Is glass a solid or a liquid?

sequoianoir

It is a solid that may have FLUID properties under certain conditions. The "glass flows" issue has been discussed at great length (and has led to controversial statements such as the claim that "glass is a supercooled liquid") and the consensus (supported by citations from glass experts) is that glass does not flow at room temperature. Note, however, that glass can and does 'creep' , just like crystalline solids do, in response to a load. Furthermore, in some applications (such as some laboratory thermometers), glass gets heated above the transition temperature at which it actually DOES become a supercooled liquid. This leads to them becoming incorrectly calibrated due to the movement (flow) of the glass. Mar 17 04, 12:35 PM

Linus_337

I always understood that it was a supercooled liquid, and had heard that it flows such that windows become thicker at the bottom over time. Mar 17 04, 2:17 PM

sequoianoir

Linus, this appears to be a confusion going back to the days of old. In "ancient" times, it appears window glass was made by spinning it. Used up to the mid-1800s was the Crown glass process, in which a glassblower would spin around 9 lb (4 kg) of molten glass at the end of a rod until it flattened into a disk approximately 5 ft (1.5 m) in diameter. The disk would then be cut into panes. The pieces were not, however, absolutely flat and the edges of the disk would be thicker due to centrifugal forces. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability. There is anecdotal evidence that occasionally such glass has been found thinner side down, as would be caused by carelessness at the time of installation. Mar 17 04, 3:07 PM

Hamlet.

Glass Is A Liquid? © 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. Mar 17 04, 4:00 PM

Hamlet.

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 Mar 17 04, 4:04 PM

hummerh3

An amorphous solid according to this link: http://www.urbanlegends.com/faq2k/science_37.html [Mar 17 04 9:17 PM] hummerh3 writes: "Glasses are uniform amorphous solid materials, usually produced when a suitably viscous molten material cools very rapidly, thereby not giving enough time for a regular crystal lattice to form. " "Common glass is mostly amorphous silicon dioxide (SiO2), which is the same chemical compound as quartz, or in its polycrystalline form, sand." http://en.wikipedia.org/wiki/Glass Mar 17 04, 9:12 PM

sequoianoir

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. Mar 18 04, 5:09 PM

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