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

GEA(Wiley) RIGHT BATCH Archaeomagnetic Investigation of Two Mediaeval Brick Constructions in North Belgium and the Magnetic Anisotropy of Bricks 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 top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text base of text 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 226 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 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 as- sumptions 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 rectan- gular 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 GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL 227 GEA(Wiley) LEFT BATCH top of RH HUS, ECH-CHAKROUNI, JORDANOVA, AND GEERAERTS base of RH top of text base of text 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 228 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 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.

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