The research of diachronic processes of production and transformation of rock art panels is a worldwide problem that raises intriguing challenges to approach the elucidation and identification of different painting events [1–6]. In last decades, archaeometric studies have been a powerful tool to understand the materials employed in paintings [4,7–11] as well as the natural and/or anthropic phenomena that affect them throughout time [12–17]. Also, direct absolute chronology of paintings has demonstrated to be a powerful tool in this matter [18,19]. However, the potential of physicochemical studies has not been completely exploited and there are great possibilities to contribute to the actual discussions about the historical processes and social practices that constructed and modified rock painting sites.
As a matter of fact, a painting is the result of the application of different pigmentary mixtures with determined proportions of diverse materials onto the rock surface. These mixtures have been usually described in the literature as the combination of a pigment (grounded solid colored materials or organic colorants), a binder (commonly an organic material) and additives (regularly used as extenders, fillers or charges) [12,17,20–31]. The pigments, also knew as chromophores, are the responsible to give color to the painting. Most of the archaeological literature has reported iron oxides to obtain reds and yellows, calcium carbonate and calcium sulphate for white colors, and carbon or manganese oxide compounds for black hues [4,32,33]. Then, the binders give cohesion to the mixture and have been linked to organic compounds, as animal fat or vegetable oils [34,35]. Gypsum, clays, quartz, bone and talc have been described as additives and usually are employed to give volume and/or to improve other paint properties such as adherence, coverage and durability [20]. However, the paintings are not only the sum of materials, but also the product of various preparation stages, practices and physical transformations employed to obtain desired final painting properties such as color hue, brightness, durability, texture, etc. Such activities could be grinding, sifting, mixing, heating treatments, among others [36–40]. Therefore, the combination of these activities and materials produces painting mixtures that are chemically and/or physically different from other preparations, even if the same raw materials have been originally employed.
However, these differences sometimes remain eclipsed behind the uniformity appearance of painting sets with similar morphologies and colors, promoting synchronic interpretations of rock art that misunderstand the site’s history. Could not be possible that paintings with similar hues and designs were the results of different painting preparations distanced in time? Is there any archaeometric approach that allows us to evidence these subtle differences? Aiming to address these inquires, in this work we present the potentials of new physicochemical lines of evidence for characterization and differentiation of painting mixtures. In this sense, the results obtained in the micro-stratigraphic studies of samples taken from the painted walls of Oyola, an archaeological site situated in the northwest of Argentina, are presented. These samples have been analyzed by micro-Raman spectroscopy (mRS) and scanning electron microscopy with energy dispersive X-ray (SEM-EDS). Particularly, new variables and levels of understanding have been defined for painting mixtures characterization; namely, differences in pigments and additives; and differences in materials preparation. These results, we think, will improve the understanding of the technical heterogeneity and temporal complexity of painting sets in Oyola and could be a useful methodological alternative to apply in other sites. Finally, it must be highlighted that the studies herein presented are not the result of a single case or sample but the product of a large number of samples analyzed throughout the archeological site of Oyola for the last 7 years [5,17,31,41].
The archaeological site of Oyola
At the eastern side of El Alto-Ancasti’s mountain, Catamarca province preserves one of the most outstanding groups of rock art sites in South America. Having more than a hundred caves documented, these paintings and engravings stand out by their diversity, colors, sizes and good preservation. After some decades of studies, most of the researchers attribute the rock art of this area to the La Aguada culture, chronologically located between 600-900 a.C. [42–45]. The traditional interpretation of this period describes a social integration between different groups as a consequence of a shared ideology materialized in a common repertory of designs in pottery, lithic objects and rock art motifs [46,47]. Figures of shamans, warriors, trophy heads and jaguars characterized this culture and expressed a powerful religious ideology that links and joints communities with different historical developments.
The archaeological site of Oyola is located in the middle of the eastern hillside of El Alto-Ancasti’s mountain, approximately at 700 meters above sea level (Figure 1). It is shaped by 38 rock shelters with prehispanic paintings and/or engravings executed in the roof and walls of granitic caves, with a diversity of color hues, morphologies, sizes and spatial arrangements. The last survey of the shelters allows us to identify more than 350 motifs, with the predominance of abstract designs and, among the figuratives, camelids and anthropomorphic motifs. The firsts investigations at Oyola were carried out in the 1970’s by Amalia Gramajo and Hugo Martínez Moreno [48,49]. As a consequence of this initial documentation, the authors claimed that the rock art of Oyola could be linked with La Aguada culture. Later, other researchers supported this interpretation [45]. However, they also described some motifs that could not be possible to be stylistically attributed to La Aguada arising hypothesis regarding the presence of other groups before or after this culture.
In this context, our works in Oyola started in 2009. Recovering the hypothesis of a wide chronology of the site’s paintings, we focus the studies on the application of different lines of evidence in order to approach the histories of production and transformation of rock art panels. In this way, we have performed stratigraphic excavations of the caves [50], stylistic studies of paintings [51], analysis of overlapping figures [52], spatial studies of the places selected to do the paints [51] and chemical analysis of micro-samples taken from paintings [5,17,41]. By the development of strategies to link all these evidences [53], the research group was able to reach a more complex understanding of the site’s history. In this challenge, the chemical analyses of micro-stratigraphic samples were a valuable tool to differentiate sets of paintings with similar color hues, morphologies and styles, usually interpreted as chronological equivalent and attributed to La Aguada culture.
This homogenous first appearance characterizes the rock art repertories of many caves at the site, such as Oyola 1, Oyola 7 and Oyola 34. These caves share the same archaeological problem: at a first glance, the painted panels look uniform with paintings of similar colors, forms and without overlapping. However, as we explain in this article, evidences contradicting this preliminary interpretation were detected in most of the caves. These differences were found by cross-section chemical study of samples taken from the figures and could be explained as a consequence of, at least, two situations: 1- differences in the chemical components, either in the pigments or additives used; and, 2- differences in the physical properties of the mixtures. In the next section we explain each hypothesis, present the limits of these interpretations and describe future research challenges. As we discuss below, it could be possible that the chemical and physical differences found between painting mixtures were the material expression of varied types of pictorial techniques.