ATM Is glass solid or liquid?

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

>
> However, there is no temperature at which glass becomes a solid or liquid.
> It just gets thicker and thicker as the temperature is lowered.  There is
no
> heat of fusion or other discernable change of state at any particular
> temperature.
>
You are correct that crystalline and amorphous materials differ. With
crystalline materials there is a definite melting transition with an abrupt
change of properties, and a measureable latent heat of fusion. With
amorphous materials such as glass, there is what is known as the "glass
transition temperature". Properties of amorphous materials undergo many
discernable changes at this glass transition temperature. It is usually this
temperature that is used to define the "solid" to "liquid" transition.
Unlike crystalline materials, the changes are usually subtle. Typically you
can see it as a change in the slope of the heat capacity, thermal expansion
coeffiencient, etc. vs. temperature. Many of the amorphous polymers that we
are familiar with, for example, natural rubber, have low glass transition
temperatures. They go from being stiff and brittle below the glass
transition temperature to flexible above it.

In amorphous materials, one typically measures the properties near the glass
transition temperature where motion is greater, and then extrapolates
properties farther away from it using the Time-Temperature Superposition
Principle. Basically, it allows one to calcualte the flow or deformation at
ambient temperatures over hundreds of years. The calculations I've seen
indicate that modern day glasses are essentially "solid" at ambient
temperatures. Their expected deformation is essentially that of crystalline
materials over very long periods of time. Such may not have been the case
with ancient glass formulations.

So the answer to the question of whether glass is solid or liquid depends on
the frame of reference: at what temperature and over what time? For the
glass formulations we typically use, and the temperatures we expose them to,
it is a solid.

Albert