*3.1. Red*

In the red areas, the presence of Fe-based pigments prevails (Table 1). In the darker shades, Raman spectroscopy identified red ochre, with the addition of a few hematite (Figure 4a, right, line 2). The bright red of Dario's daughter's dress at the left edge of the east wall is characterized by Fe (XRF, Figure 4a, left, line 1), but Raman spectroscopy individuates only calcium carbonate with its band at 1085 cm−<sup>1</sup> due to the symmetric stretching of C-O bonds [8], while hematite results instead in the dark red areas, such as the skirt of Alexander (Figure 4a, line 2). In Figure 4a, the characteristic Raman bands of hematite at 498 and 610 cm−<sup>1</sup> are highlighted [9].

A similar mixture of generic Fe-based red with hematite can be found in the north wall on the red curtains of Roxane's canopy bed on the north wall and in the dress of the putto on the right edge (Table 2). The first derivative of the reflectance spectrum of the red curtains (Figure 4b, line 3) does not show differences with respect to the bright red ones of the east wall (Figure 4b, lines 1, 2, areas 1, 2), and in each analyzed point, the spectrum follows the trend of a generic iron oxide. It is worth noting that on the curtains (Figure 4b, area 3), traces of cinnabar were also found, confirmed by Raman spectroscopy [17] and by Hg from XRF. Since from the reflectance spectrum it is not possible to individuate the features of cinnabar, it is likely that it was used in a mixture and is not found enough on the surface so as to be revealed. The third wall shows again the use of Fe-based red and hematite in the skirt of the back-turned man running away behind Bucephalus (Table 3). The reflectance spectrum of this area (Figure 4b, line 4/area 4) is similar to those recorded on the other two walls.

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**Table 1.** Main results of non-invasive investigation on the east wall—*The Darius family before Alexander* (Ip: Inflection point, Max: maximum, Min: minimum).

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**Table 3.** Main results of non-invasive investigation on the west wall—*The Taming of Bucephalus* (Ip: Inflection point, Max: maximum, Min: minimum).

maximum, Min: minimum).

(west wall) Red

*The Taming of Bucephalus* 

**Wall Colored Area X-ray Fluorescence** 

Yellow

*3.1. Red* 

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**Table 3.** Main results of non-invasive investigation on the west wall—*The Taming of Bucephalus* (Ip: Inflection point, Max:

**Vis-Reflectance** 

Pb, Fe, Sb (K) Ip:450,521,627 nm Pb-antimonate

Pb, Fe, Sb Ip:449,506,627 nm Pb-antimonate

Max:440,549,608 nm; Min:500,580,637 nm; Ip:692 nm

In the red areas, the presence of Fe-based pigments prevails (Table 1). In the darker shades, Raman spectroscopy identified red ochre, with the addition of a few hematite (Figure 4a, right, line 2). The bright red of Dario's daughter's dress at the left edge of the east wall is characterized by Fe (XRF, Figure 4a, left, line 1), but Raman spectroscopy individuates only calcium carbonate with its band at 1085 cm−1 due to the symmetric stretching of C-O bonds [8], while hematite results instead in the dark red areas, such as the skirt of Alexander (Figure 4a, line 2). In Figure 4a, the characteristic Raman bands of hematite

**(Vis-R) Raman Comments** 

[18,19]

[18,19]

hematite

CaCO3 [8] Smalt [16]

from Vis-R [9]

from Vis-R [9]

Mixture of red and yellow ochre; Pb-yellow not to be excluded due to the inflection at 510 nm [12]

Pb-yellow and few yellow ochre from Vis-R [12]. Naples yellow from XRF and Raman

Mostly Pb-yellow from Vis-R [12]

Hematite-based purple, possible caput mortuum [15]

Ca, Fe, K Sr Ip:582,640 nm CaCO3 [8] Generic Fe-based red

red) Ip:580,640 nm Hematite [10] Generic Fe-based red

**(XRF) Main Elements** 

Fe, Ca, Sr, K, S (dark

Blue Ca, Co, Fe, Bi,As, K, Si,

at 498 and 610 cm−1 are highlighted [9].

Pb

Orange Fe, Ca, Sr, Pb, K Ip:435,510,570,635 nm CaCO3 [8]

Purple Fe, Ca, Sr, K Ip:590,639,688 nm CaCO3 [8],

**Figure 4.** (**a**) From left to right: XRF and Raman spectra of points 1 and 2 on the east wall. (**b**) Comparison of the first derivative reflectance spectra measured on selected red areas (lines 1–4). **Figure 4.** (**a**) From left to right: XRF and Raman spectra of points 1 and 2 on the east wall. (**b**) Comparison of the first derivative reflectance spectra measured on selected red areas (lines 1–4).

