The colors of the iron oxides result from several ligand-field transitions within the visible wavelength range. Iron oxides weakly reflecting in the ultraviolet region and are strongly reflecting in the visible region. Average reflectance spectra from red and yellow motifs and rock supports are shown in Fig 4 . Yellow motifs show weak absorption bands that correspond to the characteristics bands of the goethite (649, 480 and 434 nm) within the visible wavelength range [ 22 , 26 ]. Absorption bands of the hematite (649, 529, 444, 404 and 380 nm) or other red iron oxides are unappreciated in the reflectance spectra of the red motifs, probably due to the proportion and nature of other red iron oxides or minerals associated with the used red raw materials [ 26 ]. Red and yellow motifs could be differentiated from the rock supports in function of their respective reflectance spectra in the visible region. Red motifs show lower reflectance values than the yellow motifs for the wavelength range above the 500 nm and a sharp positive slope higher than 560 nm in contrast to yellow motifs that present the sharp positive slope at lower wavelengths. On the other hand, the dominant wavelength (λ D ) varies between 583 and 599 nm for red pigments, between 578 and 582 nm for yellow pigments and between 575 and 584 nm for unpainted rock supports. Therefore, we can observe that λ D values differentiate the red and yellow pigments but theλ D values for supports and yellow pigments are overlapped.

According to our data, the color differences ΔE*ab between the painted motifs and the underlying rock supports was significantly for all of the plaquettes (above the 4 ΔE units). Appreciable variability were found in ΔE*ab values from the red motifs ( Table 2 ), the most distinct color differences correspond to the painted surface of the plaquette 16246 (ΔE: 30.81) and the least to the zoomorphic motifs of the plaquettes 16112 and 18728 (face A) (ΔE: 4.6 and 4.41, respectively). Minor variability in color difference was found in yellow motifs ( Table 3 ): values ranging between 9.93 (delimitated painted surface of the plaquette 18885 (face B)) and 21.98 (painted surface of the plaquette 18037 (face B)).

After the calibration performed in the spectrophotometer, three readings were taken on each analyzed point of the red and yellow motifs and on the unpainted roc surface of the plaquettes. From the average values of these readings, the CIEL*a*b* coordinates were calculated and the spectral curves were registered. Tables 2 and 3 give the colorimetric L*a*b* coordinates and the dominant wavelength for 191 measurement points (82 from red motifs, 14 from yellow motifs and 95 from the rock supports). Red and yellow pigments from the Parpalló plaquettes had positive values of the colorimetric coordinates with L* ranging from 24.81 to 50.32 for red and from 50.18 to 60.08 for yellow; a* ranging from 9.62 to 23.08 for red and from 10.67 to 16.31 for yellow; b* ranging from 9.66 to 22.61 for red and from 25.03 to 41.72 for yellow. The CIEL*a*b* coordinates of the rock supports present a noticeable differentiation with respect to the pigments with L* ranging from 35.26 to 64.80, a* ranging from 1.16 to 18.08 and b* ranging from 5.03 to 30.33. Fig 3 shows that red and yellow pigments can be differentiated in the a*-b* color space. Yellow pigments are separated from red pigments by their significantly higher yellowness (b*) and lower redness (a*). For pure hematite the a* and b* values ranging from 15 to 30 and from 2 to 30, respectively, whereas for pure goethite the a* and b* values ranging from 5 to 15 and from 22 to 48, respectively [ 22 ].

EDXRF analysis

We recorded a total of 128 EDXRF spectra on red pigments and 22 EDXRF spectra on yellow pigments from the 67 selected plaquettes with red motifs and 14 selected plaquettes with yellow motifs (Tables 2 and 3), as well as 106 EDXRF spectra on unpainted zones of the plaquettes. In order to determine the composition of the rock support, powdered samples from unpainted rock substrates were analyzed by X-ray diffraction (XRD). These analyses reveal the presence of calcite, dolomite and quartz as main crystalline phases that are characteristic of the limestone-sandstone sedimentary rocks [16]. The elemental composition of these crystalline phases is reflected in the EDXRF spectra that show intense fluorescence lines of Ca, minor intense fluorescence lines of Fe, Mn, Ti, K and Si and, in some cases, traces of Sr and Zr. On the other hand, most of these elements are present in red and yellow iron based pigments as hematite, goethite and other iron oxides where Fe is the “key element” characterized by an intense fluorescence signal that is easy to detect and does not interfere with lines from other elements present in the pigments.

However, the presence of a common set of elements (with different concentrations) in the red and yellow painted zones and in the unpainted zones of the plaquettes, make it difficult to discriminate the elemental composition of the pigments from the rock surface background. Therefore, in order to distinguish between the elemental composition of the superficial pigment layers and that of the underlying rock, the recorded spectra from painted areas were compared with spectra from unpainted areas. On the basis that the EDXRF spectrum of a colored zone with an iron based pigment presents higher iron peaks than the unpainted rock surface of the plaquette, a discussion based on the Fe/Ca fluorescence peak ratios was made.

