Chemical separation of acrylic color components enabling the identification of the pigment spectroscopic response

Abstract

Acrylic colors are mixtures of several components that can be identified as pigments, binders, and fillers, so that, when analyzed, the characteristic response of the different components may not be recognizable. This limits the accuracy of spectroscopic techniques, nonetheless particularly useful as they are noninvasive and can be applied in situ on real artworks. Here, a method is proposed to chemically separate and identify the different components of acrylic colors, in order to be able to study their spectroscopic response separately, in particular by ultraviolet–visible–near-infrared diffuse reflectance. The results clearly show that the chemical and analytical method developed here is fully reliable, with the advantage of clearly separating the response of the different components without any change of their chromatic/chemical properties. As a case study, the new method is applied here to original acrylic colors used by the Italian artist Ico Parisi, in view of building a spectra database.

Separating pigments and fillers from the polymer matrix in acrylic colors subjected to natural aging

“…The broad absorption over the complete Vis range corresponds to the typical response of brown colors, including, e.g., ochre, with Fe 3+ -related broad bands around 650 nm and 850 nm [10] (minima in the DR spectrum, corresponding to absorption maxima). In the NIR, the typical low intensity bands due to the polymer matrix [8,11,12] are visible before treatment around 1700 nm, 1900 nm, and 2200 nm, related to the second overtones of strong chemical bonds like -CH and -OH. After the sample treatment, the same spectrum is clearly observed in the UV-Vis range, while the response of the polymer in the NIR range is no longer detectable.…”

“…Figure 1b shows the ATR-FTIR spectra of the same Raw Siena samples, before and after the thermal treatment, definitely confirming the elimination of the polymer component after the thermal treatment. Indeed, the characteristic sharp peak at 1740 cm −1 , attributed to the stretching mode of the -C O bond of the acrylic group [13], as well as several minor peaks at lower wavenumbers and wider bands at higher wavenumbers, again related to the polymer matrix [8], are strongly reduced. Some bands are still detected, to be attributed to inorganic fillers (in particular calcite [8,14,15], with the known three main peaks at 713 cm −1 , 874 cm −1 , and 1400 cm −1 ).…”

“…Nonetheless, when acrylic colors are concerned, the separation of the different components, first of all the pigments from the polymeric matrix, is often a fundamental step in view of their identification and study. Certainly, when acrylic colors are subjected to aging, as in the artworks or on the palette or in the original tubes by the artists, separation can become difficult, being often based on specific treatments of fluid colors [7,8] or on rather complex techniques, e.g., the analytic pyrolysis [2,7]. The availability of an easier, more straightforward, and more widely applicable separation method, then enabling a comprehensive spectroscopic analysis of the color components, should therefore be worthy in several cases.…”

“…The first color is Raw Siena made by Liquitex, an acrylic polymer matrix containing the inorganic pigment PBr7 (various Fe oxides). The second one is a Cobalt Blue by Liquitex, which contains the inorganic pigment PB28 (Co aluminate blue spinel, CoAl 2 O 4 ); the good preservation of the tube permits to recover some fluid samples, already analyzed [8], but, for the purpose of the present study, we took some aged, dry samples. The third color is Phthalocyanine Green, containing the organic pigment PG7 (chlorinated Cu-phthalocyanine).…”

Reflectance Spectroscopy as a Novel Tool for Thickness Measurements of Paint Layers

“…Bands from calcium sulfate (1007 cm −1 ), calcium carbonate (1085 cm −1 ) and barium sulfate (985 cm −1 ) are observed in each acrylic paint and attributed to the fillers. Additional bands at 482, 600, 620, 837, 841, 1106, 1150-1200, 1240, 1305, 1449, 1452, 1728, 2411 and 2800-3100 cm −1 are attributed to the constituents of the polymeric binder [27][28][29][30][31][32]. LIBS results reported in Table 1 show the presence of the main paintings markers such as Co for cobalt blue (CB), Cd for cadmium red (CR) and cadmium yellow (CY), Cu and Ti for permanent blue light (PBL), Cu for permanent green light (PBC and PGL), CN and C 2 for primary red magenta (PRM), Ti, CN and C 2 for primary yellow (PI), Ti for titanium white (TW), and Zn for zinc white (ZW).…”