Identification of Pre-1950 Synthetic Organic Pigments in Artists’ Paints. A Non-Invasive Approach Using Handheld Raman Spectroscopy

Abstract

There is little information on the actual use of early synthetic organic pigments (SOPs) in art objects, especially those from before 1950. Their presence can, however, pose a challenge to conservation because their chemical composition, as well as their lightfastness and sensitivity to solvents, are often unknown. Here, a study on the non-invasive identification of SOPs in historic pre-1950 varnished paint-outs from artists’ materials manufacturer Royal Talens is presented. The paints were analysed using a handheld Raman device. Spectra were evaluated by recording the spectra of the same samples with a benchtop instrument. This study demonstrated that the identification of SOPs in varnished oil paints with a non-invasive approach is possible and rather straightforward. The handheld Raman device allowed us to identify fourteen SOPs from eight pigment classes. Besides the occurrence of expected and the known SOPs of this time period, there were also some surprising results, like the detection of the triarylcarbonium pigments PG2 and PB8, and the monoazo Mordant Yellow 1.

Analytical studies on commercial artists’ colour charts from Das Deutsche Farbenbuch (1925)—identification of synthetic and natural organic colourants by Raman microscopy, surface-enhanced Raman spectroscopy and metal underlayer ATR-FTIR spectroscopy

“…Over the past years, numerous researchers have started to study systematically such references from twentiethcentury material collections, artists' estates, or factory archives, many with a focus on SOPs, e.g. [11].…”

An in-and-out-the-lab Raman spectroscopy study on street art murals from Reggio Emilia in Italy

“…The Raman bands before and after the degradation experiments were contrasted on the basis of normalization with the unaltered bands. Pause et al in 2021 [50] evaluated the BRAVO Raman system for the identification of SOPs in varnished paints. Micro-Raman spectroscopy was also used for comparison reasons.…”

“…For the murals discussed in the current study, if the spectral resolution is not responsible for the unresolved bands, broadening, shifting, and merging can be the final product of either natural exposure to the outdoor environmental conditions or laser degradation. For the Bravo Raman system, the laser power is fixed (less than 100 mW) with literature reporting that laser powers of 45 mW [50] and 50 mW [51] are reaching the sample surface. For the other mobile Raman systems also elevated lasers powers were used.…”

Colour Analysis: An Introduction to the Power of Studying Pigments and Dyes in Archaeological and Historical Objects

“…These materials represent a challenge for scientists and conservators for many reasons, including the limited information available on their chemical composition and degradation. Hence, characterisation studies, such as the contribution by Pause et al [6], are fundamental to building databases and enhancing our knowledge of these materials. In their study, the authors test the use of a handheld Raman device for the non-invasive identification of synthetic organic pigments in historic pre-1950 varnished paint-outs from manufacturer Royal Talens.…”