These are traditional earthen pots that are entirely made of Iga's heat-resistant clay.
As Iga's potter's clay contains many carbonized plants, when burnt, it turns into a porous surface.
Therefore, the whole body of the earthen pot tightly retains heat and slowly heats ingredients to their core, drawing out the flavors of foods.
https://www.shokunin.com/en/matsuyama/tsuchibai.html (Tsuchibai Hanten Donabe)
https://www.shokunin.com/en/tojikitonya/koiga.html (Koiga Donabe)
https://www.shokunin.com/en/tojikitonya/iga.html (Katate Cocotte)