Glass is not a slow liquid

Window glass is an amorphous solid. Medieval panes are thicker at the bottom because they were made that way, not because the glass has flowed.

Glass has a disordered structure, like a liquid that was caught. At room temperature the time required for that structure to flow is longer than the age of the universe, by a ridiculous factor. Edgar Zanotto calculated it for cathedral glass. The windows of the Middle Ages have not creeped under their own weight in any amount a person could measure.

The thick bottoms are from the manufacturing. Crown glass was spun; cylinder glass was blown and flattened. The panes were often installed thick-edge down, because that is the stable way to set an uneven piece. Old glass is lumpy because it was hard to make flat, not because it is a liquid with patience.