Assessing the Reliability of the Pseudo-Thellier Estimates in Fired Ceramics and Source Clays

The geomagnetic field's intensity is key to understanding the Earth's core dynamics and their surface impacts. While the Thellier–Thellier method remains the standard for recovering absolute paleointensity from baked materials, it is time-consuming and may alter magnetic mineralogy due to heating cycles. The pseudo-Thellier method offers a non-thermal, faster alternative based on coercivity spectra, but its application to thermally magnetized ceramics remains unexplored. It is important to highlight that using the pseudo-Thellier method to retrieve absolute intensity requires calibration of relative data. Here, we assess the reliability of pseudo-Thellier method using ceramics and decanted clay from modern production sites in Turmalina (MG) and Cunha (SP), southeastern Brazil. Relative paleointensity estimates were compared with known intensity field values and analyzed in relation to magnetic mineralogical properties. Five samples passed all selection criteria and yielded a mean calibration factor of 2.20 ± 0.59, consistent with values obtained for volcanic rocks. Our results show: (a) Relative paleointensity reliability depends on combined statistical criteria (i.e., NRM fraction, MAD, linearity and curvature of adjustments, and pseudo-checks); (b) pseudo-Arai analysis alone (visual or statistical) are insufficient for a robust paleointensity estimation; (c) high-coercivity components may result to unreliable intensities, therefore an S-ratio > 0.90 is suggested to pre-select samples; (d) remanence anisotropy significantly affects pseudo-Thellier estimates and must be corrected using ARM anisotropy tensor, with a recommended maximum anisotropy tolerance of 25% (0.75 < f_ani < 1.25). We propose here that the pseudo-Thellier method must be systematically incorporated into archeointensity studies, even though as a complementary approach, to improve data reliability and strengthen methodological frameworks.