For centuries people have maintained that glass is really an ultra-slow-flowing liquid, and some science textbooks maintain this position.

The basis for the claim is the noticeable difference in the thickness of windows at the bottom of stained glass windows in old churches. This gives the appearance that the glass has, over the centuries, slowly flowed downward, thickening the lower portion of the pane.

In reality, this phenomenon is caused by the way windows were made in medieval times. Glassmakers started with a lump of molten glass, then rolled and flattened it before spinning the glass into discs, which were then cut into panes. This process made the sheets thicker around the edges, and for stability’s sake the thicker, heavier edges were installed at the bottom of the windows.

The theory about flowing glass was finally put to rest in 2013 through a study of a 20 million-year-old piece of amber, possessing the same properties as medieval glass.

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