It is claimed that cathedral glass is thicker at the bottom because
glass flows. Is it a property of glass to flow in the time frame of a
century or so?

Hi - thanks for the question.
Glass does flow, but the increased thickening at the bottom
of old windowpanes is not caused by this effect; 
That - nonuniform thickness - is an artifact
of the old, now obsolete manufacturing process.

This question (in the form Is glass liquid or solid?) is a FAQ
and people had long, too long arguments about this in usenet groups:

Is Glass Liquid or Solid? Philip Gibbs, Usenet Physics FAQ 
http://hypertextbook.com/physics/matter/resources.shtml
pointing to
http://www.weburbia.demon.co.uk/physics/glass.html

This question even found its way into the urban legends collection:
http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualberta.ca/~bderksen/florin.html

which adresses your question as follows:
The Antique Windowpanes Story

 The question of antique windowpanes has been addressed by Plumb,
1989[2]. He noted the following:
[...W]hy are the panes of antique window glass thicker on the bottom
than the top? There really are observable variations in thickness,
although there seem to have been no statistical studies that document
the frequency and magnitudes of such variations. This author believes
that the correct explanation lies in the process by which window panes
were manufactured at that time: the Crown
 glass process. 

That author offers a selection from technical papers, in which physicists
differ in their views. It may be too much to read, and so, 
 lets summarise: Urban Legend collection is correct on the
Crown process, but (sorry to say) wrong in conclusion that glass is a solid:

This is the correct answer:
Different disciplines define terms 'solid' and 'liquid' differently. 
Answer to question (is it a liquid?) depends on definition of liquid.
However, proper discipline for this issue is a bit esoteric field of
material science called Rheology (science of flow).
http://dmoz.org/Science/Physics/Rheology/

According to this discipline, glass is a 'viscoelastic liquid' with a very
high viscosity. Viscosity strongly depends on temperature and can be measured.
Value is such in the geometry of a window pane you will not see the flow in
a century or two, at the room temperature.
But in geological times - that is a different story - on that scale even the
rocks do flow. Read about the so called

 Search Terms: Deborah number

 The Deborah number is a dimensionless number which
 characterizes how "fluid" a material is. Even solids "flow" if ...
 ://www.google.com/search?hl=en&ie=ISO-8859-1&q=Deborah+number&btnG=Google+Search



Hedgie

Thanks

Analysis shatters cathedral glass myth
http://www.sciencenews.org/pages/sn_arc98/5_30_98/fob3.htm

A new study debunks the persistent belief that stained glass windows
in medieval cathedrals are thicker at the bottom because the glass
flows slowly downward like a very viscous liquid.

Edgar Dutra Zanotto of the Federal University of Sao Carlos in Brazil
calculated the time needed for viscous flow to change the thickness of
different types of glass by a noticeable amount. Cathedral glass would
require a period "well beyond the age of the universe," he says.

Suffice it to say that the glass could not have thickened since the
12th century. Zanotto reports his finding in the May American Journal
of Physics.

The study demonstrates dramatically what many scientists had reasoned
earlier. "You would have to bring normal glass to 350° Celsius in
order to begin to see changes," says William C. LaCourse, assistant
director of the NSF Industry-University Center for Glass Research at
Alfred (N.Y.) University.

Viscosity depends on the chemical composition of the glass. Even
germanium oxide glass, which flows more easily than other types, would
take 1032  years to sag, Zanotto calculates. Medieval stained glass
contains impurities that could lower the viscosity and speed the flow,
but even a significant reduction wouldn't alter the conclusion, he
remarks, since the age of the universe is only 1010  years.

The difference in thickness sometimes observed in antique windows
probably results from glass manufacturing methods, says LaCourse.
Until the 19th century, the only way to make window glass was to blow
molten glass into a large globe then flatten it into a disk. Whirling
the disk introduced ripples and thickened the edges. For structural
stability, it would make sense to install those thick portions in the
bottom of the pane, he says.

Later glass was drawn into sheets by pulling it from the melt on a
rod, a method that made windows more uniform. Today, most window glass
is made by floating liquid glass on molten tin. This process,
developed about 30 years ago, makes the surface extremely flat.

The origins of the stained glass myth are unclear, but the confusion
probably arose from a misunderstanding of the amorphous atomic
structure of glass, in which atoms do not assume a fixed crystal
structure. "The structure of the liquid and the structure of the
[solid] glass are very similar," says LaCourse, "but thermodynamically
they are not the same."

