Huge writes:

On 2009-02-23, Alan Braggins wrote:

Or you could save time by looking at a piece of volcanic glass a few
million years old, and noticing it still has sharp edges. On a slightly
smaller timescale, you can see sharp engraving on Roman glass in museums.

My wife has two pieces of Roman glass (she collects "art glass"). Neither
of them show any evidence of flow, and both are well over 1000 years old.

Do they show evidence of devitrification? (I'd expect the
glass to be a little cloudy or even have visible crystals.)

What's always struck me about the myth that glass (which
does have the molecular organisation of a liquid) flows is
how, supposing that it flowed, could the rearrangement of
molecules that that entails avoid hitting the more stable
crystalline arrangement, at least microscopically? And once
it starts crystallising, it'll keep going, just as if you
have super-cooled water, a tiny disturbance can make it turn
to ice in a twinkling (not that undisturbed glass is going
to do anything quickly).

--
Jón Fairbairn
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2009-01-31)