Very old church windows have been observed to be thinner at the top and thicker at the bottom: slowly oozing down...
That is a common misconception. IIRC, before glass-forming machinery was made, glass blobs were spun at the end of the pipe sort of like pizza dough is flattened. I think the rim was thicker than the center. Then it was cut, and mounted with the thicker part down. But with a little research you can verify this.