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© Copyright 1998, Jim Loy
Did you know that glass, at standard temperature and pressure, is a liquid? Glass looks and acts like a solid, doesn't it. But it is a liquid. Or is it?
You may know about viscosity. Viscosity is the thickness of a liquid. Water has a low viscosity. Molasses in January has a high viscosity. Well, the following is a story which I have heard from a couple of sources.
Glass is so highly viscous, that it would take hundreds (maybe thousands) of years to "run" into a puddle. This may seem a little far fetched. But, window panes a hundred years old, or so, are usually noticeably thinner at the top, than they are at the bottom. They have run down-hill some.
The above story seems to be false. Glass does not flow at normal temperatures. There is a chance that glass may flow, as far as I can determine from some sources, but this takes at least thousands of years to flow noticeably.
The window panes which are thicker on the bottom than on the top actually exist. But, this may have been accidentally done during manufacture. I have also heard that it was done intentionally, to prevent bowing and bending of the glass, under its own weight.
Scientists do insist that glass is a liquid (see addendum), because it has no melting point. A normal solid has a temperature at which it melts, from an obvious solid to an obvious liquid. As the temperature rises, glass just becomes less viscous. It behaves the same way that other, not quite so viscous, liquids do. They also become less viscous at higher temperatures.
How about plastic, or quartz? These seem a lot like glass. But, they are clearly solid (not liquid) at standard temperature and pressure, and they do have melting points.
Addendum:
I received several emails, informing me (and quoting sources, too) that the flow of glass, under normal temperatures, is a myth. And we find that the ripples in old glass, often taken as proof of flow, is caused by acid rain eating away at the glass, or maybe poorly made sheets of glass. By the way, we find that a glass does not have to be made of silicon dioxide. In fact, there are carbon-based glasses.
Take a survey of encyclopedias; what do they say? Well they say "Glass is a liquid," and "Glass is an amorphous solid," and "Glass is neither a solid nor a liquid." There is not a lot of consensus. There is an entire recent book called Glass, which apparently does not mention the words "solid" or "liquid" (according to the index). I can only deduce that "glass" is an amorphous word. I am leaning toward classifying glass as an amorphous solid, that is a solid that does not have a crystalline structure. And glass still does not have a well-defined melting point. That is still one of its defining properties.