Questions about glass jar lids

Factual Questions
1

  1. How do they make those metalic screw-on jar lids like this one?

  2. These usually have a button-like thing in the middle that pops out when the can is opened. How do they keep the button compressed?

2
  1. The jam is put into the jar while it’s hot and the cover attached. As the jam cools, the air above it contracts, lowering the pressure and causing the button to compress. The same thing works with home canning.
3

Yup, in home canning it’s more the whole surface of the lid (which is a separate piece from the rim) rather than a small “button” portion, but the same thing happens. You can even hear a metallic ping sound when each lid seals. There is a substance around the bottom of the lid that adheres to the rim of the jar to make a tight seal.

Home canning also involves a long “processing” time (either under high pressure, or immersed in boiling or near-boiling water) to heat up the jars and their contents, and sanitize both and avoid bacterial growth. This also helps the process of forming the nice seal once the jars cool.

4

The air above is called headspace and is critical for proper canning. Check out the National Center for Home Food Preservation for more info.