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Glass - Solid or Liquid?
Posted Monday, May 02, 2005 5/02/2005 10:38:00 PM

I pretty much remember only one thing from my high school physical science class. You know, the class before chemistry and physics where they try to teach you about the scientific method, states of matter, and planets without a single equation. I remember very clearly the day the teacher informed us that glass was not a solid but in fact super-cooled liquid. I was skeptical at first but I was told that there is evidence. You see, I was told, window pains on medieval buildings have become thicker at the bottom then at the top because the glass is actually flowing very slowly. I was shocked and amazed. I thought this would be a great topic to bring up at cocktail parties when I grow up. "Hey there pretty lady, did you know that champagne glass you're holding is actually a super-cooled liquid?" I never had a chance to use that line but I'm sure it would have worked.

The problem: he was wrong. The consensus these days is that glass is not a super-cooled liquid but in fact a amorphous solid. The confusion comes about for two reasons:

  1. Structurally, glasses are similar to liquids. Many solids have a crystalline structure on microscopic scales with molecules arranged in a regular lattice. The molecules in glass have a disordered arrangement, but sufficient cohesion to maintain some rigidity. In this state it is often called an amorphous solid or glass (see reference).
  2. Because of the molecular structure of glass there is no first order phase transition as it cools (in other words it doesn't appear to have a distinct melting point). In a solid there is a sharp distinction between the solid and the liquid state, that is separated by a first order phase transition. Some people claim that glass is actually a super-cooled liquid because there is no first order phase transition as it cools. In fact, there is a second order transition between the super-cooled liquid state and the glass state, so a distinction can still be made (see reference).

As for the window panes. It appears that the glass making techniques of the time were not very good and resulted in noticeable thickness differences across a sheet. When placing the glass in the windows, builders would place the thicker side on the bottom for greater support. "If medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more—but this is not observed" (see reference). Egyptian glass would now be puddles of glass.

In essence the question of what state of matter glass is in doesn't have an easy answer. The general consensus among those who know is that it is an amorphous solid (see reference). Perhaps glasses should be placed into a new state of matter rather then try to define it based on the "classes" of matter invented by ancient Greek scientists 2500 years ago (see reference).

References:

  1. Matter: States of Matter
  2. Wikipedia - Glass - Does glass flow?
  3. The "Glass Flows" Myth (It just ain't true)
  4. Is glass liquid or solid? by Philip Gibbs
  5. Glass: Liquid or Solid -- Science vs. an Urban Legend
  6. Analysis shatters cathedral glass myth by C. Wu

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