### A similar mixture of generic Fe-based red with hematite can be found in the north *3.2. Purple*

*3.2. Purple* 

wall on the red curtains of Roxane's canopy bed on the north wall and in the dress of the putto on the right edge (Table 2). The first derivative of the reflectance spectrum of the red curtains (Figure 4b, line 3) does not show differences with respect to the bright red ones of the east wall (Figure 4b, lines 1, 2, areas 1, 2), and in each analyzed point, the The composition of the purple areas by XRF is the same as the red ones on each of the three walls (see Tables 1–3). Raman spectroscopy instead individuates stronger hematite signals with respect to the red ones in all the purple zones, such as in the dress of Dario's wife on the east wall (Table 1).

spectrum follows the trend of a generic iron oxide. It is worth noting that on the curtains (Figure 4b, area 3), traces of cinnabar were also found, confirmed by Raman spectroscopy [17] and by Hg from XRF. Since from the reflectance spectrum it is not possible to individuate the features of cinnabar, it is likely that it was used in a mixture and is not found enough on the surface so as to be revealed. The third wall shows again the use of Fe-based The dress of Darius' wife Statira II appears as red/purplish (Figure 5, area 2) and has the same elemental composition as the red dress of Darius' daughter (Figure 5, area 1) depicted next to her (Table 1, bright red), but Raman spectroscopy indicates a strong presence of hematite only in the former (Table 1, dark red).

red and hematite in the skirt of the back-turned man running away behind Bucephalus (Table 3). The reflectance spectrum of this area (Figure 4b, line 4/area 4) is similar to those

The composition of the purple areas by XRF is the same as the red ones on each of the three walls (see Tables 1–3). Raman spectroscopy instead individuates stronger hem-

The dress of Darius' wife Statira II appears as red/purplish (Figure 5, area 2) and has the same elemental composition as the red dress of Darius' daughter (Figure 5, area 1) depicted next to her (Table 1, bright red), but Raman spectroscopy indicates a strong pres-

This difference of hue is hardly appreciated in the reflectance spectra but becomes clearer when considering their first derivatives. Indeed, for the dress of Darius' wife Statira II (Figure 5, line 2), there is a shift of the maximum of about 10 nm towards the red, and at the same time, the narrowing of the minimum to 640 nm and the consequent steep climb up to 700 nm with respect to the daughter's red dress (Figure 5, line 1). These features recur in the purple areas of each of the three walls, and more particularly, in the already discussed robe of Statira II on the east wall (Figure 5, line 2), of the half-naked Hephaestion on the north wall (Figure 5, line 4/area 4), and on the skirt of a running, backturned figure on the third wall, the Taming of Bucephalus (Figure 5, line 5/area 5).

ence of hematite only in the former (Table 1, dark red).

recorded on the other two walls.

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The purple/greyish dress of Darius' mother Sisigambi (Figure 5, area 3) consists of

Smalt is indicated by XRF from the Co presence and its associated impurities, such as Bi and As. The presence of smalt is not identifiable in the reflectance spectrum, which follows exactly those of Co-free, hematite-based purples, so this area was probably

hematite and smalt according to a common technique concerning frescoes [15].

painted with different superimposed layers, with purple hematite on the surface.

**Figure 5.** Comparison of first derivative reflectance spectra measured on selected red and purple areas (lines 1–5). **Figure 5.** Comparison of first derivative reflectance spectra measured on selected red and purple areas (lines 1–5).

*3.3. Yellow*  The yellow areas are quite articulated, being a mixture of two or three different yellow pigments. Their composition is mostly of two types: one containing Fe and Pb, and the other Fe, Pb, and Sb. The simultaneous presence of Sb and Pb indicates the use of Pbantimonate, also known as Naples yellow [21,22], and always coincides with lighter yellow hues. On the east wall, the use of Naples yellow is found on the lighter parts of the dress of Darius' daughter (Figure 6, area 1), where XRF indicates Fe, Pb, and Sb (Table 1). The first derivative of its reflectance spectrum is mostly that of yellow ochre (Figure 6, line This difference of hue is hardly appreciated in the reflectance spectra but becomes clearer when considering their first derivatives. Indeed, for the dress of Darius' wife Statira II (Figure 5, line 2), there is a shift of the maximum of about 10 nm towards the red, and at the same time, the narrowing of the minimum to 640 nm and the consequent steep climb up to 700 nm with respect to the daughter's red dress (Figure 5, line 1). These features recur in the purple areas of each of the three walls, and more particularly, in the already discussed robe of Statira II on the east wall (Figure 5, line 2), of the half-naked Hephaestion on the north wall (Figure 5, line 4/area 4), and on the skirt of a running, back-turned figure on the third wall, the Taming of Bucephalus (Figure 5, line 5/area 5).

