Archaeomagnetic Investigation of Two Mediaeval Brick Constructions in North Belgium and the Magnetic Anisotropy of Bricks

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Jozef Hus,1 Souad Ech-Chakrouni,2 Diana Jordanova,3 and Raoul Geeraerts1 1Centre de Physique du Globe, 5670 Dourbes (Viroinval), Belgium 2Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium 3Geophysical Institute, Bulgarian Academy of Science, Acad. G. Bonchev Str., bl. 24, 1113 Soﬁa, Bulgaria

Archaeomagnetic dates derived from geomagnetic field direction records in baked materials are proposed for a mediaeval brick kiln (from inclination I and declination D) and for bricks from a brick wall (from I only) in northern Belgium. They are used to verify whether a brick chronology based on the format of bricks is feasible in Flanders. The brick kiln yielded a highly reliable average magnetization direction corresponding to an archaeomagnetic date around A.D. 1650, using the British and French geomagnetic field secular variation curves as a reference, at least half a century younger than expected from historical data. The fidelity of the geomagnetic records was controlled by measuring the magnetic anisotropy of the bricks. Anisotropy of magnetic susceptibility (AMS) measurements demonstrate that the bricks have a shape related magnetic fabric, which is induced during the molding process. A test to control whether AMS can substitute for the anisotropy of thermo-remanent magnetization (ATRM) failed because of induced changes during laboratory heating. ᭧ 2003 Wiley Periodicals, Inc.

INTRODUCTION According to Arntz (1971), mediaeval bricks did not appear in NWEurope before the second half of the 12th century and were not produced in Flanders, East Fries- land, or England before the early part of the 13th century (see also Devliegher, 1957; Hollestelle, 1961). Mediaeval bricks can be distinguished from Roman and Carolingian tegulae by their form and compactness, the former being smaller but thicker than the latter. Another innovation in the Middle Ages is that low-grade, more coarse-grained clays were used to manufacture bricks (Arntz, 1971). One of the main objectives of the archaeomagnetic investigation of a brick kiln (site 1: Steendorp) and a brick wall (site 2: Kemzeke) is to provide archaeomagnetic dates for the bricks that were produced in the former and for the bricks used to construct the latter (Figure 1). This may verify the possibility of establishing a brick short chronology for Flanders based on the format of bricks. This is not only important standard

Geoarchaeology: An International Journal, Vol. 18, No. 2, 225–253 (2003) ᭧ 2003 Wiley Periodicals, Inc. Published online in Wiley Interscience (www.interscience.wiley.com). 10.1002/gea.10059 GEA(Wiley) LEFT BATCH

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Figure 1. Map showing location of the two examined sites: the brick kiln in Steendorp (Temse) and the brick wall in Kemzeke (Stekene).

for the archaeologist but also for the archaeomagnetist because our knowledge of the time variation of the inclination of the geomagnetic field during the last 1 Ky relies partly on archaeologically dated bricks (Thellier, 1981). Indeed, there are indications that in Flanders the format of bricks is no longer related to time after the 15th century (Van Hove, 1994). Instead, the format seems rather to be linked to the kind of buildings they were intended for and/or perhaps to respond to an increasing demand as well as the properties of the clay utilized. To our knowledge, archaeomagnetic investigations of brick kilns are relatively rare, although kilns to produce bricks are very common in Europe from the Middle Ages up to recent times (Abrahamsen et al., 1982). The brick kiln of Steendorp was discovered not far away from a manor, known as the “Blauwhof,” constructed during the second half of the 16th century. Whether the brick kiln was indeed in operation during the construction of the manor can be verified by comparing its archaeomagnetic results with our knowledge of the time variation of the geomagnetic field in Belgium (Hus and Geeraerts, 1998) and in neighboring France and the United Kingdom (Bucur, 1994; Tarling and Dobson, 1995). On the other hand, a comparison between the last firing time of the kiln and short standard

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top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text the construction period of the Blauwhof may help refine the reference secular base of text variation (SV) curves of the geomagnetic field in our region. Bricks collected from archaeological sites are used to retrieve the inclination I of the ancient geomagnetic field direction when their firing position is known, or on the other hand, to determine their firing position when the inclination is known (Aitken, 1974; Thellier, 1981; Goulpeau, 1984; Langouet et al., 1986; Lanos, 1983). The pieces of brick from the collapsed portions of the Steendorp kiln walls as well as the bricks from the last production found inside the kiln can help verify assumptions about using scattered bricks to recover the ancient field inclination. This test was performed by comparing the mean inclination derived from the scattered bricks with that of the bricks still in situ in the kiln wall. The magnetic anisotropy of baked materials may give rise to deviations between their remanent magnetization direction (field record) and the inducing field (geomagnetic field) itself. If a magnetic fabric occurs during the molding of bricks, its effect should be evaluated. In addition to correcting for the influence of a magnetic fabric related anisotropy, anisotropy may also inform us about the molding process itself.

STUDY SITES This particular brick kiln was discovered in the village Steendorp (Temse) (N 51.14Њ, E 4.26Њ) in a clay pit, during the excavation of marine clay of Rupelian (Oligocene) age by a brick factory (Figure 1). Unfortunately, the kiln was nearly destroyed during the exploitation of the clay pit; of what was once a large rectangular kiln, only the northern wall, 8.5 m long and about 1 m high, oriented more or less in the E-Wdirection, was left (Figure 2). The wall is around 0.9 m thick and consists of different layers: at least four vertical layers of horizontally piled up, cemented bricks, some in large pieces or some complete, and on the outside, a layer of natural silt baked in situ. The kiln is less than 100 m from the site “Hof van Leugenhaeghe,” also called Blauwhof. The Blauwhof, which was a manor, is still under archaeological excavation by the “Archeologische Dienst Waasland” and, as far as we know, was the only building in the immediate vicinity of the kiln. This suggests that the kiln very probably produced bricks for the construction of this manor. In its foundations, different brick formats (27, 21, and 17 cm long) were recovered during excavation. In the kiln wall, brick formats of 27 cm were used while inside the kiln brick formats of 21 cm were also found. This suggests that the larger format was used to construct the kiln and that the smaller format was produced in the kiln. The Mediaeval brick wall in the commune Kemzeke (Stekene) (N 51.23Њ, E 4.06Њ) was present in a pottery kiln site under excavation, but it had already been torn down before our arrival to allow the construction of a highway intersection (Figure 1). The bricks used for its construction have dimensions of about 27 ϫ 12.5/13.0 ϫ 5 cm, a format that was produced between A.D. 1375 and A.D. 1400 in this area. short standard

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Figure 2. View of remaining northern wall of the brick kiln in Steendorp (Temse) at the border of a clay pit. The wall consists of several cemented brick layers and at the outside a layer of silt baked “in situ,” clearly visible at the right side of the picture. (Photograph courtesy of Rudiger Van Hove.)

Historical Context and Time Constraints The oldest written documents mentioning the manor of Leugenhage, owned by Gilles van der Loghenhage, date from the 14th century. In 1579, the Portuguese family, Ximenez van Arragon, bought the manor and transformed the farm of the court into a Hof van plaisantie or country-house. The Blauwhof was mentioned in a written document in 1597 (Van Hove, personal communication, 1999). Hence, the country-house must have been built between 1579 and 1597. An engraving of the site in Flandria Illustrata by Sanderus in 1641 gives us a good idea of the disposition of the buildings, surrounded by a large ditch, which is still visible in the ﬁeld. Finally, in 1763 or 1766, the Blauwhof became a possession of the noble- man Libouton, who dismantled the buildings in 1770 and returned the land to farm- land. The brick wall in Kemzeke was visible in a pottery kiln site but not related to it. The kiln site operated presumably in the 13th century, but the brick wall was probably erected in the 15–16th century to retain a mill motte (Van Hove, personal short communication, 1999). standard

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top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text METHODS base of text The archaeomagnetic analysis of the baked materials from the kiln determines the ancient earth magnetic field direction, defined by its declination D and inclination I at the firing place at the last firing-cooling cycle. In baked materials, the field is recorded as a thermo-remanent magnetization (TRM), which is acquired during cooling after heating at high temperature, by ferrimagnetic grains in the presence of a magnetic field. In homogeneous, isotropic materials, the direction of the remanent magnetization vector coincides with the field direction. Baked materials can thus be considered as magnetic memories of the ancient geomagnetic field (Aitken, 1974; Thellier, 1981; Tarling, 1983; Sternberg et al., 1999). Archaeo- magnetic directional analysis and dating involve following steps: 1. Sampling and sample preparation 2. Measurement of natural remanent magnetization (NRM) 3. Magnetic stability tests (viscous remanent magnetization test, stepwise alternating field and thermal demagnetization) 4. “Cleaning” or removal of spurious magnetization components 5. Statistical analysis 6. Comparison of directional results with reference SV curves I(t) and D(t).

