Spectroscopic characterization of commercial pigments for pictorial retouching

Abstract

A set of 27 commercially available pigments by Kremer Pigmente (Aichstetten, Germany), including traditional and synthetic compounds, has been characterized in detail from a chemical and mineralogical point of view in the frame of a wide research project about pigments and colorants involving different spectroscopic techniques such as reflectance spectroscopy (in both visible and near‐infrared intervals), Fourier‐transform infrared spectroscopy (in both attenuated total reflection and reflection modes), and Raman spectroscopy. These pigments are currently employed for pictorial retouch purposes of both ancient and contemporary works of art, and a deep comprehension of their composition is fundamental in order to better understand their long‐term behavior after application.This work presents Raman results found analyzing the set of pigments in order to identify existing phases and comparing them with datasheets available by Kremer Pigmente: some inconsistencies were found, especially concerning correspondences with Chemical Abstracts Service (CAS) numbers, pigment codes, and color indexes, together with the absence of some compounds in the records.Most Raman signals were attributed to specific vibrational modes, even if in some cases, only related chemical species were identified, whereas information regarding individual vibrational modes was not found in the literature. The whole volume of data generated in this project will represent a useful tool for the study of artworks permitting both the interpretation of spectral information obtainable mostly from portable instruments and for the evaluation of materials to be used for pictorial retouching.

Fiber Optic Reflection Spectroscopy–Near-Infrared Characterization Study of Dry Pigments for Pictorial Retouching

“…2a) shows five reflectance minima (absorbance maxima) at 445, 484, 545, 584 and 635 nm, respectively. Concurrent Raman investigation showed, in accordance with the company data sheets, peaks referable to willemite, an orthosilicate containing both Zn and Co. 2 The position here identified for these absorptions are in accordance with literature, reporting willemite values at 490, 540, 590, and 640 nm. 31,33 The attributions for the three main minima (545, 584, and 635 nm) correspond to those reported for the cobalt-based pigments discussed in the following (44100, 44151).…”

“…29 The FORS technique was not able to detect undeclared compounds in the Azurite pigment powder such as fillers, in particular quartz, identified by Raman analysis. 2 Therefore, reflectance and Raman spectroscopy resulted complementary techniques compensating each other in the obtainable information. Specifically, because of the employed laser source wavelength (785 nm), the Raman spectrum of pigment 10207 did not reveal any feature referable to the coloring phase, being Azurite, and the same applies also to Malachite (see discussion below).…”

“…Literature studies on Co 2þ often focus on the glassy structures of smalt, for which a blue shift of its three absorption maxima is normally detectable with respect to the studied pigment, in which the ion is within a crystalline structure, 35,36 as demonstrated by previous data. 2 The Raman spectrum of this pigment, as well as previous literature studies, 37 revealed a quite complex situation, with the coloring species being CoTiO 3 2 where the Co 2þ ion exhibits a tetrahedral coordination that generates wide absorption bands in the visible and near infrared ranges (not detectable with the employed instrument) due to d-d transitions 35 (584 nm: spin allowed; 626 nm: two spin forbidden transitions very close in energy; 624 nm: spin forbidden). Each of these two bands is further subdivided into three sub-bands.…”

“…This goal was obtained by analyzing them through various analytical techniques such as fiber optic reflectance spectroscopy (FORS), near-infrared spectroscopy (NIR), Raman, attenuated total reflection Fourier transform infrared (ATR FT-IR), and diffuse reflection infrared Fourier transform spectroscopy (DRIFTS). While the results obtained by Raman spectroscopy have been already published, 2 the presented paper focuses on results obtained by means of reflectance spectroscopy (RS) in both the visible (VIS) and NIR spectral ranges. As discussed in the following sections, such technique demonstrated its complementary but important role in pigment characterization and permitted to obtain useful information about fillers, when the NIR interval was investigated, and to ascertain the main coloring phase, when other vibrational methods could only reveal the features of additional ones.…”

Applications of Raman spectroscopy in art and archaeology

“…Corradini et al [ 13 ] have reported the spectroscopic characterization (Raman, visible and near infrared reflectance, and FTIR in the ATR mode spectroscopies together with high‐resolution microphotography) of 27 commercial pigments currently used for retouching purposes. Repeatability was checked by measuring five spots for each pigment using μ‐Raman spectroscopy.…”