**4. Discussion**

Some common features emerged from the non-invasive analyses of the three walls, which are worthy of discussion. The presence of hematite, identified by Raman spectroscopy, seems to always coincide with a purplish hue. In fact, if we exclude the cases in which hematite is added to a Fe-based pigment to obtain a darker red tone, all the other areas rich in hematite are purple. No organic compound ascribable to a dye has been identified in these purple hues, which show the same composition and the same impurities as the red ones without hematite, and compared to these, show a definitely different shade. This chromatic diversity, with the elemental composition being the same and in the absence of organic dyes, suggests the use of hematite as a purple pigment in its own right, perhaps in the form of caput mortuum [15]. The use of this hematite-based purple pigment occurs in each of the three walls. The yellow areas show a very articulated composition. They are all characterized by the presence of yellow ochre to which other yellow pigments are added. First, Pb-antimonate, also known as Naples yellow, is easy to individuate from Sb and Pb by XRF. Furthermore, from the joint analysis of XRF and visible reflectance, a Pb-based yellow was also revealed, namely litharge, which would otherwise be impossible to identify by the elemental analysis alone when used together with Pb-antimonate. In fact, the contemporary presence of Pb and Sb from XRF may have distracted from considering other Pb-based yellows, but the Vis-reflectance revealed the spectral shape of litharge also in those areas where Pb-antimonate has been found. The presence of the Pb-antimonate or Naples yellow occurs in the yellow areas in the lighter hues, with a cold tone. Naples yellow is one of the synthetically produced pigments known and lost and rediscovered a number of times throughout history [21]. It comes from the glassmakers' tradition and its use as a pigment at the beginning of the XVI century was very early and unexpected [22]. It is worth noting that Naples yellow has already been individuated in Villa Farnesina, in the Loggia of Cupid and Psyche, both in the figurative part of the scenes and in the fruits of the vegetable festoons [23].

In Alexander and Roxane's Wedding Room, it is used in the walls painted by Sodoma to lighten the yellow tones, affording a lighter and cooler shade.

Additionally, on the third wall, the same three yellow pigments were used, but with an interesting difference. On the walls by Sodoma, the Naples yellow is always detectable by reflectance, and this means that it is on the surface, as a "final touch" in the lighter shades. On the third wall, Naples yellow is present in the yellow areas because it is individuated by XRF, but the reflectance does not always detect it, with litharge or yellow ochre prevailing instead.

Thus, even though we found the same yellow pigments for each of the three walls, it is important to highlight the diversity in the "final touch". Indeed, Sodoma used Naples yellow to refine the light hues, while the painter of the third wall, who used the same materials as Sodoma, indifferently mixed Naples yellow with yellow ochre and litharge, without preferring one particular pigment for finishing. Furthermore, the evidence that Sb disappears in the orange areas in both walls one and two, and in the third wall as well, indirectly confirms the precise choice by both Sodoma and the unknown painter to use Naples yellow only for light yellow shades.

Smalt, which is widely used in each of the three walls studied, has the characteristic of having, associated with the Co, some impurities of As and Bi. In particular, the latter allows the pigment to be placed within a specific processing method that obtained the blue of enamel from the bismuth slag, locating its extraction both geographically, as a German manufacture from the Erzgebirge, and chronologically as well, attesting to it within the XVI century [24,25]. This means that the decoration on the third wall is realistically ascribable within the 16th century. It must be remembered that the third wall was the one that housed the double bed of Agostino Chigi and Francesca Ordeaschi, who both died in 1520, *terminus post quem* from which to start the decoration of the third wall. Documentary sources attest the presence of Sodoma in Rome to settle and open a workshop in 1521 [25], and the same sources exclude any kind of pictorial production from Sodoma in the capital from that

date, apart from a few drawings [25]. This fact does not exclude that he may have passed on his knowledge to the students in the workshop, who could have worked inside Villa Farnesina, as their Master did. This information together with the scientific evidence of the painting material support the hypothesis that the third wall is probably coeval to the other two, and painted by someone who had attended Sodoma's workshop and had learned his painting techniques and his painting materials as well.

Finally, it must also be noted that for the north wall, i.e., that depicting Alexander meeting his new bride-to-be Roxane, some precious painting materials such as gold finishes and lapis lazuli were used, which do not appear on the other two walls. These unique features only present on the north wall tell us something about the relevance of this fresco, other than the authorship. In fact, Sodoma used very precious materials for decorating this wall, which he himself did not use for the east one. This must be ascribed to the painted subject of this wall—the marriage of Alexander and Roxane—which must be the most significant of the room and therefore deserving the most precious painting materials which were circulating at that time.
