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

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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 m 3) cooled in around 12 hours and the biggest kiln (BK, 8 m 3) in around 40 hours. Thermomagnetic, hysteresis and backfield curves indicated that the main magnetic carriers were Ti-poor titanomagnetites and Tipoortitanohematites. The fraction of the TRM carried by Ti-poor titanohematites is the maindifference between the two sets of bricks. This fraction is around 5-10% in bricks from BK kilnand up to 40% in those from SK kiln. Intensities of the Earth’s magnetic field were determinedusing the original Thellier-Thellier protocol with correction of TRM anisotropy. The averageintensities overestimate the expected field intensity by 5% (SK) and 6% (BK). This resultemphasizes the necessity of the cooling rate correction. In order to have a detailed evaluation ofthe cooling rate effect, we used several slow cooling rates: 0.8, 0.4, 0.2 and 0.1°C/min. Thecorrection factors obtained with the 0.8°C/min cooling ranged between -2% and 21% and wereproportional to the TRM fraction carried by Ti-poor titanohematite. The higher proportion ofthese grains in bricks from SK kiln led to an overestimate of the correction factor and anunderestimate of the intensity by 7%. However, the expected intensity is recovered whentemperature steps higher than 580°C (i.e. in the range of Ti-poor titanohematites unblockingtemperatures) 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 contributesignificantly to the TRM, all tested cooling rates give average intensities close to the expectedvalue. Incorrectly estimating the duration of the archaeological cooling has therefore a lowimpact on the accuracy of the archaeointensity data on these kinds of material.