Taken with a Grain of Salt: Experimentation and the Chemistry of Archaeological Ceramics from Xaltocan, Mexico

Neutron activation analysis (NAA) of ceramics from Xaltocan (n=651) displays high values for sodium and potassium and low concentrations of many transition metals and rare earth elements compared to other sites in the Basin of Mexico. Given that Xaltocan was situated on an island in the middle of a saline lake, the potential reasons for this chemical signature are diverse. On one hand, if the sodium and potassium were elevated due to some behavioral aspect of the potters, the Xaltocan chemical groups provide a glimpse at the behaviors of Xaltocan potters that permit more precise source designations. On the other hand, if this chemical fingerprint arose due to contamination in a saline post-depositional environment, the Xaltocan chemical groups would not be valid references for provenance studies. To evaluate these alternative hypotheses, we employ several lines of evidence: (1) comparison of the Xaltocan ceramics to over 5,000 NAA assays of clays and ceramics from the Basin of Mexico, (2) experimental doping of clays with water of different salinities and fired to different temperatures, (3) leaching experiments of archaeological pottery sherds (n=22) recovered from the site of Xaltocan, and (4) laser ablation-inductively coupled plasma-mass spectrometry of the clay and temper fraction of a small sample of Xaltocan ceramics to determine which component is responsible for the elevated sodium and potassium values. The results suggest that the high sodium and potassium values were present in the ceramic paste before firing. We then use these newly established reference groups to better understand the role of Xaltocan in the regional economy. The type of experimentation employed in this study has proven to be an important method for determining the behaviors of ancient potters and distinguishing them from post-depositional processes.