This research aims to examination the material characteristics changes of the ancient clay depending to firing temperature and to verify the firing temperature of the Joseon potteries from the Yongam site in Ssangyongdong of Cheonan, Korea. To this study, we conducted a firing experiments using the ancient clay that was verified raw material of the Joseon potteries from the study area in the temperature range from 500 to $1,200^{circ}C$. The yellowish tint, the reddish tint and the specific gravity of firing samples increase according to firing temperature rise. But the absorption ratio and the porosity decreased under same firing temperature range. The soft potteries from the Yongam site has similar material characteristics with the fired samples at 800 to $950^{circ}C$. And the hard potteries from the Yongam site bears the same relation to the fired samples at about $1,100^{circ}C$. This result is considerably signification for the interpretation of firing temperature. The geochemical characteristics of the original clay according to the firing experiments, on the other hand, is not affected of temperature in this study.