Initially, workers used wooden mallets to tap the lid of each can to see if the can was sealed. A clear sound indicated a proper seal while a dull sound revealed an improper seal. Cans that didn’t pass the test were punctured releasing any pressure from inside. The hole was then quickly brushed clean, bonded with solder, and sent back to be cooked once more. This method of testing dwindled in the 1920s with the introduction of the sanitary can and the vacuum closing machine.