Use
In home canning, food is packed into the jar, leaving some empty "head space" between the level of food and the top of the jar, then the lid is placed on top of the jar with the integral rubber seal resting on the rim. The band is screwed loosely over the lid, allowing air and steam to escape. The jar is heat sterilized in boiling water or steam. The jar is then allowed to cool to room temperature. The cooling of the contents creates a vacuum in the head space, pulling the lid into tight contact with the jar rim and creating a hermetic seal. Once cooled, the band is removed to prevent residual water between the jar threads and the lid from rusting the band. If the jar seal is properly formed, internal vacuum will keep the lid tightly on the jar. Most metal lids are slightly domed to serve as a seal status indicator: the vacuum in a properly sealed mason jar pulls the lid down such that the dome is concave, but an improper or failed seal or microbial growth will cause the dome to pop upward. Some old Mason jars used glass lids with separate rubber rings, instead of steel, and are now considered unsafe for canning.
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