Field Methods In all, 33 oriented, decimeter-sized samples were collected from the brick kiln at Steendorp, 12 bricks and 21 samples in the silt baked in situ. Most of the baked silt samples were cut from the vertical wall of the kiln, but 6 samples were also obtained from a small part of the ﬂoor that was still intact. Each large sample was encapsulated in gypsum in order to protect it and to cast several horizontal reference surfaces for the angle of inclination. Lines of known azimuth, obtained by measuring the height of the sun at a certain instant of time with a theodolite, were drawn on the horizontal reference planes. In addition, 36 scattered bricks were collected near the foot of the wall, in order to test whether the geomagnetic ﬁeld inclination could still be recovered from them, and for rock-magnetic analysis. Twenty-two large non-oriented pieces of bricks were taken from the brick wall at Kemzeke.

Laboratory Methods From the 33 oriented hand samples of the brick kiln, 65 specimens of about 8 ϫ 8 ϫ 8 cm were cut, keeping their original orientation marks, for the archaeomagnetic analysis. Their remanent magnetization was measured in a large sample spin- ner magnetometer, which guaranteed a high angular precision. In order to isolate the stable characteristic remanent magnetization (ChRM) component, stepwise alternating ﬁeld (AF) demagnetization was chosen. Stepwise thermal demagnetization on some small 1-in. samples was done for comparison and to gain information short on the nature of the remanence carriers. Thermal demagnetization was achieved standard

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top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text by cooling the samples in a zero field, after heating in a nonmagnetic, Schonstedt base of text TSD-1 oven. Before any treatment, the 33 samples of the brick kiln underwent a classic Thel- lier magnetic viscosity test, to determine the ability of the samples to acquire spon- taneously a viscous remanent magnetization (VRM), when exposed to the ambient geomagnetic field (Thellier, 1981). Also 408 cores (diameter ϭ 2.5 cm, length ϭ 2.2 cm) were drilled in the 36 pieces of bricks picked up in the interior of the Steendorp brick kiln and 357 cores from the 22 bricks from the Kemzeke brick wall for a study of their magnetic fabric. The shape of a brick is a rectangular parallelepiped with six faces and perpendicular adjacent edges of different length. Each core was drilled perpendicular to the great- est faces of the bricks and oriented relative to one of the longest edges for the Steendorp bricks and relative to one of the shortest edges for the Kemzeke bricks. Their anisotropy of low-field magnetic susceptibility (AMS) was measured in a KLY- 3S Kappabridge, and the principal susceptibilities and their directions obtained with the Agico Anisoft program. The most important anisotropy parameters, lineation L, foliation F, degree of anisotropy, P, and shape factor T were calculated according to Jelinek (1981).

The variation of low-field magnetic susceptibility Ktot with temperature was measured on different brick samples to gain information on magnetic mineral content. Five brick samples were selected by color: red-brown STEEN 132, brown-yellowish STEEN 100 and STEEN 133, and brown STEEN 126 and STEEN 112. Bulk magnetic susceptibility Ktot versus temperature was obtained in the CS-3 oven of a Kappa- bridge KLY3 by heating in air to 700ЊC, followed by cooling to room temperature. The anisotropy of the remanence of the Steendorp brick kiln was evaluated for its possible influence on the field record. Because the remanent magnetization of a brick is a TRM acquired during field cooling, this requires examination of the anisotropy of a TRM imparted to a sample in a laboratory field. The ATRM can be represented by a second-order symmetric 3 ϫ 3 tensor; hence, at least 6 heating and cooling cycles in 6 different sample directions are needed. Six cores from the Steendorp bricks were heated to 700ЊC in a TSD-1 oven and cooled in a field of 50 ␮T, applied along one of the axes of the orthogonal system of sample coordinate axes. Hereafter, the remanence components along the three sample axes were measured in a 2G cryogenic magnetometer and the procedure repeated with field cooling along the other sample axes. Mineralogical changes were monitored by measuring the room temperature magnetic susceptibility (MS) after each heating step and by comparing the NRM and induced TRM.

RESULTS Archaeomagnetic Results from Oriented Samples The viscosity test of the kiln samples shows that the VRM acquired over 3 weeks in a ﬁeld of 47 ␮T is low. The ratio of the VRM to the NRM ranges between 1% and short 7% and except for 4 samples is less or equal to 5%. standard

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Figure 3. Alternating ﬁeld demagnetization curves, showing the ratio of the residual remanence Mr to the initial remanence Mro (in %) in function of the AF peak value, for samples of the Steendorp brick kiln. The plateau in low AF’s and the convex shape of the demagnetization curves point to SD behavior.

The NRM is quite stable, which is expressed by the convex shape of the demagnetization curves and by the relatively high median destructive field between 20 and 35 mT, or high AF value needed to randomize half of the initial remanence (Figure 3). The remanence of the bricks is more stable than the baked silt samples. That the VRM overprint is negligible can be seen from the Zijderveld diagram, which shows the directional changes of the magnetization vector in a horizontal and a vertical plane during demagnetization (Figure 4). The magnetization vector displays a single-component behavior after the second demagnetization step and decreases straight towards the origin without any significant changes in inclination or declination. The magnetization direction of the kiln was obtained by averaging all the sample magnetization directions after cleaning in an alternating field of 20 mT, assuming a Fisher distribution. Table I lists average declination Dav, the average inclination

Iav, and average magnetic moment Mav (the magnetization intensity could not be determined with precision because the samples were covered with gypsum) as well ␣ ␪ as the statistical parameters, k, 95, and 80. Only samples with a magnetization direction with an angular deviation of less than 10Њ from the average direction, or 61 samples in all, were considered. Four samples, corresponding with bricks grouped together in the wall and adjacent to each other had to be omitted, but the short reason for their large deviations in remanence direction remains unclear. An in- standard

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Figure 4. Zijderveld diagram, displaying directional changes during stepwise alternating ﬁeld demagnetization for two samples of the Steendorp brick kiln. Dots represent the orthogonal projection of the endpoint of the magnetization vector respectively on the horizontal plane (solid data points) and vertical plane containing the NS direction (open data points). The samples show a single component behavior.

complete resetting of the original remanent magnetization acquired during their production is highly improbable. The scatter of the individual magnetization directions of the other samples around the average direction is low, expressed by the high value of 1405 for the precision factor k. Also the error at the 95% level of ␣ Њ confidence or 95 is low and less than 0.5 and hence the confidence and reliability of the average direction very high. The average magnetization directions of the brick samples and baked silt samples, calculated separately, are statistically indistinguishable. First of all, the precision estimates of the two groups are similar (i.e., ϭ ϭ ϭ they have similar k-values). We have F k1/k2 3120/1121 2.78 for the two Ϫ ϭ Ϫ ϭ Ϫ ϭ Ϫ ϭ separate groups with 2(N1 1) 2(21 1) 40 and 2(N2 1) 2(40 1) 78 degrees of freedom. The F-value exceeds the statistical table value of 1.7 at the significance level of 95% probability and hence passes the null hypothesis. Appli- cation of the F-test proposed by McFadden and Jones (1981) determines whether the two groups have been drawn from the same population or from different pop- ulations. The mean vectors R1 and R2 (R is the resultant vector obtained by re- short placing each magnetization vector by its unit vector and summation of all the unit standard

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top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text Table I. Average magnetization directions of ChRM after AF cleaning in 20 mT recorded in different base of text materials and parts of Steendorp brick kiln.a ␣ ␪ Dav Iav Mav 95 80 Steendorp Brick Kiln N (Њ) (Њ) (10Ϫ3 S.I.) k (Њ) (Њ) Bricks 21 12Њ 20Ј 72Њ 55Ј 0.95 3120 0.56 0.93 Baked silt 40 12Њ 15Ј 72Њ 04Ј 0.293 1121 0.67 0.85 Baked silt wall (E-side) 18 14Њ 27Ј 71Њ 43Ј 0.128 1068 1.02 Baked silt ﬂoor (E-side) 6 08Њ 59Ј 70Њ 03Ј 0.157 1404 0.5 Total 61 12Њ 17Ј 72Њ 22Ј 0.519 1405 0.48 0.65 a ϭ ϭ ϭ ϭ N number of samples, Dav average declination, Iav average inclination, Mav average magnetic 2 ϭ ␣ ϭ ␪ ϭ moment in A. m ,k precision factor, 95 semi-angle of cone of conﬁdence, 80 radius of circle enclosing 80% of directions.

