Archaeomagnetism of four pottery kilns in central Portugal: Implications for secular variation and dating

We report archaeomagnetic results from four pottery kilns in Portugal which are thought to belong to the period of Roman rule (3rd Century BCE-4th Century CE). Very few details have been published to date, so this broad assignment is based on the general archaeological context at each site. Our motivation was to see if a more precise chronology could be established by means of archaeomagnetic dating. Concomitant goals were to compare these results from Portugal to their counterparts in Spain and to expand geographic coverage of the regional geomagnetic secular variation reference curve. Experimentally, all the samples behaved in a very coherent manner during progressive alternating-field demagnetization and yielded high-precision mean archaeomagnetic directions (a95<3°) for each site. The results suggest that two of the kilns, Castelo de Vide and Peniche, were most likely in use during the late 1st/early 2nd centuries CE, whereas the two kilns at Seixal are somewhat younger, dating to the late 2nd to early 4th centuries CE.