You cannot use or reuse ordinary glass jam jars for heat processing because they don’t have thick enough glass to withstand the heat. The best jars to buy for waterbathing are Kilner or Le Parfait jars, which come with rubber seals and clips to allow the steam to escape. Alternatively, you can buy Ball or Mason glass jars with two-part lids, which consist of a flat disk held in place by a screw band; these are ideal because the disk part forms a vacuum when it is secured correctly, meaning you can check the seal before you tighten the screw band. Check that rubber seals are not damaged in any way before using. New metal disks for Mason jars should be used each time. All jars should be examined before use to make sure there are no chips or cracks in the glass. Also ensure that the new lids are not damaged in any way.