
Glass Doesn't Flow (2011) - Thevet
http://www.cmog.org/article/does-glass-flow
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4ad
This is not such a great article. It's true that the meme "glass is a liquid"
is tiresome and incorrect, and it is true that glass doesn't flow at usual
temperatures, but glass is very special and interesting because it doesn't
undergo a state transition from liquid to solid state. So in a very specific
technical sense, glass can be considered liquid, but there are many
definitions of liquid and solids and this one doesn't align with people's
naive definition.

I hoped would article would touch these special properties of glass.

[http://en.wikipedia.org/wiki/Glass#Formation_from_a_supercoo...](http://en.wikipedia.org/wiki/Glass#Formation_from_a_supercooled_liquid)

[http://en.wikipedia.org/wiki/Glass_transition](http://en.wikipedia.org/wiki/Glass_transition)

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Bognar
More generally, materials that don't have definite liquid-solid transitions
and don't have long-range crystal structure are called amorphous solids.

[http://en.wikipedia.org/wiki/Amorphous_solid](http://en.wikipedia.org/wiki/Amorphous_solid)

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whoopdedo
I was about to say something like this in response to the parent comment.
Instead of saying "glass panes are actually more like a very thick liquid than
it is a solid" perhaps we should be saying "molten glass is more like a very
flexible solid than it is a liquid."

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cdibona
As a side note, the corning museum of glass is totally worth the trip. Amazing
place.

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knodi123
My girlfriend and I met another couple to go canoeing one day on town lake.
The guy in the other boat was a chemist. He trotted out the old "glass is a
liquid" thing, and I explained no, that's not true, and I went into great
detail. But he stuck to his guns and insisted, and then played the
"professional chemist" card, saying "I _think_ I know a little bit more about
chemistry than a guy who makes webpages."

I was furious, but my dogged insistence that I was right was starting to draw
glares from both of the women, so reluctantly, in the name of peace, I dropped
it. Then the ass had the gall to say "Haha, don't worry, I'm sure there's a
lot of things about webpages that you could correct me on!"

The worst part is, if I sent him this article, and any number of others
proving the same point, he'd just say "what? why are we talking about glass?"
That conversation was nothing to him, and I'm sure he's already forgotten it.

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bostonpete
Sounds like an episode of Seinfeld.

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muaddirac
It's kind of funny that even in glassblowing circles this myth gets
propagated...where the old methods of making panes are generally well known
and understood (see the picture of the crown glass in the article).

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Florin_Andrei
If glass did flow at room temperature, telescopes would be a mess.

All the error you're allowed when making the mirror of a reflecting telescope
for visible light is 0.1 microns, or 100 nm. That's the top limit; a good
telescope needs to push that down at least 2x or 3x, if not more. The mirror
must remain within that limit for the entire lifetime of the instrument.

Now picture this big hunk of glass, resting on 3, 9, or 27 support points (or
much more than that for the large professional scopes). It's not sitting on a
flat table; it's sitting only on a few narrow points on the back, with weight
distributed equally among all points. It is so thick so as to prevent it from
sagging under its own weight and distorting the active surface via elastic
deformation.

If glass was a "liquid" to the extent that cathedral windows would thicken
noticeably at the bottom over centuries, how long would a telescope mirror
remain within 100nm? A few minutes before it got irreversibly warped like
icecream in the summer? Yet there are many telescopes still in use after more
than 100 years - and the optics are just as good now as they were in the
beginning.

\---

Before anyone tries to respond with a counter-argument: first try and realize
what 100 nm really is. If you take one strand of human hair and cut it in two,
and look with a microscope at the fresh section - if the surface of that fresh
cut was the size of the rug in your living room, 100 nm would be the size of a
large coin on that rug.

Think about that.

\---

(I make telescope mirrors and I've lost track how many times I've told this
argument whenever there's a discussion about "glass is a liquid".)

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abecedarius
Great way to connect this to other things we know about -- thanks.

 _a few minutes_ : I'd guess more like a few days: 1 mm/100 years = 10
micron/year = .1 micron / 3.65 days.

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UrMomReadsHN
I never thought glass was a liquid until 2-XL told me so and got me believing
the myth because why would a toy robot LIE to me!?

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bdcs
I think it is better to say that glass DOES flow, but it is a solid! For
example, it would take several universe lifetimes to observe any appreciable
flow (or more quantifiably, the viscosity is circa 10^19 Pa-s). Similarly,
lead flows and to a much lesser degree, steel too.

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mendort
It seems like from the definition of viscosity and the estimate of glass
viscosity provided we should easily be able to observe some glass flow over
the course of 10^12 years or so.

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lisper
Glass doesn't flow, but pitch does:

[http://en.wikipedia.org/wiki/Pitch_drop_experiment](http://en.wikipedia.org/wiki/Pitch_drop_experiment)

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pionar
Pitch has a viscosity of 3x10^10 P. That's still way less than glass, which
(according to this article) is 10^20 P.

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Dylan16807
That's within an order of magnitude of lead!

 _Does_ lead flow?

The article dismisses it but only in a casual way.

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jdmichal
Well, we could test it. Put lead into a funnel, suspend the funnel above a
beaker, and wait for a drop. Assuming the scale is linear, you should only
have to wait around 80 to 130 years based on pitch experiments:

[http://en.wikipedia.org/wiki/Pitch_drop_experiment](http://en.wikipedia.org/wiki/Pitch_drop_experiment)

More seriously, there's possibly a viscosity at which gravity is no longer
enough acceleration to overcome whatever mechanisms are holding the atoms
together. If we assume that metallic lead does not exhibit any flow on Earth,
then we can estimate this as being between 10^10 and 10^11P at _g_.

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markbnj
I was taught the exact same thing in science, sometime in the sixties or early
seventies, and I believed it for years and repeated it to many people.

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falcolas
This technical explanation of why glass "can not" flow, does nothing to try
and explain the phenomenon where old glass panes are thicker on the bottom
than the top.

A lot of things once thought impossible have been proven to be possible.
Offering an explanation of why it's impossible adds nothing to the table.

[EDIT] Missed the bit in the middle. That said, I still find the use of jokes
and second hand experiences a bit weak in evidentiary terms. Would have been
much more convincing argument if there was a count or documentation detailing
how many panels were truly installed one way or the other.

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lisper
The article explains this:

"Half of the pieces in a window are thicker at the bottom, he said, but, he
added quickly, the other half are thicker at the top. My own experience has
been that for earlier windows especially, there is sometimes a pronounced
variation in thickness over a distance of an inch or two on individual
fragments. That squares with the experience of conservators and curators who
have handled hundreds of panels. Although the individual pieces of glass in a
window may be uneven in thickness, and noticeably wavy, these effects result
simply from the way the glasses were made."

~~~
drzaiusapelord
Also, if you're a glass installer, you might feel its a best practice to put
the heavier end on the bottom. It'll make the pane feel more stable because
being top-heavy is like asking for gravity to take your glass and shatter it.

With today's production methods and quality control, we really don't have this
issue. Glass panes are pretty consistent, but from the point of view of some
18th century carpenter, you probably had to deal with a lot of inconsistent
glass panes.

