3.1. In Situ Analysis of the Rock Art: Paintings and Backdrop
Based on digital photographs, enhanced photographic images and vector drawings, the rock art catalogue was produced prior to data collection by spectrophotometry and Raman spectroscopy.
Figure 4 shows digital photographs of the different coloured rock facies found in the outcrop (SW:
Figure 4A; SBB:
Figure 4B; SB:
Figure 4C) and digital photographs, enhanced photographic images and vector drawings of the panels and the motifs (M). The location of the Sampling point (Sp) is shown where colour spectrophotometry and Raman spectroscopy were applied.
On panel 1, paintings were found on two different sectors (
Figure 4D,E,G,H). On the right, an assemblage of eleven dots painted with the fingertips was detected (
Figure 4E,F) and on the left, a surface below exhibits three inconspicuous and weathered painted lines (
Figure 4F,H). Panel 2, found 50 cm to the right of the former, is equally an open-air surface, yet even more exposed to the environment (
Figure 4I–K). This explains the extremely weathered conditions in which the paintings were found. The panel shows, on the left-hand side, dots and bars typical of the Schematic Art tradition and on the right, remains of paintings forming a group of lines barely visible to the naked eye and that were only able to be recorded by enhanced digital photography. Regarding the northern area, panel 3 is equally exposed to the environment and exhibits, on the right, a human figure with a triangular-shaped head and, on the left, patches of pigment that may have belonged to a second motif now decayed (
Figure 4L–O). These figures showed a colouration slightly more orange than the rest of the motifs. Panel 4 corresponds to a white quartz intrusion in the quartzite outcrop and sits in a more sheltered area (
Figure 4P–T). This seems to have contributed to a better preservation of the pigment for it shows the brightest and most solid colour of the whole assemblage. Nevertheless, these motifs were difficult to classify due to weathering. On the left-hand side, two vertical bars were recognised, one of them attached to two others set at an angle and, on the right sector of the panel, there is a degraded coat of paint superimposed on an indeterminate figure (
Figure 4T). Panel 5 shows a small oval-shaped form likely to have been produced by a fingertip and a shapeless patch of pigment (
Figure 4U–W).
Figure 5 and
Table 2 shows the colour data obtained; L*, a* and b* measurements for each Sp, but also, the measurements for the most representative colours detected on the natural surface of the outcrop (
Figure 5A,G,M): white (SW), brown-black (SBB) and dark grey (SB). A lighter colour (higher L*) for the SW sample was identified compared to those for the SBB and SB samples (
Figure 5A), confirming its white colouration. For the SW and SBB samples, they had positive a* (
Figure 5G) and b* (
Figure 5M) values, showing a slight orange colouration of the main white and black colouration, respectively. However, negative values of these parameters were detected by SB due to its dark colour.
Moreover, for each panel, the natural colour of the rock lacking painting is also shown in
Figure 5 in order to compare with that of the motifs; the colour of the stone is identified by an R (from rock) added to the panel’s ID (P1SpR1 and P1SpR2 for each sector of panel 1, P2SpR1, P3SpR1, P4SpR1 and P4SpR2, because the rock on panel 4 showed two different colourations and P5SpR1). Comparing the colour parameters measured (L*, a* and b*) of the Sp with those of the rocks lacking paintings, although nuances were not very intense, it was possible to distinguish different trends:
For the Sp from panels 1 (
Figure 5B,H,N) and 5 (
Figure 5F,L,R), the parameter a* was the one with more statistically significant differences comparatively to the value recorded on the original rock (P1SpR1, P1SpR2 and P5SpR1). The a* increase confirms the presence of a red painted layer on the surface.
For the motifs from panels 2 (
Figure 5C,I,O) and 4 (
Figure 5E,K,Q), L* was the parameter showing more statistically significant differences when compared with its values for the unpainted rocks (P2SpR1, P4SpR1); this fact reveals a darkening of the surface. Although L* changes were negligible in most of the cases considering the standard deviations, the coloured surfaces tended to show L* decreases (darkening).
For the motifs from panel 3 (
Figure 5D,J,P), the parameter b* showed more statistically significant differences regarding the value of the rock (P3SpR1); b* increases are associated with a more orange tone, which was also detected under the naked eye.
