So I had a good wood fire going yesterday, and threw in 5 clay balls. They are still warm this morning (it didn't get above freezing). No idea what temp they got to. The balls are very light weight.
We also had success with kiln firing, to cone 016, which is about 1500. Thanks to Lisa Roll Pottery for the kiln time, and the 'after' picture. Https://www.etsy.com/shop/LisaRollPottery
Note the color changes, the gray clay fires up red.