Casting cores used to craft large bronze masterpieces of the Florentine Renaissance and Mannerism

We investigated the casting cores of twelve large bronze masterpieces of the Florentine Renaissance and Mannerism (artworks by Ghiberti, Donatello, Verrocchio, Rustici, and Danti). Materials were characterized in their mineralogical-petrographic features, chemical composition, organic content and fossil traces. The results achieved provide information on the main recipes employed for manufacturing the casting cores, which largely agree with written sources of the time, and shed light on the art foundry processes between the 15the16th centuries. The data collected point out a pronounced affinity of natural raw materials used in all the casting cores investigated, consisting in silty-sandy clays with a suitable natural fine-grained temper component. A large use of organic fibres and matters of both animal and vegetal origin and a minor use of gypsum in selected workshops (Rustici, Verrocchio) were highlighted. Textural analyses allowed gaining information on direct vs indirect methods of casting, while firing temperatures up to about 1000 C were estimated from mineralogical and microstructural observations. Despite the strong similarities, significant differences among the various samples analysed were pointed out, providing some initial analytical clues that suggest different practices in separate workshops. Finally, the whole petrographic, geochemical and paleontological data gave information on the provenance of the clayey raw materials. This work represents the first step of a systematic approach to the classification of the core materials used in Florentine foundries along the centuries and shows the significant contribution the petrographic analyses can provide to the interpretation of the execution processes of large bronzes.