Earthen modeling in the depths of the Sierra Mixe of Oaxaca, Mexico: investigating the production process of unfired clay reliefs and sculptures in Mesoamerica’s underworld

• The study highlights the first evidence of a vast repertoire of unfired clay modelling in Mesoamerica’s underworld. • A multi-technical approach was employed to comprehend the material’s composition, structure, and modeling pressure. • Endokarstic clay was used to create unfired clay reliefs and sculptures, reflectling artisans’ knowledge of the material’s properties. • Manganese deposits in situ were identified, exploited and used as black pigment by ancient artisans. • The initial radiocarbon dating of charcoal fragments places the end visit to the cave between 600 and 900 CE. This paper addresses the mineralogical and chemical composition of earth pastes used in the production of selected unfired clay reliefs and sculptures modeled within a cave in today’s Mixe (Ayuuk ja’ay) territory of Oaxaca, Mexico, through a multi-technical approach comprising Optical microscopy (OM), Portable X fluorescence spectrometry (p-XRF), Energy Dispersion Microanalysis by Electron Probe Micro Analyzer (EPMA), UV-fluorescence microscopy, Micro-Raman Spectroscopy, X-ray fluorescence (XRF), X-ray powder diffraction (XRPD), Differential scanning calorimetry (DSC) and thermogravimetry (TG). The analyses demonstrated that no external materials were added to the mix and that manganese deposits were exploited in situ and used as black pigment. In addition, by studying the differences in the natural stratigraphic structure between clay relief samples and those of the terraces on which the clay figures were shaped, we attested that such terraces are natural sediment deposits. The study also includes the first systematic radiocarbon dating and identification of charcoal fragments, thus providing an initial chronological framework for the production of unfired clay figures and the use of this Mesoamerican sacred underworld space.