Critical evaluation of in situ analyses for the characterisation of red pigments in rock paintings: A case study from El Castillo, Spain

Abstract

Paint technology, namely paint preparation and application procedures, is an important aspect of painting traditions. With the expansion of archaeometric studies and in situ non-destructive analytical methods, a renewal of technological studies is being observed in rock art. In situ analyses have several limitations that are widely discussed in the literature, however. It is not yet clear whether they provide accurate information on paint technology, except under certain conditions. Here, we evaluated digital microscopic and pXRF in situ analyses for the characterisation of a large set of red and yellow paintings from the El Castillo cave, Cantabria, Spain. We have set experiments and used statistical methods to identify differences between paint components and determine factors impacting pXRF measurements. We found that the compositional heterogeneity of the paintings’ environment, especially variations in secondary deposits, was responsible for most of the differences observed between the pXRF signals recorded on the paintings. We concluded that the El Castillo cave environment is not suitable for non-destructive technological studies, but that more favourable contexts might exist. Following previous works and our own results, we advocate a combination of both in situ and laboratory invasive analyses for the study of paint composition and paint technology. Our research protocol, based on the comparison of rock paintings, their substrate, experimental paintings and Fe-normalisation of the signals can improve the reliability of pXRF results. We also propose to include more systematic characterisation of rock wall heterogeneity and the use of microscopic analyses in non-destructive approaches.

A multi‐analytical approach to understand El Alto‐Ancasti's painted pottery (north‐western Argentina): Non‐invasive and micro‐invasive analyses of vessel's paintings from El Taco 19

“…The latter are chiefly connected to the unintentional analysis of underlying strata below paintings. This problem has been pointed out by many researchers in the study of pottery and other archaeological materials using portable XRF (Chanteraud et al, 2021;Dayet et al, 2022) and could be reinforced by our work. As has been noted, readings performed in painting motifs also contain signals of pastes and slips.…”

“…Another depth of data interpretation could be reached by statistical processing of XRF results. Among different types of unsupervised data analyses, principal components analysis (PCA) is one of most used in non‐invasive XRF studies (Chanteraud et al, 2021; Dayet et al, 2022). As can be seen from Figure 2 (a), the exploratory analysis of white‐coloured pottery does not show clear clustering of any particular style.…”

The Sustainability of Rock Art: Preservation and Research

“…The most applied devices are portable XRF (pXRF) and Raman. Portable XRF can read elements heavier than Al or Si; it is, therefore, possible to identify pigments in rock art palettes such as Fe from red hematite ochre and Mn in black manganese oxides [103]. Raman identifies both minerals and organic substances but encounters difficulties when identifying organic binders in situ due to fluorescent background materials; moreover, the possible diagenesis of organics may hamper the possibility to distinguish between primary constituents and neoformations.…”

“…Microsamples also offer the opportunity to conduct laboratory experiments on rock substrate stability after artificial aging [147]. In a recent experiment with pXRF, Dayet et al (2022) [103] observed that the heterogeneity of paintings systems and the variability in primary and secondary minerals hamper the possibility of investigating rock art only following a non-invasive approach and concluded that the best solution for rock art research is a combination of in situ and laboratory analyses. From this perspective, a micro-invasive approach that guarantees rock art preservation and makes available a small quantity of pigments and/or rock substrate is mandatory.…”

Characterizing paint technologies and recipes in Levantine and Schematic rock art: El Carche site as a case study (Jalance, Spain)

“…Whether or not the mineral mixtures identified are natural or intentionally made by the artists is not yet proven. Other researchers have already discussed that different paint mixtures could either represent different paint recipes, or natural formations from similar or different raw material sources [ 42 ].…”