ϩ Ϫ 2 ϩ Ϫ Ϫ vectors) are compared by: [R1 R2 R v/(R1 R2)]/2(N R1 R2), which should 1/NϪ2 be greater than [(1/P) –1](Rv is the vector sum of R1 and R2). The null hypothesis that two samples have a common mean direction may be rejected at the level of significance P when the first expression is greater than the second expression. For P ϭ 0.05, the left expression is equal to 0.049, which is smaller than the value of 0.052 for the right expression. Hence, the two groups are indistinguishable and can be combined at the 95% level of confidence. The scatter of the individual magnetization directions of the brick samples is less than the scatter for the baked silt samples. Notice that the average magnetic moment of the former is about three times higher than the latter (Table I). In the baked silt, differences occur between the wall and floor samples, especially for D and with a mean I of nearly 2Њ shallower in the latter (Table I). The slight decrease of I, often observed in kiln floors, may be explained by magnetic refraction (Schurr et al., 1984; Hus and Geeraerts, 1998).

Archaeomagnetic Dating Archaeomagnetic dating is not commonly applied (Clark et al., 1988; Eighmy and Sternberg, 1990), because reference curves for the SV of the field must be available or established for the area under study. They must be based on archaeomagnetic results from baked structures dated independently by other methods. Fortunately, master curves for the SV of I and D exist for Great Britain and France. Because Belgium is relatively close to the reference places Meriden and Paris (Ͻ 300 km), we can use the British and French SV curves if we assume that the SV is similar. One difficulty is that I and D of the palaeofield direction recorded in the structure, or site data, must be transformed into the corresponding reference site values. Assuming that SV at the reference sites and the examined site are always the same, the site data can be corrected to Meriden and Paris referents, using the present day differences in field direction from direct instrumental observations. The archaeomagnetic field direction of the Steendorp kiln has been corrected to Paris and Meriden referents, taking into account differences in the present day field, and short drawn on the master curves (Figure 5). The data plot lies close to the British master standard

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␣ Comparison of the average direction recorded in the Steendorp brick kiln with the standard secular variation curves for Great Figure 5. Britain (a) after Tarling andrespectively Dobson to (1995) and Meriden France and (b)circle Paris after corresponding as Bucur to (1994). referents, The taken average into I account and present D values day of ﬁeld the differences, kiln and have been are corrected indicated by a full dot within a short standard

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top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text curve but also near a crossing of two loops, giving two possible solutions. This base of text yields an age of A.D. 1650 if we assume that bricks were not produced in this area before the early part of the 13th century (Arntz, 1971). Referring to the French master curve, we obtain three solutions because the data are close to a turn of the SV loop. Two of them can be excluded for the same reason, and the remaining solution is also close to A.D. 1650. The ages were obtained by shifting the data and their confidence limits to the nearest point on the reference curves. In the case of the master curve of France, this was to the west side of the SV loop. From this, we may conclude that the brick kiln ceased operation near the middle of the 17th century. The preciseness of an archaeomagnetic date, based on directional data, depends on several factors: 1. Orientation and measurement errors. 2. The accuracy of the reference curves D (t) and I (t) 3. The nonparallelism of the field-induced TRM and the ancient field direction 4. The nonvalidness of the assumptions made: the presence of a non-dipole component and spatial changes of the secular variation. The high remanence intensity and magnetic stability of the Steendorp kiln en- sures that measurement errors are negligible. Also orientation errors are low and have been estimated at less than 1Њ. We are as yet unable to adjust for factors 2 and 4; the possible effect of factor 3 will be examined later.

Archaeomagnetic Results from Nonoriented Brick Samples The average inclination of nonoriented, scattered pieces of brick from the Steen- dorp brick kiln was compared to that of the oriented bricks taken from the remaining wall. Fifteen pieces of brick, in which at least six 1-in. cores could be drilled, were used. The samples were cleaned by partial AF demagnetization at 20 mT. If we assume that the oven ﬂoor was horizontal, and that the bricks were piled up horizontally, then the ﬁeld inclination must correspond to one of the angles between the remanence vector and the orthogonal faces of the bricks. We denote by ␣, ␤, and ␥ the three angles between the remanence vector and respectively the largest face, the second largest face, and the smallest face of the brick. The angles are calculated from the remanence components Mx,My, and Mz along the three orthogonal sample axes x, y, and z (x ϭ longest edge, y ϭ second longest, z ϭ shortest) using the following formulas:

␣ ϭ Ϫ1221/2ϩ ␤ ϭ Ϫ1221/2ϩ tn [Mxy /(M M) z ], tn [M yx /(M M) z ], and short ␥ ϭ Ϫ1221/2ϩ tn [Mzx /(M M) y ]. standard

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top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text Table II. Angles between ChRM vector, after AF cleaning in 20 mT, and the base of text three orthogonal faces of nonoriented brick samples of Steendorp brick kiln.a Steendorp Nonoriented Bricks ␣ ␤ ␥ Sample No. N (Њ) (Њ) (Њ) 100 10 70.25 14.91 54.68 103 13 72.73 3.22 54.83 111 8 75.10 7.51 66.54 114 11 72.56 8.62 67.43 115 10 76.35 10.07 57.44 117 6 74.50 7.21 69.05 122 7 74.69 10.06 61.49 128 18 75.01 10.27 59.43 134 7 74.63 12.12 49.43 135 8 72.03 13.49 55.44 Mean 73.8 9.8 59.6 s.d. 1.8 3.4 6.5 102 12 6.89 76.61 5.02 106 18 6.65 81.05 4.45 107 15 10.35 76.37 7.07 131 19 5.56 70.46 4.44 132 20 10.42 70.20 8.09 Mean 8 74.9 5.8 s.d. 2.3 4.6 1.7 a ␣, ␤, and ␥, are respectively the angles between the remanence vector and the largest faces, second largest faces, and smallest faces of a brick.

