Physics and Chemistry of the Earth 27 (2002) 1319–1331 www.elsevier.com/locate/pce Origin of magnetic fabric in bricks: its implications in archaeomagnetism J. Hus a,*, S. Ech-Chakrouni b, D. Jordanova c a Centre de Physique du Globe, 5670 Dourbes (Viroinval), Belgium b Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium c Geophysical Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 3, 1113 Sofia, Bulgaria Abstract Comparison with the magnetic anisotropy of unbaked (only dried) and baked loam bricks, hand moulded in a rectangular frame, reveals that the same moulding technique had been applied to produce the bricks of a Medieval brick kiln that was archaeomag- netically dated at 1650 AD [Geoarchaeology (to be published)]. Anisotropy of magnetic susceptibility (AMS) measurements show that the unbaked and baked bricks have a shape-related magnetic fabric, induced during the moulding process, with average Kmax occurring in the greatest faces along the direction of the longest edges and Kmin perpendicular to the greatest faces of the bricks. The anisotropy of thermoremanence (ATRM) is high, indicating that the remanence directions of bricks may accuse large deviations from the geomagnetic field direction responsible for it. However, anisotropy seems unlikely to be the cause for the apparent dis- crepancy between the archaeomagnetical and archaeological date of the brick kiln, the latter presumably about half a century older. Besides AMS, also the anisotropy of anhysteretic remanence was examined as a possible substitute for ATRM and to obtain in- formation on the magnetic state of the minerals contributing to the remanence anisotropy. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Archaeomagnetism; Anisotropy of magnetic susceptibility; Brick; Brick kiln; Magnetic anisotropy; Mediaeval 1. Introduction cessively along six different sample axis. This investiga- tion was not very successful, because of mineralogical Magnetic anisotropy of hand-moulded, unbaked changes induced during thermal treatment (Hus et al., to (only dried) and baked bricks made of loam was ex- be published). Not entirely unexpected, as the kiln was amined for several reasons. The archaeomagnetic ex- made of recuperated, misfired bricks showing great amination of a Medieval brick kiln made of bricks, variance in their magnetic properties. Anisotropy of discovered in the village Steendorp (Belgium) in North magnetic susceptibility (AMS) of 408 samples from 36 Belgium (N 51.14°, E 4.26°), yields a very highly reliable pieces of bricks of the brick kiln, cored perpendicular to average magnetisation direction with an a95 less than the greatest faces and oriented relative to the longest 0.5° (Hus et al., to be published and Table 1). edges, revealed the presence of a magnetic fabric. AMS However, the archaeomagnetic date of 1650 AD ob- is relatively weak, less than 5%, with an average of only tained, using the French and British secular variation 1.5%, but clearly a magnetic fabric is present with (SV) curves as a reference, is about half a century average Kmax occurring in the greatest faces and oriented younger than expected on historical grounds (Bucur, along the greatest edges of the bricks and Kmin perpen- 1994; Tarling and Dobson, 1995). As anisotropy could dicular to the greatest faces (Hus et al., to be published be one of the reasons for this discrepancy in age, the and Table 2). Lineation is weak and AMS is mainly anisotropy of thermoremanent magnetisation (ATRM) determined by foliation and can be represented by a was investigated by heating some brick samples until slightly oblate ellipsoid. 700 °C followed by cooling in a field of 0.05 mT, suc- This suggests that the bricks were moulded in a rectangular frame and that the magnetic fabric was in- duced during the moulding process. Indeed, a common * Corresponding author. brick moulding technique in the Middle-Ages, as can be E-mail address: [email protected] (J. Hus). seen in an etching of the Dutch artist Jan Luiken at the 1474-7065/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S1474-7065(02)00126-2 1320 J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331 Table 1 rectangular, wooden frame with internal dimensions of Average magnetisation direction of characteristic remanent magneti- 26 Â 12 Â 6 cm. The brick material consists of Pleisto- sation, isolated after a.f. treatment, recorded in different materials of cene loess of the loess stratotype in Kesselt, excavated in the Mediaeval brick kiln discovered in Steendorp (Belgium) a yard behind the brick factory, without any additive, N D (°) I (°) k a (°) h (°) av av 95 80 except for a small amount of white sand and sawdust Bricks 21 12.3 72.9 3120 0.56 0.93 to avoid desiccation cracks (Gullentops, 1954; Juvignee, Baked 40 12.3 72.0 1121 0.67 0.85 silt 