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Canning jar lids are available in many sizes in order for them to fit a variety of canning jar styles, including quilted, jelly jars and wide-mouthed jars. Basic types of lids include the self-sealing lid, glass lid and zinc lid. Many older canning jars used glass lids in combination with rubber seals, and many of the modern, self-sealing canning jar lids also employ a rubber ring seal. The self-sealing lid is popular with modern canners due to its superior sealing abilities. Some lids and seals are single-use only, while others can be reused.
Home-canning jar size is identified by the size of the opening at the top of the jar. The canning jar lids are sized to fit the corresponding sized jars and proper sized rings as well. The rings are used to hold the canning jar lids in place while the jars are being used in either a water bath or a pressure canner.
Contrary to typical thinking, the rings do not seal the canning jars, as this is the function of the canning jar lids. The sealing rings are used to hold the canning jar lids just tight enough to keep the canning water outside of the jar. At the same time, the seals keep the liquid inside of the jar from escaping into the canning water as the jars are heated.
As the temperature inside the canning jar rises, the rubber sealing ring beneath the canning jar lid becomes soft. The seal is formed by a vacuum as the jars begin to cool. Once completely cool, the rings can be removed from the jars and the lids will remain tightly sealed. Flat canning lids can be removed with a can opener and disposed of, while the jars and rings can be cleaned and reused many times.
The zinc style, one-piece canning jar lids are placed over a canning jar's mouth with a rubber gasket placed between the jar and the lid. The lid is tightened snugly prior to being placed in the canner. Once the canning process has been completed and the jars are removed from the canner, the one-piece lids must remain in place on the jars. It is important that the lids not be bumped while the jars are being stored since the rubber gaskets are easily displaced. Bumping can lead to unsealed jars and spoiled food. For this reason, this style of jar lid receives a lesser rating from many home-canning experts.