From Table II, listing the values of angles ␣, ␤, and ␥ after cleaning, it is clear that not all pieces belong to bricks that had been fired with the largest face horizontal, shown by the small ␣ angle for several pieces. Only pieces of bricks fired with the largest face horizontal were used to calculate the ancient inclination because the inclination must be compared with the average inclination of the oriented bricks taken from the wall of the kiln and which were laid with the largest face horizontal (Figure 2). Because samples from the nonoriented bricks were drilled perpendicular to the largest face, the dip is fairly well defined, but surface irregularities and deformations mean the angle between the reference line and the longest edges of the bricks are less well defined. Furthermore, we may expect larger errors for I retrieved from bricks fired with the second largest face horizontal (fired on edge or edgewise), because the vertical angles will be more scattered than for bricks laid with the largest face horizontal (see also Abrahamsen [1973]). At our latitude, during the last 2000 years, the inclination of the field varied between 50Њ and 75Њ, and bricks certainly postdate the Roman period (see Figure 5). Therefore, we need not consider angles falling outside this range. Five bricks short yield only ␥ angles within this range, proving that they were fired on edge. For the standard

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Figure 6. Histogram of the angles between the ChRM vector and the largest faces (␣), the second largest faces (␤), and smallest faces (␥) of bricks of the Kemzeke brick wall. The distribution indicates that nearly all the bricks were ﬁred with the second largest face horizontal (on edge).

first 10 bricks in Table II, a and ␥ fulfill the condition, signifying that they have been fired with the largest faces either horizontal or upright, although the latter is improbable. If we assume that they were fired with the largest faces horizontal, then the average inclination (73.8Њ; s.d. ϭ 1.8Њ) of these bricks is not significantly different from the average inclination (72.9Њ; s.d. ϭ 0.9Њ) of the oriented bricks taken from the kiln wall (Table I). This suggests that they were indeed fired with the largest faces horizontal and that they belong to collapsed parts of the kiln walls. However, the other five bricks, with an average inclination of 74.9Њ (s.d. ϭ 4.6Њ), may have been part of the kiln walls or may have been produced in the kiln itself; some bricks at the bottom of the wall have been deliberately set on edge (Figure 2; see also Thellier, 1938; Abrahamsen, 1973). The samples from the Kemzeke brick wall were prepared in the same way as the nonoriented brick samples from the Steendorp kiln except that they were oriented on one of the smallest edges of the brick. In each piece of brick, between 12 and 15 1-in. cores were drilled, perpendicular to the largest face of the brick. A histogram of the angles ␣, ␤, and ␥ after cleaning in an AF of 20 mT shows that, except for two outliers, only ␣ reaches values from 50Њ to 75Њ (Figure 6). Most of the bricks must have been stacked with the second largest face horizontal (on edge) during their production. Omitting the two outliers, the average inclination of the remaining short 20 bricks is 62.6Њ (s.d. ϭ 3.7Њ). The surface irregularities and roughness of the standard

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top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text bricks, as well as the fact that several among them were strongly deformed during base of text ﬁring, explains the relatively high standard error. Comparison with the SV curve of I for Paris and Meriden leads to a probable age of A.D. 1400 to A.D. 1550 if we accept that bricks did not appear before the second half of the 12th century in Flanders.

Rock Magnetic Properties of Steendorp Brick Kiln The bricks from the Steendorp brick kiln show a great variety of hues, suggesting that the walls were built with recycled and misﬁred bricks produced from different source materials. The high variance observed in several rock magnetic properties supports this conclusion: for instance, the behavior of the NRM intensity during stepwise AF demagnetization (Figure 3) and during stepwise thermal demagnetization; and the large range of NRM intensity and MS values (Figure 9).

The Ktot –T curves of the samples STEEN 100, 112, 126, 132, and 133 point to magnetite (Fe3O4) as the main magnetic mineral, as can be seen from the temperatures where Ktot vanishes on the heating curves, near the Curie temperature of 580ЊC of pure magnetite (Figure 7). The shape of the curves differs depending on the color of the samples. This reﬂects clearly that different kinds of brick materials were used in the kiln construction. MS increases with temperature in samples STEEN 100, STEEN 132, and STEEN 133. Samples STEEN 132 and STEEN 133 display a broad Hopkinson peak just below the Curie temperature, with an enhancement in Ktot by a factor of about 1.5–2.5, pointing to a large blocking temperature spectrum (Dunlop and O¨ zdemir, 1997). On the other hand, MS decreases with increasing temperature in samples STEEN 112 and TEM126.

Magnetic Anisotropy Anisotropy of the magnetic properties of baked materials may be responsible for significant deviations between the direction of the acting field and the field record. Moreover, the anisotropy of bricks may reveal the technique used to produce them. The stereographic projection plots, displaying the principal MS directions of all the samples cored in the Steendorp and Kemzeke bricks show a scatter, but the emerging pattern demonstrates without any doubt that a statistically significant oriented magnetic fabric is present in both series (Figure 8a). On average, Kmax and

Kint determine a plane corresponding to the largest brick face, and Kmin is perpendicular to the largest face. Because the scatter for a single brick is high, the anisotropy parameters of the bricks have been calculated using only pieces that yielded at least 10 specimens, a total of 20 bricks for the Steendorp brick kiln and 22 bricks for the Kemzeke brick wall (Table III). Changes in MS ranged between 0.2 ϫ 10Ϫ3 and 11.4 ϫ 10Ϫ3 SI for Steendorp with an average magnetic susceptibility ϫ Ϫ3 Kav of 3.73 10 SI. For the Kemzeke bricks, the range is much narrower, and Kav is 3.41 ϫ 10Ϫ3 SI. The degree of anisotropy P is low, less than 2% for Steendorp and less than 7% for Kemzeke but with an average of less than 2% and 4.5%, respectively. short Foliation F is in general higher than lineation L and has an average value of 1.011 standard

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Figure 7. Bulk magnetic susceptibilty Ktot versus temperature T for powdered samples of Steendorp short brick kiln. Full dots and circles correspond respectively with heating and cooling cycles. Notice broad standard Hopkinson peak in sample STEEN 132 below the Curie temperature, where Kav vanishes.

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Figure 8. (a) Stereoplot of principal directions of AMS tensor of samples from the Steendorp brick kiln ϭ ᭿ and Kemzeke brick wall. (b) Stereoplot of average principal directions of AMS per brick (Kmax , ϭ ᭡ ϭ ᭹ Kint ,Kmin ). The average direction of Kmax coincides with the longest edges of the bricks, indicated by the solid arrows, and the average direction of Kmin with the direction perpendicular to the largest faces of the bricks.

short standard

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top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text base of text 0.065 0.385 Ϫ (Continued) 1.017 0.006 1.011 0.005 a LFP T 0.003 min Њ I K Њ D int Њ K I Steendorp Brick Kiln Њ D max Њ I K Њ D SI) av 3 Ϫ K 3.729 153 4 1.007 198 3 1.002 160 84 0.991 1.005 (10 AMS parameters according to Jelinek (1981), of bricks from the Steendorp brick kiln and Kemzeke brick wall.

short 100101102 13103 13106 11.38 12107 13 3.276108 18 6.043 209109 15 5.202113 22 59 0.2914 212114 1 12 0.3175115 19 10 12 115 0.2054 1.011124 5 11 181 6.337 1.002125 10 0 5.839 9 299 1.009126 10.250 10 5 1 151 276127 18 1.011 4.024 1.005 122 183 2128 15 176 2 0.3588 1.006 5129 5 14 282 206 0.2747 338 1.002 7131 5 18 1.005 0.2662 0 91 1.002 1.004132 33 2 9 0.5789 2 1.001 1.013133 0 85 19 275 7 358 0.3356 1.011 257 186 2 20 1.003 195 2 0.3626 1.002 341 1.009 88 12 15 93 266 1 3.596 70 8 19 1.003 5 103 1.007 308 165 5.616 82 1 248 0.987 1.007 10.040 1.000 0 0.996 1.007 6 289 1.001 81 1 81 272 30 0.99 1.006 1 176 1009 5 156 99 1.002 355 1000 1.000 88 88 0.986 1.010 0.993 55 105 3 1 1.003 1007 1.000 1.015 88 0 7 0.992 1.005 4 83 289 1007 2 83 0 1003 1.001 1.004 255 1 1.024 0.994 1.011 1.008 74 1.003 1.006 1003 1.006 84 85 0.995 1.002 1.018 3 1.009 128 300 1.001 2 266 1.019 1005 85 218 0.273 0.988 1.011 0.984 180 65 1.025 1003 1.004 0.89 87 1.012 331 1 1.000 0.991 1.006 77 4 0.217 1.014 1011 82 1011 125 0.992 1.006 1 88 1.002 0.428 0.567 51 1.011 0.99 1009 1.004 0.991 1.012 87 1.018 1.009 1006 1.004 0.599 0.993 176 88 1.008 79 1004 0.115 1.023 0.986 1005 1.029 162 1.009 88 1005 0.302 0.997 1.018 86 1.013 1005 1.011 86 1.016 0.994 0.035 1.008 0.243 1003 0.988 1.017 1.016 0.99 1.019 1003 1.013 0.193 1.003 1004 1.024 0.554 1002 1.008 0.386 1.006 0.185 1.016 1.011 1.014 0.548 1.020 0.052 1.017 0.449 1.021 0.706 No. N s.d. Brick Mean Table III. standard

