3.3.2. Pigments
The main chemical elements obtained through XRF analysis of the original pigments in the selected areas are Ca, Mn, Fe, Co, Cu, Hg, Pb, Sn, Ag, and Au. These elements help identify the inorganic pigments used in the polychromy of the analysed sculptures. Based on the results, the palette corresponds to the traditional one for the time [
6,
7,
25,
27,
28].
Table 2 summarises the results, indicating the colour of each proposed pigment, its chemical formula, and the chemical elements used for its identification. Subsequently, we will explain how these proposed pigments were applied in the different areas of each sculpture.
The XRF analyses identified two types of flesh tones. For the Child, the Virgin, and the angel, a blend of lead white in substantial amounts and vermilion was employed. The Hg content of the vermilion varies between 37.6 ± 0.5 cps and 61.9 ± 0.6 cps to achieve diverse shades of pink. These results are confirmed by cross-sections (A) and (B) in
Table 1, although vermilion was not found in the stratigraphy of the angel in the selected area, probably because the sample was taken from a part of the skin very close to the clothing. On the other hand, the slightly darker flesh tones of the shepherd and Saint Joseph include some ochre identified by Fe peaks, see
Figure 5. Due to the difficulty of positioning the XRF equipment close to the relief, only the mouth and eyes of the angel could be analysed. The predominant presence of Hg and Cu was found in these areas, indicating the most probable use of vermilion and azurite, respectively. Azurite was one of the most used blue pigments in Spain, not only for its beautiful colour and many different hues possible to obtain by finer or coarser grinding, but also due to the location of several copper mines in the Iberian Peninsula; therefore, it was not difficult to obtain [
25,
29]. This can be also supported by the signed contracts that specifically required the use of the highest quality pigment, which would be azurite.
The hair of the figures presents a variety of brown tones achieved through the combination of ochre, umber, vermilion, and quite possibly a copper-based green; green was often added to hair colour mixtures [
30]. For the darker hue, there is an increase in the amount of umber (from 2.7 ± 0.2 cps to 8.7 ± 0.3 cps of Mn) and ochre (from 42.9 ± 0.5 cps to 72.7 ± 0.6 cps of Fe), accompanied by a decrease in vermilion (from 32.6 ± 0.5 cps to 8.8 ± 0.3 cps of Hg). However, the quantity of lead white remains relatively stable, likely originating from the primer, as illustrated in
Figure 6. Additionally, Zr peaks originate from the detector’s collimator and remain consistently present across all measurements, while Sr peaks typically appear alongside high Ca presence, suggesting a calcium-based primer impurity [
31]. The intensity of the Sr peaks varies in relation to the intensity of the Ca peaks. In Saint Joseph’s hair, which presents the darkest brown tone, the highest peaks of Ca were observed throughout the relief, suggesting the potential use of bone black, which offered more excellent opacity compared to other pigments used at the time [
32]. Unfortunately, confirmation of its usage via phosphorus detection is not feasible, as this region of the spectrum is affected by the M-Pb peaks and electronic noise, and no sample could be taken for SEM-EDX analysis.
The vestments show a wide variety of colours: reds, browns, blues, and greens. A blend of vermilion with a small amount of a Cu-based pigment (azurite or a Cu-based green pigment) was predominantly used for the red clothes. Despite the inability to identify the Cu-based pigment by XRF analysis, the colour suggests the probable use of a green pigment; as a complementary colour to red, the artists usually selected it in such mixtures. Combining red with green results in a darker hue, while red with blue would create a purplish tone, which is not present here. A significant amount of ochre and umber was used for the brown attire, with traces of Cu-based green and lead white. This composition extends to other brown-coloured relief areas, such as the crib, architecture walls, and the stable, alongside the ox and the mule, although with varying pigment ratios to achieve different tonalities. The sheet beneath the Child and the sheep received a similar treatment, predominantly lead white and a touch of ochre.
Moving on to the blue and green clothing, there are notable peaks of Cu, which most probably identify the azurite and a copper-based green pigment, respectively. As stated above, azurite was a highly appreciated pigment, obtained from a semiprecious mineral, and the mostly used blue one in Spanish polychromy of the time, strongly recommended also by Pacheco [
25]. The characterisation of the Cu green pigment is more complicated, since there were several in use (malachite, verdigris, copper resinate, etc.) and cannot be distinguished by XRF alone, as they are all characterised by the same chemical element, Cu. Molecular analysis would offer more precise identification, but such methods were inaccessible during the examination. Furthermore, these pigments could have been applied over a brownish or greyish layer to enhance colour intensity in order to save up the expensive pigment, as well as they could be darkened adding a black or brown (mostly iron oxide) pigment [
33,
34]. An iron oxide-based pigment must have been used as the underlayer or added to azurite in the studied polychromy, as high peaks of Fe are also detected, see
Figure 7. Azurite was most likely used also to paint the sky, as already suggested by Pacheco [
25,
29] and most probably also for the blue areas of the angel’s wings, according to Cu peaks.
