Does glass flow? No. This common myth spins off from the similarity of glass with liquids. However, once cooled, most glasses are a species of their own. Since they lack a regular structure, they are not similar to crystals. The term for this is amorphous.
Glass makers did not have the technique to make constant-thickness glass till the modern age. Furthermore, it would have been slightly advantageous to make a glass plate thicker at the bottom to create stability.
The argument against the cooled flowing glass theorem, formulated by an early chemist, lies with century old Roman glass in burial tombs and antique telescopes, both which should have shown signs of degradation.
http://www.glassnotes.com/WindowPanes.html
http://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glass
Source: cmog.org
