Analytical techniques for characterizing polychromated coatings on quartzite samples from a prehistorical cave

Abstract

The most relevant aspects in cave paintings conservation is the accurate determination of the nature and origin of the pigments used in its polychromy. Several analytical techniques determine whether the presence of pigments is due to natural or human causes. This is important to define conservation methodologies for cave paintings heritage. This chapter discusses different analytical techniques employed in the mineralogical, morphological, and physicochemical characterization of a quartzite sample extracted in the vicinity of an archaeological site. It focuses on the identification of pigments deposited on the sample surface to determine their anthropological origin. Morphological aspects were studied by optical stereomicroscopy, Nomarski microscopy (OM), and scanning electron microscopy (SEM). Sample characterization was carried out using powder X-Ray diffractometry (XRD), microRaman spectroscopy, and infrared spectroscopy (IR). The main components of the sample were determined by means of energy dispersive X-ray fluorescence analysis (SEM+EDX). Physicochemical characterization, depth of pigmentation, and chromaticity in the stone sample were done by laser induced breakdown spectroscopy (LIBS), cathodoluminescence (CL), and Raman microprobe of stratigraphic profiles. Morphologic observation using optical stereomicroscopy (OM) shows only one pigment layer less than 100μm thick. Mineralogical phases forming pigments have been determined by XRD, FTIR, and microRaman spectroscopy indicating the presence of kaolinite hydroxyls, and hematite. Microprobe analyses by SEM+EDX reveal that Fe, Si, and Al are the major chemical constituents of the pigments. Minor amounts of Ca and K have also been detected. Analytical techniques with a high spatial resolution have been proven to be unique tools to analyze the interaction between pigments and the quartz substrate. LIBS spectra indicate the nonexistence of a multi-layer structure. These techniques provide significant advantages: reduction in time of analyses, sensitivity, specificity, spatial resolution, and immunity to interference. The chapter provides tables and graphs to illustrate the results obtained from different techniques. .