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

Abstract

Natural minerals and earths with coloring properties have been widely used as artistic pigments since prehistoric times. Despite being extensively studied, the complex chemistry of earth pigments is still unsatisfactory described with respect to their mineralogical and structural variability and origin. In this study, a large group of earth pigments from various geographical locations was investigated using easily accessible spectroscopic techniques and multivariate analysis with the aim to identify distinctive mineralogical and chemical characteristics of natural pigment sources. Portable X-ray fluorescence (p-XRF), Fourier transform infrared spectroscopy (FTIR) and fiberoptic Raman spectroscopy were used for the elemental, molecular and structural characterization of the investigated pigments. Diagnostic spectral features and chemical patterns (fingerprints) were identified and discussed with respect to their geological sources. Due to the occurrence of similar accompanying minerals, it was observed that the differentiation of red and yellow ochers is more challenging compared to green, brown and black pigments. However, for some of the investigated pigments, the presence of certain accessory minerals and/or of certain chemical patterns can have diagnostic value. Principal component analysis (PCA) of the FTIR and XRF data matrices showed promising results in terms of geological attribution, highlighting a promising tool for provenance research. The results of the study demonstrate the potential benefits of this rapid and nondestructive approach for the characterization and differentiation of earth pigments with similar hues coming from different geological sources.

A Multi-Analytical Study of a 17th-Century Wallachian Icon Depicting the “Mother of God with Child”

“…As inferred from the XRF analysis, a Fe-based material like an ochre seems to have been used for the Virgin's robe (sample S1), intense Fe lines being registered for this area (Figure 8a). This hypothesis is confirmed by FTIR analysis, characteristic absorptions for red ochres [46] rich in kaolinite (aluminosilicates) being registered at 3694 and 3615 cm −1 (OH stretching modes), 1030 cm −1 (Si-O stretching) and 915 cm −1 (Al-OH bending), this last peak appearing as shoulder band. Low amounts of quartz (frequently found in association with kaolinite) were identified by the doublet observed at 798 and 780 cm −1 (Si-O stretching), along with calcium carbonate identified via the characteristic vibration modes of the carbonate ion observed at 872 cm −1 (asymmetric bending, ν 2 ) and 712 cm −1 (symmetric bending, ν 4 ).…”

“…Low amounts of quartz (frequently found in association with kaolinite) were identified by the doublet observed at 798 and 780 cm −1 (Si-O stretching), along with calcium carbonate identified via the characteristic vibration modes of the carbonate ion observed at 872 cm −1 (asymmetric bending, ν 2 ) and 712 cm −1 (symmetric bending, ν 4 ). Iron oxide present as hematite was found via the diagnostic bands at 530 and 465 cm −1 (Fe-O lattice modes), confirming the use of a red ochre [46]. Small amounts of Cu, Ti, and Mn, also detected, can be linked to various types of accessory and clay minerals and/or impurities typically present within the iron oxide compound [46,47].…”

Rheological Investigation of Welding Waste-Derived Graphene Oxide in Water-Based Drilling Fluids

“…The spectrum of welding wastes shows a weak absorption band at 3748 cm −1 corresponding to the O-H bond of Si-O-H. Bands appeared at 2986 cm −1 , showing the C-H bond, and at 1700, showing C=O; multiple bands appeared in the range of 1660 cm −1 -1541 cm −1 , corresponding to aromatic C=O configurations. Peaks at 1058 cm −1 and 955 cm −1 show an Si-O stretching vibration and Si-O-Al vibrations, whereas the bands appearing between 800 and 500 show metals-oxygen bonds [4,34]. These results show that the welding waste consists of silicates, aluminates and oxides of various metals, along with some proportion of graphite.…”

The “Restoration of the Restoration”: Investigation of a Complex Surface and Interface Pattern in the Roman Wall Paintings of Volsinii Novi (Bolsena, Central Italy)

“…The peaks at 1319 and 770 cm −1 can be associated with Ca-oxalates, and those at 3528, 1435 (partially overlapped with that of calcium carbonate), 1053, and 833 cm −1 can be associated with lead white [29,30]. Lastly, the peaks at 1030, 670, 647, 601, 548, 524, and 453 cm −1 are due to various silicates, including those constituting green earth [31,32].…”