Origin of Magnetic Fabric in Bricks: Its Implications in Archaeomagnetism J

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 archaeomagnetically 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 discrepancy 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 information on the magnetic state of the minerals contributing to the remanence anisotropy. 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 investigation 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 induced 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

Table 1 rectangular, wooden frame with internal dimensions of Average magnetisation direction of characteristic remanent magneti- 26 12 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––average 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. 16 0.004 0.006 0.004 0.143 Baked 1 72 1363 1.008 1.017 1.026 0.373 1 2 271 4 123 86 2 72 1236 1.008 1.027 1.037 0.561 177 1 87 1 303 89 3 72 1288 1.007 1.024 1.032 0.566 1 3 91 5 239 84 4 72 1367 1.01 1.024 1.036 0.409 9 2 278 17 104 72 5 72 1283 1.011 1.032 1.045 0.494 171 4 81 8 289 81 6 72 1208 1.012 1.026 1.039 0.35 171 1 81 7 272 83 7 72 1336 1.016 1.021 1.037 0.141 180 4 270 3 30 85 Mean 504 1297 1.01 1.024 1.034 0.309 179 0 89 1 300 89 s.d. 61 0.003 0.005 0.006 0.148

0 N––brick number, 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.

MS of the baked bricks is about five times higher AMS ellipsoid of the unbaked and baked bricks are very compared to the unbaked ones, suggesting the forma- similar to those of the bricks of the Steendorp brick kiln, tion of new magnetic phases during firing. However, the a strong argument that the same moulding technique degree of anisotropy P 0 of AMS decreased during firing was used to produce the latter (Hus et al., to be pub- to about half the value before baking. AMS of the baked lished and Table 2). The presence of very thin, shallow bricks is also determined by foliation, much greater than grooves on one of the greatest faces of the bricks of the lineation. Remarkable is that the shape and orientation Steendorp brick kiln, as witnesses of the wire cutting of the AMS ellipsoid remains nearly unchanged after action, confirms this. Other moulding techniques would baking. Notice however that the scatter of the principal have resulted in different fabrics, such as a more prolate directions is slightly higher in the baked bricks (Fig. 1). fabric due to dominant lengthening in case of linearly The AMS ellipsoid of the baked bricks is also slightly extrusion (Hrouda et al., 2000). oblate with Kmax lying in the greatest faces and Kmin perpendicular to the greatest faces. In case of formation 3.2. Thermal investigations of new magnetic phases during baking, this suggests that either their contribution to the anisotropy is negligible In order to explain the increase in MS but decrease in or that they mimic the original anisotropy. In either P 0 in the fired bricks, MS was examined in function of case, the magnetic fabric revealed in the bricks suggests temperature on samples from the unbaked and baked that it had been induced during the moulding process. materials. The change of Ktot during heating until 700 C Indeed, lineation is higher but Kmin no longer perpen- and cooling at medium rate in air was followed in the dicular to the greatest faces (but distributed in a girdle CS-3 oven of a Kappabrige KLY-3S. During the first perpendicular to Kmax) for samples containing material heating–cooling cycle of the unbaked material, Ktot re- that was in contact with the frame and foliation higher mains nearly constant during heating until about 270 in samples from the interior of the bricks (Fig. 2). This C. At 270 C, Ktot suddenly decreases, corresponding holds as well as for the unbaked as for the baked bricks. with the dehydroxylation of iron-oxyhydroxides and Hence, AMS clearly reflects the strain induced during continues to decrease until about 400 C (Fig. 3a). Both the moulding process, which consisted in flattening a heating and cooling curves point to the presence of lump of kneaded clay by hand, pressing it against the magnetite as is indicated by the Curie temperature close sides of a rectangular frame and shearing it by cutting to 585 C, corresponding with pure magnetite. After the surplus with a wire. The shape and direction of the the first heating and cooling cycle Ktot changes nearly 1322 J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331

