Is the Ball Mason Jar Worth It for Canning and Storage?

Here's the bit that actually matters: the two-piece lid. A flat metal disc coated in a rubbery sealing compound sits under a separate screw band, and when you process a jar in a hot water bath, the air inside heats up and pushes out past that lid. As the jar cools on the counter, the trapped air contracts, and that flat disc gets pulled down and locked against the glass rim, the same shape a ball jar has used for generations, which is why bands and lids from an old batch fit a brand new jar without you ever checking. That rim has to be flawless glass, no chips, no unevenness, or the vacuum never forms, which is also why the jars are built to handle the swing from a hot water bath to a cool countertop without cracking. Once sealed, the jar doesn't need refrigeration until you break the seal to open it. Freeze one instead, leave a little headspace so the contents can expand, and the same jar handles cold just as easily as it handled heat.