Glass Dynamics

June 5, 2005 5:40 PM

I heard a long time ago that glass in old houses is thicker at the bottom of the pane because it's essentially a (very slow) liquid, and over time, it "melts" down to the bottom. Then, I heard that the story of glass "melting" was an urban legend. I thought I read this in "The Straight Dope", but I can't find it. Can anyone point me toward a definitive statement on this?
posted by interrobang to home & garden (23 comments total)

I'm pretty sure Bill Bryson mentions this in a debunking sort of way in A Short History of Nearly Everything, but I can't remember the exact quote.
posted by goodnewsfortheinsane at 5:52 PM on June 5, 2005

"Glasses are amorphous solids. There is a fundamental structural divide between amorphous solids (including glasses) and crystalline solids. Structurally, glasses are similar to liquids, but that doesn't mean they are liquid. It is possible that the "glass is a liquid" urban legend originated with a misreading of a German treatise on glass thermodynamics."
posted by Nothing at 5:58 PM on June 5, 2005

I've heard about this property of glass before (defined as viscoelasticity or creep - I'm not sure which). I heard that a plate of glass fixed at one end in a horizontal position would slowly start to sag. It would be more evident in this configuration than the thickening thing. It may be one of those things that is theoretically possible but not something that you could even measure.
posted by spirograph at 6:00 PM on June 5, 2005

Searched the index, but no luck. Googled up:

It is common for books to state that glass is more like, or just like, a liquid than a solid. This is moderately true. Like a liquid it just gets thinner and thinner (less viscous) as the temperature rises. Unlike most solids we encounter, there is no sharp melting point, unlike say ice which is solid at 31.9F and liquid at 32.1F and can be found with little beads of liquid on the solid. Glass changes from a solid that will shatter to a liquid that will flow over a range of temperature that is often several hundred degrees. In between glass is soft and squishy and can be pushed around or slowly sagged. [...]

The other factoid leading to the discussion is window glass found in old cathedrals, which is thicker at the bottom than the top. Some people use this to support the idea that glass flows over time. There are two obvious problems with this idea: if glass flowed, there would be an open gap at the top where glass moved down and objects much older than these windows, like the hundreds of pieces of Roman Era glass that are around, would be distorted and they aren't.

posted by goodnewsfortheinsane at 6:00 PM on June 5, 2005

Heh, I got into a discussion about this recently. Glass is _not_ a liquid - it's an amorphous solid. Amorphous solids are structurally pretty different from solids with a crystal lattice, but they're still emphatically solid. Wikipedia actually has a bit of the glass article specifically devoted to debunking that myth.
posted by ubersturm at 6:01 PM on June 5, 2005

I seem to recall reading about 200+ year old windows being thicker on the bottom in something recently, which I assumed to be true.

But it looks like from a google search on the phrase, that the majority opinion seems to be that it doesn't flow, and that old windows are oddly shaped due to the old processes.
posted by mathowie at 6:01 PM on June 5, 2005

More google-fu for you: Glass: Liquid or Solid -- Science vs. an Urban Legend
Spurious, unreliable research methods say that "glass is a liquid" (720 google hits) rather than "glass is a solid" (336 hits).
posted by boo_radley at 6:16 PM on June 5, 2005

As an explanation of why panes of window glass are thicker at the bottom than at the top, it's been suggested that that's because the glaziers installed the panes that way -- it's easier to balance a vertical plane on a thicker edge than on a thinner edge.

But I dunno, maybe that's just crazy talk. ;-)
posted by GrammarMoses at 6:19 PM on June 5, 2005

Well, i just did a google search on glass liquid and came up with this very well put together page. It is a very interesting read. From the conclusion:

There is no clear answer to the question "Is glass solid or liquid?". In terms of molecular dynamics and thermodynamics it is possible to justify various different views that it is a highly viscous liquid, an amorphous solid, or simply that glass is another state of matter which is neither liquid nor solid. The difference is semantic.

posted by chrisroberts at 6:20 PM on June 5, 2005

Hmmm. I can only speak to 90 year old glass. I'm currently reglazing some of our 90 year old windows with the original glass (the glazing putty had broken down over decades.)

