## 3 Results and Discussion
Ca was also detected in association with these purple/reddish areas (Figure 3) and might be related to the gypsum preparation layer, as discussed earlier. However, it cannot be excluded that it might also be due to the use of glass as an additive in the painting layer. Powdered glass, acting most likely as a drier/siccative of the oil medium, was indeed used extensively by artists, particularly between the fifteenth and sixteenth centuries in Italy and in Northern European works [42; 43; 44]. It was particularly suitable for red lakes, as it does not affect the typical transparency of these materials. In Netherlands paintings, calco-potassic glasses were common, in which the main constituents (other  
Figure 3: (**a**) Detail of the red/amaranth robe of [PERSON], with relative Cu(K\(\alpha\)) and Ca(K\(\alpha\)) XRF maps; (**b**) detail of the red/amaranth robe of [PERSON] (lower part), with relative Cu(K\(\alpha\)), Fe(K\(\alpha\)), Hg(L\(\alpha\)), and Ca(K\(\alpha\)) XRF maps. than silica) are variable amounts of Ca and K due to ashes and/or lime used in the production process [45], with lower amounts of Mn in comparison to glasses used in Italian paintings [46]. Interestingly, it is documented that finely ground Ca-rich glasses were also used as additives by [PERSON] [33]. The red garment of [PERSON], together with other small red areas of the robe of one of the two Mars in the left side of the background landscape, are characterised by the presence of Hg, indicating the use of vermillion (Figure 4) as well as red damask decorations on [PERSON]'s robe (Figures 3 and S1, at higher spatial resolution). Modelling was obtained by means of Cu- and Fe oxides/hydroxide-based compounds (both present in low quantities), as similarly happens in other garments (Figure 4). The red roof of the building on the right side of the background was painted with a mixture, or overlay, of vermillion with Fe-based compounds (See Figure S3). Vermition areas are associated with Zn traces, as is commonly attested to this pigment [47] (see Supplementary Materials S4). It is worth noting that Ca was detected in association with red draperies and robes, especially in the deepest shadows, and is well visible in the red robe of Saint [PERSON] and in the red sleeve of [PERSON] (Figures 4 and S4). The presence of Ca in these areas may suggest anew, as previously discussed, the possible presence of Ca-rich glass as an additive in the red lakes used in the modelling of red draperies, as also attested in other works by [PERSON] [25]. Cu is also characteristic of the green garment of [PERSON] (Figure 5). A wide range of Cu-based greens was available in those times and cannot be conclusively identified by means of XRF alone. It is worth noting, however, that verdigris and copper sulphates were detected in other paintings by [PERSON] [38; 42]. Sn was also detected in association with this Cu-based green robe, attesting the likely use of lead-tin yellow in modelling  
Figure 4: (**a**) Detail of the red robe of [PERSON], with relative Hg(L\(\alpha\)), Ca(K\(\alpha\)), Cu(K\(\alpha\)), and Fe(K\(\alpha\)) XRF maps; (**b**) detail of the red sleeve of [PERSON], with relative Hg(L\(\alpha\)) and Ca(K\(\alpha\)) XRF maps. (Figure 5). Lead-tin yellow was also employed in the golden damask robes of [PERSON] and [PERSON] of Arimathea, where vermili and Fe-based compounds were employed for modelling (Figure 5, showing [PERSON] robe). Green vegetation, grass, and trees were painted with Cu-based compounds and lead-tin yellow as well, not necessarily in mixture but possibly employed in layers to build the volumes of the landscape (Figure 5). Small amounts of Fe-based compounds were also detected, likely not in association with K and thus in principle excluding the use of green earth. Ca was also detected in some cases in association with trees, a presence that could not be conclusively understood (see Figure S5).