Canning Jars It is important to not use just any jar for canning. You must use a proper canning jar with the proper 2-piece lid to ensure an air tight seal. Discard any jars that are nicked on the thread or are cracked Lids The 2-piece lid consists of a sealing cap and a screw cap. The screw cap can be reused but always use new sealing caps to ensure an air-tight seal. Boiling the lids is no longer required. Heat to 180*F/82*C only. They must be placed on the jar hot for a proper seal. Ther