A multi-analytical diagnostic approach to study patinas, trace of colours and degradation process on the stone surface of a Renaissance Piety from the Museum of Ancient Art (Castello Sforzesco, Milan)

Abstract

Alteration dark patinas and traces of colours present on the surface of a marble Piety dating to the Renaissance period and stored at the Castello Sforzesco-Museum of Ancient Art (Milan, Italy) were studied and chemically characterized. For this purpose, a multi-analytical approach based on both portable non-invasive (XRF and colorimetric measurements) and micro-invasive techniques (FTIR/ATR and SEM-EDS), has been applied. The statue has been recently submitted to a restoration before an exhibition at the Louvre Museum and the original appearance of the marble surface recovered thanks to the patina removal. Through the analytical characterization carried out before and after the marble cleaning, the presence of a degradation layer composed by gypsum was evidenced on the stone surface. The origin of this layer is probably ascribable to the exposure of the statue to outdoor environment and interaction with atmospheric pollution. The chemical nature of the coating applied at the end of 19th century and responsible for the patina formation was hypothesized. Finally, the use of pigments originally applied by means of an organic binder was also highlighted.