Eli5, why do Mason jar lids typically come in 2 parts, threads and seal, as opposed to nearly every other commonly used household container?

Question

Eli5, why do Mason jar lids typically come in 2 parts, threads and seal, as opposed to nearly every other commonly used household container?

In: 7899

Answer 1

In addition to being a single use item, the lid (flat) has a distinct function when the jars are processed. As the contents of the jars heat up, air in the jars is expelled under the lid and a vacuum is formed (more on this below). At about this time, the sealing of the lid takes place and the ring keeps everything in order until cooling. Then the ring is removed.

When adding the lid and ring to the jars, the instructions are for a “finger tight” seal of the ring. If the ring is too tight the lid won’t let air escape and can jeopardize the seal, and the vacuum.