Why does a Mason jar ring loosen more easily when held under hot water?

Question

Why does a Mason jar ring loosen more easily when held under hot water? If metal expands when heated it seems there's a case to say the ring should get tighter. Does the metal expand more in the horizontal direction than the thickness direction? I was just musing while canning the other day.

Answer 1

This is one of the paradoxical things about heat based expansion. A hole in an object gets larger when heated. It's easy to think about how the metal should expand inward, making the hole smaller, but what actually happens is the whole ring expands outwards, and this outward effect is greater than the inward effect.

If you mentally think about a square ring rather than a circular one, you can see it a little easier. Circles can sometimes make it a bit harder to intuit about things. Say it's a 1mm thick square band around a 50mm jar opening (if a square jar opening existed). If the metal expands by 10%, the lengths of each side increase by 10%: 5mm. The thickness of the metal expands by 10% too, but that's only 0.1mm of expansion. So the 5mm increase in size of the whole lid dwarfs the 0.1mm expansion inward of the ring.