the baked material are highly reversible (Fig. 3b). The small increase in MS between room-temperature and 200 C is probably due to a slight lowering of Ktot because of adsorption of humidity once fired bricks are exposed to the air and loss of this physically adsorbed or intercalated water at 100–200 C during the first heating cycle. Besides a Curie temperature close to the one of pure magnetite, there are clearly other stable ferrimagnetic phases present, responsible for the decrease in MS between 150 and 320 C, with a clear break in slope at 325 C. There is a second slope break at 445 C, followed by a decrease between 440 and 500 C. As room-temperature MS of the unbaked bricks does not change very much after heating at 700 C and even decreases slightly, it is clear that the ferrimagnetic phases revealed in the baked bricks must have been formed during the intense firing reaching a maximum temperature of 1075 C. Stepwise thermal demagnetisation of a TRM induced in a field of 0.05 mT in baked brick samples, indicates that the new phases are able to carry a stable remanence (Fig. 4). The change in AMS of three unbaked bulk samples, with temperature, was followed by heating them in air successively at 150, 300, 450, 600 and 700 C with heating and cooling times of about 30 min each. MS measured at room-temperature (MS(To)) increases dur- Fig. 1. Stereoplot of average principal directions of AMS tensor of ing the first heating steps with a maximum change as unbaked and baked hand-moulded loam bricks per brick (Kmax ¼ j, high as 30% after step 450 C (Fig. 5). After heating at K ¼ m, K ¼ d) . The average direction of K coincides with the int min max higher temperatures, MS(To) decreases, reaching final longest edges of the bricks, indicated by the solid arrows, and the aver- values that are only slightly different or less than the age direction of Kmin with the direction perpendicular to the greatest faces of the bricks. initial ones. The total change in degree of anisotropy is less than 3%, accompanied by only minor changes in the principal susceptibility directions and shape of the AMS ellipsoid. Afterwards the samples were heated during 5 h reversible with temperature, but with the cooling curve at 700 C. This resulted in a decrease of both MS and P 0 slightly below the heating curve. Except for some dif- with about 20% and 5% respectively. Consequently, the ference below 200 C, the heating and cooling curves of large increase in MS for the bricks fired at 1075 C must

Fig. 2. Stereoplot of principal directions of AMS of samples of unbaked and baked hand-moulded loam bricks (Kmax ¼ j; Ã; Kmin ¼ d, s). Closed symbols correspond to samples taken in the interior of the bricks, open symbols with material that was in contact with the frame. The Kmin directions cluster along the perpendicular to the greatest faces of the bricks in the former but distribute in a girdle perpendicular to the greatest edges in the latter. J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331 1323

Fig. 3. (a) Variation of low-field MS Ktot versus temperature T for powdered samples of unbaked loam bricks, during first and second heating– cooling cycle in air. Full dots and circles correspond respectively with heating and cooling cycles. The Curie temperature near 585 C indicates magnetite as the main magnetic phase. (b) Variation of low-field MS Ktot versus temperature T for powdered samples of loam bricks fired at 1075 C, during first and second heating–cooling cycle in air. Full dots and circles correspond respectively with heating and cooling cycles. The inflections at about 325 and 445 C point to the formation of new magnetic phases during firing at 1075 C. be attributed to new magnetic phases, probably due to along six different sample directions. The degree of breakdown of the Fe-bearing silicates at very high ATRM is between 17% and 48%, with an average of temperatures. The decrease in P 0 already occurs at lower 38%, much higher than the degree of AMS (Table 4). temperatures and is probably related to the trans- Although the number of samples examined is relatively formation of iron-oxyhydroxides, oxidation of the low, the orientations of the TRM and MS anisotropy original magnetites and breakdown of Fe-bearing phyl- ellipsoids are not very different (Tables 2 and 4). As for losilicates. MS, the maximum TRM axis lies in the greatest faces and is oriented along the greatest edges of the bricks and 3.3. Anisotropy of thermoremanent magnetisation the minimum direction is perpendicular to the greatest faces (Fig. 9). There is a strong linear correlation be- 0 0 2 The ATRM of seven baked brick samples was mea- tween PTRM and PMS with R ¼ 0:91 and between the sured by heating them in a zero field in air, from room- normalised principal susceptibilities and normalised temperature until 700 C, followed by cooling in a principal thermoremanences with R2 ¼ 0:97. The slope constant field of 0.05 mT that was successively applied of the best fitting straight line through the latter is 1324 J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331

value of 0.2 expected for multidomain (MD) and pseu- do-single domain (PSD) grains than to the value 0.5 for SD grains (Stephenson et al., 1986).

