Glass is a liquid?

I could have sworn I read this on here years ago but, alas, I cannot find it.
Am I the only person who still encounters people who insist glass is, in fact, a liquid? It seems to be one of those little-known "facts" that people like to bring up to show everyone how smart they are, even if it's completely irrelevant.
I don't know why this bothers me so much, possibly because I've been drawn in to several silly arguments over it.
I propose a new theorem: Whenever the subject of glass is brought up in public, in any context, eventually someone will claim it is a liquid.

[ganzfeld]

The answer is not very easy. The definition of "liquid", if you think about it, is kind of vague for things that are extremely viscous. However, there are a few myths about glass being a liquid that can be kicked to the curb quickly. Ordinary glass in windows don't warp or change shape over hundreds or thousands of years, no matter what anyone tells you. Old panes have irregularities because of how they were made, not because they are old. They aren't thicker on the bottom than on the top. If you had a bunch of glass in a container, it would never in a million years fill the bottom of that container.

Glass has all the properties of a solid that we would ordinarily associate with solids so there is no reason at all to consider it a liquid. From an ordinary engineering point of view, it's definitely a solid at room temperature. Engineers can measure its ordinary solid material properties just as they would any other solid.

A very few materials engineers and physicists use a different definition of solid and liquid in which "supercooled liquids" include glass. It has to do with their molecular structure and their lack of a definite melting point and does not really have anything to do with the ordinary definition of a liquid. Glass is a solid, not a liquid for all except a very few esoteric purposes. Here is a good reference:
http://math.ucr.edu/home/baez/physic...ass/glass.html

[lynnejanet]

I have seen old panes of glass that are thicker at the bottom, and I've always beleived the UL that glass slowly "flows" due to gravity. I really had no idea it wasn't true. Thanks, Ganz & Halfshell.

[Nice to know that after several years at snopes, I'm still learning. Or maybe that just means that I came here with a whole lotta stupid!]

[Barns & No Bull]

Originally Posted by lynnejanet

[Nice to know that after several years at snopes, I'm still learning. Or maybe that just means that I came here with a whole lotta stupid!]

Maybe we can learn some more...

I have seen old panes of glass that are thicker at the bottom, and I've always beleived the UL that glass slowly "flows" due to gravity. I really had no idea it wasn't true. Thanks, Ganz & Halfshell.

I presume you have also seen old glass panes that don't really look any thicker at the bottom than at the top. But do you recall ever seeing an old pane that was thicker at the top?

[Johnny Slick]

It seems like selection bias is also at play here. Most of the time the way you notice when part of a window is thicker than another part is when you break it. If you break a window that's thicker at the bottom than on the top, the top part breaks and the bottom is left there to observe. If you break a window that's thicker on the top than the bottom, the weight of the top bearing down on the space where the bottom used to be will possibly cause the entire thing to shatter.

[Barns & No Bull]

I would agree that observational bias probably applies, and that's where I'm going with this. But I cannot agree with your analysis of what is causing the bias.

Do you not think that many people have observed 'thicker at the bottom' when looking at unbroken panes?

[Cervus]

My college chemistry teacher insisted that glass was a liquid, citing the "proof" that old panes of glass are thicker on the bottom than at the top because gravity causes the glass to slowly flow to the bottom. He was a rather grumpy and nasty person so I didn't feel like arguing with him.

[Troberg]

A very few materials engineers and physicists use a different definition of solid and liquid in which "supercooled liquids" include glass.

Or, using the more common term, it's an amorphous solid. A fancy word for "not having a distinct melting point".

http://en.wikipedia.org/wiki/Amorphous

I remember having a science teacher who insisted that glass was a fourth state of matter, and in her simple understanding that meant it had to be a plasma.

The same teacher changed the test questions because, "Oh I don't know where this solar particle stuff is from, I learned FIFTY years ago that the Aurora is sunlight reflecting off the ice caps!"

[Flyer22]

Interesting discussion. I didn't realize that I'm in the minority: I consider glass to be a definite liquid, albeit a supercooled one.

