[M-IS04] Geomaterials in cultural heritage: investigation techniques, weathering, and conservation

[MIS04-02] Salt weathering in the archaeological site of Yoshimi Hyaku Ana (Saitama, Japan): the petrographic and environmental background

*Luigi Germinario^1,2, Chiaki T. Oguchi¹ (1.Department of Civil and Environmental Engineering, Saitama University, Japan, 2.Department of Geosciences, University of Padova, Italy)

Keywords:Subterranean cultural heritage, Cave, Tuff deterioration, Soluble salts, Deliquescence, Microclimate monitoring

This contribution reports on the investigation of underground stone decay in the archaeological site of Yoshimi Hyaku Ana. It is a tomb complex from the 6^th-7^th century that also includes a subterranean gallery network from the WWII era, located in Saitama Prefecture in Japan. The site is affected by intense salt weathering, with efflorescences and crusts composed of mixed soluble sulfates: gypsum, alunogen, alum-(Na), halotrichite, epsomite, polyhalite, tamarugite, thenardite, and mirabilite. The driving forces of salt weathering were explored by investigating the stone properties and the surrounding environment, and by researching how those variables interact with one another. The soluble salts derive mainly from the dissolution of pyrite, glass, and other rock-forming minerals from the volcanic tuffs into which the underground galleries are excavated; the tuffs are characterized by changing chemical-mineralogical composition that controls the salt variability. This is also influenced by the microclimate: in the most isolated areas, where relative humidity is extremely high (~100%), salts and crusts do not form; near the surface, salt weathering is evident instead, with the temperature and humidity fluctuations favoring the cyclic and seasonal nature of salt crystallization/dissolution and hydration/dehydration.