Invasive and Non-Invasive Analyses of Ochre and Iron-Based Pigment Raw Materials: A Methodological Perspective

Abstract

Naturally occurring and deeply coloured iron-bearing materials were exploited very early on by human populations. The characterization of these materials has proven useful for addressing several archaeological issues, such as the study of technical behaviors, group mobility, and the reconstruction of cultural dynamics. However, this work poses some critical methodological questions. In this paper, we will review ochre studies by focusing on the analytical methods employed, the limits of non-invasive methods, as well as examples of some quality research addressing specific issues (raw material selection and provenience, heat treatment). We will then present a methodological approach that aims to identify the instrumental limits and the post-depositional alterations that significantly impact the results of the non-invasive analysis of cohesive ochre fragments from Diepkloof rock Shelter, South Africa. We used ochre materials recuperated in both archaeological and geological contexts, and we compared non-invasive surface analyses by XRD, scanning electron microscopy coupled with dispersive X-ray spectrometry (SEM-EDXS), and particle-induced X-ray emission (PIXE) with invasive analysis of powder pellets and sections from the same samples. We conclude that non-invasive SEM-EDXS and PIXE analyses provide non-representative results when the number of measurements is too low and that post-depositional alterations cause significant changes in the mineralogical and major element composition at the surface of archaeological pieces. Such biases, now identified, must be taken into account in future studies in order to propose a rigorous framework for developing archaeological inferences.

The Sustainability of Rock Art: Preservation and Research

“…A wide range of geochemical techniques have been used to characterize the composition of rock art and ochre materials ( [107,119] and references therein), in which the mineral composition could be identified by thin section petrography, X-ray diffraction (XRD), Raman spectroscopy and Fourier transformed infrared spectrometry (FTIR). Elemental analysis could be carried out by semi-quantitative methods such as scanning electron microscopy coupled with dispersive X-ray spectroscopy (SEM-EDX) and X-ray fluorescence (XRF), or the concentration of elements could be determined by inductively coupled plasma and atomic emission spectroscopy (ICP-AES), neutron activation analysis (NAA), and laser ablation inductively coupled mass spectroscopy (LA-ICP-MS), in which NAA and LA-ICP-MS are considered the most sensitive to trace elements [120]. Among these, the portable version of XRF and Raman are non-invasive methods that could be executed without sample removal and preparation, while the attenuated total reflectance (ATR) mode of FTIR and LA-ICP-MS require tiny volumes or micro-samples.…”

Assessment of Easily Accessible Spectroscopic Techniques Coupled with Multivariate Analysis for the Qualitative Characterization and Differentiation of Earth Pigments of Various Provenance

“…As it is well documented in the literature, the complex chemistry and the occurrence of multicomponent mixtures makes the analysis of earth pigments challenging, and thus, for a complete characterization, a multi-analytical approach is most often necessary [35,36]. Earth pigments have been intensively studied over time via a wide range of analytical techniques .…”

Ancient Pigments in Afrasiab Murals: Characterization by XRD, SEM, and Raman Spectroscopy

“…In general, studies that investigate the types of materials and technology used in creating the ancient murals employ techniques such as Raman spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy plus energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffractometry (XRD) to identify pigments and organic materials used in conservation treatments [14][15][16][17][18][19]. In particular, in recent years, nondestructive analytical methods based on portable X-ray fluorescence (P-XRF) spectrometry, particle-induced X-ray emission (PIXE), and false-color infrared imaging (FCIR), combining spectrophotometric colorimetry and spectral analysis, have been improved and applied to research on murals [13,[20][21][22][23][24][25][26][27][28][29][30]. These techniques are noninvasive and, therefore, allow us to obtain the composition of the main elements on the sample surfaces at several points, as well as create maps.…”