Re: ATM glass is not solid

["Albert Highe" <ahighe@ix.netcom.com>]

Glass - Solid or Liquid?

The more I think about it, the more I find the discussion about whether
glass is a liquid or solid to be amusing. For those who just want to know
the answer - it's a solid. For those who still dispute this or are unsure,
read on.

Man invented language to describe the world around him. He also found a need
to classify what he experienced in relationship to himself. He observed
there were materials that he could touch, prod, and move about. These
materials kept their shape - stones, trees, animals and his fellow humans.
He classified these materials as "solid".

Man also observed materials that he could not hold in his hand. These
materials would take the shape of the container they were put into and flow
downhill - water, milk, and honey. He classified these materials as "liquid"
.

After a hearty meal of beans, man also experienced a class of materials that
he could not hold in his hand or in a container. They would always expand to
fill whatever size cave he lived in. He classified these materials as  "gas"
.

Those are the definitions of solid, liquid, and gas. From those definitions,
glass is a solid. It holds its shape and does not flow or expand to fill
available space. What could be more simple?

Some people seem to think that to be classified as a solid, a material must
be crystalline. Not true. There are crystalline solids and there are
amorphous solids. Many (not all) crystalline solids can transform into
another state of matter (liquid or gas). In doing so, they undergo a first
order transition from the ordered, crystalline state, to the disordered
liquid or gaseous state (CO2 for example goes directly from a solid to a
gas).  As a consequence of being a first-order transition, physical
properties, such as density, strength, thermal and electrical conductivity,
etc. take a discontinuous jump at the transition temperature. These changes
of physical properties are usually dramatic, making it easy for us to
distinguish clearly between the two states. There is always a measurable
heat of fusion, and in the case of water, it is quite large.

Amorphous solids also can go through phase changes between the solid and
liquid state. Their transition is second-order and the changes are smaller,
yet still measurable. Its straightforward to measure the change with
temperature of specific heat, thermal and electrical conductivity, density,
etc, above and below the transition temperature. Because the transition is
second order, there is no latent heat of fusion. So what? The definition of
solid is determined by measuring its physical properties. It has nothing to
do whether it undergoes first- or second-order phase transitions or no
transitions at all.

Bear in mind, again, that the definitions of states of matter were/are
important in defining the relationship of materials to us. The
classifications are valid for defined conditions - one atmosphere pressure
and typical ambient temperatures on Earth.

It is amusing to me that we have no difficulty thinking of ice as a solid
when an ice cube will last half an hour on the kitchen countertop. On the
other hand, how can we speculate that glass may be a liquid when sharp
shards from the Roman Empire still litter the ground, 2000 years later.
These shards show no evidence of flow.

Why does this confusion persist?

I think, in part, it stems from a combination of human nature and training.
Most solids in our experience are opaque. Water, the most common liquid, is
clear. In addition, most of us have been taught that glass is not
crystalline, like ice, or salt, or sugar. We have been taught that the
arrangement of its atoms is random as are the molecules in a liquid. We are
told that we can think of glass as a frozen, or supercooled liquid where the
crystalline structure was not allowed to form. A nice mental picture, but it
has led to confusion about the nature of glass. It is not a supercooled
liquid.

Ceramics have a similar random structure as glass. I don't think we have any
difficulty thinking of clay pots, bricks, and toilet bowls as solids. No one
expects to find the bricks in the walls of the schoolhouse to have grown
thicker at the bottom upon returning from summer vacation.  I doubt
archeologists in the future will speculate whether our toilets are thicker
at the base because they flowed over the last 200 years.

Add to this the urban legend that glass panes in old buildings have "flowed"
so that the bottom of the panes are thicker than the top. I don't dispute
the evidence. Some window panes in some buildings that are at least a
hundred years old are, in fact, thicker at the bottom. The logic that has
led to this conclusion is flawed. One assumption is that the artisans of the
day could make glass with the same uniform thickness that we can today. This
is not the case. In those days, processes for making glass produced glass
with non-uniform thickness. After cutting the panes to size, it is far more
likely that it was standard practice to place the thicker section at the
bottom. Consequently, there should be little surprise that, today, we find
the glass of that period is thicker at the bottom. In comparison, buildings
made of stone frequently have bases that are thicker than at the top of the
wall. No one would suggest that the stone must have flowed.

So why do we care whether glass is a liquid or a solid? As ATMer's, I think
we want some assurance that the mirrors we labor so long to make 1/20 wave
won't flow and become ¼ wave mirrors over time. Rest assured, glass is far
more solid than is ice or salt or sugar. Unless we plan to do some extended
observing at temperatures exceeding 1000 degrees, our mirrors will retain
their dimensions down to the angstrom level long after our grandchildren
have children. If that doesn't fit the classification of a solid, nothing
else will.

Albert

----- Original Message -----
From: "Eric Claussen" <etclaussen1@yahoo.com>
To: "Ian Stirling" <root@mauve.demon.co.uk>
Cc: <atm@shore.net>
Sent: Wednesday, July 04, 2001 12:54 PM
Subject: Re: ATM glass is not solid


>
> I saw a show on the Discovery channel that was about very old buildings,
and
> how they were built.  According to the show,  panes of glass were often
> oriented so the thick edge was at the bottom for extra strength.   I have
> know idea how true that is,  but it sounds reasonable.
> We could do an experiment.   Measure a piece of glass in all dimensions,
and
> then bury suspended inside a time capsule.   In a thousand years (yeah
> right, like anyone would find it)  someone could open it and measure it
> again...
>
> Eric Claussen
> http://www.ericclaussen.com
> ----- Original Message -----
> From: "Ian Stirling" <root@mauve.demon.co.uk>
> To: <atm@shore.net>
> Sent: Wednesday, July 04, 2001 7:31 AM
> Subject: Re: ATM glass is not solid
>
>
> >
> > >
> > >
> > > Richard go to the "Old Village" in Copenhagen, there you will see some
> multi
> > > century windows where the glass has flowed over the lower frame some
> extent,
> > > some panes have small holes in the top half that look like a melted
hole
> in
> > > a very thin membrane, but without a beaded edge, if get my drift. A
> better
> > > desciption is if you take a blob of blu-tak and now stretch it in
every
> > > dirrection in the same plane eventually it gets so thin that it
> ruptures,
> > > this is what I'm getting at. Try it.
> >
> > Unfortunately, this is wrong.
> > Glass for windows was made by several processes, one popular one was to
> > dip a rod into some molten glass, get a large blob, spin it into a
> pancake,
> > then chop it up into panes.
> > This leads to uneven panes, possibly even ones with flaws in the edge.
> > Glaziers then install these heavy end down, as it's a bit easier.
> >
> > Also, if you examine a few of the windows, you will often find a
> > pane or two for whom "gravity" is to the left, right, or top.
> >
>
>
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