This paper reports on the characterization of the polychromy and alteration films found on the big relief Madonna con Bambino and Angeli Adoranti on the spire of the 14TH -century Porta dei Cornacchini of the Florentine Cathedral. Recently a multidisciplinary study has been undertaken to characterize the polychrome finish layer, discovered for the first time on the sculptures, and at the same time to define the composition and the origin of the alteration film observed on marble surface. In situ non-destructive investigations (XRF) were carried out and samples were analyzed by different instruments (optical microscopy, FT-IR, EDX connected to a scanning electron microscope). Finally, cleaning tests have been carried out through laser technique and aqueous and mechanical medium. Therefore, the diagnostic investigation was fundamental in defining the methodological choices and above all the level of cleaning during the restoration.