"It is sometimes said that glass in very old churches is thicker at the bottom than at the top because glass is a liquid, and so over several centuries it has flowed towards the bottom. This is not true. In Mediaeval times panes of glass were often made by the Crown glass process. A lump of molten glass was rolled, blown, expanded, flattened and finally spun into a disc before being cut into panes. The sheets were thicker towards the edge of the disc and were usually installed with the heavier side at the bottom. Other techniques of forming glass panes have been used but it is only the relatively recent float glass processes which have produced good quality flat sheets of glass."

-- http://math.ucr.edu/home/baez/physics/General/Glass/...

"There is a widespread opinion that glasses are supercooled liquids and therefor 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., quoted in http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualb...