Standard, Metal, Jar lids are self-sealing using a process that is simple but effective. The circular, metal lid is crimped around the edge, and into the groove thus created is placed a colored sealing compound. When ready for use, the lid is placed atop the readied jar of food, gasket side down, and held there by the metal screw band. Once the canning processing has begun, the heat being generated causes the gasket compound to soften. In this conditions it flows slightly, moulding onto the jar-sealing surface of the glass rim while still allowing air to escape from the jar. As the jar cools, however, the gasket is drawn even tighter onto the glass, forming an air tight seal. This is because as jars cool, the contents in them contracts, pulling the self-sealing lids firmly against the jar rims to form a high vacuum.
It is said that gaskets in unused lids can work well for a period of several years after the date of manufacture. The gasket compound in older, unused lids, however, may show their age through an increased tendency to fail to seal on their jars. It is recommended that after five or six years have passed that they should be replaced with newer product.