gauss
2002-05-13, 12:28 AM
Alright. So, as expected everyone came out of the wood work with arguments for and against glass being liquid. I am psyched that you all are at least interested in the subject. I too, find materials science very interesting.
So there are pretty much two arguments people use to make the case that glass is liquid. The first is that it has no melting point. As one of you pointed out, that is not to say it doesn't melt. It only means that there is not a specific temperature that it will transition from liquid to solid and vice versa. Instead glass moves from liquid to solid over a range of temperatures. It is very tempting, then to make the argument that since there is no melting temperature, it never actually becomes solid. This isn't true though. If you plot viscocity against temperature you will see that the viscocity becomes so high around room temperature that really the plot is only an interpolation of points for viscocity at higher temperatures. If you want to consider glass a liquid, then its viscocity is so high that nobody has ever been able to devise a way to even detect viscous behaviour. There are several other things that fall into this category. The are called solids.
The other quite famous argument comes from people looking at the windows of old churches and noting that the glass is thicker at the bottom. In fact, this may be true but it doesn't prove anything. In older times glass obviously couldn't be produced with the same quality as now. It is very reasonable to expect that the glass produced then would be thicker on one end. It may be reasonable to note that this end should be placed down as that is a more stable placement of the glass. But whether you believe the last sentance or not, the fact is no one recorded the placements and thicknesses at the time they were placed, so the shoot from the hip conclusions about it flowing have no scientific basis. There is another similar story about a very old university. Recently a professor went into the basement and saw some glass rods that were for students to use in the labs. They were on this rack that supported two sides, and the very old rods were sagging down in the middle. Excited he ran to his collegue, and brought his there and said something like, "Look! finally conclusive evidence that glass at room temperature continues to flow." While the two were down there, a student came down and grabbed one of the rods to use. The student stopped and said, "Oh this one is bend," put it back on the rack, and grabbed a straighter one. I guess you guys see what was happening, It is just as likely that the rods diddn't bend, as that for dozens of years students took the straightest rods from the stock until mostly ones that were produced bent remained.
So here are the facts. If you interpolate the viscocity for glass down to room temperature, it is so high you are left with nothing more than a gimmick to tease your friends with. In no engineering application would a viscocity this high ever need to be taken into account. Glass does not have a specific melting temperature. Glass does in fact go through 5 stages from liquid to solid. I no longer remember them, but will look them up. In fact at high temperatures, metals and ceramics exhibit viscoelastic strains. This is known as creep. Creep is where a solid, still far below its melting temperature, undergoes plastic deformation, still under its yield strength. These materials are still considered solid. In metals this is still below the solidus temperature by a lot. Even if glass does turn out to flow a few microns per 10 years under its weight 500 degrees below its glass transition temperature, that makes it no more liquid then a block of iron which exhibits the same behaviour well below its melting temperature.
I know a lot of you won't believe me because I am just some punk kid, and it is no fun to think of glass as solid. But talk to a different material scientist, and they, if informed on the subject will paint you the same picture I think.
-gauss
So there are pretty much two arguments people use to make the case that glass is liquid. The first is that it has no melting point. As one of you pointed out, that is not to say it doesn't melt. It only means that there is not a specific temperature that it will transition from liquid to solid and vice versa. Instead glass moves from liquid to solid over a range of temperatures. It is very tempting, then to make the argument that since there is no melting temperature, it never actually becomes solid. This isn't true though. If you plot viscocity against temperature you will see that the viscocity becomes so high around room temperature that really the plot is only an interpolation of points for viscocity at higher temperatures. If you want to consider glass a liquid, then its viscocity is so high that nobody has ever been able to devise a way to even detect viscous behaviour. There are several other things that fall into this category. The are called solids.
The other quite famous argument comes from people looking at the windows of old churches and noting that the glass is thicker at the bottom. In fact, this may be true but it doesn't prove anything. In older times glass obviously couldn't be produced with the same quality as now. It is very reasonable to expect that the glass produced then would be thicker on one end. It may be reasonable to note that this end should be placed down as that is a more stable placement of the glass. But whether you believe the last sentance or not, the fact is no one recorded the placements and thicknesses at the time they were placed, so the shoot from the hip conclusions about it flowing have no scientific basis. There is another similar story about a very old university. Recently a professor went into the basement and saw some glass rods that were for students to use in the labs. They were on this rack that supported two sides, and the very old rods were sagging down in the middle. Excited he ran to his collegue, and brought his there and said something like, "Look! finally conclusive evidence that glass at room temperature continues to flow." While the two were down there, a student came down and grabbed one of the rods to use. The student stopped and said, "Oh this one is bend," put it back on the rack, and grabbed a straighter one. I guess you guys see what was happening, It is just as likely that the rods diddn't bend, as that for dozens of years students took the straightest rods from the stock until mostly ones that were produced bent remained.
So here are the facts. If you interpolate the viscocity for glass down to room temperature, it is so high you are left with nothing more than a gimmick to tease your friends with. In no engineering application would a viscocity this high ever need to be taken into account. Glass does not have a specific melting temperature. Glass does in fact go through 5 stages from liquid to solid. I no longer remember them, but will look them up. In fact at high temperatures, metals and ceramics exhibit viscoelastic strains. This is known as creep. Creep is where a solid, still far below its melting temperature, undergoes plastic deformation, still under its yield strength. These materials are still considered solid. In metals this is still below the solidus temperature by a lot. Even if glass does turn out to flow a few microns per 10 years under its weight 500 degrees below its glass transition temperature, that makes it no more liquid then a block of iron which exhibits the same behaviour well below its melting temperature.
I know a lot of you won't believe me because I am just some punk kid, and it is no fun to think of glass as solid. But talk to a different material scientist, and they, if informed on the subject will paint you the same picture I think.
-gauss