Ancient building practices in the Phlegraean Fields: material characterization of the Tempio di Apollo (Italy)

Giovanna Montesano¹, Piergiulio Cappelletti^2,3, Renata Esposito⁴, Sossio Fabio Graziano⁵, Luigi Guerriero², Maria Laura Iadanza⁶, Martina Mercurio², Fabio Pagano⁶, Marida Salvadori⁶ & Concetta Rispoli^2
1Dipartimento di Ingegneria e Geologia (INGEO), Università di Chieti-Pescara G. D’Annunzio, Chieti, Italy.
²Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università degli Studi di Napoli Federico II.
³Centro Musei delle Scienze Naturali e Fisiche, , Napoli, Italy.
⁴Indipendent Archaeologist Researcher - collaborator of the Ministry of Culture, Napoli, Italy. ⁵Dipartimento di Farmacia, Università a degli Studi di Napoli Federico II, Napoli, Italy.
⁶Phlegraean Fields Archaeological Park, Rione Terra, Palazzo De Fraja, Pozzuoli, 80078, Napoli, Italy.
Corresponding author e-mail: piergiulio.cappelletti@unina.it

This study investigates the building materials and techniques used in the Tempio di Apollo, located in the Phlegraean Fields (southern Italy), an area of active volcanoes with caldera-forming eruptions in the past and also characterized by present-day bradyseism. The availability of pyroclastic deposits strongly addressed the local building practices. Mortar and brick samples were collected from selected structural sectors of the monument, adopting a minimally invasive sampling approach. Analytical investigations were carried out through Polarized Light Microscopy (PLM), X-ray Powder Diffraction (XRPD), and Scanning Electron Microscopy (SEM). The mortars are lime-based and show variable degrees of cohesion. The binder appears micritic to cryptocrystalline, with frequent lime lumps. The aggregates are mainly composed of volcanic fragments such as pumice, with occasional ceramic fragments. XRPD data confirm calcite as the dominant binder phase, reflecting the original choices in raw material; the presence of gypsum and halite reflect subsequent alteration processes. These results underline the deliberate use of local volcanic products and illustrate the technological strategies adopted by Roman builders to ensure durability and structural performance. The study contributes to a broader understanding of ancient construction practices in dynamic volcanic landscapes.

Roman mortars, hydraulicity, ceramic.