Are ceramics and bricks reliable absolute geomagnetic intensity carriers?

• Published in: Physics of the earth and planetary interiors 2011-08, Vol.187 (3), p.310-321
• Main Authors: Morales, Juan, Goguitchaichvili, Avto, Aguilar-Reyes, Bertha, Pineda-Duran, Modesto, Camps, Pierre, Carvallo, Claire, Calvo-Rathert, Manuel
• Format: Article
• Language: English
• Subjects: Archeointensity; Bricks; Earth Sciences; Environmental Sciences; Geophysics; Global Changes; Open kiln; Physics; Pottery; Sciences of the Universe; Western Mexico
• ISSN: 0031-9201; 1872-7395; 0031-9201
• DOI: 10.1016/j.pepi.2011.06.007

► Synthetic archeointensity experiment on an original artisan kiln from Michoacán. ► Study on pottery baked following traditional techniques. ► Significant thermal gradients within a single oven. ► Raw-intensities of pottery/bricks over/under estimate the expected site intensity. ► Observed scatter could arise from the differentiated cooling rate within the oven. A detailed rock-magnetic and archeointensity study was carried out on materials baked by a western Mexican artisan following traditional techniques to produce faithful reproductions of archeological pieces of the Michoacán region (Western Mesoamerica). The field strength at the site (41.0 ± 0.5 μT) was measured with a fluxgate magnetometer and the temperature of the furnace during the baking process was monitored continually by means of a thermocouple placed in the middle of the baking cavity. Rock-magnetic experiments performed on the raw material (clay and paste) and on in situ prepared baked ceramics and bricks included measurement of thermomagnetic curves (susceptibility and strong-field magnetization versus temperature), first-order reversal curves (FORC), anisotropy of magnetic susceptibility (AMS) and anisotropy of thermoremanent magnetization (A-TRM). Magnetite and probably hematite are present in the samples as carriers of the remanence. Hysteresis ratios suggest that the samples fall in the pseudo-single-domain grain size region, which may indicate a mixture of multi-domain and a significant amount of single-domain grains. Ceramic pieces and brick fragments were subjected to the Thellier-Coe archeointensity method and to an alternative paleointensity experiment, with a TRIAXE magnetometer, in order to check whether they are faithful recorders of the local geomagnetic field strength. Mean raw-intensity of sample M1 (pottery) overestimates a 7% the expected site intensity, while those corresponding to the brick samples (LQ1 and LQ2) underestimate it 15%. Brick sample LNQ shows a slightly lower intensity (7%), but agrees with the expected site intensity within the experimental uncertainty. The intensity retrieved from the volcanic fragment also included closely reproduces the expected intensity. After A-TRM and cooling-rate corrections, all mean raw values move closer to the expected intensity. Measurement of temperatures at different parts inside the kiln (bottom and upper parts of both central and peripheral parts) revealed the existence of significant thermal gradients, similar to those