1), but a little broader, according to Naples yellow features [12]. In the yellow skirt of Hephaestion, XRF indicates Fe and Pb but no Sb. Since there is no evidence of lead white The purple/greyish dress of Darius' mother Sisigambi (Figure 5, area 3) consists of hematite and smalt according to a common technique concerning frescoes [15].

in this area, Pb is probably due to litharge, a lead monoxide, as can be inferred in the first derivative of the reflectance spectrum from the maximum at 530 nm [12], mixed with yellow ochre, which can be seen in its weakened maximum at 450 nm (Figure 6, line 2). An abundant use of litharge still accompanied by a background of yellow ochre, as suggested by Fe presence, is even clearer in the landscape on the north wall (Figure 6, area 4), where Smalt is indicated by XRF from the Co presence and its associated impurities, such as Bi and As. The presence of smalt is not identifiable in the reflectance spectrum, which follows exactly those of Co-free, hematite-based purples, so this area was probably painted with different superimposed layers, with purple hematite on the surface.

### XRF indicates only three main elements: Ca, Pb, and Fe (Table 2), and the reflectance *3.3. Yellow*

shows, in its first derivative, mostly the spectrum of litharge but little modified by yellow ochre, recognizable for the maximum at 450 nm (Figure 6, line 4). The yellow dress of Roxane's maid on the left edge is characterized by Fe and Pb, and the first derivative of its reflectance spectrum essentially shows yellow ochre, so in this case, the source of Pb should be other than litharge (Figure 6, line 3/area 3). All three types of pigments, yellow ochre, litharge, and Naples yellow, can be found in Roxane's dress (Figure 6, area 5), where Naples yellow is used to paint lighter, cold tones. The main elements found from The yellow areas are quite articulated, being a mixture of two or three different yellow pigments. Their composition is mostly of two types: one containing Fe and Pb, and the other Fe, Pb, and Sb. The simultaneous presence of Sb and Pb indicates the use of Pbantimonate, also known as Naples yellow [21,22], and always coincides with lighter yellow hues. On the east wall, the use of Naples yellow is found on the lighter parts of the dress of Darius' daughter (Figure 6, area 1), where XRF indicates Fe, Pb, and Sb (Table 1). The first derivative of its reflectance spectrum is mostly that of yellow ochre (Figure 6, line 1), but a little broader, according to Naples yellow features [12]. In the yellow skirt of Hephaestion, XRF indicates Fe and Pb but no Sb. Since there is no evidence of lead white in this area, Pb is probably due to litharge, a lead monoxide, as can be inferred in the first derivative of the reflectance spectrum from the maximum at 530 nm [12], mixed with yellow ochre, which can be seen in its weakened maximum at 450 nm (Figure 6, line 2). An abundant use of litharge still accompanied by a background of yellow ochre, as suggested by Fe

presence, is even clearer in the landscape on the north wall (Figure 6, area 4), where XRF indicates only three main elements: Ca, Pb, and Fe (Table 2), and the reflectance shows, in its first derivative, mostly the spectrum of litharge but little modified by yellow ochre, recognizable for the maximum at 450 nm (Figure 6, line 4). The yellow dress of Roxane's maid on the left edge is characterized by Fe and Pb, and the first derivative of its reflectance spectrum essentially shows yellow ochre, so in this case, the source of Pb should be other than litharge (Figure 6, line 3/area 3). All three types of pigments, yellow ochre, litharge, and Naples yellow, can be found in Roxane's dress (Figure 6, area 5), where Naples yellow is used to paint lighter, cold tones. The main elements found from XRF, Fe, Pb, and Sb, suggest the presence of Naples yellow and yellow ochre, but from Vis-reflectance, litharge can also be individuated. *Heritage* **2021**, *4* FOR PEER REVIEW 10 XRF, Fe, Pb, and Sb, suggest the presence of Naples yellow and yellow ochre, but from Vis-reflectance, litharge can also be individuated.