3.4. Anisotropy of anhysteretic remanent magnetisation

In order to obtain more information on the status of the magnetic minerals, responsible for the magnetic fabric of the bricks, also the anisotropy of partial and total anhysteretic remanence (APARM and AARM) was examined on unbaked and baked brick samples. For the baked bricks this allows also to test whether AARM can be used as a substitute for ATRM. The AARM tensor is a symmetric second-order 3 3 tensor when anhysteretic remanent magnetisation (ARM) changes linearly with the direct field. Linearity was examined by imparting an ARM to an unbaked and baked sample in an a.f. of 100 mT and a stepwise increasing steady field from 0.01 mT until 0.1 mT, with 0.01 mT Fig. 4. Stepwise, thermal demagnetisation of TRM imparted to baked increments, applied parallel to the a.f. axis. The linear loam bricks in a steady field of 0.05 mT. The distribution of blocking correlation between ARM and biasing steady fields until temperatures indicates the presence of several ferrimagnetic phases. 0.1 mT is high and better than 99% (Fig. 7). From the change of ARM acquired in a steady field of 0.05 mT, in function of the maximum a.f. value applied, follows that saturation is not reached in the highest peak value of 100 mT applied, neither for the unbaked bricks nor for the baked ones (Fig. 8). This indicates that both series of bricks contain a soft and hard fraction. Anisotropy of the soft fraction, or partial anhysteretic remanence PARM, was examined by applying first of all a constant field of 0.05 mT along one of the sample axis and a decreasing a.f. starting from a maximum value of 30 mT (PARM (d:c: ¼ 0:05 mT, a:f: ¼ 30 mT)). The procedure was repeated along six different sample directions, corresponding with the three orthogonal cube edges. Before each step, the sample was demagnetised, successively along three perpendicular sample axis at 100 mT, ending with the direction along which the PARM was given. The decay rate of the a.f. was kept constant at 0.25 nT per half cycle. Afterwards, AARM was obtained after applying the maximum alternating field of 100 mT at- tainable, called total anhysteretic remanence hereafter, Fig. 5. Change in room temperature, low-field magnetic susceptibility following the same procedure. The total anhysteretic (MS(To)) in unbaked loam bricks after stepwise heating at increasing remanence TARM (d:c: ¼ 0:05 mT, a:f: ¼ 100 mT) was temperatures in air until 700 C. each time demagnetised in an a.f. of 30 mT and the remaining PARM acquired in the a.f. interval between 30 and 100 mT measured before imparting a new TARM positive and equal to 0.16, much different from 1, and in another sample direction. Table 4 shows that the with an intercept equal to 0.28, different from zero (Fig. AARM is mainly determined by the soft fraction in both 6). This signifies that the TRM ellipsoid is more aniso- the unbaked and baked bricks. The shape of the an- tropic than the susceptibility ellipsoid and that both el- isotropy ellipsoid of TARM and of the soft fraction is lipsoids are of different shape. Consequently, although slightly oblate, but more oblate in the hard fraction. the principal directions coincide, the condition of pro- The effect of the hard fraction is therefore to lower the 0 portionality between the anisotropy ratios is not satis- overall degree of anisotropy. The relation between PAMS 0 fied and AMS cannot be used as a substitute for ATRM and PAARM is poor with a linear correlation coefficient in this case. The value of the intercept is closer to the R2 ¼ 0:13 for the baked bricks. J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331 1325

Table 4 Average principal anisotropy parameters of ARM in unbaked and baked brick samples and TRM in baked brick samples

0 Steady ﬁeld Alternating ﬁeld N RM LFP T Kmax Kint Kmin (mT) (mT) D () I () D () I () D () I () Magnetic anisotropy of anhysteretic remanent magnetisation AARM Unbaked 0.05 00–30 7 1.946eÀ4 1.038 1.095 1.142 0.418 15 2 105 6 267 83 s.d. 0.018 0.017 0.009 0.277 0.05 30–100 7 2.580eÀ4 1.029 1.052 1.086 0.203 12 3 282 2 158 87 s.d. 0.017 0.032 0.034 0.42 0.05 00–100 7 4.454eÀ4 1.031 1.076 1.114 0.438 12 0 102 3 281 87 s.d. 0.019 0.016 0.018 0.312

Baked 0.05 00–30 7 5.092eÀ3 1.081 1.255 1.374 0.507 185 2 275 4 62 86 s.d. 0.043 0.035 0.043 0.231 0.05 30–100 7 3.362eÀ3 1.054 1.092 1.160 0.327 124 0 214 12 32 78 s.d. 0.059 0.04 0.064 0.617 0.05 00–100 7 8.388eÀ3 1.067 1.186 1.277 0.456 184 3 274 3 53 86 0.028 0.032 0.033 0.213

Magnetic anisotropy of thermoremanent magnetisation ATRM Baked 0.05 7 1.965eÀ02 1.080 1.262 1.383 0.504 7 3 277 4 134 85 s.d. 0.04 0.081 0.098 0.19 N––number of samples, for other deﬁnitions see Table 3.

PARMii/PARMt, for the partial anhysteretic remanences obtained in a maximum a.f. of 30 mT, plot on a straight line with slope 0.99 and passing through the origin (Fig. 10). Hence, the shape of the TRM and the PARM (d:c: ¼ 0:05; a:f: ¼ 30 mT) ellipsoids may be considered as identical. This means that this PARM can be used as a substitute for TRM to deduce the direction of the field responsible for the TRM. If the PARM ratios of the hard fraction are used instead, a high scatter is obtained. Consequently the shape of the TRM and TARM ellipsoids are not completely identical and the slope of the best fitting straight line through the normalised TRMii/TRMt ratios against TARMii/ TARMt is different from 1 and no longer passes through the origin (Fig. 10). Stephenson et al. (1986) recom- mended to use low-field isothermal remanent magneti- sations as a substitute for TRM as longitudinal ARMÕs, with the steady field applied along the alternating field axis, may lead to the production of a gyromagnetic re- Fig. 6. Comparison of normalised principal axes of the susceptibility manence (GRM) in addition to the ARM (see also, ellipsoid and TRM (0.05 mT) ellipsoid of seven baked brick samples. Stephenson, 1993). In the present case there was no The best fitting straight line has a positive slope and intercept of 0.28. evidence of a disturbing GRM. ARM was preferred This signifies that both ellipsoids are of different shape and that the here, as its properties are closer to a TRM compared to TRM ellipsoid is more anisotropic than the susceptibility ellipsoid. an IRM and because of the good linearity with field.