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top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text base of text 0.246 0.253 Ϫ minimum ϭ min 0.02 1.044 0.01 1.028 LFP T 0.007 intermediate susceptibility, K ϭ int min Њ I K Њ D shape factor of susceptibility ellipsoid. maximum susceptibility, K ϭ ϭ int Њ max K I Kemzeke Brick Wall Њ D degree of anisotropy, T ϭ max Њ I K foliation, P Њ average magnetic susceptibility, K D ϭ ϭ av SI) av 3 Ϫ K 3.410 177 4 1.016 201 6 1.002 162 82 0.982 1.016 lineation, F (10 ϭ (Continued) number of samples, K ϭ 123 144 145 21 2.3436 13 6.0527 17 1.275 2728 19 9.1409 16 91 4.481 276 0 13 1.903 260 18 0 2.546 277 1.012 1 3.965 2 92 1.024 5.894 1.015 3 86 2 266 1.021 181 5 262 1.021 7 6 16 170 2 1.015 9 186 5 1.011 10 1.012 1.003 2 1.006 1.026 2 9 355 0.999 173 176 1.007 359 171 0.998 182 1 88 8 8 81 28 10 80 1.005 24 0.985 1.001 1.000 0.971 87 0.996 0.986 257 81 1.010 217 1.022 58 0.972 1.017 20 85 0.981 80 1.022 72 1.042 1.017 78 0.981 1.017 1.021 0.988 1.032 0.988 1.065 1.045 0.978 1.012 1.035 1.034 1.010 1.014 0.296 1.062 0.279 1.031 1.028 1.056 1.018 0.001 1.025 0.288 1.018 1.040 1.028 0.189 1.039short 1.050 0.377 0.245 0.258 101112 2513 1214 15 2.43715 19 4.56216 16 1.44517 12 92 3.71118 16 72 5.049 26619 15 1 1.47920 10 25 82 2.64821 9 16 87 1.012 2.045 263 1.01822 13 3 2.763 1.012 14 86 2 3.443 340 260 1 1 14 1.024 3.649 357 269 1.024 0 2.471 1.008 9 2 173 8 84 1.710 281 5 6 357 1.019 172 1.006 1.009 17 1.002 78 3 0.997 1.020 0 356 85 4 10 208 170 0.997 186 1.019 4 114 359 1.017 1.004 4 1 1.003 77 342 82 4 354 1.007 80 191 6 197 1.004 1.006 0.976 72 0.986 1.002 1 168 3 0.99 1.001 85 2 178 175 1.021 0.979 1.012 80 17 1.000 3 1.010 137 0.971 1.008 86 1.041 1.027 3 1.021 0.989 86 219 1.000 82 1.018 1.021 0.977 12 1.063 1.001 1.018 1.033 1.007 0.988 86 294 0.979 1.029 1.047 1.017 88 341 1.045 1.015 0.3 1.008 0.981 0.257 85 1.015 1.069 1.042 0.976 87 0.228 1.022 1.022 0.14 1.023 0.993 1.047 1.060 1.015 0.993 0.32 1.029 1.030 1.007 0.354 1.063 1.036 1.006 0.405 1.054 1.013 0.428 1.052 1.010 0.449 1.021 0.129 1.017 0.378 0.277 0.224 No. N N s.d. Brick susceptibility, L Table III. a Mean standard

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᭺ Figure 9. Degree of anisotropy P versus mean magnetic susceptibility Kav for Steendorp ( ) and Ke- ᭹ mzeke bricks ( ). The bricks of the Steendorp brick kiln show a large range of Kav values, but lower degree of anisotropy, compared to bricks of the Kemzeke brick wall.

(Ϯ0.005) for Steendorp and 1.028 (Ϯ0.01) for Kemzeke. The anisotropy of MS of all but a few bricks can be represented by a slightly oblate ellipsoid with Kmax and

Kint determining the largest brick face and Kmin perpendicular to the largest face (Figure 8b). The degree of anisotropy P seems to increase with the mean susceptibility Kav (Figure 9). According to Henry’s mixing model, this may indicate the superposition of the effects of a low MS matrix with weak anisotropy and a highly anisotropic ferrimagnetic component (Henry, 1983a, 1983b; Borradaile and Henry, 1997). The increase in anisotropy may be due to an increase in the concentration of the more anisotropic component (Hrouda and Henry, 1996).

Anisotropy of Remanence The degree of ATRM of brick samples of the Steendorp brick kiln varies between short 4% and 26% and is in all the samples higher than the degree of anisotropy P of MS standard

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top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text (Table IV). The foliation F varies between 2% and 15% and is in nearly all the base of text samples higher than the lineation L that changes between 2% and 11%. The ratio between the TRM acquired during the first heating and the NRM is not constant but varies between 1.02 and 1.19, with the highest ratios corresponding to the highest observed degrees of ATRM. Room temperature MS changes appreciably in some samples after the first heating to 700ЊC, between Ϫ4% and ϩ18%. A decrease of less than 4% occurs during the subsequent five heating cycles. Nevertheless, samples with minor mineralogical changes show degrees of ATRMs between 4% and 8%, higher than the degree of anisotropy of MS. The correlation between MS and P is high (r ϭ 0.95) for the six Steendorp brick samples, but there is no correlation between the TRM intensity and the degree of anisotropy of TRM (r ϭϪ0.20), although there is some apparent tendency for samples with the weakest TRM to have the highest anisotropy. The principal TRM directions differ appreciably from the directions of the principal susceptibilities (Figure 10 and Table IV). Obviously, there is no relation between the principal susceptibilities MS and the principal values of TRM, neither between the shape of the ATRM and AMS ellipsoids.

DISCUSSION Archaeomagnetic Dating The coincidence between the field direction recorded in the baked silt and bricks of the Steendorp brick kiln can be taken as a strong argument in favor of a highly reliable field record. From the directional results, we may conclude that this kiln was abandoned near the middle of the 17th century. This implies that bricks with a longest edge of 21 cm were still produced around A.D. 1650. Blauwhof was probably built between A.D. 1579 and A.D. 1597, making the archaeomagnetic age of the kiln younger then expected unless the kiln was used over a longer period, in this case more than half a century, which seems unlikely but not impossible. Pro- duction may have ceased after Blauwhof was built, but the kiln may have started up again later. In addition, the assumption that the SV at the site, Meriden, and Paris was the same may be wrong. Yet another possibility is that the kiln belongs to a site with a number of brick kilns, which may have been in operation for a long period. Site surface surveys and trial trench observation are positive, but only further excavation can prove this. It is, therefore, important to examine if steepening of the inclination did occur because of anisotropy or magnetic refraction (Aitken and Hawley, 1971). Indeed, a steepening of about 5Њ in I would give an age at the turn of the 16th century after correction. That average inclinations of the baked silt samples and bricks are indistinguishable may just be a coincidence (Table I). Further, tightly grouped directions do not guarantee that the anisotropy effect is absent (Lanos et al., 1999). The wall of the kiln is oriented 70ЊE, or nearly east to west, so we may expect higher I values due to magnetic refraction. The shallowing of I in the baked silt samples short from the floor partly compensates for the effect of steepening of I in the wall of standard

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Њ top of text base of text min K )I( Њ D( ) Њ int C and room temperature. See K Њ )I( Њ D( ) Њ T between 700

␮ max K )I( Њ D( a 0.5090.63 347 1810.016 190.15 320 235 8 347 84 46 85 1 53 92 40 77 0 38 280 4 143 49 247 6 86 Ϫ Ϫ Ϫ Ϫ average thermo-remanence acquired in a ﬁeld of 50 ϭ av 2.28E-023.27E-021.98E-04 1.0396.13E-04 1.1104.55E-04 1.008 1.0128.63E-04 1.035 1.1331.57E-02 1.002 1.002 1.051 3.97E-02 1.059 1.052 1.2576.37E-03 1.021 1.003 1.010 4.95E-02 1.048 1.057 1.0893.77E-03 1.008 1.016 0.09 1.0056.84E-02 1.022 1.148 1.119 1.003 1.022 0.187 1.037 1.019 343 1.029 0.238 1.203 1.005 1.030 34 1.021 1.051 66 93 0.493 1.008 0.457 1.041 15 0.559 149 139 10 0.244 274 0.05 158 328 23 70 190 11 7 343 65 58 241 337 37 16 28 5 299 139 19 351 5 97 20 32 86 1 245 51 1 231 22 99 6 191 209 4 79 74 53 av av av av av av av av av av av av TRM TRM TRM TRM TRM TRM Anisotropy parameters of ATRM of samples from Steendorp brick kiln. average magnetic susceptibility, TRM

