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The preserving jars may have glass or metal lids which rest on a rubber band. Either clips or screw-bands are used to hold the lids tightly in place while the jars are cooling. When the jar is cold, the vacuum formed inside holds the lid in place.
Always examine the jars carefully before use, as any ridges or chips on the mouth of the jar or lid will prevent it from sealing. Wash all bottles thoroughly, rinse in clean hot water and invert them to drain. The rubber bands must fit properly and be soft and flexible. They should be soaked in boiling water just before they are put on the jar.