Fig 5 shows the scatter-plot of the normalized K-lines of iron and calcium from the analyzed points of the red and yellow motifs. In both cases, the intensity of the Fe peaks from colored zones is greater than in the nude rock and the linear tendency of the Fe versus Ca signals shows a negative slope with significant differences between painted and unpainted zones. The anti-correlation between Ca and Fe observed in the rock substrates is characteristic of the weathering processes as iron replaces calcium in the rock surface layers. The anti-correlation in the pictorial motifs will be associated with the presence of red and yellow iron based pigment layers on top of the calcareous rock substrates.

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larger image TIFF original image Download: Fig 5. Normalized net areas of the Fe K-lines versus the normalized net areas of the Ca K-lines from the painted and unpainted zones of the Parpallo’s Cave plaquettes. (a) Motifs with red pigments. (b) Motifs with yellow pigments. https://doi.org/10.1371/journal.pone.0163565.g005

Nevertheless, a number of EDXRF measurements from pigmented motifs present iron levels similar to the unpainted rock surfaces. In these cases, the EDXRF spectra of the pigmented zones are undistinguishable of the unpainted rock surfaces (Fig 6) and both spectra present similar low Fe/Ca ratios with intense signals of calcium and low signals of iron (see Fig 7 and S3 and S4 Tables). This fact would be due to the expected fluctuations of the fluorescence signals in thin or deteriorated pigment layers or in pigment layers highly absorbed into the rock support. Such is the case of the signs and zoomorphic motifs from the plaquettes 16126, 16112, 16322, 16753, 17960, 18704 (face A), 20345 (face B), the ramiform motif from the plaquette 20045, the red dot of the plaquette 16127 (face B) or the weak red line presents in the plaquette 17416.

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larger image TIFF original image Download: Fig 6. Plaquette 16126. EDXRF spectra of the light red pigment (legs of a zoomorphic motif) and the rock support. The fluorescence peaks at 3 keV (Ag) and 8 keV (Cu) come from the X-ray tube and the detector framework. https://doi.org/10.1371/journal.pone.0163565.g006

Conversely, most plaquettes with red motifs and all the plaquettes with yellow motifs present significant differences in Fe content when we compare the EDXRF spectra from the red or yellow motifs and the underlying rock surfaces. In these plaquettes the Fe/Ca ratios in the pictorial motifs are higher than in the bare rock (see Fig 7 and S3 and S4 Tables). These high iron levels are characteristic of well-defined motifs with prominent pigment layers and intense colorations, and confirm the presence of iron based mineral compounds. As representative examples, in Fig 8 we show the EDXRF spectra of the red snout of a zoomorphic motif recorded on the plaquette 16168 and the yellow horse head recorded on the plaquette 18465. In these cases, the intensity of the iron signal and the Fe/Ca ratio are more intense in the pigment layer than in the underlying rock and the calcium peaks of the rock support are attenuated by deep red or yellow pigments with a dense pictorial layer.

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larger image TIFF original image Download: Fig 8. Comparison between EDXRF spectra of red and yellow iron based pigments and that of the rock support. (a) Red zoomorphic motif on plaquette 16168. (b) Yellow zoomorphic motif on plaquette 18465. (The fluorescence peaks at 3 keV (Ag) and 8 keV (Cu) come from the X-ray tube and the detector framework). https://doi.org/10.1371/journal.pone.0163565.g008

With respect to the raw material used as red or yellow pigments in the plaquettes of the chronological sequence included in this work, the EDXRF analyses only confirm the use of iron based pigments (oxides and/or hydroxides for red and yellow pigments) but these analyses cannot specify the type (molecular composition) of raw materials that were used as pigments. However, some red and yellow motifs of several plaquettes show different spectra for certain elements like manganese, arsenic or lead. Manganese was detected as component of the iron based red pigments in zoomorphic motifs of the plaquettes 18704 (face A), 18788 (face A) and the painted surface of the 20004 (face A). Arsenic was detected in the zoomorphic motif of the plaquette 16735. Lead was detected in the iron based yellow pigment on the painted surface of the plaquette 18929. In these plaquettes, the net areas of the manganese, arsenic or lead fluorescence lines of the pigments are significantly higher than in the rock supports, whereas for the rest of plaquettes these elements have not been detected or the net areas of these elements in the painted motifs and the rock supports are similar (S3 and S4 Tables). The presence of Mn, As and Pb could be associated to pigment raw materials from different ores [27]. In relation to the elemental composition of the rock supports, strontium and zirconium are discriminatory elements in a certain number of plaquettes (S3 and S4 Tables) that indicate differences in the geogenesis of the rock supports included in this study. As conclusion, we can consider that the artists used specific raw materials with different geological fingerprints characterized by the presence or absence of few elements (manganese, arsenic or lead into the red/yellow iron oxide matrix; strontium and zirconium into the rock support).

Pictorial motifs that are recognizable as anthropomorphous, zoomorphous, or well defined geometric figures would be catalogued as artistic or symbolic representations and we can postulate the anthropic origin of the decoration. However, if the pictorial motifs are vague forms and these forms are difficult to catalog or interpret, we cannot exclude that the pigmented areas are the result of a surface patination of the plaquettes associated with geogenic, biogenic, weathering, firing or post-depositional processes in the archaeological site. In any case and if sampling is allowed, additional microscopic, elemental, molecular and geological analysis should be performed to complement the EDXRF analyses and provide a holistic information about the nature and composition of the red and yellow pigmented areas of the plaquettes.