5:15 PM - 7:15 PM
[MZZ43-P05] Archaeomagnetic Study of Medieval Haji Ware Excavated from the Ruins of Irino Castle in Kuroshio Town, Kochi Prefecture
The ruins of Irino Castle are the remains of a medieval mountain castle located in Kuroshio Town, Kochi Prefecture. Based on historical documents and other sources, the site is identified as the residence of the Irino clan, a powerful local ruling family from the 15th to 16th centuries. In response to a planned relocation of residential areas to higher ground, an emergency excavation was conducted from 2016 to 2017. This excavation was prompted by the reassessment of potential tsunami heights following the damage caused by the 2011 Great East Japan Earthquake, particularly concerning the anticipated impact of a future Nankai Trough earthquake. As a result of the excavation, various structural remains, including those in the Nino-dan and Tsume-no-dan areas, were uncovered. Among the artifacts, a large number of Haji ware pottery fragments were reported.
In this study, various archaeomagnetic analyses were performed on pottery fragments excavated from three locations: Nino-dan, Tsume-no-dan West, and Tsume-no-dan East. Through these analyses, we attempted to evaluate the excavated artifacts and structures based on a scientific approach distinct from conventional archaeological methods. Especially, in an effort to explore new ways of utilizing cultural heritage, this study focused on numerous pottery fragments that are considered relatively less valuable from an archaeological perspective—specifically, fragments that are not from easily identifiable parts such as rims or bases.
Stepwise thermal demagnetization was performed on two samples from each site. The results showed that four samples exhibited linear demagnetization of natural remanent magnetization (NRM), while two samples displayed curved or kinked demagnetization behavior around 300–460°C. These demagnetization patterns likely reflect the firing techniques used in pottery production and the actual usage of the pottery, possibly as candlestands or other objects.
Magnetic hysteresis analysis revealed that while the samples from Tsume-no-dan East showed slightly different tendencies, the samples from Nino-dan and Tsume-no-dan West exhibited similar hysteresis parameters. This suggests similarities in the raw clay material and firing environments of the latter two groups. When rock magnetic properties are similar among samples, the ratio of NRM to magnetic susceptibility can serve as an indicator of the geomagnetic field intensity at the time of pottery firing. This ratio tended to be higher in the Nino-dan samples compared to the Tsume-no-dan West samples. Previous studies indicate that geomagnetic intensity around Japan showed a decreasing trend from the 15th to the 16th centuries. Based on this, the formation period of the structures at Nino-dan may be relatively older than that of Tsume-no-dan West.
In this study, various archaeomagnetic analyses were performed on pottery fragments excavated from three locations: Nino-dan, Tsume-no-dan West, and Tsume-no-dan East. Through these analyses, we attempted to evaluate the excavated artifacts and structures based on a scientific approach distinct from conventional archaeological methods. Especially, in an effort to explore new ways of utilizing cultural heritage, this study focused on numerous pottery fragments that are considered relatively less valuable from an archaeological perspective—specifically, fragments that are not from easily identifiable parts such as rims or bases.
Stepwise thermal demagnetization was performed on two samples from each site. The results showed that four samples exhibited linear demagnetization of natural remanent magnetization (NRM), while two samples displayed curved or kinked demagnetization behavior around 300–460°C. These demagnetization patterns likely reflect the firing techniques used in pottery production and the actual usage of the pottery, possibly as candlestands or other objects.
Magnetic hysteresis analysis revealed that while the samples from Tsume-no-dan East showed slightly different tendencies, the samples from Nino-dan and Tsume-no-dan West exhibited similar hysteresis parameters. This suggests similarities in the raw clay material and firing environments of the latter two groups. When rock magnetic properties are similar among samples, the ratio of NRM to magnetic susceptibility can serve as an indicator of the geomagnetic field intensity at the time of pottery firing. This ratio tended to be higher in the Nino-dan samples compared to the Tsume-no-dan West samples. Previous studies indicate that geomagnetic intensity around Japan showed a decreasing trend from the 15th to the 16th centuries. Based on this, the formation period of the structures at Nino-dan may be relatively older than that of Tsume-no-dan West.