Finally, the entire sculpture surface was gilded, except for the flesh and hair areas. Colour layers were applied over the gold and, next, partially removed by sgraffito technique. Sgraffito involves scratching, scoring, or engraving the polychromy with a stylus to reveal the burnished gold underneath the colour, creating brocade textile design [
25]. The analysed areas showed Au peaks with varying intensity, depending on the superficial colour layer (ranging from 6.2 ± 0.4 cps to 128 ± 1 cps). Moreover, the presence of Fe was observed, indicating the use of red bole (soft clay applied as a base for gold) under the gold leaf. The cross-section prepared from the sample extracted from a golden area (
Figure 4 and
Table 1) reveals that some parts were intervened and refreshed with a new layer of gold leaf. The original layer, which lies underneath (layer 4 in the stratigraphy), was applied over the bole (layer 3), made of earth pigments, primarily clays, with traces of calcium carbonate and gypsum. Above this layer, two noticeably thick layers are observed: one white layer with a preparation of calcium carbonate and gypsum (clearly distinguishable from the original ground layer) and another orangy layer, composed of earth pigments and a bit of calcium carbonate, serving as a base for the gold. The variation in the intensity of the Au peaks is also likely due to one or two layers of gold, depending on whether we analyse an original or intervened area.
The flesh tones of Saint Peter and Saint Paul share a similar composition to those found in Saint Joseph and the Shepherd of the Nativity: lead white, vermilion, and ochre. However, these flesh tones include a small amount of a Cu-based pigment, which might be blue azurite or a Cu-based green pigment, according to old receipts for some flesh areas [
25].
The estofado of the clothing of both saints is based on gold leaf, confirmed by the high peaks of Au. Gold is detected in every analysed point of both sculptures, excluding the flesh and hair areas, as observed in the relief. It was also applied over the bole, as indicated by prominent Fe peaks.
Saint Paul’s red cloak was made mainly of vermilion mixed with ochre, supplemented possibly with a Cu-based green due to its darkened hue. On the other hand, the green colour of the tunic was painted with a Cu-based green (unidentifiable between malachite, verdigris, copper resinate, etc.) mixed with ochre and a bit of umber, while the violet shades may be obtained by combining azurite with red earth and/or red lake.
Looking at the sculpture of Saint Peter, the colour of the cloak was primarily obtained with a substantial amount of ochre, identified by intense Fe peaks, mixed with a small amount of umber and Cu-based green (more likely than Cu-based blue pigment for this colour). The blue tunic is painted most probably with azurite and a bit of smalt, the combination explained in Pacheco treatise for darkening of azurite [
24,
28]. The blue pigment could have been mixed with ochre or overlaid on a layer of ochre to intensify the colour (
Figure 8) [
25].
Finally, the golden hues adorning the key, sword, and books carried by both figures were made with ochre, probably a Cu-based green, and a small amount of umber, while the sword presented silver.
Different areas of the flesh tones, hair, and cross were analysed. The flesh tone consists of a significant amount of lead white, blended with ochre, and a minimal amount of lead-tin yellow. Unlike the other sculptures analysed, vermilion was not detected in Christ’s flesh tone, as it represents a lifeless body. Furthermore, it is the only figure whose flesh tone presents the addition of lead-tin yellow, giving it a lighter hue.
The blood running through Christ’s body was painted with vermilion and applied over the flesh colour layer. In the shaded areas of the crown, hair, and cross, there is a decrease in the amount of lead white compared to the flesh tone. Christ’s hair and the cross, both rendered in dark brown hues, were prepared with ochre, umber, a Cu-based green pigment [
30], and some vermilion to give it a reddish undertone. Next, in the darker areas of the cross, high peaks of Ca were observed, indicating the use of bone black, further corroborated by a small amount of P. Lastly, the crown’s ochre colour with greenish tones was mainly achieved with a Cu-based green pigment mixed with earth pigments (see
Figure 9).