## 3 Results  
### Characterization of the Reference Panel  
The capabilities of STEAM have been tested by applying the protocol to a panel containing a set of samples whose characteristics were known (see Table 1). Table 3 reports the series emerging from the GS obtained from the spectra of the reference panel and ranked following the abundance criterion. The list establishes the order of removal for  
Figure 3: Statistically Tailored Elemental Angle Mapper (STEAM) flow chart. the series (ranking equal to zero, \(q\) = 0, means that no channels have been deleted from the spectra before the evaluation of the corresponding set of similarity maps, \(\left[A\right]_{k_{0}}\)). The first places of the ranking are a prerogative of the series of the Pb and this is straightforward: Lead should appear in nearly all the analyzed spectra since the pigments' sequences have been spread on a panel prepared with a layer of White Lead over gypsum. On the other hand, the peaks related to the X-ray lines of elements such as Hg (L\({}_{\alpha}\) at 10.0 and L\({}_{\beta}\) at 11.8 keV) or Fe (K\({}_{\alpha}\) at 6.4 keV), which characterize, respectively, cinnabar and the couple hematite/red ochre (red ochre is a clay colored by a variable amount of hematite), come toward the end of the list because their contributions have been detected in a limited number of sequences. Figure 4 shows the sets of similarity maps, \(\left[A\right]_{k_{q}}\), resulting from the comparison between the spectra of the database (the Giotto's gable) and those collected on the reference panel. Figure 4A shows the results obtained without manipulating the spectra. It is possible to observe the presence of different groups of similarity distributions within \(\left[A\right]_{k_{0}}\), but any partition is easily ascribable to the features reported in Table 1. Being hematite/red ochre and red lake pigments with very different composition, the sequences they belong should be distinguishable, but this does not arise from \(\left[A\right]_{k_{0}}\) even if hematite/red ochre is characterized by Fe while red lake does not contain elements detectable by either the spectrometers. Cinnabar-based sequences cannot be clearly recognized too; even if Hg should be a distinctive element, from Figure 4 it is quite difficult to suppose a significant difference between the sequences 15 (Cinnabar) and 5 (Carmine/Red Lake) or argue that 15 and 10 both contain Cinnabar (Hg). The fact that Hg and Fe do not constitute a peculiar feature is not due to the low sensibility of the instruments, otherwise, these elements would not be noticed within the global spectrum; it could be that the counts for the peaks at 6.4 (K\({}_{\alpha}\) emission of Fe), 10, and 11.8 keV (respectively, L\({}_{\alpha}\) and L\({}_{\beta}\) emissions of Hg) are too low for making iron or mercury a distinctive characteristic of the related spectra. Essentially, Hg and Fe cannot be a real signature because mercury and iron's X-ray main series have low counts in comparison with Lead which, in turn, dominates and flattens the \(\left[A\right]_{k_{0}}\) similarity maps as foregone since the panel has been prepared with White Lead. Figure 4B shows what happens not considering the 10.5 and 12.6 keV peaks (respectively, L\({}_{\alpha}\) and L\({}_{\beta}\) emissions of Pb) in the data. Three principal groups appear and strikingly they fit the description of Table 1 quite at all: 2, 10, 13, 15, 16, and 19 reflect the presence of cinnabar; 3, 11, 12, 14, and 17 individuate the hematite/red ochre group, while 1, 5, 6, 9, and 20 correspond to Red Lake. The remaining sequences, 4, 7, and 8, deserve further discussion. Sequence 4 is Red Ochre over White Lead, therefore, it should be characterized by Fe and resemble the similarity distribution of the Hematite/Red Ochre group. Probably, the intensity of K\({}_{\alpha}\) of Fe for sample 4 is particularly weak in comparison with the intensity of Pb's lines even if the principal lines in Pb L series have been already removed.