Quality control of archaeomagnetic determination using a modern kiln with a complex NRM

A modern (1959) brick kiln from western Spain has been studied in order to conduct a quality control test of the archaeomagnetic method in a partially heated structure. The kiln exhibits two stable natural remanent magnetisation components: a low-temperature component (150–620 °C) acquired during kiln use and a randomly oriented, high temperature component (500–680 °C) acquired during the original firing of the bricks. A detailed rock magnetic study revealed a magnetic mineralogy dominated by non-stoichiometric magnetite and hematite, both of which contribute to the characteristic remanent magnetisation. Both the direction and intensity of the characteristic magnetisation have been compared with the known values for the geomagnetic field at the site location. Irrespective of the method used to determine the mean archaeomagnetic direction (principal component analysis, great circle analysis, with or without hierarchy) the results are statistically indistinguishable from each other and from the known field direction. In all cases the direction is within 5° of the expected value. Thellier-style archaeointensity determinations have been carried out on a smaller specimen set, with the mean result consistent with the known field value. The results demonstrate the reliability of the archaeomagnetic method in determining the features of the geomagnetic field in the past, even in cases with a complicated magnetic history. Furthermore, they highlight the importance of a uniform sample distribution in order to obtain truly representative values for the direction and intensity of the ancient geomagnetic field.