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DO
CATHEDRAL GLASSES FLOW?
-
- From:
PHYSICS NEWS UPDATE - The American Institute of Physics Bulletin
of Physics News
- Number
370 May 6, 1998 by Phillip F. Schewe and
Ben Stein
Reportedly
some stained glass windows from 12th century cathedrals are thicker at
the bottom than at the top, suggesting that glass is a liquid which
flows (albeit slowly) downward under the force of gravity.
Surprisingly,
the scientific literature on this point is scant, and Edgar Zanotto at
the Federal University of São Carlos in Brazil (dedz@power.ufscar.br)
investigated this
issue by modeling several different kinds of glass. He determines that
if glass flows it must on a time scale of billions of years and not
mere centuries. Zanotto points out that glass vases several thousands
of years old do not show the effect of any downward flow. He argues
that some cathedral glasses might be larger at the bottom because of
the old manufacturing process in which the glasses were blown into
cylinders and then flattened manually. (American Journal of Physics,
May 1998).
- From:
ScienceNOW
- May
1998, by Erik Stokstad
Cathedral
Glass Myth Shattered
The
popular notion that medieval cathedral windows have thickened at the
bottom - by slowly flowing like a liquid-doesn't hold water. Even
after considering the specific chemical composition of stained glass
windows, according to a report in the May issue of the American
Journal of Physics, it would take longer than the age of the Universe
for the glass to sag appreciably.
When
materials scientist Edgar Zanotto of the Federal University of São
Carlos, Brazil, first heard about sagging medieval windows, "I
thought it was just a local [Brazilian] myth." But then he heard
the same tale from colleagues in Argentina, and found echoes of it in
textbooks and even in the Encyclopedia Britannica. Although glass isn't
supposed to flow at room temperature, old glass has many impurities
that might help it ooze. So Zanotto sat down to do the calculations.
Zanotto
looked up the chemical compositions of some 350 medieval glasses and
calculated a typical viscosity. The old glass should flow a little
more easily than modern glass, he found, but only at temperatures
above roughly 200 degrees Celsius (modern glass has to be hotter than
about 250ºC to flow). Below 200ºC, the molecules would remain
gridlocked. Zanotto also considered an extreme case: germanium oxide
glass, which is thought to flow even at bitterly cold temperatures.
But even a germanium oxide window would hold its shape, he concluded.
Such glass would visibly sag, Zanotto says, "but only if you wait
for 10 E33 years."
The
paper goes a long way to laying the legend of flowing windows to rest,
says Jonathan Katz, a materials scientist at the Washington University
in St. Louis. Katz, Zanotto, and others think that cathedral glass
makers centuries ago were unable to make even panes and that builders
put the fat end on the bottom for stability.
If the
cathedral glass really is sagging, Katz says, it could be spotted with
an interferometer in the lab. "It would be a great graduate
student project," he says. Still, there's good evidence the
thesis would be a short one: If glass moves at room temperature, he
adds, "your camera wouldn't focus right after 10 years on a shelf."
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