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Why is it necessary to remove the clamps!!

After processing, the spring action of the clamps is replaced by the natural force exerted on the vacuum inside the jar from the pressure of the air outside the jar. If you were to leave the spring clamps on the jar, you would not, by trying to lift off the lid, be able to test whether the jar was properly processed and sealed or not. 

This simple, but extremely important seal-test ( the "lid-lifting" test) cannot be performed when using jars with a thread type, a wire-bail type of closure or any other mechanical sealing devices.