Figure 6 shows the Raman spectra of the motifs and rocks from each panel. In the Raman spectra of the SBB and the SB samples (
Figure 6A), strong peaks were identified that could be assigned to Si-O-Si at 206 cm
−1 and 466 cm
−1 [
24,
25,
26,
27] due to the presence of quartz. Moreover, in the SB and also in the black layer detected on the white sample (SW-black layer in
Figure 6A), weak peaks at 1360 and 1580 cm
−1 were found; they are attributable to amorphous carbon [
6]. It is important to highlight that these particular Raman peaks assigned to amorphous carbon can be modified due to the different conditions during the measurement [
28]. Raman peaks in the spectral region 1000–1600 cm
−1 are attributed to the presence of charcoal or soot likely derived from the combustion of vegetable materials [
3,
6,
16]. Therefore, these black layers on the white rock can be attributed to fire ignitions that took place in this more sheltered area over time [
6,
13]. Moreover, the high fluorescence detected in the Raman spectra of the black layer suggested its organic composition. The orange patches detected on the rock surfaces (orange stains in
Figure 6A) were analysed by Raman spectroscopy in order to distinguish between natural features and the painted motifs, because macroscopically, they may be mistaken. The Raman spectra of the orange stains detected peaks at 226 cm
−1, 293 cm
−1, 412 cm
−1 and 612 cm
−1 (this latter almost inappreciable), suggesting the presence of hematite (Fe
2O
3) [
29]. However, this Raman spectrum did not show fluorescence revealing, therefore, the lack of organic compounds in them. Therefore, using Raman spectroscopy, it was possible to distinguish between the natural orange stains in this stone and the red paintings.
Considering the Raman spectra from the Sp, in addition to the peak at 206 and 466 cm
−1 revealing the presence of silicate minerals, a highly intense peak of the hematite at 293 cm
−1 was detected in most of them; this peak of greater or lower intensity was found in the spectra of all the Sp from panel 1 (
Figure 6B), panel 4 (
Figure 6E) and panel 5 (
Figure 6F) and those of Sp2, Sp3, Sp5 to Sp8 from panel 2 (
Figure 6C). Goethite, whose main Raman peaks are 301 cm
−1 and 386 cm
−1, was identified mainly through the latter, on panel 2 (Sp6 and Sp7 with higher intensity and Sp8 and Sp4 with lower intensity) and panel 3.
Moreover, it is also important to highlight the presence of peaks at 1360 and 1580 cm
−1 for the Raman spectra of the Sp from panels 2 (Sp3), 3 (Sp2) and 4 (Sp5), assigned to amorphous carbon [
6]. On panel 3, for Sp2, two Raman spectra are depicted in
Figure 6D, because of the heterogeneity in its composition: one spectrum suggesting the presence of amorphous carbon in the darkest part (P3Sp2a) of the motif and one spectrum suggesting the presence of goethite (P3Sp2b). P3SpR1 showed a Raman spectrum with a broad peak in the range 1200–1600 cm
−1 (
Figure 6D), which is assigned to amorphous carbon [
6], showing higher intensity than that detected in the SB sample (
Figure 6A). It is worth reiterating that the presence of amorphous carbon can be related to recent wildfires or hearths whose remains are still visible nearby. Moreover, making a comparison between the Raman spectra of the backdrop, P3SpR1 was the unpainted surface revealing both the highest fluorescence and highest presence of organic matter.
3.2. Laboratory Characterisation
Stereomicroscopy allowed the characterisation of the appearance and extension of the samples collected (
Figure 7). Regarding the coloured surface samples found in excavation, it was possible to find an intense red colouration on the surfaces mainly on PE (
Figure 7A). The coloured layer showed a variable thickness.
The stones collected, despite belonging to the rock outcrop, showed different colourations and textures. SW was covered almost completely by a discontinuous black-brown-coloured layer (
Figure 7B) with variable thickness. Under this layer, it was possible to identify an orange-coloured layer. SBB (which was attested to show colourations from brown to black) displayed the characteristic opal-A speleothems (
Figure 7C), specifically identified as flowstone [
30,
31,
32]. SB was the darkest one (
Figure 7D) and showed a black colouration with brown iridescence. Moreover, incipient opal speleothems were also detected.
Considering the possible colorant materials exhumed from the excavation, the samples collected exhibited different textures. PGMT1 showed laminated structures (
Figure 7E), PGMT2 was composed of a granulated structure, while PGMT3 (
Figure 7F) and PGMT4 were composed of a granular structure but more compact than PGMT2 (
Figure 7G).
