"Glass flows?" This is a misunderstanding!

If you visit old churches that are hundreds of years old, you may find a strange phenomenon. There are usually very old glass windows in the church. Looking out of the glass windows, you will find that things become distorted, a bit like the effect of a funhouse mirror. If you look closely, you will find that the bottom of such glass windows is obviously thicker than the top.

At this time, the tour guide will usually say that this is because glass is actually a very slow-flowing liquid. If it is left standing in the church for a long time, it will slowly flow down and pile up at the bottom, so the glass at the bottom becomes thicker.

This statement is actually a big misunderstanding. Glass is not a liquid, and hundreds or thousands of years will not make it flow.

How do we know this? Because humans have been using glass for a long time. There are some glass cups in the museum that are 2,000 to 3,000 years old. If you have the chance, you can go to the museum to see that these cups still retain their original shape without any deformation. If the church glass looks like this because it flows, these glass cups should have flowed into a pool long ago.

Scientists estimate that a piece of glass could survive billions of years—longer than the universe has existed to this day—without noticeable change in its shape.

Then you might ask the second question: since glass is not liquid, why is the glass at the bottom of the old church thicker?

If you really have the opportunity to visit many old churches and look carefully at their glass, you will find that it is not always like this. Occasionally, there are a few pieces of glass that are not thick at the bottom, but thick at the top. The laws of physics will not make mistakes and there are no exceptions. So when you encounter this kind of situation, you should consider that it is not a physical problem, but a human problem.

In fact, this is true. Many glasses in the Middle Ages were made with one side thinner and the other side thicker. Workers often installed the thick side downwards, which was more stable. But some people didn't care, or installed it incorrectly. Therefore, the thickness of some glasses is reversed.

You may want to ask a third question: Why is the thickness of medieval glass uneven?

This is because the glass technology at that time was relatively backward. Glass workers usually made a cylinder first and then flattened it. The resulting glass was not very uniform. After being cut into pieces of glass windows, one side was often thicker and the other side was thinner.

Later, as glass craftsmanship improved, glass workers would pour molten glass onto molten metal, letting it float and spread out, thus obtaining very uniform glass.

Now you know that glass is not liquid, and hundreds or thousands of years are not enough for it to flow. The reason why the glass windows in the church are uneven in thickness is not because they flow, but because they are uneven when they are manufactured.