The brightest colors: A Fourier‐transform Raman, surface‐enhanced Raman, and thin‐layer chromatography‐surface‐enhanced Raman spectroscopy study of fluorescent artists' paints

Abstract

Among the so‐called “special effect” pigments, fluorescent ones play an important role, thanks to their application to a wide range of objects in everyday life. Also in the artistic field, they have been exploited since 1950s by several painters (among others Andy Warhol and Frank Stella), thus representing a particular class of materials with which conservators must confront themselves. Not much information is available about the complete composition of commercial fluorescent paints that are based on fluorescent dyes dispersed in a transparent and brittle resin. Herein, Fourier‐transform Raman spectroscopy and surface‐enhanced Raman spectroscopy (SERS) were used to recognize the main dye components of the paints, whereas thin‐layer chromatography was coupled with SERS to better distinguish further chromophore mixes responsible for the color and/or the fluorescence emission due to the presence of optical brighteners. This methodology results suitable to more fully define the composition of such fluorescent paints originally used by artists in their works, thus providing valuable information to conservators for restoration and retouching efforts.

Special-Effect and Conventional Pigments in Black Light Art: A Multi-Technique Approach to an In-Situ Investigation

“…In particular, in the case of Earthquake , the Raman spectrum of area 1 shows the typical bands of rhodamine 6G at 1647, 1530, 1510, 1360, 1344, 1309, and 1182 cm −1 [ 11 ] ( Figure 1 e and Table 2 ). The remaining Raman bands observed for the painting and also listed in Table 2 are due to: (i) the resin in which the rhodamine dye was dispersed to obtain the fluorescent pigment, corresponding to those reported in the literature [ 9 ] for a copolymer of p -toluene sulfonamide, melamine, and formaldehyde; (ii) the binder of the fluorescent paint, identifiable as polyvinyl acetate (PVA) thanks to the bands at 1728, 1450 and 1380 cm −1 [ 21 ]; (iii) titanium white in rutile form (intense bands at 608 and 444 cm −1 , [ 22 ]; (iv) calcium carbonate as dolomite (band at 1098 cm −1 ) [ 23 ], probably present as an extender.…”

“…The Raman spectrum obtained on the yellow area 2 of the painting Earthquake by Maria Cristiana Fioretti ( Figure 3 b) shows several bands in common with the spectrum of the reference fluorescent paint Light Yellow Y173 [ 11 ], among which those at 1588, 1548, 1429, 1235, 1203, and 692 cm −1 are characteristic of the coumarin dye Solvent Yellow 160:1 ( Table 2 ), while the others are due to the melamine–formaldehyde- p -toluene sulfonamide resin. The bands due to the vinyl binder to calcium carbonate and rutile are also observed ( Table 2 ).…”

“…Some authors had already analyzed the composition of these paints from the point of view of coloring materials by the FT-Raman, SERS, and TLC-SERS techniques [ 11 ]. Table 1 lists the analyzed materials, together with their coloring components.…”

“…To acquire the emission and visible reflectance spectra of these paints, in addition to the Raman spectra already reported in reference [ 11 ], they were spread both on canvas and glass slides.…”

“…It should be emphasized that the composition of commercial luminescent pigments, especially fluorescent ones, is usually not disclosed by manufacturers and, for this reason, several analytical techniques have recently been applied for the identification of their components. In particular, the techniques used include: spectrofluorimetry, both coupled with thin-layer chromatography (TLC) [ 7 ] and three-dimensional (3D) [ 8 ]; Raman spectroscopy with near-infrared excitation at 785 or 1064 nm [ 9 , 10 , 11 ]; surface-enhanced Raman spectroscopy (SERS) also coupled with TLC [ 11 ]; high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS) [ 12 , 13 ]. Raman spectroscopy and HPLC-MS have also been applied to micro-samples taken from works of art to identify the materials used by the artists [ 9 , 12 , 13 ].…”