[DocB]

Originally Posted by Troberg

Or, using the more common term, it's an amorphous solid. A fancy word for "not having a distinct melting point".

http://en.wikipedia.org/wiki/Amorphous

Small nitpick: "amorphous" is actually a fancy word for "not having a regular crystal structure".

[lynnejanet]

Originally Posted by Barns & No Bull

Maybe we can learn some more...

I presume you have also seen old glass panes that don't really look any thicker at the bottom than at the top. But do you recall ever seeing an old pane that was thicker at the top?

The two that I remember were very old glass from a barn window, and glass that was in the local pioneer museum. By very old, I mean old in North American terms, so it was probably no more than 130 years old. The pioneer museum had an old frame with one side removed, so that you could see the variation in thickness. It wasn't great, but it was noticeable. I very clearly remember being taught about glass "flow" during museum tours, as a kid.

I don't think I've ever seen an old pane that was thicker at the top, but I've never really looked.

The house that we live in was built in about 1880, and it has original windows, for the most part (I think some of the storms have replacement glass, but it's still at least 80 years old). There are lots of faults and bubbles in the glass, but it's not noticeably thicker at the bottom. It's also getting very fragile, and breaks easily, although it may always have been that way.

Where are we going with this, Barns? Are you contending that glass does flow? Or are you reinforcing the idea of observational bias?

[Towknie]

While it doesn't provide a definitive answer to the question at hand, this cite gives a reasonable explantion to the antique window pane thing.

The action of heat and centrifugal force combined is soon visible. The nose of the piece, or hole caused by the removal of the blowing pipe, enlarges, the parts around cannot resist the tendency, the opening grows larger and larger; for a moment is caught a glimpse of a circle with a double rim; the next moment, before the eyes of the astonished spectator, is whirling a thin transparent circular plate of glass which, but a few minutes before was lying in the glass-pot, an indistinguishable portion of the molten mass. The sound of the final opening of the piece has been compared to that produced by quickly expanding a wet umbrella. In this way a flat circular disc, nearly sixty inches in diameter, or sometimes more, is produced, of almost uniform thickness, except at the point of attachment to the ponty, where there is a swelling called, as already stated, the bull's-eye. The glass at the edge of the disc is also in some cases a little thickened. Still whirling, the table, as it is now called, is carried off, laid flat upon a support called a whimsey, detached by shears, or otherwise, from the ponty, lifted into the annealing kiln upon a fork and piled upon its edge against the preceding table.
The glass was amazingly flat and uniform in thickness for a hand-fabricated disk 5 ft in diameter, but not in comparison to sheet glass drawing processes of recent years and float glass processes widely used now. Anyone who has attempted any glass blowing will recognize the difficulties inherent in the process pictured.
After the tables were cooled, panes were cut. Quoting Muspratt further,

The cutting of a circle into rectangular sheets, must necessarily be attended with waste, while the bull's-eye confines those sheets to comparatively small sizes. Uniformity of thickness also, except by the most skillful manipulation, is difficult of attainment... The splitter carefully examines each table before splitting it, and turns it round till he has brought it into the position in which he may split it to the best advantage, announcing at the same time it's quality. The first quality is called best - the next, seconds - then thirds - fourths - CC - CCC or Irish - and the tables containing any very glaring defect, come under the denomination of coarse... No wonder that tables of the best quality are few and far between, in some manufacturies a forlorn hope never to be realized.

The explanation being put forward here as to why antique window planes are thicker at the bottom than the top is not yet complete. Why at the bottom rather than the top? A window glazier's instruction book of the period, if one could be found, might contain the answer. It would certainly make good sense to install the glass with the thick edge down!

[Andrew of Ware]

I have read that larger panes of old glass were made by passing the molten between metal rollers. These expanded slightly as the hot glass passed between them thus making parts of the pane thicker than others. Naturally the glass was then placed with the thicker part at the bottom.

