Calcium X-ray absorption near edge structure (XANES) spectra: A thermometer for the firing temperature of ceramics?

Test samples have been fabricated similar to Poverty Point Objects (PPOs) using silty loam from the UNESCO World Heritage Site Poverty Point. In an oxidative atmosphere these objects were fired in a defined way to 300 degrees C, 500 degrees C, and 800 degrees C respectively. For all samples, including a non-heated one, X-ray absorption near edge structure (XANES) spectra were measured at the Ca-K-edge. Two of the characteristic features of these spectra were used for obtaining for the first time a calibration curve for determining the firing temperature of ceramic objects. Using this calibration curve, the "firing temperature", i.e. the highest temperature these objects have been used at of two Poverty Point Objects found at smaller sites (Terra' Lewis, Caney Mounds) could be estimated at similar to 500 degrees C-a typical temperature for a wood fire. This result supports in a direct way the hypothesis that at least some of the Poverty Point Objects have been used as cooking stones.