curious enough to read the occasional book but I don't burden myself with any real or useful knowledge.
* Note that chemistry is just a small branch of physics. That should piss off chemistry gurus out there.