I just popped upstairs to measure all sides...same thickness all the way around. Single pane, old fashioned "wavy" glass. So if it gets thicker over time, it has to be a heck of a lot more time than 90 years. :)
posted by jeanmari at 6:27 PM on June 5, 2005

Also, it seems that glass can be explosive all on its own (video)
posted by boo_radley at 6:33 PM on June 5, 2005

The thick side is down because that was the proper way to install it. It's in the instructions. Glass usually had a thicker side due to the relatively crude manufacturing technologies
posted by Ken McE at 6:33 PM on June 5, 2005

I've read somewhere that glass will, over millions of years, "flow". So, it takes a lot more than a few hundred or thousand years for this to happen but it will eventually happen.
posted by taumeson at 6:50 PM on June 5, 2005

very old window glass (americans can generally find this in new england churches & such) was blown in large rondels (essentially bowls spun out with centrifugal force into plate shapes) and then cut into panes. getting that completely flat was pretty challenging.

and yes, glass can explode if it isn't annealed properly - the crystalline structure isn't stable if it cools too quickly.
posted by judith at 7:34 PM on June 5, 2005

Thanks, everyone.

This vindicates me, by the way.
posted by interrobang at 8:01 PM on June 5, 2005

Ken McE -- you say the practice of hanging glass thick-side-down is "in the instructions". Somewhere, once, reading up on this subject, I read (MGFIFM) that it would be nice to locate some such instructions, contemporary to some of the old buildings we see with fat-bottomed glass, but that no one had found such a citation. Can you direct me to (Roman, medieval, colonial) instructions like this?
posted by xueexueg at 8:12 PM on June 5, 2005

In the "olden days," panes of glass were manufactured using the Crown glass process, where the molten glass was rolled flat and spun on a disc before being cut for final fitting. The centrifical force of the rotation caused the edges of the glass to bulge. When it was installed, it was more stable to install it heavy-side-down.

Later on, glass was "rolled"--large cylinders would flatten out the molten glass, but inconsistencies in the temperature of the cylinder and glass would give it a wavy texture.
posted by Civil_Disobedient at 8:30 PM on June 5, 2005

Glass is a supercooled liquid. It is that by definition, because it has no melting point. Old glass gets thicker at the bottom over time moreso than newer glass would over the same amount of time because the older glass, being more cystalline in quality, contains a lot of lead.
And just by the by, the 1/2-life of glass (wrong expression but you know what I mean) is infinity. Meaning that it will never ever ever ever ever breakdown. Hence, it's the most important of all the products we use that should be recycled. Just sayin'..
posted by peacay at 9:09 PM on June 5, 2005

Obsidian, which is a kind of glass, does break down over time: water enters the crystal structure and it turns into clay or the like. Of course, this happens over geological timescales, not human timescales, unless you look very closely.

Man-made glasses have some forms of breakdown which are rapid enough to damage historical artifacts. Alkaline environments can make glass break down faster. And don't forget plain old weathering by abrasion.
posted by hattifattener at 11:14 PM on June 5, 2005

peacay: I think it's funny that the first hits on the Google search you linked to says that glass is not a supercooled liquid at room temperature.

Physicists and materials scientists don't refer to glass at room temperature as a "supercooled liquid." The term amorphous solid, or sometime just "glass" is used to refer to the material's phase.
posted by grouse at 12:06 AM on June 6, 2005

grouse...I definitely sit corrected. I was unaware that my chemistry professor was teaching urban legends a decade ago -- I was mouthing off from my recall (poor appeal to Authority I guess) -- that's why I didn't review those links in the googlehit. I am happy to become better edumactated.
posted by peacay at 2:17 AM on June 6, 2005

hattifattener I had a look around and the only figure I kept seeing was that a glass bottle takes a million years to break down. I have a feeling that's not particularly scientific - mostly kids informational and recycling sites (no, I didn't look for long). I'm sure that abrasion from wind and water will work slowly but that last link you posted didn't really mention anything much at all about it. The point I was making in perhaps a hyperbolic fashion is that glass above everything else should be recycled.
/derail /stupidity
posted by peacay at 2:40 AM on June 6, 2005

Windows in churches bow at the bottom under the immense weight of the glass, grout and lead. The lead is such a soft metal that if it isn't fully pre-streched before being used it will strech under the pressure of the glass. But in time, especially if it's an intricate peice, it will strech anyway unless there is adaquate reinforcement. That's why the larger windows require giant support beams to support the weight. There needs to be rebar or internal steel for every twenty inches of leaded glass if you don't want eventually bowing and even then you'll probably get a little.
posted by princelyfox at 2:49 AM on June 6, 2005