What is the exact temperature that the kiln needs to be set to for kashering used China? Thank you!

Shalom, Thank you for your question. As I wrote in the original answer, the Shulchan Aruch (the source should read 451,1) rules that one may kosher clay vessels by returning them to the furnace. You ask what level heat that would be. From my research a kiln needs to reach around 1400 C for making porcelain vessels. This then would be the temperature needed for china dishes. However, this law only applies to using a real furnace that is used to make pottery vessels – and not just a very hot oven. Also, there are opinions that state that our practice today is not to use this form of koshering at all (see Hagalat Ke’lim, R’ Cohen, chapter 13, law 90, and footnotes there [Hebrew]). If you are planning to use such furnace, be aware that there is a good chance that the plates will crack and break. Blessings.