ϭ short av K Sample No. L F P T also legend of Table III for other deﬁnitions. Steen 108.10Steen 112.05 K Steen 170.06 K Steen 118.05 K Steen 132.5a K K Table IV. Steen 100.11 K a standard

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Figure 10. Stereoplot of principal directions of ATRM tensor of samples from the Steendorp brick kiln (see also legend of Figure 8). baked silt, which increases slightly from east to west. Higher inclinations from refraction may be expected in the bricks because their remanence is much greater. However, the difference in I between bricks and baked silt is less than 1Њ (see Table I), which indicates that this is not the case. Some compensation may have occurred due to anisotropy; most bricks were cemented with the largest face horizontal, resulting in some inclination shallowing. The archaeomagnetic investigation of the bricks from the brick wall of Kemzeke shows that most of them were ﬁred on edge and that they may represent a single production series. The average inclination I corresponds with an age between A.D. 1400 and A.D. 1550, if we assume that they postdate the 12th century. The ﬂoor of the kiln in which they were produced, however, must have been perfectly horizontal. Further, averaging must compensate for the surface irregularities of the bricks, short which resulted in imperfect vertical layers of the brick piles. standard

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top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text Our ﬁndings support the trend of a decreasing mediaeval brick format with time base of text in Flanders: the format of 27 cm was still produced in the 15th perhaps until the middle of the 16th century while the format changed to 21 cm in the 17th century. In the 13th and early 14th centuries, large bricks with lengths between 29 and 32 cm were made to replace natural stones (Hollestelle, 1961). Once bricks became a more common construction material, the format was reduced to about 23 ϫ 11 ϫ 5.5 cm around A.D. 1500 in Brugge (Devliegher, 1954). The same trend occurred in Aardenburg, now in the Netherlands, but at that time part of county Flanders. According to Trimpe Burger (1962–1963), the format of 30 ϫ 14 ϫ 7 cm was in use around A.D. 1300 in this locality, while, in the middle and second half of the 14th century, it was replaced by smaller formats of 28.5/25.5 ϫ 13/12 ϫ 6/5.5 cm. According to Van Hove (1994), the derdelingsteen, or bricks with a length of 20 cm or smaller, were produced around A.D. 1450. Indeed, examination of the Holy Cross church in Vrasene (East Flanders) shows that this format appeared here between A.D. 1400 and A.D. 1481 and the format 18/20 ϫ 9/10 ϫ 4/4.5 between A.D. 1448 and A.D. 1481 based on the dating of the Gothic church. According to the same author, bricks with lengths between 23 and 25 cm were currently in use in the middle of the 15th century and probably produced during the ﬁrst half of century. After this, the format of bricks did not systematically decrease, but different formats were produced at the same time, probably depending on builders’ needs.

Magnetic Properties Magnetic measurements conﬁrm that the bricks used for the Steendorp kiln construction are recycled bricks of different origins. This is reﬂected by the large range of remanence and MS values, their different behavior with regard to alternating

ﬁelds (Figure 3) and temperature, and different Ktot (T) curves (Figure 7). Interest- ing is that the archaeological excavation of the Blauwhof revealed that the lower parts of the walls of the buildings were intentionally pulled down. They were made of poor quality bricks or deformed (misﬁred) ones, which puzzles the archaeolo- gists, but may explain why the bricks were not reused.

The rock magnetic properties, in particular Ktot versus T curves, and stepwise thermal demagnetization curves (not shown here) and stability of NRM point to single domain (SD) magnetite as the principal remanence carrier. Indeed, the alternating ﬁeld demagnetization curves of NRM show a small plateau in low ﬁelds, and Ktot increases with T and displays large Hopkinson peaks typical for SD magnetites. In multidomain (MD) magnetites, we would rather expect a small and nar- row Hopkinson peak (Dunlop, 1974).

Anisotropy of Magnetic Susceptibility Hand-molded bricks, a common molding technique in the Middle Ages (see, for instance, etchings by Johannes Luiken in 1690 and 1694 depicting a tilemaker [Lui- ken, 1694]), may have an oriented magnetic fabric induced by alignment of aniso- short tropic particles during molding. The anisotropy of magnetic particles may have standard

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top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text several origins, of which the most important ones are shape and magnetocrystalline base of text anisotropies (Tarling and Hrouda, 1993). Magnetocrystalline anisotropy arises because the magnetization occurs along certain crystallographic directions called easy magnetization directions. Shape anisotropy results from the difference in demagnetization factors in different directions in nonequidimensional grains. Because the anisotropy of magnetic remanence may cause deviations between the ﬁeld record and the acting ﬁeld, archaeomagnetic directional and intensity results must sometimes be corrected for this effect. This requires examining the anisotropy of thermo-remanent magnetization of the bricks, with the risk of mineralogical changes during heating. In order to avoid mineralogical changes, AMS has been proposed in the past as a substitute for ATRM. Although the degree of AMS is low (less than 5%), the bricks of the brick kiln and brick wall reveal a magnetic fabric with average principal MS directions related to the shape of the bricks. For comparison, high AMS values (up to 13%) have been reported by Jordanova et al. (1995) for Greek bricks of the 17th and 19th century.

On average, Kmax coincides with the longest edges, and Kmin is perpendicular to the largest faces of the bricks. The fabric is induced during the molding of the bricks and consequently reﬂects and depends strongly on the molding process.

There is a strong linear relationship between the principal components Kii and the mean MS, with regression lines passing through or intersecting close to the origin, allowing separation of the bulk (matrix) and ferrimagnetic tensors when certain conditions are fulfilled. First, the matrix and ferrimagnetic fabric must re- main constant in orientation and intensity from one sample to the other. Second, the variation in Kav and Kii should be due only to a change in concentration of the ferrimagnetic contribution, while the matrix contribution remains constant. Hence, we assume that a single magnetic phase is responsible for the anisotropy and that composition (concentration) rather than strain or grain size (variable amounts of MD and SD grains) controls the variation in degree of anisotropy. However, the total AMS must be seen as the sum of the anisotropies of each magnetic phase weighted by its susceptibility. The weak positive correlation between P and Kav in Figure 9 must rather be seen as an indication of a composite fabric with both para- and ferrimagnetics contributing to the anisotropy degree. Indeed, the maximum P values present are far below the maximum expected for paramagnetic phyllosilicates. The range of P values in samples with low MS, near the upper limit of 5 ϫ 10Ϫ4 for phyllosilicates, cannot be due to a single strong ferrimagnetic component but rather linked to the degree of deformation in this case. The principal directions correspond to the stress directions, which gave form to the bricks. Because the bricks were handmade, some bricks, or in other production sites where other molding techniques (like extrusion) were in use, the principal MS directions are less or not related to the symmetry of the bricks (see Figure 8a; also Garcia [1996]). A comparison between hand-molded, unbaked (only dried), and baked bricks of loam (Hus et al., in press) demonstrates that the magnetic fabric is present before short firing and is not much modified during baking. The principal MS directions before standard

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top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text and after heating are indistinguishable, but the degree of anisotropy decreases after base of text heating. This experiment indicates that the magnetic fabric is induced during the molding process (see also Kovacheva et al. [1998]). The presence of very shallow thin grooves on one of the largest faces of the Steendorp bricks, running more or less parallel with the longest edges of the bricks, reﬂects how these bricks were molded. The molding process probably involved throwing a lump of kneaded clay into a rectangular frame, pressing it by hand into the frame, and scraping the excess off in a single movement. Compression of the lump during ﬁlling of the frame and shearing when scraping off the excess results in small-scale deformations. Never- theless, the molding is responsible for overall grain alignment and the observed overall magnetic fabric. Hence, the magnetic fabric will vary considerably inside a single brick. An analysis of the AMS data of each brick from Steendorp and Ke- mzeke in detail shows that lineation is higher in the material that was in contact or close to the molding frame and foliation higher for the material in the interior of the brick. Taking into account the production rate of a brick molder, which was several thousand a day, and the fact that the brickmaker’s movement would be economical, it is not astonishing that grain alignment is far from perfect (complete). Hence, a large number of subsamples must be measured to reveal the small-scale changes of the fabric inside the brick and to obtain a meaningful average fabric.