**Figure 6.** Comparison of first (**top**) and second (**bottom**) derivative reflectance spectra, recorded on selected yellow areas (lines 1–5)**. Figure 6.** Comparison of first (**top**) and second (**bottom**) derivative reflectance spectra, recorded on selected yellow areas (lines 1–5).

Litharge's presence can be argued by the first derivative (Figure 6, line 5), where Roxane's dress shows the main features of both yellow ochre and Naples yellow: namely, the relative maximum at 445 nm and the broader shape of the main band, respectively. Nevertheless, a further growth at 509 nm (Figure 6, line 5) suggests the presence of litharge, otherwise impossible to establish on the basis of XRF data alone, due to the joint presence of Pb and Sb. Litharge features of Roxane's dress are more evident if the second derivatives are checked, in the maximum at 494 nm when compared with the landscape area, which was the one with the most evident presence of litharge (Figure 6, lines 4, 5). In fact, if we consider the second derivatives, the mixture of yellow ochre and Naples yellow (Fig-Litharge's presence can be argued by the first derivative (Figure 6, line 5), where Roxane's dress shows the main features of both yellow ochre and Naples yellow: namely, the relative maximum at 445 nm and the broader shape of the main band, respectively. Nevertheless, a further growth at 509 nm (Figure 6, line 5) suggests the presence of litharge, otherwise impossible to establish on the basis of XRF data alone, due to the joint presence of Pb and Sb. Litharge features of Roxane's dress are more evident if the second derivatives are checked, in the maximum at 494 nm when compared with the landscape area, which was the one with the most evident presence of litharge (Figure 6, lines 4, 5). In fact, if we consider the second derivatives, the mixture of yellow ochre and Naples yellow (Figure 6,

ure 6, line 1) does not account for the trend between 465 and 550 nm of Roxane's dress

also appear in this spectrum, facilitating the identification of litharge in a situation difficult to decipher due to the overlapping of common elements. Naples yellow is also easily individuated by its Raman features at 510 and 655 cm−1 [11,12], together with calcium carbonate [8] (Figure 7). The 655 cm−1 Raman band of Naples yellow is present when there is

an excess of lead and it is specific to a non-stoichiometric Pb:Sb molar ratio [11].

line 1) does not account for the trend between 465 and 550 nm of Roxane's dress (Figure 6, line 5), which can be explained by the presence of litharge already seen in the landscape (Figure 6, line 4) with the two relative maximums at 494 and 529 nm, which also appear in this spectrum, facilitating the identification of litharge in a situation difficult to decipher due to the overlapping of common elements. Naples yellow is also easily individuated by its Raman features at 510 and 655 cm−<sup>1</sup> [11,12], together with calcium carbonate [8] (Figure 7). The 655 cm−<sup>1</sup> Raman band of Naples yellow is present when there is an excess of lead and it is specific to a non-stoichiometric Pb:Sb molar ratio [11]. *Heritage* **2021**, *4* FOR PEER REVIEW 11

**Figure 7.** Raman spectrum of Roxane's yellow dress showing the presence of lead antimonate and calcium carbonate. **Figure 7.** Raman spectrum of Roxane's yellow dress showing the presence of lead antimonate and calcium carbonate.

The presence of Naples yellow is also found on the third wall, as revealed by XRF and Raman spectroscopy (Table 3). However, in these areas, the first derivative of reflectance spectra almost exclusively shows the shape of litharge (Figure 8, lines 3, 4), as can be seen from Figure 8 by comparison with the reflectance spectrum on the landscape of the north wall (mostly litharge and yellow ochre, Figure 8, line 2) and with the dress of Roxane's maid, which in reflectance shows mostly yellow ochre (Figure 8, line 1). The striking presence of litharge allows for its otherwise difficult identification, due to the simultaneous presence of Pb and Sb. The presence of Naples yellow is also found on the third wall, as revealed by XRF and Raman spectroscopy (Table 3). However, in these areas, the first derivative of reflectance spectra almost exclusively shows the shape of litharge (Figure 8, lines 3, 4), as can be seen from Figure 8 by comparison with the reflectance spectrum on the landscape of the north wall (mostly litharge and yellow ochre, Figure 8, line 2) and with the dress of Roxane's maid, which in reflectance shows mostly yellow ochre (Figure 8, line 1). The striking presence of litharge allows for its otherwise difficult identification, due to the simultaneous presence of Pb and Sb.
