I'm guessing the magic jar is made by Flamme. Flamme flashback, the region is old, and Flamme statue nearby. Though, a jar would probably break over the many centuries.
God, finishing this manga gave my body some huge tingles. I was curious if it was a documented phenomenon and it is! It's called Frisson, aesthetic chills, and psychogenic shivers. This manga is honestly one of the best manga I've ever read!