3.5. Comparison of ATRM and AARM of baked bricks 4. Discussion There is a good coincidence between the directions of the ATRM and AARM ellipsoids within a few degrees 4.1. Origin of anisotropy (Table 4 and Fig. 9). For four samples both ATRM and AARM were measured on the same sample and hence The magnetic fabric of the unbaked loam bricks is the remanence anisotropies can be compared. The nor- mainly determined by magnetite and Fe-bearing phyl- malised ratios TRMii/TRMt (with i ¼ 1; 2; 3) against losilicates. Previous magnetic investigations and KðT Þ 1326 J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331

Fig. 8. Acquisition of ARM in function of alternating field in a biasing steady field of 0.05 mT, and alternating field demagnetisation of ARM (d:c: ¼ 0:05, a:f: ¼ 100 mT) in an unbaked (open symbols) and baked (full dots) hand-moulded loam brick sample. The sigmoid shape of the acquisition and demagnetisation curves and abscissa of the intersection, or R-ratio according to Cisowski (1981), near 0.5, point to non- interacting SD grains.

Fig. 7. Acquisition of longitudinal ARM in function of the biasing steady field superposed on an alternating field of 100 mT, in an unbaked (a) and baked (b) brick sample. curves in Fig. 3 indicate that the most important ferrimagnetic mineral is magnetite but that also oxidised magnetites and goethite are present (Hus and Geeraerts, 1986). The clay mineralogy of the Pleistocene loess deposits in Belgium, examined by Thorez et al., in 1970, consists mainly of montmorillonite, mixed-layer clays and illite. Upon heating at high temperatures the clay minerals contained in the loess, including the clay-sized Fe- Fig. 9. Stereoplot of principal directions of TRM, ARM and PARM bearing silicates (mainly phyllosilicates), iron-oxides and of samples of baked hand-moulded loam bricks, (Kmax ¼ j, Kint ¼ m, iron-oxyhydroxides, will undergo transformations in Kmin ¼ d). The orientations of the TRM and AARM anisotropy el- chemical composition and/or crystal structure depend- lipsoids are not significantly different. ing on the firing conditions: temperature and envi- ronment (i.e. reducing or oxidising). As most of the as dehydration, oxidation or reduction, dehydroxyla- phyllosillicates and the iron-oxyhydroxides contain the tion, the formation of new phases and even vitrification hydroxyl group OH and some also adsorbed water, at very high temperatures. The temperatures at which different chemical and structural changes will occur such these changes occur depend on many factors such as the J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331 1327

