At some point during your science studies, you would have been introduced to the idea of The Three States of Matter: solid, liquid, and gas. As you may have realised when thinking about, for example, the melting of glass, or contemplating the nature of a flame, this three state model doesn’t tell the whole story. Solid, liquid, and gas are more like three categories into which more specific states of matter fit. This series explores some of these states which perhaps don’t seem to fit neatly into the three states model as you may have learned it.
There is a pervasive, persistent, and entirely incorrect idea that glass is an extremely high viscosity liquid. The reason this comes about makes a certain amount of sense: glass has the same chemical composition as crystalline quartz, but has a liquid-like lack of long-range order.
The associated claim that glass does flow over time is incorrect. “Evidence” is given by pointing out that early glass windows tend to be thicker at the bottom. This “evidence” is actually incorrectly attributed: glass panes with anything resembling a truly uniform thickness are a very recent advance. Until the early twentieth century, the only way to make a nearly flat sheet of glass was to start by blowing a globe or cylinder, then squashing it flat while it is still hot, either directly down onto itself or after cutting the shape open. The result is a glass pane with significant thickness variations, including typically one edge being thicker than the rest. Logically, these windows were usually installed with the thicker, heavier edge at the bottom. The idea that the glass of the windows has flowed down over time is completely contradicted by the fact that there are occasional examples of these glass panes being installed with the thick edge at the top or on one of the sides.
Glass does not flow until you heat it up past its glass–liquid transition temperature (almost the same as a melting point, more on that shortly.) Glass has both a fixed volume and a fixed shape. The only thing that glass has in common with a liquid is that disorder of its atoms. Even then, the atoms vibrate around fixed positions, as is typical of a solid, rather than moving relatively freely as they would in a liquid.
Glass is a solid.
It is, however, part of a class of solids that often get neglected. It is a perfect example of an amorphous solid.