Effect of Supersaturated Solid Solution of Oxygen on Fine Structure of Rust of Old Japanese Nails

The oxygen concentration in old Japanese nails was measured to be 0.15 mass% to 0.38 mass% by EPMA. These nails were one with bended top used for the main hall of the Anumi shrine in 1835, one with rolled top for the gate of the Saidai-ji temple in ca. 1300, one with rolled top for the eaves of the main hall of the Amanosan-kongo-ji temple in 1085 similar to 1124 and one with bended top in the five-storied pagoda of the Bicchu-Kokubun-ji temple before 800. At the measurement point of EPMA, no oxide with the size more than 10 nm was detected by HRTEM. Oxygen in iron base of the nails is in the state of supersaturated solid solution. Oxygen was absorbed in molten iron with white sparks, "Wakibana", during the processes of Tatara smelting, decarburization in Okaji process and forge-and-welding, followed by rapid solidification of iron for oxygen to be supersaturated in solid solution. This is the common characteristics of pre-modern ironmaking process in the world. A thin layer of Fe3O4 was observed in the interface between FeO layer and iron base of nail, which was formed by the reaction of oxygen supersaturated in solid solution at the interface during forging. Most of inclusions in nails were FeO silicate slag with FeO fine particles. As there were FeS particles in silicate slag inclusions in the nail of the Bicchu-kokubun-ji temple, the nail was probably manufactured from iron produced using the mixture of iron sand and iron ore by Tatara smelting in the south of Okayama prefecture.