## 3 Results
### XRF Data  
The results of the XRF analysis are summarized in Table 2, in which the detected elements are listed in order of decreasing abundance. The data revealed the joint presence of calcium (Ca), iron (Fe), rubidium (Rb), strontium (Sr) and zirconium (Zr), attributable to the constituent materials of the plaster, based on lime (to which calcium and strontium can be referred) and of a material of volcanic origin such as pozzolana (characterized by the presence of iron, rubidium and zirconium). The analysis is not able to identify the possible presence of an additional silicate filler (due to the impossibility of detecting silicon) or a carbonate one (the calcium of the charge would not be distinguishable from that of the binder). The ubiquitous presence of lead (Pb) could suggest the presence of this element in trace amounts in the plaster filler. Alternatively, the same element could be related to the use of modest quantities of lead white (or another lead-based pigment, for example minium/red lead) by the painter. In the blue points, a constant presence of arsenic (As), cobalt (Co), nickel (Ni) and bismuth (Bi) was detected, which allow us to identify the use of malt. Residues of this pigment are also correspondingly present in the lacunae of the pictorial layer in the [PERSON]'s mantle.  
The presence of Co, As and Ni is associated with the minerals used for producing blue malt, such as smaltite (in the Middle Ages) and later erythrite and cobaltite (in the seventeenth and eighteenth centuries) [35]. According to some authors, the presence of bismuth associated with Co, As and Ni suggests that the minerals came from the Erzgebirge mines in Saxony [36; 37].  
In the red-colored areas, as well as for the red component of the skin tone of [PERSON], the XRF analysis indicates the presence of mercury and iron, which indicates the use of vermilion/cinnabar (HgS) and earth pigments. On the other hand, vermilion/cinnabar seems absent from the red brushstrokes of the blood that gushes from the wounds on [PERSON]'s legs, painted with red earth pigments. In the tracts of blood with a more orange and brilliant color, the increase in intensity of the lead fluorescence line and the simultaneous decrease of that of iron might suggest that minium was also employed. However, it is necessary to consider that XRF analysis cannot reveal the presence of organic dye pigments, which could have been used for the making of the blood.  
At some points, XRF detected traces of arsenic, such as in the white in the crescent moon on which the [PERSON] stands, in the brightest yellow light of the mantle of [PERSON] (whose color is attributable to yellow ocher) and in the dark part of the red robe of the [PERSON]. Here, the presence of arsenic could be due to the use of a very small quantity of malt blue (for which cobalt, nickel and bismuth would be in quantities smaller than the  
Figure 7: Result of the multispectral similarity tool (**A**) and cluster analysis applied to the IR1, IR2 and IR3 bands (**B**).  
detection limits of the instrument used in the configuration analysis), or to impurities of this element in some earth pigments, or to the use of orpiment (As\({}_{2}\)S\({}_{3}\)) and/or realgar (As\({}_{4}\)S\({}_{4}\)). The latter hypothesis seems, however, rather unlikely, considering the low photon counting rate related to arsenic, which is not sufficient to provide a chromatic perception in the painting.  
Lastly, an arsenic contamination could also be traced back to the residues of sanitizing treatments for wooden elements, such as the extensive ones on the ceiling of the room hosting the wall painting.