AN ARCHAEOMAGNETIC STUDY OF A ROMAN BATH IN SOUTHERN FRANCE* M. L. HAMMOND and M. J. HILL† Geomagnetism Laboratory, School of Environmental Sciences, University of Liverpool, Oliver Lodge Laboratories, Oxford Street, Liverpool, L69 7ZE, UK PH. LANOS Institut de Recherche sur les Archéomatériaux (CNRS UMR 5060 IRAMAT-CRPAA) and Géosciences-Rennes (CNRS UMR 6118), Université Bordeaux-Montaigne & Université Rennes 1. Laboratoire d’archéomagnétisme ’Géosciences-Rennes’, Campus scientifique de Beaulieu, CS 74205 35042, Rennes CEDEX, France and F. COLLEONI Université Rennes 2, LAHM - UMR 6566 CReAAH, Place du recteur Henri Le Moal, CS 24307 35043, Rennes CEDEX, France Absolute Thellier–Thellier archaeointensity determinations were conducted on 137 tiles and sedimentary blocks from five different structures from the ‘Site de la Molère’, southern France, to constrain the archaeological interpretation of a relative age difference between structures and add new data to the French secular variation database. We present 89 new archaeointensity and 20 new directional results, with averages of 57 (±8) μT in the second century AD and 68 (±7) μT in the third century AD. The overall data trend is consistent with the limited available data, indicating rapidly increasing field intensity from the second to the third century AD. KEYWORDS: ARCHAEOMAGNETISM, ARCHAEOINTENSITY, SITE DE LA MOLÈRE, ROMAN PERIOD, TILES, CHRONOLOGY INTRODUCTION The field of archaeomagnetism is based on the principle that burnt archaeological materials such as tiles, bricks and fireplaces record the magnetic field at the time of their last heating. Depending on the magnetic mineralogy of the studied material, heating to as little as 120 °C can produce ma- terial suitable for archaeomagnetic study. If the material is well dated, it is possible to use the geomagnetic field information recorded within the material to contribute to the local secular variation (SV) record. SV records are crucial for the investigation of physical processes occurring deep in the Earth, as well as being used for archaeomagnetic dating. In practice, it is possible to conduct archaeomagnetic dating in France for the last three millennia by using inclination and declination parameters. This is achieved by comparing the magnetic record contained within burnt, undated, archaeological material with the existing, well defined French SV curve (e.g., as demonstrated by Le Goff et al. 2002). Archaeomagnetism is well established in Western Europe and France in particular (e.g., Thellier 1981; Bucur 1994; Chauvin et al. 2000; Genevey and Gallet 2002; Gómez-Paccard et al. 2012; Genevey et al. 2013; Hervé et al. 2013a). *Received 7 November 2014; accepted 5 January 2016 †Corresponding author: email m.hill@liverpool.ac.uk Archaeometry ••, •• (2016) ••–•• doi: 10.1111/arcm.12240 © 2016 University of Oxford