The restoration of polychrome sculptures is an extremely complex activity and simultaneously tackles different conservation problems, on supports of a non-ordinary nature such as oil painting applied to marble, or the gilding of sculpted surfaces. These are very delicate finishes, compromised by clumsy attempts at maintenance and by centuries of oblivion.

Certainly one of the most faced and important topics in the restoration is the cleaning phase. With this "fundamental" activity, the restorer tries to re-establish a balance between the visible nature of the work and its conservation, implementing a series of actions that aim at the rediscovery of the "colors and the original substrate" and at the conservation of the work in time. We are used to thinking that works, whether pictorial or sculptural, are simply "dirty" or damaged before a restoration. It is not so. The history of a work must absolutely contemplate the maintenance activities, the rethinking that occurred in the various eras, the attempts at "modernization" and other activities carried out to try to preserve it, update it, prolong its life or change its appearance. The sculptures of the Gothic portal, weighed down by more than 600 years of history, appeared to be covered with a complex layer of dust, mixed with the black smoke of the candles, layers of protective waxes of various kinds and drying oils mixed with pigments. These substances were superimposed on the pictorial finishes, on the very delicate drawings and spread into the interstices of the precious carvings of the sculptures. The restoration operations began by gently lightening incongruous deposits, working layer by layer and testing each step with caution and behind a continuous punctual check to check the result.