And the high cost of wasting jam makes local experimenting unwise.
And if you mess this up, food poisoning can blind and/or kill you. Food sitting around at room temperature for 6 months or 2 years is not really a process you want to be experimenting with...
I guess the cans have the "clicky click" to ensure safety. But this is actually life-or-death: you don't want to mess this up at any step of the process.