Re: pinhole Glass: a liquid? a solid?

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Sat Mar 27 2004 - 13:39:40 PST 

Message-Id: <l03110709bc8ba3480de5@[192.168.111.176]>
Date: Sat, 27 Mar 2004 13:39:40 -0800
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole Glass: a liquid? a solid?

Hi Craig

The Corning Museum of Glass is a good resource on this issue.
The reference can be found in the middle of my Urban legends of Science Page
http://www.exo.net/~pauld/popularerrors/popularerrors.html

Basically a solid resists shear stress and returns to its original shape
when a shear stress is released. A liquid shears easily when a shear stress
is applied and the molecules in the liquid do not return to their original
positions when the stress is relieved. . There are in-between cases like
warm plastics that return almost to their original shape but not exactly,
i.e. they flow some.

So

Glass at earth surface temperatures is a solid.

On a scale where water has a viscocitty of 0.01 poise, and SAE 30 motor oil
is 1 poise, glass is 10^20 poise.
This means that it takes billions of years for glass to deform its shape by
less than a wavelength of light. (If you heat glass up to a few hundred
degrees its viscocity drops and it will fllow under gravity.)

After the original publication that ancient cathedral glass was wider at
the bottom other researchers found that many cathedrals had windows that
were thicker at the top or the sides.

Paul D

>So is glass a liquid or a solid? In high school we were all told that glass
>is an "amorphous liquid" of very high viscosity that takes centuries to
>flow. Ancient cathedral windows with thicker bottoms than tops were given as
>evidence. In recent times, however, I have heard this "myth" disputed with
>the idea that ancient glass production was less than perfect and uneven
>panes would always be installed with the thickest end down.
>
>Is glass a liquid (of any type/viscosity)?
>
>Does glass "flow"? If so, at what rate?
>
>Are the ancient panes with thicker bottoms evidence of "flow" or building
>design?
>
>Is the basic question (liquid/solid) complicated by differences in ancient
>and modern glass "recipes"?
>
>Are there any credible resources that would be helpful on this matter?
>
>
>Craig Childress
>Technology/Science Instructor
>Hillsdale High School
>(650) 378-8170
>(650) 574-4173 FAX
>
>_________________________________________________________________
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