If one placed the china in a kiln which was brought up to the standard temperature used to produce the china, would that not negate any non-kosher or even any milk-meat essences and basically be as if the china was just produced? If a Jew did this, would it even require immersion in a mikva? Of course, there is a distinct possibilty that the china finish could be negatively impacted, but I can’t say this from any experience, just conjecture.
Shalom, Thank you for your question. There is much Rabbinic discussion about koshering china and clay vessels by returning them to the furnace. The ruling of the Shulchan Aruch (Orech Haim, 551,1) is that one may kosher such vessels by returning them to the furnace they were fired up in (but the heat of a regular oven would not suffice). You are correct that there is a danger that the vessel will crack in the process. Even if a Jew refurnaced the vessel it would not be considered that a Jew had manufactured it, just that they had koshered it, and so it would still need immersion in a mikvah if made by a non-Jew. Blessings.