Melting Glass...

[Sylvester Graham]

Ok, just wondering. Perhaps I shouldn't have been so artsy-fartsy about it. Everyone's shown off their intelligence nicely.

Although... my chemisty teacher enjoyed telling that urban myth quite often, accompanying the example of glass with wax.

[Dave_B]

It was not his fault. The Society Committee on Education of the American Chemical Society included it in a sourcebook for High School Chemistry teachers. They were wrong. He had been actively mislead by a group of experts he had every reason to trust. He should be forgiven for his error.
Cheers,
Dave B.

[Steven Barall]

Finally, a technical topic I can finally claim expertise on. I am a part time glass blower and I have talked about this exact issue with the now retired chief glass making engineer for Corning Incorporated. He was responsible for all the glass that Corning melted all over the world.

I asked him specifically about whether or not glass sags over time due to gravity. He told me that at Corning they did an accelerated age test for the affects of gravity. The test approximated many hundreds of years of gravity on a technically perfect rod of glass. The test itself took, I believe, something like 12 or 15 years. The thing just basically sat there in a corner of the office in a sealed chamber for all that time. The results are that gravity does not affect glass.

There are in fact still many factories that still make glass panes in the way that Ole described although with some variation to how he described the process. Most of the glass used in the stained glass industry is made this way.

There is another and much older process for making glass panes and its probably how the glass in question was made. Because of the way the glass blowing works, it's sort of easy to make round flat disks called rondels. An inherent result of the process is that the glass is much thicker in the center of the disk than the edges which is where the term "Bullseye" glass comes from. So you make one of these rondels that is perhaps four or five feet across and then simply cut rectangles out of it. The edges of the individual panes that were closer to the center of the original disk are thicker than it's opposite edges.

So that's it for my technical contribution to the site and I suspect that I won't be having another for some time to come.

[GPS]

...
The test approximated many hundreds of years of gravity on a technically perfect rod of glass. The test itself took, I believe, something like 12 or 15 years. The thing just basically sat there in a corner of the office in a sealed chamber for all that time. The results are that gravity does not affect glass.
...
So that's it for my technical contribution to the site and I suspect that I won't be having another for some time to come.

Don't be so sure, Steven -here is one for you... While I agree with your conclusion I doubt the technical details of the test. In what way the 12-15 years of just sitting there approximated many hundreds of years of gravity?? That doesn't make sense. Usually if you have this kind of tests, for ex. corrosion tests, fatigue tests etc. you have to make the leading cause of the tested characteristic exaggerated in some way or the other. Either you expose the part to stronger corrosive ambiance (in the above test) or the part gets more strain in a given time (the famous IKEA chairs tested in front of customers with constant weight up and down on them) etc. That would not be the case of this perfect rod at all. The way it could be tested for gravity would be if you put the rod in a centrifuge and rotated it for higher gravity under a shorter time (rather than 100 years). As you described it, it wouldn't say anything about sagging in 100 years.

[GPS]

Unless... they had some super duper method how to measure a submicroscopic change of the rod shape - even smaller than in the nanotechnology realm... hm...

[Dave_B]

The way you do such a test is that you keep gravity the same and measure very small changes in dimension and then do an extrapolation. If things are going to move a cm over centuries they must move ~.1mm over a period of years. The folks at Corning labs are quite clever and can easily measure ten to the minus twelve (a trillionith) of a meter. In a pinch they can probably measure ten to the minus fifteenth of a meter. You can also pick a geometry for the test that magnifes the effect of any flow that might occur. The failure to see any changes with such precise measurements over a period of 15 years can then be used to put an upper bound on the magnitude of any possible flow effect. Such tests are routinely done to measure changes in size with temperature, magnetic field, pressure, gravity, electric field, etc. One can measure both reversible and irreversible effects.
Cheers,
Dave B.

[GPS]

Sure, even if here we have changes that don't make even a mm in 100 years. Still, some kind of greater gravity imitation would be easier to measure. Hm. Would be interesting to know their measuring method.

[Bruce Watson]

Assuming the disfigurment is indeed a product of time, and not of the poor production standards in the 18th century (I could be wrong), I'm fairly sure the reason windows warp is because glass is in fact a supercooled liquid, or some sort of amorphous solid. I'm a little patchy on the chemistry but you get the idea. Under the pull of gravity and over a long period of time, the molecules in a pane of glass move perceptibly.

My training in engineering, in particular strength of materials, indicates that your assumption is wrong. Typical window glass at normal environmental temperatures does not change shape over time due to gravity. Perceptibly or imperceptibly.

What you are seeing is old glass plate production techniques. They didn't have float glass manufacturing 300 years ago.

The structure of glass is amorphous. In this case that means that it does not form a crystal structure like most solids do. Yet, it has sufficient cohesive attraction to reach a rigid state. Put another way, its viscosity is sufficiently low at room temperature that it's considered a solid.

So don't worry about your camera lenses. OTOH, it's better to use them for their intended purpose than to leave them laying around for hundreds of years!

[Richard Årlin]

I did not bother to read the whole string even. If your house had stood since the last glaciation maybe and if your measuring devices were good enough (very good) you'd perhaps be able to measure the infiniesmally slight thickening in the bottom. It's a myth quite simply. Windowglass was mouthblown at the time and cut open to cool as sheets. Naturally such glass has not an absolute equal thickness and would obviously be put in the framework with the thickest and thus heaviest edge downwards... gee

Richard

[Ed Richards]

> The way you do such a test is that you keep gravity the same and measure very small changes in dimension and then do an extrapolation.

Or you put a weight on the rod to simulate a higher force of gravity.

I am certainly willing to listen to better scientific evidence on this, like a Corning scientist, but Wikapedia does not qualify.:-)