Solid-State NMR and Resonance Raman Studies of Ultramarine Pigments
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SOLID-STATE NMR AND RESONANCE RAMAN STUDIES OF ULTRAMARINE PIGMENTS Eleonora Del Federico1, Wolfgang Schoefberger2, Rajeev Kumar2, Wen Ling2, Sofia M. Kapetanaki2, Johannes Schelvis2 and Alexej Jerschow2 1 Department of Mathematics and Science, Pratt Institute, 200 Willoughby Ave, Brooklyn, NY 11205 2 Chemistry Department, New York University, 100 Washington Square East, New York City, NY, 10003.
ABSTRACT We report on a study of ultramarine pigments via Colorimetry, resonance Raman, and Al, Si solid-state NMR spectroscopy. NMR parameters are shown to correlate well with the intensities of Raman signals corresponding to the chromophores S3-. and S2-.. Further, a correlation is established between the colorimetric parameters L* (lightness) and C* (chromaticity) and the paramagnetic shift in NMR spectra for both 27Al and 29Si. The parameter h (hue) appeared not to vary over the range of paramagnetic host concentrations studied. Preliminary results on faded pigments in both acidic and basic media show that the concentration of diamagnetic guest molecules in the sodalite lattice rises, and some of the paramagnetic species are replaced. 27
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INTRODUCTION Lazurite, the natural ultramarine pigment and the source of "royal blue", has been a popular choice for painters since the late 13th century when it was introduced to Europe by Marco Polo. Because it was even more precious than gold those who commissioned its use were primarily those who wanted to convey their high status. In addition, Lazurite was "frescocompatible", as opposed to other blue pigments available at the time, and therefore feasible for usage in lime-mortar environments. From a conservation standpoint, it is important to note that some instances of the pigment fading in alkaline media have been reported [1]. Lazurite's synthetic counterpart was first synthesized in 1828 by furnacing kaolin, sodium carbonate and sulfur in an oxygen-free atmosphere. By controlling the furnacing temperature and mixture composition, different colors of the pigment (blue, turquoise, green, violet, pink) became available. Ultramarine pigments are aluminosilicates characterized by a sodalite framework with the generic formula [Al3Si3O12]3- (also known as the "β-cage"). Strong paramagnetic (S3-., S2-..) and diamagnetic (S4 or S3Cl) guests are enclarthrated inside the cages and are responsible for the color of these pigments [2]. S3-., is a blue chromophore, whereas S2-. is yellow, and S4 (or S3Cl) is red. The relative concentration of these species determines the final color of the pigment. Although many studies have been published about the aluminosilicate framework structure, scarce information is available on the chormophore concentration and occupancy [3]. To better understand the structure, color and fading mechanisms of these pigments, it is necessary to determine the concentration and occupancy of these chormophore species.
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Solid-state NMR is an ideal technique for the study of these systems as it provides detailed information on the local structure ar
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