hydroxylation of the silicates and finally a gradual decrease of MS at still higher temperatures, due to oxidation of magnetite and further transformation of goethite towards hematite. Heating during 5 h at 700 C in air results in a 19% decrease in MS accompanied with a 5% decrease in P 0 but no significant changes in the principal susceptibility directions. Even in the bricks fired at 1075 C no significant changes in the orientation nor in the shape of the MS ellipsoid occur, although there is a fivefold increase in MS and a 50% decrease in P 0 (Table 3). Both lineation and foliation are halved. The increase in MS can be explained by the formation of strong magnetic minerals (Fig. 3b), resulting from the structural breakdown of the Fe-bearing silicates at high temperatures. The new phases are likely Si–Al spinel phases but their exact nature could not be determined yet. It cannot be excluded, but it is hardly to conceive that the new ferrimagnetic minerals mimic the shape of the Fig. 10. Comparison of normalised principal axes of the ARM ellipsoid and TRM ellipsoid for four baked brick samples. The best fitting platy phyllosilicates, which would have resulted most 0 straight line of the normalised principal axes of the PARM (d:c: ¼ probably in an increase of P and foliation. It rather 0:05, a:f: ¼ 30 mT) and TRM ellipsoids passes through the origin and seems that the contribution of the newly formed ferri- has a slope near to one; hence PARM can be used as a substitute for magnetic phases to the anisotropy of the fired bricks TRM. is low and that the latter is mainly determined by the phyllosilicates and magnetites which, although trans- formed and oxidised, partly survived the intense heating chemical and mineralogical composition of the clay, the (Fig. 3). Hence, breakdown of the silicates, oxidation of temperature and heating time, pressure and the atmo- the magnetites and change in grain size and shape due to sphere under which heating is carried out. Phyllosilicates sintering may explain the strong decrease in anisotropy. such as illites will loose their physically adsorbed or intercalated water at about 100–200 C. In an oxidising atmosphere divalent iron will oxidise. At higher inter- 4.2. Magnetic state of minerals mediate temperatures, at about 400–550 C, dehydroxylation or the loss of structural hydroxyl will occur The sigmoid form of the a.f. demagnetisation curves and at around 900–1000 C structural breakdown sets in of ARM (d:c: ¼ 0:05; a:f: ¼ 100 mT) in the unbaked with the formation of new phases like Si–Al spinel and baked brick samples and of IRM (100 mT) in the phases (Murad and Wagner, 1998). baked ones, or integrated microcoercivity spectra, point Heating of samples to moderate temperatures below to the presence of SD grains (Figs. 8 and 11). On the 350 C has been proposed in the past to enhance the other hand the IRM (100 mT) demagnetisation curves magnetic fabric in sediments (Tarling and Hrouda, of the unbaked samples have an exponential form, 1993). Commonly, heating results in the formation of characteristic for MD grains, determined by barriers new magnetic minerals and/or transformation of weakly opposing wall motion (Fig. 11). As domain walls are less magnetic paramagnetic minerals into minerals with a strongly pinned compared to SD moments, the MD higher MS. One assumes that the fabric enhancement is spectrum is softer than the SD spectrum. From Fig. 12 related to the new ferromagnetic minerals which mimic follows that the ARM and SIRM coercivity spectra and the crystallographic structure and shape of the phyllos- demagnetisation curves of the baked samples are ap- ilicates (Urrutia-Fucugauchi and Tarling, 1983; Xu proximately the same, indicating that these remanences et al., 1991). In the present study, in unbaked bricks are carried by nearly the whole grain ensemble. Ac- a slight increase in room-temperature MS occurs after cording to the modified Lowrie-Fuller test (1971) by heating between room-temperature and 300 C, which Johnson et al. (1975), which compares the stability of can be attributed to stress release in oxidised magnetites a.f. demagnetisation of SIRM and ARM in case of (Van Velzen and Dekkers, 1999), the loss of adsorbed magnetite, the grains would be predominantly in the SD water, the remanence unblocking in goethite and trans- grain size range when ARM is more stable than SIRM, formation of goethite (Fig. 5). Heating above 300 C which is the case here (Fig. 12). According to Cisowski results in a much higher increase in MS, reaching a (1981) this is only valid for strongly interacting SD maximum at about 450 C, corresponding with the de- grains but not for non-interacting or weakly interacting 1328 J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331

much lower than the value 0.5 corresponding with non- interacting SD particles (Fig. 11). Indeed, the abscissa of the point of intersection is an approximation of the remanence coercive force field Hcr, since the IRM acquired in a steady field corresponding to this field value equals the undemagnetised SIRM at the alternating field corresponding to the same field value. According to Wo- hlfarth (1958) the ratio of the SIRM, demagnetised in a field corresponding to the remanent coercive force value, to the undemagnetised saturation remanence equals 0.5 or SIRMðHcrÞ/SIRMð0Þ¼0:5. While there is no evidence for negative interactions in the ARM (Fig. 8), the low R-value for the IRM is an indication of negative interactions, making it more difficult to impart an IRM than to demagnetise it. The remanence coercive force for the unbaked samples is about 50 mT, much higher than Fig. 11. Acquisition of isothermal remanent magnetisation and alter- the value of 20 mT for the baked ones. In well-annealed nating field demagnetisation of IRM acquired in a steady field of 0.3 T magnetites, as is probably the case here for the baked in an unbaked (open symbols) and baked (full dots) hand-moulded bricks, one may expect to observe SD-behaviour for loam brick sample. The abscissa of the intersection of the acquisition grain sizes beyond the theoretical SSD-MD threshold and demagnetisation curves, much lower than 0.5, suggests the pres- (see also Heider et al. (1992) for hydrothermal magne- ence of interacting SD grains. tites). Theoretically, the anhysteretic susceptibility Xarm ¼ dðARMÞ=dðHoÞ for non-interacting SD grains is infi- nite, meaning that even an infinitesimal small biasing field is sufficient to block all the magnetic moments parallel to Ho. Grain interaction fields in SD grains and internal demagnetising fields in MD grains are responsible for a distribution of biasing fields in our samples, rendering Xarm finite (Veitch et al., 1984 and Fig. 7). For the baked bricks ARM is less intense, by a factor 2.2, than TRM acquired in the same steady field, pointing to SSD grains close to the SSD-MD threshold or to MD particles in case of magnetite (Dunlop and OOzdemir,€ 1997). From the ARM and IRM acquisition and demagnetisation curves one may conclude that the remanence carriers of the baked and unbaked brick samples are mainly SD, but that also MD grains are present in the latter. The high Hcr value of the unbaked loam is probably related to the presence of goethite and oxidised magnetites.