The mineralogical composition detected by XRD (
Table 3) confirmed that the red deposits scraped were mainly composed of quartz (SiO
2) and hematite (α-Fe
2O
3). PE in addition showed calcite (CaCO
3) and jarosite (KFe
3(SO
4)
2(OH)
6) and PW, muscovite (KAl
2Si
3AlO
10(OH)
2) and rectorite ((Na, Ca)Al
4((Si, Al)
8O
20)(OH)
42H
2O). PE was the red-coloured surface sample showing the highest amount of hematite. Regarding the natural stones collected around the outcrop (SW, SBB and SB), they were composed mainly of quartz. SB also showed lepidocrocite (γ-FeOOH), anhydrite (CaSO
4) and jarosite. In SW, in addition to quartz, goethite (α-FeOOH) and anhydrite were identified. Note that none of the stones from the outcrop showed hematite in the composition, confirming Raman spectroscopy results. The dark layer on the white sample (SW-black layer) showed quartz and mainly, goethite. Regarding the colorant materials collected from the excavation, the four samples were composed of quartz and hematite. PGMT1 also contains plagioclase (NaAlSi
3O
8), potassium feldspar (KAlSi
3O
8), anhydrite and butlerite (Fe(OH)SO
4 2H
2O). PGMT2 also showed muscovite and PGMT3 revealed, in addition to quartz and hematite, traces of anhydrite. PGMT1 and PGMT2 include a higher hematite content than the other two colorant materials PGMT3 and PGMT4.
Regarding the FTIR spectra of the coloured surface samples, the following features were found (
Figure 8):
Observations of cross-sections by stereomicroscopy and SEM allowed the characterisation of the microtexture and composition of the coloured surface samples collected in the excavation and also the red deposit–substrate boundaries of these samples (PG:
Figure 9, PE:
Figure 9 and
Figure 10 and PW:
Figure 11). For PG, an external layer with variable thickness was detected on the surface: from a few µm until ca. 500 µm (
Figure 9A,B). This layer showed a dark red colouration (
Figure 9A). SEM allowed the identification of a clean deposit–substrate boundary, suggesting that the material was deposited on the surface (
Figure 9B). The substrate was rich in Si due to the quartzite substrate (
Figure 9,EDS1). Moreover, it was detected a filler in fissures rich in Fe and to a lesser extent Al, Si, P, K and Ti. The layer detected on the surface was composed of a mixture of different shaped microparticles (aciculae, micrometric spheres, rhombohedral particles, etc.,
Figure 9C). It was rich in C, Si, Al and, to a lesser extent, Mg, Na, P, S, Cl, K, Ti and Fe (
Figure 9,EDS3). Notice the remarkable difference in the intensity of the C-peak in the spectra of the external layer comparatively to that from the substrate (compare EDS1 and EDS3 in
Figure 9). It is also important to note the low Fe content in the external layer, comparatively to that in the filled fissures.
For the PE sample, on the compact soil block, a layer with an orange colouration (
Figure 9D) was detected. This external layer showed a similar thickness to that found in PG: from a few µm until 500 µm (
Figure 9E). The substrate is composed of quartz (SiO
2) grains (
Figure 9,EDS6) into a matrix rich in Fe and also to a lesser extent, Si, Na, Mg, Al, P, S, Cl and K (
Figure 9,EDS5). The external layer seems to also be deposited on the surface, since an interaction between the external layer and the surface was not detected (
Figure 9E). As was identified for PG, it is composed of C, Si and Al and to a lesser extent, Na, Mg, P, S, Cl, K, Ti and Fe (
Figure 9,EDS4). As reported for PG, the external layer showed higher C content than the quartz grains underneath.
Figure 10 shows the compositional map of the PE sample. The layer was composed of a mixture of micrometric particles rich in Si, Fe, K and Al. There were also isolated grains rich in Ti. The compositional map also allowed us to confirm that this layer is independent of the substrate underneath because there is a clear boundary between them.
The PW sample also revealed a red layer on the surface with a similar colour detected on the other two samples collected (
Figure 11A). However, there was not a clear limit between the red layer and the quartz substrate since hematite crystals placed over the quartz grains showed signs of deterioration (
Figure 11B,C). Platy crystals of hematite showed clear signs of alteration (
Figure 11C). In addition, the red layer was compact, showing filled fissures with certain continuity (
Figure 11B). However, the other two samples (PG and PE) did not show continuous fissures since the deposits were composed of different sized microparticles. Contrary to PG and PE, the red layer was thicker in PW (up to ca. 3 mm). Attending to the compounds of this sample, the quartz was the main component underneath the red layer (
Figure 11C,EDS3). The red layer was composed mainly of Si, Al, K and Fe (
Figure 11C,EDS2). The fillers of the fissures were rich in Fe and to a lesser extent, Al, Si and P (
Figure 11C,EDS4). Although these Fe-rich fillers may come from the platy hematite crystals (
Figure 11C,EDS1), incipient mammillary botryoidal hematite crystals (
Figure 11C,EDS5) can also contribute with Fe for the fillers. The red layer was composed of planar silicates rich in Si and Al and to a lesser extent, K and Mg (
Figure 11D,EDS6), identified as muscovite by XRD. These planes were bonded by a matrix mainly rich in Fe (in minor amounts, Al, Si and P), which seemed to come from the hematite crystals (
Figure 11D,EDS7). It should be noted that the C content reflected in the EDS spectra of the red layer (
Figure 11C,D,EDS2,6) was similar to that detected on the quartz grains (
Figure 11C,EDS3).