Glass does not have a precise freezing point; rather, it has what's
known as a glass transition temperature, typically a few hundred
degrees Celsius. Cooled below this temperature, liquid glass retains
its amorphous structure yet takes on the physical properties of a
solid rather than a supercooled liquid.

"At first, I thought that the [sagging window idea] was a Brazilian
myth," Zanotto wrote, but he soon learned that people all over the
world share the belief. Repeated in reference books, in science
classes, and recently over the Internet, the idea has been repeatedly
pulled out to explain ripply windows in old houses. "For the
layperson, it makes a lot of sense," says LaCourse.

In 1989, Robert C. Plumb of Worcester (Mass.) Polytechnic Institute
suggested in the Journal of Chemical Education that definitive proof
might require an instruction book written in the Middle Ages advising
glaziers to install glass panes with the thick end at the bottom. Now
if only such a handbook could be found.

From Science News, Vol. 153, No. 22, May 30, 1998, p. 341.
Copyright Ó 1998 by Science Service.

An amusing upshot of the glass flowing myth is that restorers have on
occasion reversed the more stable thick end down in the belief that in
a few hundred years the glass will have flowed to uniform thickness.

Glass flowing can be observed in old TV EHT tubes where the curve of
the top cap noticeably sinks. This is however only after many years at
elevated temperature.

Best

There is a book called, as I recall, "Glass Handbook" which includes
pictures from an experiment where glass rods were clamped in a bent
position for something like ten years.  When the rods were unclamped
at the end of this time, they remained curved at first, but within a
day they were again perfectly straight.

While added comments confirm and elaborate what I have written in my
answer, I feel  a comment on the comments is in order, to prevent
possible misconceptions.

First, I want to warn readers to be careful with numbers which start with 10
as in treadora's: 

 "..since the age of the universe is only 1010  years. "

That would be pretty young universe. If you look back at the original article
the number is 10.E10 years - this in scientific notation. Original
uses exponential notation - and during careless copying the
superscript gets leveled. At least 20% of large physics numbers on the
web get totally messed up like this,
     Moral is: 1) use scientific notation on the web
http://www.nyu.edu/pages/mathmol/textbook/scinot.html
               2) do not copy whole articles (even if you have
copyright holder's permission) - just give a quote and a pointer
(link, URL) of the whole.
               3) re-read what you wrote / copied

 By the way - viscosity of the glass (depending on the kind)
 is about 10.E40 Pas at room temperature,
 which comes out (when copying mechanically) as (WRONG) 1041 Pascal.seconds.

 What it means is this: You make a rod,  1 mm square in cross-section and
 use it to hang 1 kg mass.   The stress in the rod is about E7 Pa (Pascals)
and so speed of creep is E-33 inverse seconds. That is rate of strain. Strain
is dimensionless  number dl/l where l is length of your rod and dl the
elongation due to the stress. It is very slow, but it is a real flow.

 fstokens-ga seems to imply that deformation of glass is fully elastic.
 That is not so.
 All viscoelastic materials (I did provide links to rheology sites,
right?) have (spectrum of) relaxation times.  If you deform (bend) any
material and release the force, part will recover - that was elastic
deformation, and part will stay bent permanently- that was plastic
part of the deformation.
  The ratio of the two will depend on ratio of relaxation time to the
time material was bent. This is more visible in polymers (commonly
called plastics) then with glass (and this ratio is called Deborah
number) of the test.

  There was more nonsense written about this 'flow issue', than I have
time or patience to mention or refute, but just in brief:

Glass transition temperature (property of all platics) has nothing much
to do with this (it is rubbery to brittle state transition) 

 Issue of amorphous vs crystalline has very little to do with this:
 both solids and liquids can be both amorphous or crystaline (see e.g. LCDs) 
and this property (crystalinity) is not either/or - but a matter of degree. 
This property does  not determine what is liquid nor what the viscosity
 will be.
By and large (this is moral 2):
 -considering the interest and passions which these
issues tend to raise, I believe we should teach elements of Rheology
in elementary schools.
 Lacking that, I recommend that interested parties look at the
rheology links I provided. (particularly if they intend to write about
it). It is understandable, interesting, and agrees with common
experience.
The classical materials sciences - theory of elasticity and
hydrodynamics - are abstract, limiting cases (of zero and infinity
Debrah number) which  exist
mostly in academia, rarely in real life,  and have just few special
engineering applications.


hedgie