Glass is a fluid?

[Sayonara]

I read an article back in 9th grade that glass was an uber viscous fluid. Apparently they measured the glass murals of ancient cathedrals and found that the bottom of the glass was thicker than the top of the glass. Any one know how or what contributes to its viscousness?

It's the way it was manufactured.

 

We've been over this before in an old thread - I'll try dig it up.

[Sayonara]

http://www.scienceforums.net/forums/showthread.php?t=2124

 

Unfortunately the link that is posted several times and explains the whole thing no longer works.

[Lance]

Wiki is indeed God.

 

Glass as a liquid

 

One common belief is that glass is a super-cooled liquid of practically infinite viscosity when at room temperature. Supporting evidence for this position is that old windows are often thicker at the bottom than at the top. It is then assumed that the glass was once uniform' date=' but has flowed to its new shape.

 

One possible source of this belief is that when panes of glass were commonly made by glassblowers, the technique that was used was to spin molten glass so as to create a round, mostly flat and even plate (the Crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk would be thicker because of centripetal forces. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability. Also, the sparkle is greater and the visual effect stronger when the thicker side is down. There is anecdotal evidence that occasionally such glass has been found thinner side down, as would be caused by carelessness at the time of installation.

 

Writing in the American Journal of Physics, physicist Edgar D. Zanotto states "...the predicted relaxation time for GeO2 at room temperature is 1032 years. Hence, the relaxation period (characteristic flow time) of cathedral glasses would be even longer" (Am. J. Phys, 66(5):392-5, May 1998). In layperson's terms, he wrote that glass at room temperature is very strongly on the solid side of the spectrum from solids to liquids.

[edit']

 

Evidence against glass flow

 

* If medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more—but this is not observed.

* If glass flows at a rate that allows changes to be seen with the naked eye after centuries, then changes in optical telescope mirrors should be observable (by interferometry) in a matter of days—but this also is not observed. Similarly, it should not be possible to see Newton's rings between decade-old fragments of window glass—but this can in fact be quite easily done.

* Likewise, precision optical lenses and mirrors used in microscopes and telescopes should gradually deform and lose focus. This is also not observed.

[ecoli]

damn... I hate all those ignorant people who spread urban legends out of...uh...ignorance. It serves me right to listen to tour guides at the Corning Glass museum.

[CanadaAotS]

EDIT: I didn't see the 2nd page lol, this is in reply to the super-cooling post

 

oh, you can do that? kinda like super-heating it.

 

Theres an urban myth attached to super heating. Its said that if you heat up water in a ceramic cup for too long in a microwave it super heats and if you take it out to quick the water "explodes" and you could get 100+ degree centrigrade water all over your hands arms etc.

 

They tested it out on myth busters and found that it really does happen and could be very dangerous :|

 

but back on topic lol, super cooling, what happens to water thats super cooled? does it behave differently?

 

and glass never solidify's? have they tried lowering its temperature to like a few kelvins?

[doG]

Theres an urban myth attached to super heating. Its said that if you heat up water in a ceramic cup for too long in a microwave it super heats and if you take it out to quick the water "explodes" and you could get 100+ degree centrigrade water all over your hands arms etc.

Actually, when you think of boiling water, you think of bubbles coming to the top. Where do the bubbles come from? They form at a nucleation site. Usually this consists of a small flaw in the container or a seam--say, where the sides meet the bottom--but it could also be turbulence in the water due to convection. Such a flaw or turbulence is common in teapots or saucepans used to boil water on the stove. However, when you are boiling water in a microwave, you are probably using a ceramic mug, which is much smoother and may not have a good starting point for nucleation. On top of that, a microwave oven heats more or less uniformly, so the amount of convection and turbulence in the water is greatly reduced. Without nucleation, the water just gets hotter and eventually becomes superheated; that is, it exceeds the normal boiling point for water without actually boiling.

 

This is when things can get touchy. If you move the cup around a bit, or drop a teabag into it, or put your spoon in to stir it, or whatever, you can provide the nucleation site the water has been looking for. So it turns from a superheated liquid to a boiling one--quickly. Usually, this will just mean it boils over the top of the cup, in which case you'll only be burned if you happen to be holding it (such as if you took it out of the microwave and jostled it enough to start it going). It could also splatter a bit, just as a pot of boiling water occasionally throws up some scalding drops. But it's not going to "blow up" and throw all the water into your face.

 

See the Straight Dope for more....

[ecoli]

Actually' date=' when you think of boiling water, you think of bubbles coming to the top. Where do the bubbles come from? They form at a nucleation site. Usually this consists of a small flaw in the container or a seam--say, where the sides meet the bottom--but it could also be turbulence in the water due to convection. Such a flaw or turbulence is common in teapots or saucepans used to boil water on the stove. However, when you are boiling water in a microwave, you are probably using a ceramic mug, which is much smoother and may not have a good starting point for nucleation. On top of that, a microwave oven heats more or less uniformly, so the amount of convection and turbulence in the water is greatly reduced. Without nucleation, the water just gets hotter and eventually becomes superheated; that is, it exceeds the normal boiling point for water without actually boiling.

