Why is Glass a Liquid?

When I was in eighth grade, my well-meaning physical science teacher taught a lesson about viscosity. He explained that some liquids, such as water, have a low viscosity and flow freely. Others, such as mollasses, have a high viscosity and flow very slowly. To conclude the lesson with a stunning factoid, the teacher then announced that glass is, in fact, a liquid-- but that its viscosity is so high that it resembles a solid.

This bugged me. The same teacher had taught us that ice cream, gelatin, and other liquidy substances were amorphous solids. I could see no reason that glass would be regarded as a liquid while Jell-o remained a solid. When I pressed my teacher for more information, he was unable to provide it. I assumed that I just didn't "get it" and spent the next ten years wondering why glass is a solid.

Then the mood struck me to consult the almighty University of Google to find out the truth about glass: Is it a liquid or a solid? And, if it's a liquid, why?

As I found out an article by Florin Neumann, authored in 1996, there is no explanation for why glass is a liquid-- because it's not.

As anyone with a basic understanding of the states of matter can tell you, liquids have a definite volume but no definite shape. Solids, on the other hand, have both a definite volume and a definite shape. I don't know about you, but I've never been able to pour a window into a cup. If it were even remotely possible, even on a theoretical level, it would take thousands of years to accomplish for one reason-- glass is solid.

Some people cite the fact that glass "flows" slightly toward gravity over an extended period of time, as evidence that glass is actually a liquid. While its tendency to collect makes it an amorphous, rather than crystalline, solid, that does not make it a gas. Human skin also sags over time due to the effects of gravity, but no one has ever stated that our faces are liquids.

This common misconception, which is unintentionally perpetuated by teachers worldwide, results from a mis-reading of physicist Gustav Tammann's statement that "glass is a frozen supercooled liquid."

What Tammann meant by this is that sand must be melted into a liquid and then allowed to cool, or freeze, at room temperature. At any reasonably temperature, glass remains frozen. Like any other solid, it will either melt or incinerate under extreme heat. But the properties of liquid glass are completely different than the glass involved in windows or mirrors.

Ultimately, there is no reason why glass is a liquid-- because it isn't. It holds none of the basic properties necessary to be regarded as a liquid.