Hierapolis of Phrygia was one of the main Hellenistic, Roman and Byzantine cities in South-Western in Asia Minor (Pamukkale, Denizli province, Turkey), protected by UNESCO since 1988, was built in the third century B.C. and, even if damaged several times by earthquakes, it has survived for over a millennium. The site is also known for the white travertine waterfalls made of calcareous deposits of thermal waters which gave the name to the site ("Pamukkale" means "cotton castle") [1-3]. During the antiquity, Hierapolis was also a famous pilgrimage centre because of the presence of the Ploutonion, the sanctuary dedicated to Hades-Pluto and his wife Kore-Persephone, visited by numerous ancient writers, such as Cicero and Strabo [4]. The sanctuary was monumentalized during the early Imperial period (1st century B.C. -1st century A.D.) exactly above the entrance to a natural cave, from which thermal waters and poisonous gases emerge. It was used for sacred rituals: sacrifices, incubation, healing. As referred by Strabo (XII, 4, 14),the Ploutonion grotto is characterized by the emission of gases, which were toxic to the animals approaching it during sacred rituals, while the Galli, the eunuch priests of the goddess Cybele, miraculously entered the cave and remained inside without any harm, demonstrating their supernatural power [4][5][6]. The gas composition is mainly characterized by CO 2 , that is colorless, odorless, and tasteless [7][8]. During the last years, the excavation conducted by the Italian Archaeological Mission, brought to light the entire sanctuary, discovering the grotto and the theatre, and numerous findings [4][5]. The internal walls of the natural cave were found entirely blackened and covered by manganese-based encrustations [9]. In this study, the characterization of different materials found in the excavation area of the Ploutonion is reported.
