The Misappliance Of Science

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Amorphous SOLID, got it?

The notion that glass flows at normal ambient Earth temperatures, only very slowly, is taught by so many poorly-qualified elementary school science teachers, and comes to so many kids so early in life, that it becomes a matter of utmost faith. Renounce the idea that glass flows and you might as well uncork the ocean and let it flow down the drain. It also seems to feed peoples' sense of irony to be around glass and to think they are in the presence of something weird and counter-intuitive.

Urbanlegends.com has collected many fine cites on the topic, with a variety of ways of attacking the myth, and the collection of articles can be found here.

Debunked: You can see glass flow in the windows of old buildings.

No, you see uneven glass, and you see it getting thicker in all directions, not just downward. Years ago, window glass was made by what is known as the Crown Glass process, where a balloon of glass was blown, and then spun out into a disc. Window panes were cut from this disc, which was thicker in the middle, getting thinner as it got to the edge of the disc. When you occasionally see a thick pane with a circular ripple, you see the center of the disc, where the "punt" that spun the disc out was attached.

Some people erroneously debunk this legend by saying that glaziers purposely put the thicker portions of glass towards the bottom of the pane, to balance it or something. This is incorrect, as can be observed by looking at many old windows. Glaziers put the panes in however they were cut to fit.

A good description of various glass making techniques can be found here.

Debunked: Large telescope mirrors often become distorted due to glass flow.

Large telescope mirrors get distorted by weight shifting, not flow. They can sag under their own weight when tilted, but when righted, the sag goes away. If the glass had flowed, the shape would be permanently changed. Large mirrors would rapidly become completely unusable.

The 200 inch Hale Telescope mirror at Palomar Observatory in California (along with other large reflector telescopes) uses a complex set of support actuators in chambers inside the mirror, constructed to a ridiculously high degree of precision, to provide additional variable support according to the current angle of the mirror. This sort of support technique would not be possible if the mirror kept permanently changing shape by flowing.

Debunked: Glass is not a crystal, therefore it is a liquid and flows.

Glass is defined by material science experts as an "amorphous solid", meaning that it has no long-range, crystalline structure, but that it nevertheless is a solid. A solid is defined as a material with a viscosity of greater than 10^14.6 poise. Glass is a solid.

If a puling little window pane could exhibit visible flow, then there are many pieces of glass in the world that are much older than the oldest window, and by rights these should be "puddles" by now. There are obsidian chips from the time of the dinosaurs that are still razor sharp.

If you need figures to back this up, try this. If you'd be fine with an expert opinion, try this letter from Prof. C. Austin Angell, material scientist who wrote a review article for Science in 1995.

References:

• Urbanlegends.com collection of non-flowing glass cites
• Various types of hand-blown glass
• Scott Sehlhorst knows flow
• Justin Bukowski goes to the expert