5:15 PM - 6:45 PM
[HQR05-P03] The Geological Age-Environmental Indicator Potential of Plastic Toys (KINKESHI capsule toy series) in Modern and Contemporary Contexts
Keywords:Anthoropocene, Underwater archaeology, PVC, Plasticizer, Lake Hibara, X-ray fluorescence analysis (XRF)
With the recent acceleration of human activity, discernible traces of human presence have recorded in seabed and lake-bottom sediments. Artificial radionuclides from nuclear testing, plastic pollution, and spherical fine-grained coal residues stemming from fossil fuel encompass indicators of human activity. Additionally, the presence of domestic and industrial waste materials buried in sediment offers further insights into past human activities in local area. In the realm of archaeology, historical background is reconstructed based on morphological features of objects such as pottery and porcelain fragments, as well as stratigraphic data. However, there remains a dearth of detailed studies focusing on artifacts produced during and after the modern period, which lie beyond the conventional scope of archaeological inquiry.
A land-based excavation survey was conducted along the shoreline of Lake Hibara as part of the investigation into the 'Hibara Village Ruin', submerged following the eruption of Mount Bandai in 1888. During the research process, a human-shaped toy, seemingly composed of plastic, was discovered amidst modern and contemporary artifacts such as ceramic fragments, glass shards, and nails. Given its outstanding weather resistance, plastic is likely to retain its physical shape and chemical characteristics for an extended period following disposal or burial. Therefore, we explored the potential of plastic products as geological fossils for geochronological dating and environmental change assessment based on the physical properties measurement and conducting chemical analyses of plastic toys.
The plastic toy was excavated from within a gravel-mixed silt layer at a depth ranging from 5 cm to 15 cm below the surface. It measured 39.6 mm in length, had a mass of 3.4 g, and exhibited a density of 1.59 g/cm³. The toy's morphology suggested resemblance to an earlier version of the Kinkeshi capsule toy, a product based on the character of Kinnikuman from the Weekly Shonen Jump manga series, manufactured by Bandai Co., LTD. However, the original toy measured 46 mm in length, had a mass of 4.5 g, and a density of 1.47 g/cm³, the excavated relic appeared slightly smaller in size and denser in composition, casting doubt on its authenticity. To confirm this uncertainty in detail, material characteristics of the relic and the original were compared using various analytical techniques. Observations of the microscopic internal structure were conducted utilizing a 3D X-ray microscope, and element mapping analysis was performed using XRF and μXRF. Mineral identification was carried out using μXRD, and evaluation of non-crystalline materials was achieved through FT-IR.
The original Kinkeshi was comprised of PVC, calcium carbonate, and phthalate plasticizer in proportions of 27:40:33, respectively. 3D X-ray microscopic analysis revealed a hole approximately 0.1 mm in diameter at the center of the body of the relic Kinkeshi. XRF analysis indicated an increase of chlorine, lead, and silicon, and a decrease of calcium compared to the original Kinkeshi. μXRD and FT-IR confirmed that the relic Kinkeshi was composed of the same materials as the original. Kinkeshi with more recent production years tended to exhibit higher chlorine concentrations, lower calcium concentrations, and lower densities compared to older toys. Additionally, older toys utilized phthalate-based plasticizers, whereas newer toys employed non-phthalate-based alternatives.
These findings raise the possibility that the relic Kinkeshi could be counterfeit, as their physico-chemical characteristics differ from those of the originals. However, an alternative explanation is that these differences result from post-burial diagenetic processes. Dissolution of the plasticizer and filler, as well as ion-exchange reactions between lead in the soil and calcium in the filler, could account for the observed variations. The dissolution of the plasticizer is irreversible, attributed to reactions with microorganisms and organic matter present in the soil and lake water. A slightly elevated concentration of lead was detected in the lake sediments located some distance from the excavation site. Notably, a dumping ground of the mine site (Hibara Mine), extracted lead and operated until 1943, was situated near the excavation site. Therefore, it is plausible that ore from the dumping site may have influenced alterations in the characteristics of plastic relics.
Acknowledgments: BANDAI CO., LTD. provided valuable information and assistance. This work was supported by JSPS Grant-in-Aid JP22H00028.
A land-based excavation survey was conducted along the shoreline of Lake Hibara as part of the investigation into the 'Hibara Village Ruin', submerged following the eruption of Mount Bandai in 1888. During the research process, a human-shaped toy, seemingly composed of plastic, was discovered amidst modern and contemporary artifacts such as ceramic fragments, glass shards, and nails. Given its outstanding weather resistance, plastic is likely to retain its physical shape and chemical characteristics for an extended period following disposal or burial. Therefore, we explored the potential of plastic products as geological fossils for geochronological dating and environmental change assessment based on the physical properties measurement and conducting chemical analyses of plastic toys.
The plastic toy was excavated from within a gravel-mixed silt layer at a depth ranging from 5 cm to 15 cm below the surface. It measured 39.6 mm in length, had a mass of 3.4 g, and exhibited a density of 1.59 g/cm³. The toy's morphology suggested resemblance to an earlier version of the Kinkeshi capsule toy, a product based on the character of Kinnikuman from the Weekly Shonen Jump manga series, manufactured by Bandai Co., LTD. However, the original toy measured 46 mm in length, had a mass of 4.5 g, and a density of 1.47 g/cm³, the excavated relic appeared slightly smaller in size and denser in composition, casting doubt on its authenticity. To confirm this uncertainty in detail, material characteristics of the relic and the original were compared using various analytical techniques. Observations of the microscopic internal structure were conducted utilizing a 3D X-ray microscope, and element mapping analysis was performed using XRF and μXRF. Mineral identification was carried out using μXRD, and evaluation of non-crystalline materials was achieved through FT-IR.
The original Kinkeshi was comprised of PVC, calcium carbonate, and phthalate plasticizer in proportions of 27:40:33, respectively. 3D X-ray microscopic analysis revealed a hole approximately 0.1 mm in diameter at the center of the body of the relic Kinkeshi. XRF analysis indicated an increase of chlorine, lead, and silicon, and a decrease of calcium compared to the original Kinkeshi. μXRD and FT-IR confirmed that the relic Kinkeshi was composed of the same materials as the original. Kinkeshi with more recent production years tended to exhibit higher chlorine concentrations, lower calcium concentrations, and lower densities compared to older toys. Additionally, older toys utilized phthalate-based plasticizers, whereas newer toys employed non-phthalate-based alternatives.
These findings raise the possibility that the relic Kinkeshi could be counterfeit, as their physico-chemical characteristics differ from those of the originals. However, an alternative explanation is that these differences result from post-burial diagenetic processes. Dissolution of the plasticizer and filler, as well as ion-exchange reactions between lead in the soil and calcium in the filler, could account for the observed variations. The dissolution of the plasticizer is irreversible, attributed to reactions with microorganisms and organic matter present in the soil and lake water. A slightly elevated concentration of lead was detected in the lake sediments located some distance from the excavation site. Notably, a dumping ground of the mine site (Hibara Mine), extracted lead and operated until 1943, was situated near the excavation site. Therefore, it is plausible that ore from the dumping site may have influenced alterations in the characteristics of plastic relics.
Acknowledgments: BANDAI CO., LTD. provided valuable information and assistance. This work was supported by JSPS Grant-in-Aid JP22H00028.