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Glass Flow

richardm

Philosopher
richardm
Joined
Aug 6, 2001
Messages
9,248
Does it? I thought not, but recently I've seen it repeated in a pop science book - glass is a liquid. The example given is that of mediaeval cathedral glass. If you remove a pane, you'll find that it is thicker at the bottom than at the top.

To me, this speaks more of the way mediaeval glass was made - almost impossible to make perfectly flat sheets by hand. So they naturally put the thicker ends at the bottom to make the most of the strength (as opposed to putting the thinner edge where the weight of the pane would rest on it). But according to this book and others, it has got like that because over the centuries it has flowed down there.

But what's the deal? Does glass flow or not?
 
Glass is a super cooled fluid or to put it another way it is just about a liquid. The bit about medivel panes of glass is also true.
 
I cannot speak to the accuracy of this page, but the author provides references:

http://www.ualberta.ca/~bderksen/florin.html

edited to add: The most relevant cite in this page seems to be:

Plumb, R. C. (1989) Antique windowpanes and the flow of supercooled liquids. Journal of Chemical Education, 66(12): 994-996.

who argues that the glass does not flow, but that the thickness is due to the manufacturing process.
 
geni said:
Glass is a super cooled fluid or to put it another way it is just about a liquid.
Click to expand...

In what way is it a super cooled fluid? Are rocks considered super cooled fluids too? What about metals? After all, if you heat iron enough that turns liquid too.
 
Glass is an amorphous substance meaning that it has no specific melting point and that it is not a true solid since it does not have a crystalline structure.

Kind of like butter. If butter is put into a freezer, it becomes quite hard. Allow the butter to come to room temperature, it becomes far less hard. Further increase the temperature, it becomes softer. Increase the temperature enough and it will gradually melt.

Glass works the same sort of way. It will flow at room temperature, but very, very slowly.
 
From Oxtoby et al., Chemistry: Science of Change, 2nd ed.:
Glasses are amorphous solids of widely varying composition. Outwardly, glasses often resemble crystalline solids and have mechanical properties similar to those of crystals. On a molecular level, however, glasses resemble liquids in structure, but with the diffusional motions of the molecules brought to a halt.
Click to expand...
If memory serves, the consensus is that glass either does not flow at all unless heated to its melting point, or if it does flow, the flow would not be observable over a few hundred years. Either way, the anomalies in old windows are not due to glass flow.

There is some concern about glass flow in the context of containment of nuclear waste, which can be hazardous for tens of thousands of years (or longer).
 
bewareofdogmas said:
Without heating glass would take trillions(1,000,000,000,000)
of years to flow.
Click to expand...
I recall seeing an article in which the author (a physical chemist) calculated the time it would take for glass to "flow" to the extent it would have to in order to produce the thickness variation typical in medieval window panes. He came up with a minimum flow time of at least 17,000 years. Unfortunately, I can't recall at present where I saw the article.

I did a google search, and found the following sites that say, no, glass doesn't flow:

http://www.glassnotes.com/WindowPanes.html

http://www.sciencenews.org/sn_arc98/5_30_98/fob3.htm (This one claims it would take more than the age of the universe to flow so much, as noted above by bewareofdogmas)

http://www.maths.adelaide.edu.au/Applied/staff/ystokes/windows.html
 
Very slowly?

I've seen various estimates on the amount of time that it would take glass to perceptibly flow, from may millions of years on the low end of the scale, to bewareofdogmas's statement of trillions of years, to a calculation I saw showing something like 10^39 years (10^39 years compared to the age of the universe is the same as the age of the universe compared to a few picoseconds).

Since glass has never flowed at room temperature, I think it stretches the truth to answer with a "yes," qualified by "very slowly" or not.
 
Glass doesn't flow at room temperature on any timescale at all relevant to humans.

The best proof of this is the lenses, mirrors and prisms of telescopes. If glass flowed even a tiny amount it would ruin the optics. Needless to say, this has never been observed, even with very old lenses.

Old window glass was thicker on one side due to the manufacturing process. It was framed with the thickest side down for stability.
 
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