Bit of a necrobump but my understanding (and personal experience) is that the water layer is to allow the CO2 produced through fermentation to escape, while keeping molds and other pathogens out. Using a sealed jar of any kind would result in a messy, stinky explosionWe age it! Lol!
I was think about he jars themselves though.
Note the deep water groove, I read something about the water allowing certain bacteria through, but not oxygen blah blah blah...
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"Fermentation", don't know how wide that definition actually is.
Sorce
Fermentation is pretty cool, honestly .. turning cucumbers into pickles or cabbage into sauerkraut with nothing but salt and water is very satisfying. Highly recommended for anyone who's been considering it!