Recent advances in archeomagnetism have resulted in sophisticated databases for published archeomagnetic data. For example, the GEOMAGIA50 database (Donadini et al. 2006; Korhonen et al. 2008) involves about 8,000 geomagnetic field directions and intensities for the past 50 kyr. The ARCH3k database by Donadini et al. (2009) contains 2,671 declination, 4,174 inclination, and 2,670 intensity data from archeological artifacts and lavas for the past 3 kyr. Genevey et al. (2008) compiled the Archeoint database for the past 10 kyr which has 3,648 archeointensity records reported from archeological artifacts and lavas.
Concerning the Archeoint database, 70% of the records are from Europe while only 12% of them are from East Asia. Among the East Asia records, 188 data are from Japan: 145 data from archeological artifacts (9 data, Nagata and Arai (1963); 19 data, Sasajima and Maenaka (1966); 56 data, Kitazawa (1970); 6 data, Domen (1977); 58 data, Sakai and Hirooka (1986)) and 43 data from lavas (7 data, Nagata and Arai (1963); 2 data, Sasajima and Maenaka (1966); 6 data, Kono (1978); 1 data, Tanaka (1979); 13 data, Tanaka (1980); 3 data, Tsunakawa and Shaw (1994); 1 data, Takai et al. (2002); 10 data, Yoshihara et al. (2003)).
In broad sense, ‘archeointensity’ stands for an absolute paleointensity of the geomagnetic field during historical period which is estimated from both archeological artifacts and lavas. In narrow sense, the estimation material is limited to archeological artifacts. Generally speaking, archeological artifacts have been considered to be more reliable paleointensity recorders, because they were certainly burned/baked by our ancestors and their natural remanent magnetizations (NRMs) are definitely thermoremanent magnetization (TRM) origin with good thermal stability.
There has been a long gap in time since the last internationally recognized archeointensity result was published from archeological artifacts in Japan (e.g., Sakai and Hirooka 1986). In contrast, brand new data with modern paleointensity techniques have been published from East Asia outside Japan, for example, Korea (Yu et al. 2010; Hong et al. 2013) and China (Cai et al. 2014). In 1960s to 1970s, a group of Japanese researchers had made systematic oriented-sample collections from baked clay at many pottery kilns excavated in and around Sakai city, Osaka prefecture, Japan. The collections were associated with a lot of excavations arising from a big demand of housing land developments due to the growing economy in Japan at that time. Paleomagnetic directions for fifth to tenth centuries were intensively measured from these samples, and they were published by Hirooka (1971) and Shibuya (1980). Untreated and/or partially demagnetized samples have been reserved and stored for further archeointensity (paleointensity) researches.
A relatively large number of the reserved samples were partially demagnetized by alternating field (AF) up to 20 to 40 mT. Thus, AF-based paleointensity techniques are thought to be suitable for these samples. The Tsunakawa-Shaw method, which has been previously called the LTD-DHT Shaw method (Tsunakawa and Shaw 1994; Yamamoto et al. 2003), is one of such techniques. Its applicability and validity have been elucidated for various types of volcanic rocks from historical lava flows (e.g., Yamamoto et al. 2003; Mochizuki et al. 2004; Oishi et al. 2005; Yamamoto and Hoshi 2008) but remain unassessed for archeological artifacts including baked clay.
In the vicinity of the archeological sites, Nakajima et al. (1974) conducted a reconstruction experiment: they reconstructed a kiln which was carefully imitating an excavated kiln of the seventh century and measured paleomagnetic directions from baked clay samples taken from the kiln. These samples have been reserved and stored after partial AF demagnetization by 20 to 40 mT in 1972. In the present study, we applied the Tsunakawa-Shaw method to these samples. Because the reconstruction experiment was done after the measurement of the in situ geomagnetic field, we can compare the in situ filed with archeointensity results obtained by the Tsunakawa-Shaw method. We also conducted a suite of rock magnetic experiments and an observation with scanning electron microscope, to characterize rock magnetic properties of the samples.
Reconstruction experiment
The reconstruction experiment was conducted in Sakai city, Osaka, Japan, on January 1972 (Nakajima et al. 1974). A Noborigama kiln, which was carefully imitating an excavated one of the seventh century, was reconstructed and fired with many Sue-type earthenwares in it (Figure 1).
After cutting the trees and weeds (Figure 1a), the ground was dug up (Figure 1b). Then in situ geomagnetic field was measured by a Schmidt-type magnetometer (Figure 1c): declination (Dec) = −5.63°, inclination (Inc) = 46.78°, and intensity (Int) = 46.350 μT. This is fairly consistent with the field calculated from the model of IGRF-11 (IAGA Division V, Working Group V-MOD 2010) at the place for the year of 1972: Dec = −6.23°, Inc = 47.7°, and Int = 46.438 μT. To record in situ temperature variations during the firing, thermocouples were embedded in and around the kiln (Figure 1d,e). After the embedment, the body of the kiln was made up with bamboos and tree branches, and they were subsequently covered and coated with clay (Figure 1f). With Sue-type earthenwares, the firing was done using naturally grown pine trees and other miscellaneous woods taken around the kiln (Figure 1g,h).
It was found that the temperature very close to the floor surface (−2-cm level, three different positions) was risen up to approximately 1,000°C while that at the level 20 cm below the floor (−20-cm level) was only up to approximately 350°C during firing (Figure 2). After reaching to the highest temperature of approximately 1,000°C, the fire hole and the chimney of the kiln were blocked to make the inside reductive atmosphere. The kiln was then naturally cooled down for approximately 24 h. After the cooling down (Figure 1i,j), baked clay was carefully sampled from the floor surface (Figure 1k) and the −20-cm level (Figure 1l) using plaster of Paris for accurate orientation. Each sample was cut in laboratory into cubic shape with approximately 3.7 cm on a side.
The samples were subjected to partial AF demagnetization up to 20 to 40 mT. Nakajima et al. (1974) reported that the mean paleomagnetic directions resulted in Dec = −5.03°, Inc = 43.37°, and α95 = 2.42° for the floor surface samples (N = 10) and in Dec = −4.80°, Inc = 43.52°, and α95 = 3.38° for the −20-cm level samples (N = 5). These directions were reasonably consistent with the in situ geomagnetic field direction measured prior to the firing, though the mean inclinations were approximately 3° shallower than the in situ field.