I really loved the idea of glass being a liquid that was just moving ‘super slow.’ I first heard it from a tour guide or two in an old building somewhere, and I could swear my chemistry teacher once mentioned it. But, alas, it is not the case:
Why old European glass is thicker at one end probably depends on how the glass was made. At that time, glassblowers created glass cylinders that were then flattened to make panes of glass. The resulting pieces may never have been uniformly flat and workers installing the windows preferred, for one reason or another, to put the thicker sides of the pane at the bottom. This gives them a melted look, but does not mean glass is a true liquid.