The 'E' Type Forum

mgcjag wrote: ↑

Sat Nov 28, 2020 7:57 pm

Just for info lead will sag over time....glass will not....old glass was hand made l....Steve

Thought I posted this last night...

Glass does sag after a century or two. Technically it’s not a solid at room temp but a ‘supercooled liquid. Not sure if it solidifies when coole further.

If glass flows over the centuries, then why do bottles and other glass vessels that are hundreds or thousands of years old (i.e. Egyptian, Greek, Roman, medieval european) remain in their original shape? Shouldn't they also show plastic deformation over time? Plenty of variation in mass and material among glass artifacts in museums, but no apparent change in shape. Why is the 'flow' evidence only apparent in ancient handmade glass window panes?

Hi All....just to tidy up this thread http://forum.etypeuk.com/viewtopic.php? ... 225303080f iv moved the discussions on glass here....Steve

Same thing happens with stone flags used for roofing , after a century ( or two ) when the roof is re-done , the flags have " bent " to follow any sags or dips .

I have read with interest the comments on the linked post (my background is materials science, and more precisely amorphous materials).
Sorry to disagree with those who think that glass significantly creeps upon time or load (it's only my opinion).

Creeping depends mainly on the relative loading vs solidification temperatures and on the relative strength of the chemical bonding between the atoms or molecules of the solid that will impact on the diffusive movements of such atoms, hence on the long term possible creep.

Silicate glass becomes solid at a rather high temperature range, around 1000 °C, and the glassy/amorphous nature at ambiant temperature doesn't help this material to creep.

Lead on the other hand has a rather low solidification temperature (327 °C = 500K) : at ambiant temperature (298K), we are at nearly 60% of its melting temperature! Hence the relative softness of lead at ambiant temperature...

Ageing of glass leads to crystallisation, not creep.
Consider e.g. the old glass materials retrieved from historical antique sites: even if not always visible, the microcrystallisation at atomic level embedded in an amorphous matrix produces fragility of the material that can be broken quite easily under load.
But even such antique glasses have not crept substantially over time and the products essentially keep their shape

Hi Steve

Don’t shoot the messenger! I’m not even sure whether it’s a physics or inorganic chemistry - possibly neither (materials science, maybe?).

Either way, the concept of supercooled liquids / amorphous solids is neither new nor controversial. I remember being surprised when Raymond Baxter mentioned it on Tomorrow’s World. Maybe I’m wrong and it was Biddy Baxter on Blue Peter?

Lay people like both of us (judging by your remarks) might think glass is about as obvious a crystalline structure as one could get, but that wouldn’t make it true.

I’m sure that on further thought you will recognize the two fundamental flaws in your own argument: namely, your unprovable assumption that other centuries-old glass objects have not also subtly changed shape over time and the impossibility of taking before/after measurements to support either argument.

Glass does creep. The glass in the windscreen of my XFS crept all the way to the car park floor in one afternoon when an angry passing golfer hit it with a club.

PeterCrespin wrote: ↑

Mon Nov 30, 2020 2:45 pm

I’m sure that on further thought you will recognize the two fundamental flaws in your own argument: namely, your unprovable assumption that other centuries-old glass objects have not also subtly changed shape over time and the impossibility of taking before/after measurements to support either argument.

I agree with you Peter