🚀 go-pugleaf

Christopher Scanlan <nuke@csd4.csd.uwm.edu> writes:

>you were talking about glass flow in windows.  In older buildings
>According to my ceramics professor, this is simply because that is
>how window glass was made when these buildings were built.  The
>plates of glass were thicker near the bottom.  If you actually
>work out the calculations for how long it would take for glass
>to flow to this extent it turns out to be several centuries.

Fascinating!  I wonder if anyone else can support these calculations.

Now, since the subject of this article was "the Nature of Glass",
I would like to see if I can start a bit of controversy on the net!

It is often said that
	" Glass is a liquid "
or that
	" Glass is a supercooled liquid "

I have my own opinions on this, but first I want to see what comes out.

|  jeff@minmet.uq.oz.au	       |  Jeff Gates
|			       |  Dept Mining & Metallurgical	       _--_|\
|  Ph. direct: +61 7 365 3726  |		     Engineering      /      B
|  Ph. Dept: +61 7 365 3738    |  University of Queensland	      \_.--\_/
|  Fax: +61 7 365 3888	       |  Brisbane   Qld   4072   Australia	    v

jeff@mama.minmet.uq.oz (Jeff Gates) writes:

>Now, since the subject of this article was "the Nature of Glass",
>I would like to see if I can start a bit of controversy on the net!
>
>It is often said that
>	" Glass is a liquid "
>or that
>	" Glass is a supercooled liquid "
>
>I have my own opinions on this, but first I want to see what comes out.

I would argue that it is not a liquid.  Consider two "window panes", one
made of vitreous silica, one made of monocrystalline quartz.  In the
glass (the vitreous silica), the argument for being a liquid is that it
still can "flow", even at room temperature.

It is clear that this "flow" process in the glass at room temperature
takes place through diffusion.  Gravity would add a nearly imperceptible
bias for diffusion in the downward direction, which conceivably over
millions of years would be measurable through extra thickness at the
bottom.  (See the calculations in the related posting.)

Now consider the crystalline "window".  Vacancies are naturally created
at the surface, and because they are lighter than an ion, their
diffusion has a nearly imperceptible bias upwards.  This would result in
thickening at the bottom also.

In short, if a glass "flows" at room temperature, it is through
diffusion; but amorphous structures have no monopoly on diffusion.
Therefore a crystal can be considered to "flow" at room temperature as
much as a glass, but clearly the crystal is not a liquid; therefore,
vitreous silica at room temperature can't be considered to be one
either.  Therefore I reject the definition of a glass as a supercooled
liquid, at least at room temperature.  However, it is a useful concept.

As a working definition for *inorganic* glasses, I would offer "a
metastable supercooled melt".  Whether or not certain amorphous polymers
can be considered glasses, I have no idea.  What do you polymer people
think?

				- Rob

In article <BwyCCL.H8L@news.iastate.edu> 
locher@iastate.edu (Robert C Locher) writes:
>It is clear that this "flow" process in the glass at room temperature
>takes place through diffusion.  Gravity would add a nearly imperceptible
>bias for diffusion in the downward direction, which conceivably over
>millions of years would be measurable through extra thickness at the
>bottom.  (See the calculations in the related posting.)

  It is NOT clear.  Simply because bulk flow is very slow does NOT
mean there is none.  It is conceivable that bulk flow would
be slower than the diffusion effect you mention.  But you
haven't shown that at all.

  Corn syrup, though very viscous, does slump through bulk flow, even
when it has been in the freezer (i.e., has been supercooled.)  You
seem to have assumed that super-cooled fluids don't flow.  I
challenge this.

Peter Mardahl

locher@iastate.edu (Robert C Locher) writes:

>jeff@mama.minmet.uq.oz (Jeff Gates) writes:

>>Now, since the subject of this article was "the Nature of Glass",
>>I would like to see if I can start a bit of controversy on the net!
>>
>>It is often said that
>>	" Glass is a liquid "
>>or that
>>	" Glass is a supercooled liquid "
>>
>>I have my own opinions on this, but first I want to see what comes out.

>I would argue that it is not a liquid.  Consider two "window panes", one
>made of vitreous silica, one made of monocrystalline quartz.  In the
>glass (the vitreous silica), the argument for being a liquid is that it
>still can "flow", even at room temperature.

>It is clear that this "flow" process in the glass at room temperature
>takes place through diffusion.  Gravity would add a nearly imperceptible
>bias for diffusion in the downward direction, which conceivably over
>millions of years would be measurable through extra thickness at the
>bottom.  (See the calculations in the related posting.)

>Now consider the crystalline "window".  Vacancies are naturally created
>at the surface, and because they are lighter than an ion, their
>diffusion has a nearly imperceptible bias upwards.  This would result in
>thickening at the bottom also.

>In short, if a glass "flows" at room temperature, it is through
>diffusion; but amorphous structures have no monopoly on diffusion.
>Therefore a crystal can be considered to "flow" at room temperature as
>much as a glass, but clearly the crystal is not a liquid; therefore,
>vitreous silica at room temperature can't be considered to be one
>either.  Therefore I reject the definition of a glass as a supercooled
>liquid, at least at room temperature.  However, it is a useful concept.

>As a working definition for *inorganic* glasses, I would offer "a
>metastable supercooled melt".  Whether or not certain amorphous polymers
>can be considered glasses, I have no idea.  What do you polymer people
>think?

>				- Rob

Crystalline solids often 'flow' at room temperature as a result of
dislocation motion.

T. D. Taylor
Department of Ceramic Engineering
Clemson University
Clemson, SC USA

Glass-flow-fans,

     As children and amateur archaeologists, my brothers and I used to dig
up old bottles from a late-19th-century dump not far from our house in
Pittsburgh. Many of the bottles were warped and smashed as though they'd
been melted. None of the surrounding materials we dug up were charred, nor
were there any other indications of the kind of heat needed to melt glass
in the other stuff we dug up.

     My father (who is not a materials engineer or scientist, but just a
guy) "explained" it with the "glass is a liquid" "fact," combined with the
weight of the earth piled on top of them for around 100 years.

     Any comments?

Dave Land