Fig. 12. Lowrie–Fuller test, or comparison of ARM and IRM, in an 4.3. AMS and AARM as substitutes for ATRM unbaked (open symbols) and baked (full dots) hand-moulded loam brick sample. The higher stability of ARM compared to IRM points to the presence of stable SD particles. As anisotropy may cause serious deflections between the remanence vector and the ancient geomagnetic field direction, the anisotropy of TRM in baked clay was SD grains. As the magnetite content of the loess samples studied by several authors (Rogers et al., 1979; Veitch is low, on basis of the low-field MS, magnetostatic in- et al., 1984; Lanos, 1987; Yang et al., 1993). Unconvinc- teractions are expected to be negligible except if clusters ing results in several cases, because of experimental dif- of ferrimagnetic grains are present. The ordinate of the ficulties or thermally induced changes in the samples, point of intersection of the IRM acquisition and de- lead to the proposal to use AMS and the anisotropy of magnetisation curves of the unbaked and baked sam- room temperature imparted remanences as substitutes ples, called the R-ratio by Cisowski (1981), is about 0.36, of ATRM (Stephenson et al., 1986). J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331 1329

The characteristics of the TRM tensor can be derived demonstrating that the same moulding technique was from the AMS (and/or AARM) tensors when the fol- used. The experiment shows that the orientations of the lowing conditions are met AMS, TRM and ARM anisotropy ellipsoids are not significantly different but that the shape changes. The • The directions of the principal vectors must coincide. ATRM in bricks from the brick kiln which showed the • There must be a constant proportionality ratio be- least changes during heating is also high and compara- tween anisotropy ratios of MS and anisotropy ratios ble with that of the loam bricks (Hus et al., to be pub- of TRM (principle of proportionality of the eigen- lished). However, the archaeomagnetic results from values) or otherwise stated, the anisotropy ellipsoids bricks, summarised in Table 1, were obtained on hori- must have an identical shape. zontally laid bricks taken in the remaining wall of the brick kiln that was oriented more or less in the E–W These conditions are met for the baked bricks for a direction. Shallowing of the inclination due to aniso- PARM acquired in a maximum a.f. of 30 mT but not for tropy would result in an even greater age discrepancy MS. From the MD theory of TRM (Schmidt and Clark, when compared with the reference SV curves of Meriden 1985; Stacey and Banerjee, 1974) Cogne (1987) could and Paris (Bucur, 1994; Tarling and Dobson, 1995). derive the order of magnitude of the eigenvalue ratios Besides, no significant difference in the stable remanence of the TRM tensor from weak-field AMS measurements direction is found between the bricks and the silt baked for MD particles. The theory was generalised for an- ‘‘in situ’’ around the kiln (Table 1). It would be unlikely isotropic MD and SD particles by Stephenson et al. that the horizontally stratified natural silt accuses the (1986). They found a linear relation between the nor- same deflection. The archaeological date of the brick malised principal susceptibilities pi and the normalised kiln relies on the well-constrained construction period, remanences qi: between 1579 and 1597 AD, of a manor excavated in the immediate vicinity (less than 100 m). As bricks with pi ¼ po þ sqi ði ¼ 1; 2; 3Þ: the same dimensions were found in its foundations, ar- The theory predicts that in MD and SSD grains, TRM chaeologists are convinced that the brick kiln had been anisotropy is greater than anisotropy of susceptibility. operated to produce bricks for the construction of the For MD particles the slope s is positive, less than 1 and manor. The archaeomagnetic date is based on the as- the intercept po between about 0.12 and 0.17 when the sumption that the SV in the brick kiln site and reference particles are nearly spherical and with intrinsic suscep- sites Meriden and Paris is the same, which may be tibilities between 10 and 100. For uniaxial, aligned SD wrong. Before favouring one or the other explanation, particles the intercept is 0.5 and the slope À0.5, meaning different hypotheses remain to be tested. The kiln may that the maximum and minimum susceptibility axis are have been in use for a relative long period of time, in this interchanged with those of the TRM, resulting in an case more than half a century, which seems unlikely but inverse fabric (Potter and Stephenson, 1988). For the not impossible. Also unlikely is that production was baked brick samples the directions and shape of the stopped after construction of the manor but that the kiln TRM and PARM (d:c: ¼ 0:005, a:f: ¼ 30 mT) aniso- was started up again after a long period of rest. Another tropy ellipsoids are not significantly different and con- possibility is that the kiln is not an isolated construction sequently the latter can be used as a substitute for the but belongs to a brick kiln site, which may have been in former (Fig. 9 and Table 4). The shapes of the TRM and operation for a long period of time. Only a more ex- TARM ellipsoids are slightly different because of the tensive excavation can ascertain this. hard fraction. The conditions mentioned above are not met when AMS is compared with ATRM. 5. Conclusions 4.4. Anisotropy, cause of age discrepancy between archaeomagnetic and historical age of Steendorp brick • The hand-made rectangular bricks of loam have a kiln? shape-related AMS, with Kmax coinciding with the longest edges and Kmin perpendicular to the greatest Although ATRM reaches high values in the baked faces, induced during the moulding process. Their loam bricks and may be responsible for strong devia- AMS is very similar to the AMS previously found tions between the remanence direction and field direc- for bricks of a Mediaeval brick kiln discovered in tion, anisotropy seems to be unable to explain the Steendorp (Belgium) and archaeomagnetically dated difference in archaeomagnetic age and presumable his- at 1650 AD, indicating that the same moulding tech- torical age of the Steendorp brick kiln. The bricks of the nique was used. brick kiln have a magnetic fabric very similar to the one • The degree of AMS of unbaked bricks decreases after observed in the loam bricks with minimum MS direc- firing at 1075 C but the shape and orientation of tions perpendicular to the greatest faces of the bricks, the MS ellipsoid remain unchanged. New magnetic 1330 J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331

