The exterior of the basin is decorated with couples of figures representing moments of the ritual of the baptism, or the roles attribuited by the popes to the bishop of Saint Donnino. Between the figures, stands out a pope showing a roll with the inscription "INSTITVCIO ALEXANDRI PP. II" interpreted as the foundation act in the church of Borgo of a baptismal font, indipendent from Parma. During the recent restoration, some encrustation of limestone have been removed, probably due to the flowing of the water, so it has been supposed that the basin was originally a "fountain" above a baptismal tank, collecting the acqua viva for the rite; this is coherent also with the better visibility of the figures from below.