Q: Is it true that the stained glass windows in many old cathedrals are thicker at the bottom because the glass has flowed over the centuries?
A: No, said materials scientist Edgar Dutra Zanotto of the Federal University of Sao Carlos in Brazil, who recently calculated how long it would take the windows to flow in such a manner. He concluded that the glass is actually very stable, and such flow would require a period "well beyond the age of the universe."
The difference in thickness probably results from the way the glass was manufactured, according to Science News. Until the 19th century, the only way to make window glass was to blow molten glass into a large globe, then flatten it into a disk. Whirling the disk introduced ripples and thickened the edges, which were probably installed at the bottom of windows for structural strength. Later glass was drawn into sheets by pulling it from the melt on a rod. Today, most glass is made by floating liquid glass on molten tin.