## 3 Results and Discussion  
MA-XRF Before Restoration: Evaluating the Extension and the Conditions of the Original Pictorial Layer  
The presence of an extended intervention by [PERSON] was initially evident to conservators, and the comparison between the Cu and Fe maps strongly supported their hypothesis (Figure 2a,b). Two different modelling of the blue drapery are indeed visible, in particular on the [PERSON]'s proper left shoulder and sleeve. Moreover, in the blue portion of the mantle over the head, Cu is present on only one side; on the contrary, Fe is present in the whole area. In addition, there is a difference also on the [PERSON]'s proper left shoulder, which is slightly \"higher\" in the Fe map than in the Cu one. Moreover, few paint losses are visible in the Cu map, while the Fe map shows an intact painting layer. From these evaluations it is possible to hypothesise that Cu is most probably characteristic of the original pictorial layer composed of azurite [43] and thus painted by [PERSON], while Fe is possibly present as Prussian blue, introduced during the early XVIII century and thus available during [PERSON]'s intervention [44]. The drapery visible in the painting (before restoration intervention) matches with the distribution of Fe, confirming that this element is present in the superficial layer. It is, however, true that Prussian blue cannot be absolutely determined with XRF alone and needs other analytical methods to be conclusively identified, such as FTIR [45]. Lead white, the presence of which is suggested by the distribution of Pb (Figure 2c), was most likely employed together with Prussian blue in [PERSON]'s layer. Indeed, the modelling of the drapery visible in the Pb map matches with that of Fe. The mixture of Prussian blue, deep dark in colour, and lead white (or other white pigments) is common [46] and thus not surprising. However, it cannot be conclusively excluded that lead white is present (also) in the original pictorial layer. It has to be noted that the presence of Pb in principle may be due also to other lead-based compounds, such as massicot (yellow) or minium (orange) and cannot conclusively distinguished by XRF [47]. It is, however, true that the hypothesis of the use of lead white is the most probable. Figure 1: The _Madonna Poldi Pezzoli_ (**a**) before and (**b**) after restoration (courtesy of the Poldi Pezzoli Museum). The distribution of the Cu map has been of utmost importance for the restoration intervention. Indeed, the Cu map showed that the most likely original painting layer, with the exception of sporadic lacunas, was in a good conservation state and possibly rather intact. Together with X-ray radiography carried out by the OPD scientific laboratory [39], MA-XRF allowed researchers to determine that the percentage of lacunas overall in the supposed original layer was less than 3%. This information strongly supported the choice of removing [PERSON]'s layer. There are other elements detected which are most likely related to [PERSON]'s intervention. For instance, the presence of Co traces in the mantle of the [PERSON], particularly over the shoulder and in the border (close to the edge of the painting), may be due to the use of Cobalt blue by [PERSON] himself or to an earlier intervention with small (a blue Co-based glass pigment employed in painted works from the mid XV century [48]). Most probably, [PERSON] also added the \"marbling\" effect on the red garment of the [PERSON], realised with shell gold and visible in the Au map (Figure 3a). Similarly, the red edge of the sleeve was retouched by [PERSON] with vermilion (see Hg map, Figure 3b). In these cases, the evaluation of [PERSON]'s intervention is not directly possible with XRF, as shell gold and vermilion are materials which are coeval with [PERSON], and this evaluation was accomplished by visual and optical microscopy inspection by OPD conservators [39]. Similarly, the red sleeve cuff (visible in Hg map in Figure 3b) was considered a later addition by OPD conservators.