1996). The moulded bricks were oven-dried at 75 °C Total 61 12.3 72.4 1405 0.48 0.65 during about 40 h. After drying, seven of them were fired in the oven of the brick factory, using gas burners, N––number of oriented samples, Dav––average declination, Iav––aver- age inclination, k––precision factor, a95––semi-angle of cone of confi- with a heating–cooling period of about 3 1/2 days and dence, h80––radius of circle enclosing 80% of directions. maximum temperature of 1075 °C. Each brick was cut into 1-in. cubes, resulting in 502 cubes for the unbaked bricks (two were slightly broken and not used) and 504 for the baked bricks. The dried bricks are yellowish grey end of the 17th century, consisted in pressing a lump of before baking, 5YR 6/4 according to the Munsell colour kneaded clay in a rectangular mould and scraping off chart, and become light pinkish brown after firing (5YR the superfluous material from the exposed surface with 7/2). a stretched wire (Luiken, 1694). In order to demonstrate that the same moulding procedure had been used to produce the bricks of the Steendorp brick kiln, their anisotropy was compared with that of a series of un- 3. Results baked and baked bricks obtained using the moulding technique depicted in the engraving. Indirectly, the an- 3.1. Anisotropy of magnetic susceptibility isotropy may inform us about the technique used by the brickmakers and hence reflect the technological and AMS of all the samples was measured in a Kapp- cultural status reached in brick technology in the 17th abridge KLY-3S and the principal anisotropy parame- century. ters obtained according to Jelinek (1981): average Moreover, our knowledge of the SV of the inclination magnetic susceptibility (MS) Kav, corrected degree of 0 of the geomagnetic field during archaeological periods, anisotropy P , Lineation L, foliation F, shape factor T partly relies on inclination determinations of removed and the principal values and directions of the MS tensor bricks collected in archaeological sites (Thellier, 1981). (Table 3 and Fig. 1). The unbaked (only dried) bricks The experiment shows that ATRM in bricks may be have a magnetic fabric with an average degree of AMS, 0 large and hence responsible for large deflections of P , less than 7%, much higher than the average of 1.5% the remanent magnetisation direction from the ambient found for the bricks of the Mediaeval Steendorp brick geomagnetic field direction. kiln. It should be mentioned here that the source ma- As the ATRM results of the brick kiln are unreliable, terial of the bricks of the Steendorp brick kiln is still we examined the possibility to use the anisotropy of unknown, but probably not loess, which is absent in the anhysteretic remanence (AARM) as a substitute for sandy area of North Belgium. Also, the loess material ATRM. used to produce the bricks in the present experiment was firmly pressed by hand intentionally in order to enhance the anisotropy. Foliation, which attains an average 2. Sample preparation and moulding procedure value of about 4%, about twice as high than the average lineation, determines the anisotropy. The shape of With the collaboration of the brick factory Nelissen the AMS ellipsoid is slightly oblate with Kmax lying in Kesselt (Belgium), 14 bricks of pure loam were ob- in the greatest faces of the bricks and Kmin perpendicular tained by pressing the material firmly by hand in a to the greatest faces (Table 3 and Fig. 1). The average Table 2 Principal AMS parameters according to Jelinek (1981) of 408 samples drilled in 36 pieces of bricks from the Mediaeval brick kiln discovered in Steendorp (Belgium)a À6 0 N Kav (10 ) LFP T Kmax Kint Kmin D (°) I (°) D (°) I (°) D (°) I (°) 408 3.773 1.004 1.011 1.015 0.44 185 0 275 0 50 89 0 N––number of samples, Kav––average magnetic susceptibility, L––lineation, F––foliation, P ––degree of anisotropy, T––shape factor of susceptibility ellipsoid, Kmax––maximum susceptibility, Kint––intermediate susceptibility, Kmin––minimum susceptibility, D––declination, I––inclination. a The samples were drilled perpendicular to the greatest faces and oriented towards the longest edges of the bricks. J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331 1321 Table 3 Average principal AMS parameters per brick for unbaked and baked hand-moulded loam bricks À6 0 N° N Kav 10 SI LFP T Kmax Kint Kmin D (°) I (°) D (°) I (°) D (°) I (°) Unbaked 1 71 249.8 1.022 1.042 1.066 0.32 184 2 274 6 72 83 2 72 252 1.02 1.035 1.056 0.273 167 5 77 3 312 85 3 71 250.2 1.02 1.045 1.067 0.368 178 1 88 4 286 86 4 72 245.9 1.023 1.042 1.067 0.287 13 2 283 0 183 88 5 72 209 1.022 1.042 1.066 0.309 171 6 261 1 358 83 6 72 254.5 1.015 1.051 1.069 0.548 178 9 88 3 338 80 7 72 250.5 1.029 1.033 1.063 0.062 178 1 88 6 282 84 Mean 502 244 1.021 1.041 1.063 0.329 179 3 89 2 331 86 s.d.