 

This is when things can get touchy. If you move the cup around a bit, or drop a teabag into it, or put your spoon in to stir it, or whatever, you can provide the nucleation site the water has been looking for. So it turns from a superheated liquid to a boiling one--quickly. Usually, this will just mean it boils over the top of the cup, in which case you'll only be burned if you happen to be holding it (such as if you took it out of the microwave and jostled it enough to start it going). It could also splatter a bit, just as a pot of boiling water occasionally throws up some scalding drops. But it's not going to "blow up" and throw all the water into your face.

 

See the Straight Dope for more....

 

That explains a lot when I'm making agarose gels in a glass beaker, it only starts "bubbling" when I take it out of the microwave and mix it around.

[mrblond5311]

What would happen if we used a ceramic pot on a huge stove? Would it still have the chance to "super-heat" with the lack of a good nucleation site?

[Pleiades]

Imagine a liquid as a box of Lego blocks that aren’t snapped together; imagine a solid as a bunch of Lego blocks that have been snapped together. An amorphous solid, such as glass, is where all the Lego blocks aren’t snapped together but are still stuck to each other so that you can’t pour them like a liquid.

 

Glass is a funny thing. It can be made from pure silicon dioxide (silica) but usually contains other things. Silicon dioxide is quite a diverse chemical, in addition to being able to become an amorphous solid it has 17 crystalline forms. See http://www.minsocam.org/MSA/collectors_corner/arc/silicanom.htm

 

If glass does flow, it does it over a really long time, any variation in thickness of old glass is due to imperfect manufacturing. Modern sheet glass is made by pouring glass onto a bath of molten tin, causing it to flatten out; the process can still leave minute variations in the thickness of the glass, causing it to distort the view through it. See http://www.glassonline.com/infoserv/history.html

 

What I’d like to see is a glass rod suspended at one end horizontally to make a cantilever, with a weight hung from the unsupported end. Set this up somewhere stable and check on it every few years.

 

But it's not going to "blow up" and throw all the water into your face.

If could blow up if you added a spoon of sugar to it, creating thousands of nucleation sites.

[mrblond5311]

Is that why they say water boils "faster" if you add a little salt to it? Or does it just appear to boil faster because of the extra nucleation sites?

[Pleiades]

Since adding salt to water actually raises the boiling point, I would say that yes, the reason you get more bubbles when the salt hits the water is because it provides a lot of nucleation sites. Once the salt is dissolved, the water will sometimes stop boiling for a bit, until it reaches the new, higher boiling point.

[mrblond5311]

Lol, thanks dude you rock. I guess I'll stop adding salt to pots of water now. Do all solids take longer to heat than liquid? Say would garlic powder taker longer to heat up than garlic sauce? I know it's probally a stupid question but you never know.

[RyanJ]

Lol, thanks dude you rock. I guess I'll stop adding salt to pots of water now. Do all solids take longer to heat than liquid? Say would garlic powder taker longer to heat up than garlic sauce? I know it's probally a stupid question but you never know.

 

Hmm... garlic powder would probably take more to heat up because in the liiquid its particles are dispursed.

 

Some substaces actually cool the water when they are dissolved in it and its pretty cool to watch, an example is Ammonium Nitrate which cna actually make the water freeze, its the stuff used in thse "cold packs" they use when you hurt yourself

 

Cheers,

 

Ryan Jones

[Pleiades]

I don’t know about garlic, but for the purposes of cooking, adding salt doesn’t affect things much. As I said, adding salt raises the water’s boiling point, but it doesn’t actually change the temperature of the water. As you may know, water in a pot will stay at its boiling point, no matter how much heat you add, due to the latent heat of vaporization. Adding salt will actually allow the water in a pot to become hotter, which is probably a good thing when cooking, as long as you are aware the food will cook faster. Ammonium nitrate on the other hand actually consumes heat energy when it mixes with water

[Kyle]

I've always had a plan to get a pane of glass and lay it horizontally then set a heavy ball bearing right in the center. I thought that if I left it for years and years and years, even passing it from generation to generation, I'd eventually have a deep funnel that looks like those illustrations for space-time curvature.

 

Maybe I'm way off but I always thought it would be interesting to try -- a long-term project.

[tomgwyther]

My Grandmother's house in wiltshire England was about 350 years old, she got a guy in to rotate all her windows 180', I Guess she should ask for her money back!!!!

[RyanJ]

I've always had a plan to get a pane of glass and lay it horizontally then set a heavy ball bearing right in the center. I thought that if I left it for years and years and years' date=' even passing it from generation to generation, I'd eventually have a deep funnel that looks like those illustrations for space-time curvature.

 

Maybe I'm way off but I always thought it would be interesting to try -- a long-term project.[/quote']

 

A long term project and them some... I'd think it could happen given enough time and it would be interesting to see.... you'd only have to wait a few hundred years too

 

Cheers,

 

Ryan Jones