Old windows aren't thicker at the bottom because glass flows

A favourite classroom fact says glass is a very slow liquid that sags over the centuries. The real reason old panes are uneven is how they were made.

Look closely at a window in a very old building and you may find panes that are thicker at the bottom than at the top. For decades this was offered as proof of a strange idea: that glass is really a liquid, flowing downward so slowly that it takes centuries to show.

It is a good story, and it is wrong. Once glass has cooled and set, it stops flowing in any way that matters.

Glass is an unusual material, which is why the idea took hold. Its atoms are not lined up in the neat, repeating pattern of a crystal such as quartz or ice. They are jumbled, more like a snapshot of a liquid, and scientists call it an amorphous solid. But in how it behaves, glass is a solid in every sense: it holds its shape, resists bending, and breaks rather than drips.

How slowly could it move, if at all? In 2017, researchers estimated how stiff the medieval glass of Westminster Abbey, made in 1268, is at room temperature, and worked out the fastest it could possibly flow. Their answer was about one nanometre per billion years.

A nanometre is a millionth of a millimetre. At that rate, even the whole age of the universe would not change a pane by anything you could see.

Uneven from the start

So why are the old panes uneven? Because they left the workshop that way. One of the most common ways to make window glass, until machines took over, was crown glass. A glassblower blew a hollow globe, moved it onto an iron rod, reheated it and spun it until it opened out into a flat disc up to about one and a half metres across. The disc was thinnest at the rim and grew thicker toward the centre, and panes were cut from it. Each piece carried some slope and ripple from the start.

Truly flat glass came later, with the float process: molten glass is poured onto a bath of molten tin, where it floats and spreads into a smooth, even sheet. It was developed in Britain in the 1950s and spread around the world during the 1960s. Before that, ripples, bubbles and uneven thickness were simply what window glass looked like.

The flowing-glass myth lasts because it sounds like the kind of odd truth science is full of. The real answer is plainer: when something old looks strange, ask first how it was made.