However, the result is remarkable. For the brick kiln, the average direction Kmax of the 408 cores deviates only 5Њ from the longest edges of the bricks, and the average Њ dip of Kmin is only 1 from perpendicular to the largest faces of the bricks (Figure 8).

Anisotropy of Remanence A significant anisotropy may cause deviations between the remanence vector and the ancient geomagnetic field direction, and intensity determinations may de- pend on the direction in which the laboratory field is applied. The anisotropy of TRM in baked clay has been studied by several authors, in some cases, with un- convincing results because of experimental difficulties or thermally induced changes in the samples (Rogers et al., 1979; Veitch et al., 1984; Lanos, 1987; Yang et al., 1993). The determination of the anisotropy of TRM requires repeated heating to high temperatures, with concomitant risk of mineralogical changes. Unfortunately, room temperature AMS cannot substitute for ATRM because the conditions of coincidence of the principal directions and proportionality of the eigenvalues are not met for the bricks of the Steendorp kiln (Stephenson et al., 1986; Cogne´, 1987). Significant changes occurred in several samples during heating. Differences often occur between the orientation of the eigenvectors of the anisotropy tensors of TRM and MS (Stephenson et al., 1986; Garcia, 1996). Garcia (1996) found a linear relation between AMS and ATRM for several series of bricks and pottery. But the linear relation was different from one series to the other, even when the material comes short from the same site and has the same origin. Therefore, even when the orientations standard

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top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text of the principal axes of the AMS and TRM ellipsoids are nearly the same, the shapes base of text of the ellipsoids are different (Chauvin et al., 2000). Although ATRM tends to increase with the degree of ASM, the correlation is only 0.75 and consequently weak. To compare the shape of the ASM and TRM ellipsoids, we calculated the normalized principal axes defined by Stephenson et al. (1986) by ϭ ϩ ϩ ϭ the parameters pi Ki/(K1 K2 K3)(i 1, 2, 3) There is no linear relation between the normalized principal susceptibilty and TRM values (R2 ϭ 0.036), pos- sibly because the carriers of MS and TRM are different. Especially in samples with a weak MS, the contribution of paramagnetic phyllosilicates to AMS cannot be ignored. From the average MS, the magnetite content is estimated at less than 0.015%. At Steendorp, the low AMS of the bricks, less than 4.5%, and the agreement between the mean remanence directions of the bricks and baked silt of the kiln suggest that it may not be very useful to correct for TRM deviations induced by the anisotropy in this case. According to Nagata (1961) and Stacey and Banerjee (1974), an AMS degree of 5% may give rise to TRM deviations of 3Њ. For MD magnetite and an ATRM degree of 10%, the maximum deflection of TRM would be 2.7Њ (Dunlop and O¨ zdemir, 1997). It should be stressed that deviations will occur because of anisotropy at the blocking temperatures of the remanence carriers. In several samples, we noticed an enhancement of MS near the Curie temperature, meaning that the samples are more permeable for the field at these elevated temperatures (Figure 7). Although we ignore the exact AMS at the blocking temperatures, theoretically, it should be much lower than at room temperature. During cooling, deviations may occur because of increasing effect of demagnetizing fields in shape-aligned magnetites.

CONCLUSIONS The archaeomagnetic investigation of two brick constructions, a brick kiln and a brick wall, enabled us to verify whether a chronology based on the formats of bricks for Mediaeval Flanders is feasible. The archaeomagnetic investigation of the

Steendorp brick kiln resulted in a highly reliable average remanence direction (Iav ϭ Њ ϭ Њ ϭ Њ ϭ 72.4 ,Dav 12.3 ) with high precision (a95 0.5 ,N 61), which yields a date for the last firing around A.D. 1650 using the British and French secular variation curves as a reference. The bricks used to construct the Kemzeke brick wall were in general fired with their second largest face horizontal (on edge) and were produced between A.D. 1400 and A.D. 1550, as indicated by the field inclination alone. The age of the bricks found at Steendorp and Kemzeke support the trend of decreasing brick format in Mediaeval Flanders. The production of small and large brick formats after the middle of the 15th century, however, makes it hazardous to establish a brick chronology based on the format of bricks until the Late-Mediaeval period. Because the archaeomagnetic date of the brick kiln was at least half a century short younger than expected from historical data, the anisotropy of the bricks, which standard

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top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text may be responsible for deviations between the geomagnetic field and field record, base of text was examined. AMS of the bricks from both sites reveal a magnetic fabric related to the shape of the bricks and reflecting the molding process. On average, the maximum magnetic susceptibility direction coincides with the longest edges of the bricks, and the minimum magnetic susceptibility direction is perpendicular to their largest faces. The orientation and shape of the magnetic susceptibility ellipsoid are indicators of the molding technique and procedure. Anisotropy of magnetic susceptibility (AMS) cannot be proposed as a substitute for the anisotropy of the thermo-remanent magnetization (ATRM) of the Steendorp brick kiln, because the conditions of coincidence of the principal directions and proportionality of the eigenvalues are not met, probably because of a highly variable degree of induced changes during subsequent heating. Great variance in their magnetic properties indicates that the bricks used for the construction of the Steendorp brick kiln are recycled ones of different origin.

The authors are highly indebted to Professor H. Thoen of the Department of Archaeology and Ancient History of the Rijksuniversiteit Gent and Dr. Rudiger Van Hove and Dr. Jean-Pierre Van Roeyen of the Archeologische Dienst Waasland for their invitation to examine archaeomagnetically the Steendorp and Kemzeke sites and also for valuable information concerning the evolution of the formats of bricks during the Middle Ages. The research was supported by a fellowship of Dr. D. Jordanova granted by the Diensten van de Eerste Minister, Wetenschappelijke Technische en Culterele Aangelegenheden (DWTC), Belgium. The authors would also like to thank the owner of the brick factory Swenden for permission to investigate the brick kiln in Steendorp. We thank Niels Abrahamsen for his critical review of the manuscript, which helped us to improve the text very much, and also Yongjae Yu, who gave us some interesting hints for future research on the anisotropy of bricks.

REFERENCES Abrahamsen, N. (1973). Archaeomagnetic tilt correction on Bricks. Archaeometry, 15, 267–274. Abrahamsen, N., Hansen, B.A., & Sørensen, M.A. (1982). Palaeomagnetic investigation of four Medieval kilns from Bistrup near Roskilde, Denmark. Journal of the European Study Group on Physical, Chemical and Mathematical Techniques Applied to Archaeology, 7, 115–122. Aitken, M.J. (1974). Physics and archaeology (2nd Edition). Oxford: Clarendon Press. Aitken, M.J., & Hawley, H.N. (1971). Archaeomagnetism: Evidence for magnetic refraction in kiln structures. Archaeometry, 13, 83–85. Arntz, W.J.A. (1971). Tijdstip en plaats van ontstaan van onze middeleeuwse baksteen. Bulletin Konink- lijke Nederlandse Oudheidkundige Bond, 24, 98–103. Borradaile, G.J., & Henry, B. (1997). Tectonic applications of magnetic susceptibility and its anisotropy. Earth Science Reviews, 42, 49–93. Bucur, I. (1994). The direction of the terrestrial magnetic ﬁeld in France, during the last 21 centuries. Recent progress. Physics of the Earth and Planetary Interiors, 8, 95–109. Chauvin, A., Garcia, Y., Lanos, Ph., & Laubenheimer, F. (2000). Paleointensity of the geomagnetic ﬁeld recovered on archaeomagnetic sites from France. Physics of the Earth and Planetary Interiors, 120, 111–136. Clark, A.J., Tarling, D.H., & Noe¨l, M. (1988). Developments in archaeomagnetic dating in Britain. Journal of Archaeological Science, 15, 645–667. Cogne´, J.P. (1987). TRM deviations in anisotropic assemblages of multidomain magnetite. Geophysical Journal of the Royal Astronomical Society, 90, 1013–1023. short Devliegher, L. (1954). De opkomst van de kerkelijke gotische bouwkunst in West-Vlaanderen gedurende standard

