Evolution of renaissance and mannerism artistic foundry: a petrographic study of casting cores used to craft florentine bronze masterpieces

The petrographic and geochemical characterization of core materials used to craft bronze artworks represents a recognized tool in archaeometallurgical studies. It can provide relevant contributions to the solution of knowledge problems such as those related with the development of art foundry, as well as provenance, attribution and dating of the object under study (e.g. [1] and references therein). Notwithstanding, till today only a few investigations have been carried out on the topic, mainly concerning ancient age artistic productions. Overviews regarding core materials used in Renaissance bronzes have been reported by some Authors [2, 3, 4], but further insights are needed in order to achieve a deeper knowledge on 15th and 16th centuries art foundry, especially concerning large bronze artworks. Here we report a petrographic study of casting cores of masterpieces by Lorenzo Ghiberti (Sacrificio di Isacco, 1401, and Porta del Paradiso, 1425-1452), Donatello (David and Amore-Attis, 1430-1450), Andrea del Verrocchio (Incredulità di San Tommaso, 1466-1483), Giovan Francesco Rustici (Predica del Battista, 1506-1511) and Vincenzo Danti (Decollazione del Battista, 1570-1571). Samples were taken from these artworks and studied by means of polarized light microscopy in thin (cross) section, computerized image analysis, XRD, SEM-EDS. The core materials of these sculptures show remarkable petrographic similarities, thus suggesting a well established tradition in the selection and processing of raw materials, kept during the whole period analysed. The collected data suggest the general use of silty-sandy clays, provided from the origin with a certain content of natural temper components, to which variable contents of chamotte fragments and organic fibres have been purposely added. In some artworks, a further addition of sand can be hypothesized. Gypsum additions have been observed in Verrocchio’s and Rustici’s sculptures. Probable differences in supply localities of raw materials are testified by weak differences in composition of matrix and skeleton grains, being anyway coherent with the Florentine geological background. Colour, mineralogical association and microstructure are quite variable among different artefacts and sometimes within a single artefact, strongly depending on the distance from the metal wall (i.e. transient heating produced by metal pouring), on the presence of organic matter and on the exposition (closed or open shapes, or possible pre-cooking before the casting). General and peculiar features pointed out in the present work provide important data on the evolution of the art foundry during the Renaissance and Mannerism.