Anyone who thinks that glass is a liquid only has to go into the thousands of churches in Europe that still have medieaval glass and you will not see any glass flowing down. Further proof can be seen with Roman glass. For example, the Portland Vase in the British Museum is almost 2,000 years old, but it is still crystal clear.

[lynnejanet]

So, another, related, question. Does glass get brittle and more breakable as it gets older? Or is old window glass just more fragile, by nature, than modern glass? Or, does it just seem more fragile, because it is thinner, and less uniform than modern glass?

[Horse Chestnut]

Hmmm. Years ago I dated a glass artist, and he was the first person to tell me that glass was a liquid, and cited the example of the "thicker at the bottom" panes. He even called his glass studio "Slow Water Glassworks".

Of course, I never thought to ask why a 200 year old pane would change shape, but a 2000 year old glass vial would not. What can I say: I thought I was in love.

[Malruhn]

Originally Posted by lynnejanet

So, another, related, question. Does glass get brittle and more breakable as it gets older? Or is old window glass just more fragile, by nature, than modern glass? Or, does it just seem more fragile, because it is thinner, and less uniform than modern glass?

Finally something I can help with!!

Glass used to be made of... glass. nowadays, there are additives to make it more pliable, less prone to shattering, and sometimes there are glazes on it that didn't exist over 25 years ago, so, no, it really doesn't get more fragile. It just gets more polluted the newer it is.

Oh, and I had been told my whole life that it was a liquid as well. Crap, snopes destroys another "fact" of my life...

[Johnny Slick]

I think that on further review Towknie got it nailed. The reason you don't see a lot of thinner glass on the top of a window pane is because when a window maker has a pane that's irregular, they're going to put the heavy part on the bottom to make it easier to stick in the window-hole and also to prevent against catastrophic damage.

[charlie23]

I've worked with glass as a medium for over 20 years, and have also worked as a Glazier off and on during that time: I've repaired stained glass windows that were over 200 years old and can't even guess how many windows I've repaired in old (and new) houses and shops. Also I don't care to guess how many times I've heard the "glass is liquid" story, but the explanation of thicker bottoms and obvious bubbles and "drips" in flat glass panes as proof of the theory simply dosen't work. I've seen plenty of old panes that were thicker at the top and apparently "oozed" against gravity, although in general I think Towknie's explanation is correct; common sense & aesthetics would dictate putting the thick end down. Old time glaziers just didn't always utilize common sense, although the practicle aspect of having the thick end down in a small pane is really minimal.
The reality is simply that until fairly recently even glass window panes were hand blown:
Until the 1800's glass panes were made from rondels, as Towknie pointed out: a glassblower would simply spin a sphere of molten glass into a circular plate of the desired thickness, then cut the panes from that. Due to centrifugal force the outside edge was always thicker and of course various imperfections were inherent in the process.
Several other methods were developed in the 19th century, but primarily they blew large cylinders rather than rondels then cut and flattened those. This resulted in larger sheets, but they still had the normal imperfections.
"Plate" glass first went into real production in the 1920's, but even then they simply poured a big puddle of molten glass onto an iron table (plate) then drew it through rollers. Window ("float") glass as we know it today didn't come along until the 1950's when they started floating the molten glass on a bed of molten tin, which allows the glass to stay hot enough to settle out under it's own weight and produces a near perfect sheet.
As far as the brittleness, Malruhn is more or less correct: modern glass is less brittle because of it's composition, the fact that there are fewer flaws in it and thanks to various processes such as tempering. But glass, like any other material, is subject to stressing from repeated temperature extremes and exposure to the elements so it does in fact become more brittle with age. It's notoriously difficult to cut antique glass.

[Silas Sparkhammer]

My college physics prof taught us that if you were to take a sheet of glass and put a weight on it, the weight would cause the glass to deform, slowly, re-shaping itself around the weight and bending permanently under it. He said this was demonstrated that glass was a very high-viscosity liquid.

I never actually did the experiment; if I did, what would you expect to happen?

Silas ("A trip to the doctor's office for six stitches in the sole of my foot."