"charlie" wrote in message
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"Ed Huntress" wrote in message
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"charlie" wrote in message
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"Ed Huntress" wrote in message
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"Gunner" wrote in message
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On Sun, 24 Feb 2008 13:03:20 -0800 (PST), "
wrote:
On Feb 24, 4:54 am, wrote:
can anyone tell me that what is fludity in cast iron???should it be
more or less in c.i??
Isn't being "cast" meaning it is solid? So, there is no fluidity. Must
be trick question on a final exam!
Paul
Glass is solid, yet its considered a "slow liquid"
this is incorrect. glass is classified as an amorphous solid.
http://en.wikipedia.org/wiki/Amorphous
Maybe why all those old lathes wont hold tolerance anymore. The cast
iron has flowed a bit.
G
Gunner
regards,
charlie
http://glassartists.org/chaniarts
There's some truth in both statements, Charlie. Amorphous solids --
non-crystalline solids -- have no distinct melting point. The definition
of the "glass-transition" temperature, which is arbitrarily considered
the temperature at which an amorphous material passes from solid to
liquid, is just a viscosity number grabbed out of thin air.
did you read the link? it has nothing to do with the melting point. it has
to do with the formation of crystals and the speed of cooling.
I didn't read the link before writing, but I have now. It isn't very
helpful. However, the description of glass from the same source says this:
"Glass is generally treated as an amorphous solid rather than a liquid,
though both views can be justified."
It points to a rather detailed university reference, titled "Is Glass Liquid
or Solid?", which concludes:
"There is no clear answer to the question "Is glass solid or liquid?". In
terms of molecular dynamics and thermodynamics it is possible to justify
various different views that it is a highly viscous liquid, an amorphous
solid, or simply that glass is another state of matter which is neither
liquid nor solid. The difference is semantic."
I don't want to get into a ****ing contest over this, but I've written
entire articles about amorphous solids, including both glassy ceramics and
glassy metals, and including the thermodynamic and metastable/kinetic
properties of glass ceramics. If a material is amorphous, it doesn't form
crystals -- that's the definition of "amorphous." But glass, and other
glassy materials, can form crystals under specific conditions.
So the shorthand of calling it a "slow liquid" or a "superliquid" isn't
complete, but it isn't completely wrong, either. As I said, it's a handy
description. If you want to get into the distinctive properties of amorphous
solids with school students you're in for a very dissappointing time. It's
actually quite a complex subject, one that's still evolving.
So, to explain the behavior of glassy, or amorphous solids, teachers have
often described glass, for example, as a "superliquid." The term has no
precise meaning. It's just a handy description.
they were wrong.
As I said, it's a handy description. And if you're going to get into the
expansion and exothermic properties of glass at the supposed "glass
transition temperature," note that even water has a non-linear expansion
property at a critical temperature above crystallization that isn't much
different, physically, from the secondary "transformation" properties of
glass.
There are some 250-year-old windows in one of my family's houses that
make a strong case that glass is liquid. They look like wavy gravy at
their bottoms, and it's not a result of being blown as cylinders and then
cut and flattened (which is, of course, the way they were made). They all
get wavy at the same point, on the bottom side. You could say that
they're just especially subject to creep, if you think of them as solid.
Or you could say they're just a very, very viscous liquid.
--
Ed Huntress
that is also incorrect. your wavy windows did not creep. that again is an
old wive's tale. your windows were made that way because the craftsmen of
the time thought they would last longer with the wavy or thicker part
down, or did it because of esthetics in your case as it would look odd to
have the demarkation lines at different points in adjacent windows.
OK. I've been corrected about that before but I keep forgetting. Old wives's
tales die hard. d8-). Then, my glass-topped dining room table, which had a
hefty planter on it for years and creeped so much that you can see it with
the naked eye.
if this were true, then why aren't the glass objects the egyptians made
5000 years ago puddles in a museum about now?
Because they aren't stressed sufficiently to creep, for the viscosity of the
glass. All glass creeps under load and time. So does steel. So does
concrete. What you're saying is that the stress level for glass to creep in
human-scale quantities of time is higher than the load imposed by the
glass's own weight. That's true of most materials.
there are articles that measure the creep or thickening of glass, and the
time for this to occur to something you could see is longer than the age
of the earth.
http://www.glasslinks.com/newsinfo/supercooled.htm
http://math.ucr.edu/home/baez/physic...ass/glass.html
"The use of the term "supercooled liquid" to describe glass still
persists, but is considered by many to be an unfortunate misnomer that
should be avoided. In any case, claims that glass panes in old windows
have deformed due to glass flow have never been substantiated. Examples
of Roman glassware and calculations based on measurements of glass
visco-properties indicate that these claims cannot be true. The observed
features are more easily explained as a result of the imperfect methods
used to make glass window panes before the float glass process was
invented."
OK. But glassy solids, or amorphous solids, are still just very viscous
liquids with close-order organization that gives them properties different
from a standard liquid. Like Silly Putty, they can shatter, for example. But
physics today describes supercooled liquids as something different. The
physics of amorphous materials is not simple.
Gunner's shorthand description is arguable as a partial explanation.
"Amorphous solid," in thermodynamic terms, is an oxymoron. The implications
of calling glass a "liquid" can be misleading if it's carried too far, to be
sure. But so are most simplifications. That includes calling glass a
"solid."
--
Ed Huntress