Ken Fox
AP/IB Physics Teacher
Smoky Hill High School
Aurora, CO
Conclusion
There is no clear answer to the question "Is glass solid or liquid?." In
terms of molecular dynamics and thermodynamics it is possible to justify
various
different views that it is a highly viscous liquid, an amorphous solid or
simply that glass is another state of matter which is neither liquid or
solid. The
difference is semantic. In terms of its material properties we can do
little better. There is no clear definition of the distinction between
solids and highly
viscous liquids. All such phases or states of matter are idealisations of
real material properties. Nevertheless, from a more common sense point of
view,
glass should be considered a solid since it is rigid according to every
day experience. The use of the term "supercooled liquid" to describe glass
still persists
but is considered by many to be an unfortunate misnomer which should be
avoided. In any case, claims that glass in old windows have deformed due to
glass flow have never been substantiated. Examples of Roman glassware and
calculations based on measurements of glass visco-properties indicate that
they
cannot be true. The observed features are more easily explained as a
result of the imperfect methods used to make glass window panes before the
float glass
process was invented.
References
"Antique windowpanes and the flow of supercooled liquids", by Plumb,
Robert C. (Worcester Polytech. Inst., Worcester, MA, 01609, USA), J. Chem.
Educ. (1989), 66(12), 994-6
Ernsberger, F. M. "In Glass: Science and Technology"; Uhlmann, D. R.;
Kreidle, N. J., Eds.; Acad.: New York, 1980; Vol. V, Chapter 1.
"Physics of Amorphous Materials" by S.R. Elliott (London: Longman Group
Ltd, 1983;)
C. Austin Angell, Science, March 1995
Robert H. Brill, "A Note on the Scientists definition of glass", Journal
of Glass Studies, vol 4, 127-138, 1962
Florin Neumann, "Glass: Liquid or Solid -- Science vs. an Urban Legend"
"Do Cathedral Glasses Flow", Am. J. Phys. v66, pp392-396, May, 1998
Edgar Zanotto, American Journal of Physics 66 p392, May 1998
Paul Steinhardt, "Crazy Crystals", New Scientist, 25 January 1997