A Mason jar — named after John Landis Mason, who patented it in 1858 — is a molded glass jar used in home canning to preserve food. The jar's mouth has a screw thread on its outer perimeter to accept a metal ring or "band". The band, when screwed down, presses a separate stamped steel disc-shaped lid against the jar's rim. An integral rubber ring on the underside of the lid creates a hermetic seal. The bands and lids usually come with new jars, but they are also sold separately. While the bands are reusable, the lids are intended for single-use when canning. Largely supplanted by other products and methods for commercial canning, such as tin cans and plastic containers, glass jars and metal lids are still commonly used in home canning.
In home canning, food is packed into the Mason jar, leaving some empty "head space" between the level of food and the top of the jar. The lid is placed on top of the jar with the integral rubber seal resting on the rim. A band is screwed loosely over the lid, allowing air and steam to escape. The jar is heat sterilized in boiling water or steam and the lid is secured. 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 to create 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 used today are slightly domed to serve as a seal status indicator. The vacuum in a properly sealed Mason jar pulls the lid down to create a concave-shaped dome. An improper or failed seal or microbial growth will cause the dome to pop upward.