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
Hi gaus, you have obviously made up your mind on this topic, I on the other hand sit on the fence. Of course there isn’t just solid, liquid and gas to categories the state of a substance into, there is also the supercritical state, the liquid crystalline state and the so-called glassy and rubbery states (which are still being argued over). Not that they are important here but most people only think there are three.
I see your point of view but as I said I’m not entirely convinced either way. The ‘old glass panes thicker at the bottom’ argument is a thorn in the side for pro liquid glass people (and a weapon for pro solid glass people) don’t you think as there is just no scientific evidence to support it as you have pointed out.
Water is weirdest substance and solid water exists in a wide variety of stable (and metastable) crystal and amorphous structures.
To think we got to this from the common misuse of alloy to reference an aluminium alloy (or an aluminum alloy depending on where you’re from), we had better not mention the misuse of the word plastic, oh dear I already have
Don’t worry. My lips are sealed on the whole plastic issue. That could be a real mess. As for glass. pandora’s box has been busted open and spread to the corners of the earth so I guess that is fair game: I guess my stance on glass is that it is a solid from an engineering standpoint. The article you posted diddn’t talk in detail about the thermodynamic motivation for calling glass a liquid which I would have liked to see. However I have trouble believing that glass is a super cooled liquid, as it is not cohesive forces but bonds that hold it together. This has been interesting. I didn’t realize that there were people that were informed on the issue that believed that it could be a liquid. That whole lack of any evidence really seems to hurt the “liquid people.”
-gauss
So I went and got my old ceramics text. Here is the reference for anyone wanting to get a stronger understanding of glass. Funamentals of Ceramics. Michel Barsoum. Mcgraw-Hill 1997.
The text is often used in undergraduate and graduate courses. Chapter nine is all about glass and its properties and formation and structure. The second paragraph adresses this issue:
"Numerous X-ray diffraction studies of glass have shown that while glasses have short range order, they clearly lack long range order and can therefore be classified as solids in which the atomic arrangement is more charateristic of liquids. This observation suggests that if a liquid is cooled rapidly enough such that the atoms do not have enough time to rearrange themselves in a crystalline pattern before their motion was arrested, then a glass is formed. As a consequence of their structure, glasses do not have unique melting points but rather soften over a temperature range. Similarly viscocity increases gradually as the temperature is lowered. "
-gauss
That was intentional. I didn’t think anyone would want to read too much detail as this is a unicycle forum but then again there are allot of people here interested in materials when it comes to prototype unicycles.
I did read somewhere (here?) that glass peddles will be available soon, how about a glass frame
Being an ex-glassblower I’m in the super cooled liquid camp.
The glass pane being thicker at the bottom is a construction issue. One of the hardest things for a glassblower to learn is to make a thin neck. Glass is blown into a cyclindrical shape using various methods(stretching, swinging, blowing into molds etc. It is then broken off the blowpipe and out into an annealer. After cooling one side is cut and then heated up again. While softening the cylinder is pulled apart until it can safely fall flat. annealed and then cooled it can be cut by scoring the surface and then applying force from underneath the score with your hands or on the edge of a table etc. The thick parts are put at the bottom for stability reasons.
Just a brief hopefully not too confusing description. I have some great references from the 12th century describing how to make glass etc. both Latin and english if you’re interested…
great post.
Supercooled liquid? I am curious to hear why you think that. If it is because glass is amorphous, I want to remind you that metals can also be made to be amorphous. As I mentioned from the Barsoum text, that X-ray diffraction seems to clear up the whole issue, as you would expect it would be able to. I have found among the ceramists that I work for and with that there isn’t really any discussion on the issue. They tend to be all solidly in the solid camp.
-gauss