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL 251 GEA(Wiley) LEFT BATCH

top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text de XIIe eeuw. Bulletin van de Koninklijke Commissie voor Monumenten en Landschappen, 5, 177– base of text 345. Devliegher, L. (1957). De vroegste gebouwen van baksteen in Vlaanderen. Bulletin van de Koninklijke Nederlandse Oudheidkundige Bond, 6, 245–250. Dunlop, D.J. (1974). Thermal enhancement of magnetic susceptibility. Journal of Geophysics, 40, 439– 451. Dunlop, D.J., & O¨ zdemir, O¨ . (1997). Rock magnetism. Fundamentals and frontiers. Cambridge: Cam- bridge University Press. Eighmy, J.L., & Sternberg, R.S. (1990). Archaeomagnetic dating. Tucson, AZ: The University of Arizona Press. Garcia, Y. (1996). Variation de l’intensite´ du champ magne´tique en France durant les deux derniers milleńaires. The`se de Doctorat, Me´moires de Geósciences nЊ74, Universite´ de Rennes 1, Rennes. Goulpeau, L. (1984). Les possibilite´s de l’archeómagne´tisme des mate´riaux de´place´s. In Datation-car- acte´risation des ce´ramiques anciennes. Journal of the European Study Group on Physical, Chemical and Mathematical Techniques Applied to Archaeology, PACT, 10, 191–202. Henry, B. (1983a). From qualitative to quantitative magnetic anisotropy analysis: The prospect of finite Strain calibration. Tectonophysics, 98, 327–336. Henry, B. (1983b). Interpre´tation quantitative de l’anisotropie de susceptibilite´ magne´tique. Tectono- physics, 91, 165–177. Hollestelle, J. (1961). De steenbakkerij in de Nederlanden tot omstreeks 1560, Van Gorum’s Historische Bibliotheek No. 66. Assen, The Netherlands: Van Gorum. Hrouda, F., & Henry, B. (1996). Mathematical modelling of factors affecting the regression line method for separation of ferromagnetic AMS from total rock AMS. Acta Universitatis Carolinae Geologica, 40, 3–12. Hus, J., Ech-Chakrouni, S., & Jordanova, D. (2002). Origins of magnetic fabric in Bricks: Its implications in archaeomagnetism. Physics and Chemistry of the Earth, 27, 1319–1331. In press. Hus, J., & Geeraerts, R. (1998). The direction of geomagnetic field in Belgium since Roman times and the reliability of archaeomagnetic dating. Physics and Chemistry of the Earth, 23, 997–1007. Jelinek, V. (1981). Characterization of the magnetic fabric of rocks. Tectonophysics, 79, 63–67. Jordanova, N., Karloukovski, V., & Spatharas, V. (1995). Magnetic anisotropy studies on Greek pottery and bricks. Bulgarian Geophysical Journal, 21, 49–58. Kovacheva, M., Jordanova, N., & Karloukovski, V. (1998) Geomagnetic field variations as determined from Bulgarian archaeomagnetic data. Part II: The last 8000 years. Surveys in Geophysics, 19, 431– 460. Langouet, L., Goulpeau, L., & Lanos, P.H. (1986). Les rećents progre`s dans l’e´tude de l’archeómagne´tisme des mate´riaux de´place´s en France. Journal of the European Study Group on Physical, Chemical and Mathematical Techniques Applied to Archaeology, 15, 177–185. Lanos, P. (1983). Etude me´thodologique de l’archeómagne´tisme des tuiles et des briques. Rapport de Diploˆ me d’Etudes Approfondies, Rennes, France. Lanos, P. (1987). Archeómagne´tisme des mate´riaux de´place´s. Application a` la datation des mate´riaux de construction d’argile cuite en archeólogie. Unpublished Master’s thesis, Universite´ de Rennes I, Rennes, France. Lanos, P., Kovacheva, M., & Chauvin, A. (1999). Archaeomagnetism, methodology and applications: Implementation and practice of the archaeomagnetic method in France and Bulgaria. European Journal of Archaeology, 2, 365–392. Luiken, J. (1694). Het Menselyk Bedryf. Amsterdam. McFadden, P.L., & Jones, D.L. (1981). The fold test in paleomagnetism. Geophysical Journal of the Royal Astronomical Society, 67, 53–58. Nagata, T. (1961). Rock magnetism (2nd edition). Tokyo: Maruzen. Rogers, J., Fox, J.M.W., & Aitken, M.J. (1979). Magnetic anisotropy in ancient pottery. Nature, 277, 644– 646. short Schurr, K., Becker, H., & Soffel, H.C. (1984). Archaeomagnetic study of medieval fireplaces at Mannheim- standard

252 VOL. 18, NO. 2 GEA(Wiley) RIGHT BATCH

top of RH INVESTIGATION OF TWO MEDIAEVAL BRICK CONSTRUCTIONS IN BELGIUM base of RH top of text Wallstadt and ovens from Herrenchiemsee (southern Germany) and the problem of magnetic re- base of text fraction. Journal of Geophysics, 56, 1–8. Stacey, F.D., & Banerjee, S.K. (1974). The physical principles of rock magnetism. Amsterdam: Elsevier. Stephenson, A., Sadikun, S., & Potter, D.K. (1986). A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals. Geophysical Journal of the Royal Astronomical Society, 84, 185–200. Sternberg, R., Lass, E., Marion, E., Katari, K., & Holbrook, M. (1999). Anomalous archaeomagnetic directions and site formation processes at archaeological sites in Israel. Geoarchaeology, 14, 415– 439. Tarling, D.H. (1983). Palaeomagnetism. Principles and applications in geology. Geophysics and archaeology. London: Chapman and Hall. Tarling, D.H., & Hrouda, F. (1993). The magnetic anisotropy of rocks. London: Chapman and Hall. Tarling, D.H., & Dobson, J. (1995). Archaeomagnetism: An error assessment of fired material observations in the british directional database. Journal of Geomagnetism and Geoelectricity, 47, 5–18. Thellier, E. (1981). Sur la direction du champ magne´tique terrestre, en France, durant les deux derniers milleńaires. Physics of the Earth and Planetary Interiors, 24, 89–132. Thellier, E. (1938). Sur l’aimantation des terres cuites et ses applications geóphysiques. Annales de l’Institut de Physique du Globe, 16, 157–302. Trimpe Burger, J.A. (1962–1963). Ceramiek uit de bloeitijd van Aardenburg (13e en 14e eeuw). Berichten van de Rijksdienst voor het Oudheidkundig Bodemonderzoek, 12–13, 495–548. Van Hove, R. (1994). Archeologisch onderzoek in de H.-Kruiskerk te Vrasene (Beveren, O.-Vl.). Ro- maanse driebeukige kerk (1153–1183) en voor-romeinse bewoningssporen, In R. Van Hove (Eds.), Bijdragen van de Archeologische Dienst Waasland, deel II (pp. 146–147). Sint-Niklaas, Belgium: Scheerders van Kerchove. Veitch, R.J., Hedley, I.G., & Wagner, J.-J. (1984). An investigation of the intensity of the geomagnetic field during Roman times using magnetically anisotropic bricks and tiles. Archives des Sciences Gene`ve, 37, 359–373. Yang, S., Shaw, J., & Rolph, T. (1993). Archaeointensity studies of Peruvian pottery—from 1200 B.C. to 1800 A.D. Journal of Geomagnetism and Geoelectricity, 45, 1193–1207.

Received January 11, 2001 Accepted for publication September 16, 2002

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