Is Glass A Solid Or An Extremely Slow Moving Liquid?

Windows in older houses are thicker at the bottom because they have been pulled down by gravity. I say slow moving liquid.

David, I thought that was debunked: ye olde glass panes weren't made uniformly. So when they were installed, homebuilders of yore (who weren't stupid), put the strongest part on the bottom.

I believe that glass is generally accepted to be a supercooled liquid. It is known to flow, for example in stained glass windows that are hundreds of years old.

As I recall from reading a book by Magnus Pyke, glass does have a crystalline form, which is very brittle in comparison with its liquid form.

Alex is correct. I work in Ceramic engineering, and the story of old glass moving is total bunk. There is now way that glass can move in Human time frames. Glass is considered a "Super Cooled Liquid" and requires heat to alter viscosity. Now, can glass move over Geologic time... I have no idea, it is possible, but I am not going to wait around to find out.

I have seen many windows with the telltale "gravity slump" when I lived in San Francisco. Some would have horizontal ripples in them instead of the thicker bottom, much like skin sags from old age. I suppose that could also be a manufacturing defect.

Why does noone read anything? Glass does NOT flow due to gravity! Its a total myth.

I'm pretty sure I've read that you can sometimes find windows with the thick part at the top - where someone forgot to put the strong part at the bottom. I figured it would be on the straight dope or snopes, but a quick google search didn't find it.

Indeed as almost anyone really "into" old houses knows, the story of glass "flowing over time" is complete hogwash. Materials scientists and engineers have studied up one side and down the other on 100, 200, 400, and 1000 year old installed glass and it's completely bogus! The supposed flow lines are an artifact of the manufacturing process. In former times, the glass was handblown into large cylinders, which were then cut lengthwise and allowed to cool flat upon a polished flat piece of stone. The glass didn't dry perfectly flat though, so usually there would be very minor flow lines. These can go horizontlly (suggesting flow over time maybe) or just as easily vertically -- it just depends on how the glass was cut!

In my own 1836 house, I've got lots of original wavy glass windows and the wave patern varies from pane to pane. I've had to replace a couple small panes and always do so with salvage glass from that same period. If I couldn't find it, though, there is a supplier that makes new glass via the old method: http://www.restorationglass.com/glasstype.html

If I needed a lot of it, this would be the way to go. For a pane here or there, I can usually find some salvage stuff. There are plenty of morons who think vinyl looks OK on an old house.

Straight talk from Sid.

I haven't heard one way or the other. If a pane of glass sits in the hot sun for a hundred years, I wouldn't be surprised to hear that it might've "flowed" a bit. But, on the other hand, the opposite wouldn't surprise me either.

What did surprise me was hearing that glass didn't completely harden. Especially since I was holding a glass plate at the time that certainly FELT solid... :)

Interesting tidbit!

The ripples you see in old glass are due to the manufacturing process: they've been there since the beginning. Glass sheets were (and are) formed using a number of different methods, but all of the early technologies resulted in glass with varying thicknesses and other obvious imperfections.

My high school science teacher told me that glass does flow over time, e.g. the old windows mentioned above.

Are you saying that my strong educational foundation in public school systems in the early 1990s was fraudulent? What else could they have told me that was a complete lie?! Who can I trust if I can't trust my high school science teacher?

As a Geologist, I believe that flowing glass is not a myth. I'm not sure where people are getting their information that it was debunked. In geology, you can see solid materials that "flow" when you see rocks with curving layers. Since rocks and glass can flow, it would seem that all solids can flow to some extent. Even if glass isn't crystalline, many things that are solids are not crystalline, and and many things that are crystalline can flow.

Glass does not flow. If it did, the high precision glass lenses used in astronomy, etc. would not work after just a few days. (Well, technically they would, but the accuracy would be v. off)

The windows in old houses sag because the manufacturing techniques of the day were not nearly as good as the methods we use today. (Currently, we make glass panes today by floating molten glass on liquid metal.)

Before Snopes.com, this was all over alt.folklore.urban called "glass flow" about ten years ago (wow). As I recall the discussion went on for about a year and no one pinned it down.

Anyone here remember that?

Found it! (new window)

The key is what time frame are you considering? As said before, if it is geologic time, say millions of years, then glass flow is not unreasonable. However, on short time frames, human lifetime (tens of years), or even human civilization timeframe (hundreds of years), i would bet that the glass flow is negligible.
I'm not sure if the amorphous nature of pitch is the same as glass, but take a look at:
http://en.wikipedia.org/wiki/Pitch_drop_experiment

I had to chuckle about Floyd's high school science teacher comments. Yes, a lot of really bad information comes from teachers who are nitwits and speak from their butts. Don't get me wrong, an awesome teacher (and I've had some) really make a huge difference in kids' lives. The problem is those awesome ones (in the U.S. anyhow) don't get paid any more than the urban legend spreaders. It wouldn't be "fair" after all... Ugh. :-P

Some folks are bringing up the scale of time we are considering. Well the argument is made that glass in 200 year old buildings (like my house) has flowed under the force of gravity. That is the timescale. To be safe, take it out 1000 years or so and look at some old cathedral window glass. It would be pretty tough to find any examples of installed window glass older than that, so speaking of flow rates in geologic timeframes is extrapolating WAY beyond the limits of available samples. The assertion is that glass is visibly thicker in the bottom of old windows because of flow. That has been proven false by gifted material scientists. Any visible waviness was an artifact of production techniques. Done.

Science teachers, both good and bad ones, are also at the mercy of what is known at the time they learned the facts, AND the quality of their own teachers and texts.

Bah. Science teachers were teaching this junk decades ago when I went to school (the same time they taught us the earth was cooling and their was an ice age fast on the way ... make up your minds!) and it was disproved then. In the late 80s my Materials Science prof in college chuckled over the common misconception of waviness from window glass flowing in old buildings.

Ignorant, lazy teachers continue to spout such absurdities, but with the easy access of information we have today, it's unforgivable. No excuses for lazy deadwood teachers! Pay the good ones more and fire the rest. Take that useless Masters of Education degree and go flip burgers. If you were any good, you would have gotten a real Masters in (choose one) science, history, math, etc.

Yes old thread and old question.
Glass is not a liquid at comfortable human temperatures.
The often cited wavey glass pane windows and bluges i the panes are falicies.
Look closely at an old pane of glass that is supposedly melting. Sometimes the thickest points are the sides or top of the pane. This discounts that glass is liquid and boing pulled down by gravity. Unless of course it was mounted in one of those obsolite anti-gravity window frames.

The ones that are thickest at the bottom should show the glass melting onto and starting to wrap around or puddling up on the windows frame or stained glass lead framing. Yet it does not!

Glass panes that are a few centuries old should be paper thin at the top and all puddled up at the bottom by now, but are not.

Glass pane munufacturing was not perfected and commericalized until the late 1950s. Any glass panes you see from before then will have many optical imperfections that are noticeable when looking through them.