Molten glass - Mark Interrante - from NY Times

Molten glass - Mark Interrante - from NY Times

This is an addendum to my earlier (and popular) post about whether or not glass is a liquid.  If you haven’t read the previous post, the crux of the myth is that many of us are taught in science class that glass is a veeerry slow flowing liquid, and that’s why old windows are thicker on the bottom than the top.  News flash — actually you can find old windows that are thicker on the top than the bottom because it’s just an artifact of how glass windows were poured back in ye olde days.  But that doesn’t mean that glass is a simple thing.

A few months ago there was a great article in the New York Times Science section called “The Nature of Glass remains Anything But Clear“.  This very nice article about glass talks about how — even if it’s not a liquid — it’s a pretty complicated thing. “The arrangement of atoms and molecules in glass is actually indistinguishable from a liquid,” it says Solids tend to have atoms arranged in nice little tinker-toy stacks, whereas atoms in liquids aren’t so organized, more like someone threw the tinker toys across the floor in a rage, which is why they can flow.  The atoms in glass are more jumbled than organized.  So how come glass is so strikingly hard if its atoms don’t have a rigid order?

From the article:

“When cooled, a liquid either freezes, as water does into ice, or it does not freeze and forms a glass instead. In freezing… the molecules line up next to and on top of one another in a simple, neat crystal pattern. When a liquid solidifies into a glass, this organized stacking is nowhere to be found. Instead the molecules just move slower and slower and slower, until they are effectively not moving at all, trapped in a strange state between liquid and solid. .. This glass transition does not occur at a single, well-defined temperature; the slower the cooling, the lower the transition temperature. … By contrast, water, cooled quickly or cooled slowly, consistently crystalizes to the same ice structure at 32 degrees Fahrenheit.”

The reason glass forms is still a hot topic, with many competing theories.