Non destructive evaluation of deterioration of the rock cliff Buddha sculptures (Magaibutsus) in Oita Prefecture, Japan, based on magnetic susceptibility and moisture content

Deterioration of the rock-cliff Buddha sculptures (Magaibutsus) in Oita Prefecture, Japan, was evaluated by using portable magnetic susceptibility meter and moisture meter. Investigation was carried out for the Magaibutsus of Sugao, Ogata Miyasako, Inukai, Usuki, Monzen and Oita Motomachi. These Magaibutsus were carved in the rock-cliff of the Aso-4 pyroclastic flow sediments. There is a tendency that magnetic susceptibility is high (more than 7 0× 10⁻³SI unit) for the non-deteriorated Magaibutsus preserving pigments on the surface, and moisture content for those Magaibutsus is low (less than 3%). On the other hand, deteriorated Magaibutsus have low magnetic susceptibility and high moisture content. The results suggest that the deterioration proceeds in the following order: Ogata Miyasako, Sugao < Inukai, Usuki, Monzen < Oita Motomachi. The relationship between magnetic susceptibility and moisture content reveals that magnetic susceptibility is more sensitive than moisture content to deterioration of the Magaibutsus. Deterioration of the Magaibutsus seems to be mainly attributable to salt crystallization and freeze-thaw cycle.