No. The uneven thickness of old window panes is not evidence that glass has flowed downward over the centuries. Historical glassmaking produced sheets with variations in thickness.
Research described by Penn State examined the composition of medieval window glass and calculated flow far too slow to explain visible changes over that time. The Corning Museum of Glass also attributes uneven panes to how the sheets were made.
Glass has a disordered atomic structure and complex behaviour, but that does not mean a window noticeably drains toward its bottom. The important distinction is between extremely slow material changes and the visible irregularities already present when the glass was manufactured.
Uneven old glass reflects its manufacture, not centuries of visible downward flow.
Penn State University
Research using medieval window-glass composition found that flow remains far too slow to account for visibly thicker bottoms, even after revising earlier viscosity estimates.
Based on: Penn State University
Corning Museum of Glass
The museum explains that thickness variations in old stained-glass windows result from how the sheets were made, rather than glass flowing downward over time.
Based on: Corning Museum of Glass
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