Why do Ball mason jars have those 2-piece lids instead of a regular lid?
- 7 years agoFavorite Answer
Because the "inner" lid is the part that has the seal on it for true canning purposes. When canning, the pressure from the condensed liquid and suction from the canning process causes the seal to expand, therefore completely sealing the jar with pressure. The top ring does little to hold it in place, until the seal is broken, that's when you hear the "pop" when you open something. The tight compression of the inner lid is what keeps the freshness, the ring is mostly used to pressure the seal if you don't use it for actual "canning", with a suction machine.Source(s): Some canning experience :)
- Chris HLv 77 years ago
It saves someone money. Once you use the inner disk in a boiled process, the seal is no good. But you still have the outer ring and only need to replace the inner disk.