The AFU and Urban Legend Archive
Science
Glass Flow
 amorphous solid more




From: iayork@panix.com (Ian A. York)
Newsgroups: alt.folklore.urban
Subject: Re: Does glass sag? Definative Answer`
Date: 27 Jan 1997 15:42:03 -0500

In article <19970127194801.OAA14707@ladder01.news.aol.com>, ThnWiteDuk <thnwiteduk@aol.com> wrote:
>
>Glass is not a solid. It is an amorphous liquid.

Every definition of glass defines it as an amorphous SOLID. From Physics of Amorphous Materials_ by S. R. Elliott (London: Longman Group Ltd, 1983; ISBN 0-582-44636-8), from the definitions section, p. 5:

  1. "A glass is an amorphous solid which exhibits a glass transition"
  2. "A solid is a material whose shear viscosity exceeds 10^14.6 poise"

There is a widespread opinion that glasses are supercooled liquids and therefore have a finite viscosity at ordinary ambient temperatures. Stories are told of glasses flowing under their own weight: of ancient windowpanes that are thicker at the bottom; of glass that has sagged in storage. These observations must find other explanations, because glasses of commercially useful compositions are in fact rigid solids at ordinary temperatures. --Ernsberger, F. M. In Glass: Science and Technology; Uhlmann, D. R.; Kreidle, N. J., Eds.; Acad.: New York, 1980; Vol. V, Chapter 1.

Glass is an amorphous solid. A material is amorphous when it has no long-range order, that is, when there is no regularity in the arrangement of its molecular constituents on a scale larger than a few times the size of these groups. ... A solid is a rigid material; it does not flow when it it subjected to moderate forces. More quantitatively, a solid can be defined as a material with a viscosity of more than about 10^15 P (poises). --Doremus, Glass Science, 1973

>It flows at an incredibly slow rate.

There's only one article ("Gravity-Induced Flow of a Structural Glass at Zero Temperature", by Clare C. Yu and S. N. Coppersmith, from Journal of Non-Crystalline Solids, 131-133, 1991, pp 476-478) that even remotely supports your contention, and its very support actually refutes it, because it concludes that glass could only flow if several real-world considerations were ignored: You'd have to have a piece of glass several miles long to observe any flow, and it would take several million years for the effect to manifest itself. They dismiss with a wave of their hands the objection that a piece of glass that huge would crumble under its own weight, destroying the evidence in much less than a million years. (This is from Ray Depew's analysis of the paper; see DejaNews for his full post.)

Thus you should further qualify "incredibly slow rate" as "never, in the real world."

>Look at the windows of old buildings. See how the glass is crooked,
>curvy, and light-refractive. Same reason.

Different reason. Manufacturing process: the Crown glass process.2 The process is described vividly in an 1860 chemistry textbook (Muspratt, S. Chemistry Theoretical, Practical & Analytical as Applied and Relating to the Arts and Manufactures; Mackenz)ie: London, 1860; Vol. II, pp 21-216) by Sheridan Muspratt (founder of the College of Chemistry, Liverpool), drawing considerably from a paper presented by an artisan, Henry Chance (Chance, H. J. Soc. Arts 1856, 4, 222-231), to the Society of Arts, London. About 9 lb (#1 or 2 oz) of molten glass was collected in a lump, rolled, and blown to the shape of a florence flask, and an iron cup was attached to keep the shape centered. The flask was then expanded and flattened to the shape of an enormous decanter. Next and iron punty was attached and the blow pipe removed, leaving a hole. Finally the shape was spun in a flashing furnace, and, to quote,

The action of heat and centrifugal force combined is soon visible. The nose of the piece, or hole caused by the removal of the blowing pipe, enlarges, the parts around cannot resist the tendency, the opening grows larger and larger; for a moment is caught a glimpse of a circle with a double rim; the next moment, before the eyes of the astonished spectator, is whirling a thin transparent circular plate of glass which, but a few minutes before was lying in the glass-pot, an indistinguishable portion of the molten mass. The sound of the final opening of the piece has been compared to that produced by quickly expanding a wet umbrella. In this way a flat circular disc, nearly sixty inches in diameter, or sometimes more, is produced, of almost uniform thickness, except at the point of attachment to the ponty, where there is a swelling called, as already stated, the bull's-eye. The glass at the edge of the disc is also in some cases a little thickened. Still whirling, the table, as it is now called, is carried off, laid flat upon a support called a whimsey, detached by shears, or otherwise, from the ponty, lifted into the annealing kiln upon a fork and piled upon its edge against the preceding table.

>Glass never melts (a physical property of a solid is a melting point). It

False. Not all solids display a melting point.

>These are facts.

Your assertion does not make them facts. You need to provide a whole bunch of references, since what you say flatly contradicts the massive body of literature on glasses.

Anyone new to this discussion, please read <http://tafkac.org/science/glass.flow>,
<http://www.ualberta.ca/~bderksen/florin.html> and
<http://www.ualberta.ca/~bderksen/windowpane.html> before replying.

Ian "http://tafkac.org/science/glass.flow" York

-- 

      Ian York   (iayork@panix.com)  <http://www.panix.com/~iayork/>
      "-but as he was a York, I am rather inclined to suppose him a
       very respectable Man." -Jane Austen, The History of England



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