The forest or the trees? Behavioral and methodological considerations for geochemical characterization of heavily-tempered ceramic pastes using NAA and LA-ICP-MS

摘要

In this paper, the utility of LA-ICP-MS for instrumentally separating clay and temper within pottery samples is examined through a well-known case study that has been previously characterized through NAA and petrography. Coarse Orange pottery is composed of kaolinitic clays heavily tempered within volcanic ash from the basalt-rich Tuxtla Mountains of southern Veracruz, Mexico. We attempt to address the following topics with analyses on this pottery: 1) can LA-ICP-MS replicate the same groups previously identified by NAA and petrography: 2) can instrumental isolation of the clay fraction within the pottery replicate known patterns in raw clay source geochemistry in the Tuxtla Mountains: 3) can data for both the clay and volcanic ash fractions of each pottery sherd be combined to simulate the bulk INAA data? The results demonstrate that the clay fraction of the pottery sherds can be isolated. Furthermore, the LA-ICP-MS data for the clay fractions display nearly identical chemistry to available NAA data for raw clay samples collected from geological contexts in the region. Instrumentally isolating the clay fraction in this case eliminates the homogenizing effects of adding a common temper and increases the chemical separation among groups making it easier to identify clay procurement strategies for each specimen. However, focusing only on the clay fraction ignores significant tempering variation. Attempts to simulate the bulk NAA by combining LA-ICP-MS data for clays and temper was reasonably successful in reestablishing the tempering variation lost by exclusive focus on clays, but such an approach could not replace the more precise data generated from NAA and petrography.