Cooling rate effect on thermoremanent magnetization in archaeological baked clays: An experimental study on modern bricks

Abstract

The influence of cooling rate on the intensity of thermoremanent magnetization (TRM) and the necessity to correct archaeo/palaeointensities for this effect have long been recognized. However the reliability of the correction is still questioned. We studied 35 bricks baked in two modern kilns (SK and BK) in known experimental conditions and with measurements of the direction and intensity of the geomagnetic field at the site. The smallest kiln (SK, 0.2 m3) cooled in around 12 hr and the biggest kiln (BK, 8 m3) in around 40 hr. Thermomagnetic, hysteresis and backfield curves indicated that the main magnetic carriers were Ti-poor titanomagnetites and Ti-poor titanohematites. The fraction of the TRM carried by Ti-poor titanohematites is the main difference between the two sets of bricks. This fraction is around 5-10 per cent in bricks from BK kiln and up to 40 per cent in those from SK kiln. Intensities of the Earth's magnetic field were determined using the original Thellier-Thellier protocol with correction of TRM anisotropy. The average intensities overestimate the expected field intensity by 5 per cent (SK) and 6 per cent (BK). This result emphasizes the necessity of the cooling rate correction. In order to have a detailed evaluation of the cooling rate effect, we used several slow cooling rates: 0.8, 0.4, 0.2 and 0.1°C min-1. The correction factors obtained with the 0.8°C min -1 cooling ranged between -2 and 21 per cent and were proportional to the TRM fraction carried by Ti-poor titanohematite. The higher proportion of these grains in bricks from SK kiln led to an overestimate of the correction factor and an underestimate of the intensity by 7 per cent. However, the expected intensity is recovered when temperature steps higher than 580 °C (i.e. in the range of Ti-poor titanohematites unblocking temperatures) were excluded from the calculation of archaeointensity and cooling rate correction. In the case of the BK kiln bricks, for which Ti-poor titanohematites does not contribute significantly to the TRM, all tested cooling rates give average intensities close to the expected value. Incorrectly estimating the duration of the archaeological cooling has therefore a low impact on the accuracy of the archaeointensity data on these kinds of material.