first, some info from @peacener (shared with me elsewhere):
peacener wrote: BBS (black bone syndrome) as someone pointed out to me... is good for tea, specifically, aged tea. The kiln change happened in old dragon kilns and also happened to jianyao teabowls also fired in dragon kilns. It would convert the Iron Fe2O3 to iron magnetite Fe3O4. I have shared my teabowl and both of my zhuni pots that exhibit this characteristic.
old zhuni has a high iron content to begin with. so did the clay for the (song dynasty) jianyao teabowls. Once you dive deeper into the mysteries of zhuni and jianyao, you see how similar their firing is and how the clay has similar reduction fired color change (greyish) all simply because the clay for jianyao bowls and zhuni have such a high iron content. And if jian bowl and zhuni are fired perfectly, also exhibit similar color, of course zhuni being more red, and Jian being an off reddish-brown. It is worthy of notation though that they were both fired for a lengthy duration (2 weeks?) at a similar temperature (1200c), too, and both, as I repeat myself, in dragon kilns. dragon kilns are seriously cool. epsilon phase iron oxides and dragon kilns are something to really love!
https://ceramics.org/tag/epsilon-phase-iron-oxide
so. similarly, magnetism is present on both the left objects (clay), whereas magnetism is absent on the right objects (clay):
here's a video also from @peacener of a laozhuni pot (same pot as in the lower left pane of the pic above) exhibiting ferromagnetism:
the ovoid object dragging the lid around is a magnet.
and here's a shot of its interior: