Identification of “red ochre” in soil at Kfar HaHoresh Neolithic site, Israel: Magnetic measurements coupled with materials characterization

A lump of red ochre was found alongside various fire-related features in a Terra Rossa soil sequence near Kfar HaHoresh archaeological site. Soil micromorphology combined with rock magnetism together with XRD, XRF, FTIR, and color indices, all show the unique nature of the red ochre lump vs. the embedding soil and allow us to define this ochre lump as baked clay. In contrast to natural soil, the baked clay demonstrates a fourfold increase in magnetic susceptibility, twofold increase in saturation magnetization along with lessened frequency dependent magnetic susceptibility and coercive force. Thermomagnetic measurements confirm that the red compacted material was produced by clay firing at least at 750-800 °C, with no addition of any iron prior to firing. The raw material was likely local Terra Rossa clay which contains abundant Fe(III)-smectite and superparamagnetic Fe oxyhydroxides. Heating has resulted in clay mineral phase transformation, namely a) production of hematite and magnetite predominantly of single-domain size, b) distortion of phyllosilicate crystal structure due to dehydroxylation. Fe supply for newly formed pyrogenic magnetite/hematite was mainly provided by release of Fe ions from clay crystal structures. Complex evolution of redox conditions during clay firing in air first produced magnetite and then hematite at higher temperature. The microfabric indicates that clay was likely levigated prior to firing. Whether additives were used in antiquity remains unclear. Our study contributes to increasing data on the inception of pyrogenic clay technology in southern Levant already in the PPNB, and highlights the need of integrated magnetic methods for efficient reconstruction of ancient baked clay technology.