Further progress in the study of epsilon iron oxide in archaeological baked clays

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The occurrence of ε-Fe_(2)O_(3) in archaeological samples that have been subjected to high temperatures is gradually being detected by the use of micrometric structural characterization techniques. This work provides new information by revealing that the ε-Fe_(2)O_(3) is formed as a response to temperature, the aggregation state and the position within the baked clay with respect to the nearest heat source. In addition, depending mainly on the atmospheric environment, the temperature reached by the combustion structure, the distance from the heating source and the particle aggregation, other iron oxide magnetic phases are produced. In the baked clay studied here, hematite is found over the whole range of samples studied but its magnetic contribution is negligible. Magnetite is observed at the sample surface, probably due to local atmospheric environment closest to the combustion source. Maghemite is found at all depths up to 6 cm below the sample surface. ε-Fe_(2)O_(3) has a limited distribution, found within 2-3 cm of the sample surface. Furthermore, the viability of this compound as a palaeofield marker has been evaluated in both archaeological and synthetic samples. The results indicate that ε-Fe_(2)O_(3) is able to register the direction of the magnetic field. Linear palaeointensity plots have been obtained in synthetic samples, although the value of the palaeofield could be, sometimes, overestimated.
© 2020 Elsevier B.V. All rights reserved. The authors thank the anonymous reviewers for their constructive corrections and suggestions. The authors also acknowledge the financial support from the Spanish Ministry of Science, Innovation and Universities under the projects RTI2018-095856-B-C21, CGL2017-87015-P, CGL2017-92285-EXP, CGL2017-92285-EXP/BTE, MAT2017-86540-C4-1-R, MAT2017-87072-C4-2-P and RTI2018- 095303-A-C52, from Comunidad de Madrid NANOFRONTMAG S2013/MIT-2850 and NANOMAGCOST S2018/NMT-4321, and from the European Commission under H2020 frame by AMPHIBIAN Project ID: 720853. APO thanks the Ministry of Economy and Industry and Competitivity (PTA Contract).