Figure 12 shows micrographs and EDS spectra of the three rock facies from the outcrop (SW, SBB and SB). For the SW sample, with the dark brown layer on the surface (
Figure 7B), the visualisation with stereomicroscopy of the cross-section allowed a dark orange colouration below the black-brown surface layer to be identified (
Figure 12A,B). SEM allowed us to identify that this orange-coloured layer with a thickness up to 200 µm was composed of quartz crystals (
Figure 12B,EDS2) immersed in a Fe-rich matrix, also with Al, Si and P (
Figure 12B,EDS1). In the quartz grains composing the substrate, it was possible to find cavities coated by a mineral phase rich in Fe, which may be assigned to goethite as was identified by XRD (
Figure 12C,EDS3).
In the SBB sample (
Figure 12D,E), it was detected a superficial layer rich in Si and to a lesser extent, Fe, Na, Al, P, S and K (
Figure 12D,E,EDS4,5) belonging to the opal A speleothem. This layer was composed of two parts: the lowermost section showing a continuous and compact layer with a few µm-thickness (
Figure 12E,EDS5) and the uppermost section composed of a less compact layer with more empty spaces and higher thickness: ~60 µm (
Figure 12E,EDS4). Moreover, opal A speleothems were also identified inside the stone (
Figure 12E,EDS6).
For the SB samples which superficially showed a black colouration (
Figure 7D), it was possible to identify, below this superficial black layer, an orange colouration with variable thickness, up to 50 µm (
Figure 12F,G), rich in Si, S, Fe and K and to a lesser extent, Na, Al and Ti (
Figure 12G,EDS8), confirming the presence of opal A speleothem with a laminar growth. Below this laminar structure, it was possible to find massive crystals rich in Fe (
Figure 12G,EDS9), which can be assigned to the lepidocrocite (γ-FeO(OH)) detected by XRD. The presence of As in the lepidocrocite grains can be related to the dissolution of arsenian pyrite and subsequent mobilisation of the As
3+ or more rarely As
4+ [
37]. In the quartz grains (
Figure 12G,EDS11) of the substrate, acicular structures rich in S, Fe, K and P (
Figure 12G, EDS 10) were found and assigned to the iron-hydroxysulphate mineral jarosite (KFe
3(SO
4)
2(OH)
6) detected by XRD.
Attending to the possible colorant materials, PGMT1 (
Figure 13A,B) showed the accumulation of laminar structured silicates rich in Si, Al, K and Fe as major elements and, to a lesser extent, Mg, P and Ti (
Figure 13B,EDS1). Considering elemental composition by SEM-EDS and mineralogical composition by XRD, inside this laminar structure, quartz (indicated by an asterisk,
Figure 13B) and hematite (indicated by an arrow,
Figure 13B) grains were found.
Regarding PGMT2 (
Figure 13C–E), quartz grains (
Figure 13D,EDS2 and 13E,EDS4), laminated silicates (
Figure 13E,EDS5) showing in their composition Al, Si, Fe, K and Mg (muscovite identified by XRD) and mammillary botryoidal hematite crystals (pointed out with arrows in
Figure 13E) were mixed with micrometric particles rich in Si, Fe, Al and K (
Figure 13D,EDS3). The pitting found on the boundaries of the quartz grains reveals its intense weathering (
Figure 13E).
Conversely to PGMT1 and PGMT2, PGMT3 and PGMT4 showed a colouration tending to purple (
Figure 13F). In PGMT4, it was possible to detect a coloured layer with two different parts: the lowermost part (up to 2 mm-thick) showed a red colouration intercalated with white grains (
Figure 13F) recognised as quartz grains (pointed out with arrows in
Figure 13G,H), while the uppermost part (ca. 1 mm-thick) showed lower quartz content with the relative enrichment of particles rich in Fe, corresponding to botryoidal hematite grains (
Figure 13G,H,EDS6).