Painted plasters from the oldest basilica of Tusculum (Italy)

The examination of mortars and plasters from the oldest basilica of Tusculum—one of the earliest archaeologically documented basilicas in Italy, dating to the late 3rd – early 2nd century BCE—offers valuable insights into the technological expertise, material provenance and production techniques of ancient construction practices. A total of 11 plaster samples, 2 mortars, 2 cocciopesto samples, and the complete range of pigments were collected for analysis. The methodological approach includes optical microscopy, scanning electron microscopy, electron microprobe analysis, X-ray diffraction, inductively coupled plasma mass spectrometry and thermogravimetric analysis. The results highlight the complex layering sequence of the plasters, revealing successive interventions characterized by the use of different aggregates and binders. The outermost layer exhibited a technique rarely encountered in that period, resembling the marmorino finishing. The inner plaster aggregates deviate from conventional sand aggregates, comprising two types of pozzolanic materials selectively choiced: fine, black pozzolans in the outer layers, and coarse, red pozzolans in the inner layers. The binder composition varies across the layers, being aerial in the outer layers and hydraulic in the inner layers. As for provenance of the raw materials, comparative geochemical analysis aligns both aggregates' provenance with local volcanic districts, notably the Alban Hills. Specifically, the red pozzolans are comparable to the Pozzolane Rosse, a type of pozzolan increasingly used in Roman monuments from the late 1st century BCE due to their advantageous properties. The black pozzolans, on the other hand, showed strong compatibility with Pozzolane Nere and, to a lesser extent, Pozzolanelle. Overall, the study provides valuable insights into the historical exploitation of volcanic resources and the technological advancements in mortar production in the architecture of towns neighboring Rome in the late 3rd – early 2nd century BCE.