The gilded bronze panels of the Porta del Paradiso by Lorenzo Ghiberti: non-destructive analyses using portable XRF

The east door of the Baptistery of Florence, which is universally known with the name Porta del Paradiso, was crafted by Lorenzo Ghiberti between 1426 and 1452. This celebrated masterpiece has undergone systematic investigations since 1966, the year of the flood. The studies were mainly aimed at assessing the state of conservation, understanding the corrosion mechanisms, and defining the criteria for its the future preservation. The increasing interest for the handicraft techniques of the Reanissance art foundry also stimulated some preliminary metallurgical studies. Recently, a dedicated technological investigation project was promoted by the Opificio delle Pietre Dure, the institution responsible for the restoration work. Within this project we carried out an overall recognition of the bronze alloys used to craft five of the ten main panels of the door using a homemade portable XRF instrument equipped with 60 kV, 1.6 mA, tube and a 5 mm2 Si-Drift detector. About 230 measurements were performed mostly on the back side of the investigated panels focusing on the main casting, recasting, and plug areas. According with the few available literature data, our analyses confirmed the use of quaternary alloys. The preliminary calibration of the instrument allowed distinguishing among three different main alloy compositions while multivariate analysis of the trace elements provided information about possible different material stocks. These and other results that we discuss in the present work, represent the first objective support for a temporal collocation of the relieves and for understanding the casting and mounting sequence of Ghiberti's masterpiece.