The common self-sealing lid consists of a flat metal lid held in place by a metal screw band during processing. The flat lid is crimped around its bottom edge to form a trough, which is filled with a colored gasket material. When jars are processed, the lid gasket softens and flows slightly to cover the jar-sealing surface, yet allows air to escape from the jar.
It is best to buy only the quantity of lids you will use in a year. Never reuse lids. To ensure a good seal, carefully follow the manufacturer's directions in preparing lids for use. Examine all metal lids carefully. Do not use old, dented or deformed lids or lids with gaps or other defects in the sealing gasket.
Follow the manufacturer's guidelines enclosed with or on the box for tightening the jar lids properly.
If screw bands are too tight, air cannot vent during processing and food will discolor during storage.
Over-tightening also may cause lids to buckle and jars to break, especially with pressure-processed food.
If screw bands are too loose, liquid may escape from jars during processing, seals may fail, and the food will need to be reprocessed.
Do not retighten lids after processing jars. As jars cool, the contents in the jar contract, pulling the self-sealing lid firmly against the jar to form a high vacuum.
Screw bands are not needed on stored jars. They can be removed easily after jars are cooled. When removed, washed, dried, and stored in a dry area, screw bands may be used many times. If left on stored jars, they become difficult to remove, often rust, and may not work properly again.
Tip is from Fagor's home canning cookbook