phases created during heating are responsible for a Heider, F., Dunlop, D.J., Soffel, H.C., 1992. Low-temperature and large increase in MS but do not contribute to the an- alternating field demagnetization of saturation remanence and isotropy or otherwise mimic the original anisotropy. thermoremanence in magnetite grains (0.037 lm to 5 mm). J. Geophys. Res. 97, 9371–9381. • The degree of anisotropy of TRM and ARM are Hrouda, F., Hanak, J., Terzijski, I., 2000. The magnetic and pore much higher than the degree of anisotropy of MS fabrics of extruded and pressed ceramic models. Geophys. J. Int. but the orientation of the anisotropy ellipsoids is sim- 142, 941–947. ilar. The shape of the partial anhysteretic remanence Hus, J., Ech-Chakrouni, S., Jordanova, D., Geeraerts, R. Archaeo- (PARM (d:c: 0:05, a:f: 0–30 mT)) anisotropy el- magnetic investigation of two Mediaeval brick constructions in ¼ ¼ north Belgium and the magnetic anisotropy of bricks, Geoarchae- lipsoid of the soft fraction coincides with the shape of ology, to be published. the TRM anisotropy ellipsoid and consequently can Hus, J.J., Geeraerts, R., 1986. Palaeomagnetic and rock magnetic be used as a substitute for ATRM to deduce the field investigations of Late Pleistocene loess deposits in Belgium. Phys. direction which produced the TRM. The shape of the Earth Planet. Interiors 44, 21–40. AMS ellipsoid is different from the TRM ellipsoid Jelinek, V., 1981. Characterization of the magnetic fabric of rocks. Tectonophysics 79, T63–T67. and cannot be used as a substitute for the latter. Johnson, H.P., Lowrie, W., Kent, D.V., 1975. Stability of anhysteretic • The high degree of ATRM of bricks may be respon- remanent magnetisation in fine and coarse magnetite and maghe- sible for large deviations between the inducing field mite particles. Geophys. J. Roy. Astron. Soc. 41, 1–10. and field record and should be examined if ancient Juvignee, E., Haesaerts, P., Mestdagh, H., Pissart, A., Balescu, S., 1996. geomagnetic field data are retrieved from them. Reevision du stratotype loessique de Kesselt (Limbourg, Belgique). Comptes rendus de lÕAcadeemie des Sciences de Paris 323, 801–807. • Anisotropy is unlikely to be responsible for the differ- Lanos, P., 1987. Archeeomagneetisme des mateeriaux deeplacees. Applica- ence between the archaeomagnetic and presumed his- tion a la datation des mateeriaux de construction dÕargile cuite en torical age of the Steendorp brick kiln. archeeologie. Theese, Rennes: Universite de Rennes I. Lowrie, W., Fuller, M., 1971. On the alternating field demagnetization characteristics of multidomain thermoremanent magnetization in magnetite. J. Geophys. Res. 76, 6339–6349. Acknowledgements Luiken, J., 1694. Het Menselyk Bedryf. Amsterdam. Murad, E., Wagner, U., 1998. Clays and clay minerals: the firing The authors are highly indebted to the owner and process. Hyperfine Interact. 117, 337–356. Potter, D.K., Stephenson, A., 1988. Single-domain particles in rocks brickmakers of the brick factory Nelissen in Kesselt for and magnetic fabric analysis. Geophys. Res. Lett. 15 (10), 1097– the production of a special series of handmade loam 1100. bricks. The authors would like to thank Prof. H. Thoen Rogers, J., Fox, J.M.W., Aitken, M.J., 1979. Magnetic anisotropy in of the Department of Archaeology and Ancient History ancient pottery. Nature 277, 644–646. of the ‘‘Rijksuniversiteit Gent’’, Dr. Rudiger Van Hove Schmidt, P.W., Clark, D.A., 1985. Step-wise and continuous thermal demagnetization and theories of thermoremanence. Geophys. J. R. and Dr. Jean-Pierre Van Roeyen of the ‘‘Archeologische Astron. Soc. 83, 731–751. Dienst Waasland’’ for their invitation and permission to Stacey, F.D., Banerjee, S.K., 1974. The Physical Principles of Rock take samples in the Steendorp brick kiln for an archae- Magnetism. Elsevier, Amsterdam, 195 pp. omagnetic investigation. We would also like to thank Stephenson, A., 1993. Three-axis static alternating field demagnetiza- the reviewers for their suggestions and N. Abrahamsen tion of rocks and the identification of natural remanent magnetization, gyroremanent magnetisation, and anisotropy. J. Geophys. for his critical review. This research was supported by the Res. 98 (B1), 373–381. ‘‘Diensten van de Eerste Minister, Wetenschappelijke Stephenson, A., Sadikun, S., Potter, D.K., 1986. A theoretical and Technische en Culturele Aangelegenheden (DWTC), experimental comparison of the anisotropies of magnetic suscep- Belgium’’. tibility and remanence in rocks and minerals. Geophys. J. R. Astron. Soc. 84, 185–200. Tarling, D.H., Dobson, J., 1995. Archaeomagnetism: an error assess- ment of fired material observations in the british directional References database. J. Geomag. Geoelectr. 47, 5–18. Tarling, D.H., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Bucur, I., 1994. The direction of the terrestrial magnetic field in Chapman & Hall, London, 217 pp. France, during the last 21 centuries. Recent progress. Phys. Earth Thellier, E., 1981. Sur la direction du champ magneetique terrestre, en Planet. Interiors 8, 95–109. France, durant les deux derniers milleenaires. Phys. Earth Planet. Cisowski, S., 1981. Interacting vs. non-interacting single domain Interiors 24, 89–132. behavior in natural and synthetic samples. Phys. Earth Planet. Thorez, J., Bourguignon, P., Paepe, R., 1970. Etude preeliminaire des Interiors 49, 131–141. associations de mineeraux argileux des loess pleeistoceenes en Belgi- Cognee, J.P., 1987. TRM deviations in anisotropic assemblages of que. Annales de la SocieeteeGeeologique de Belgique 93, 265–285. multidomain magnetite. Geophys. J. R. Astron. Soc. 90, 1013– Urrutia-Fucugauchi, J., Tarling, D.H., 1983. Palaeomagnetic proper- 1023. ties of Eocambrian sediments in northwestern Scotland: implica- Dunlop, D.J., OOzdemir,€ OO.,€ 1997. Rock Magnetism Fundamentals and tions for world-wide glaciation in the Late Precambrian. frontiers. Cambridge University Press, Cambridge, 573 pp. Palaeogeog. Palaeoclim. Palaeoecol. 41, 325–344. Gullentops, F., 1954. Contributions a la chronologie du Pleistoceene et Van Velzen, A.J., Dekkers, M.J., 1999. Low-temperature oxidation of des formes du relief en Belgique. Meemoires de lÕInstitut Geeologique magnetite in loess-paleosol sequence: a correction of rock magnetic de LÕUniversite de Louvain 18, 125–166. parameters. Studia geophysica et geodaetica 43 (4), 357–375. J. Hus et al. / Physics and Chemistry of the Earth 27 (2002) 1319–1331 1331

Veitch, R.J., Hedley, I.G., Wagner, J.-J., 1984. An investigation of the Xu, T.-C., Ye, S.-J., Yang, F., 1991. A preliminary study of thermally intensity of the geomagnetic ﬁeld during Roman times using enhanced magnetic fabric in the Tertiary sediments from the magnetically anisotropic bricks and tiles. Arch. Sc. Geneeve 37 Qaidam Basin, NW China. Studia geophysica et geodaetica 35, (Fasc. 3), 359–373. 295–301. Wohlfarth, E.P., 1958. Relations between diﬀerent modes of acquisi- Yang, S., Shaw, J., Rolph, T., 1993. Archaeointensity studies of tion of the remanent magnetisation of ferromagnetic particles. J. Peruvian pottery––from 1200 B.C. to 1800 A.D. Geomag. Geo- Appl. Phys. 29, 595–596. electr. 45, 1193–1207.