Laser Spectroscopy of Materials Used in Paintings
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LASER SPECTROSCOPY OF MATERIALS USED IN PAINTINGS LONDA J. LARSON, KYEONG-SOOK KIM SHIN AND JEFFREY I. ZINK Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90024-1569
ABSTRACT A wide variety of natural resins, waxes, gums, drying oils and proteinaceous materials used on paintings are photoluminescent. The photoluminescence spectra of these materials are reported and discussed. The application of this technique towards the identification of these materials is considered. Both bulk materials and films prepared from selected bulk materials were studied and a comparison is made between the bulk and film samples. Temperature and excitation wavelength studies are reported and discussed for several of the samples. INTRODUCTION It has long been recognized that irradiating a painting with ultraviolet light can cause portions of it to luminesce [1,2]. Furthermore, different substances used on paintings may show different luminescence characteristics such as the color and intensity of the luminescence. Thus, this technique may be useful in determining the composition of painting layers. Luminescence spectroscopy is a well developed tool for studying molecular properties [3], but detailed studies of the luminescence properties of substances utilized on paintings have been limited. Increasing attention is being given to this technique by several workers. E. Rene' de la Rie has made progress in the analysis of the luminescence properties of some natural resins, linseed oil, some pure pigments and pigments in the presence of linseed oil [4-6]. Further work has been done by T. Miyoshi and co-workers who studied samples of poppy and linseed oil as well as various pigments [7,8] and A. Wallert who studied the fluorescence of quinone, lichen and redwood dyestuffs [9]. More recently, B. Guineau used the combined techniques of laser induced Raman and fluoromet'ic microanalysis to study organic dyes and pigments [10]. In this article, we report a study to test the feasibility of applying luminescence techniques for the identification of painting materials. The luminescence properties of a variety of samples including natural resins, waxes, gums, drying oils and proteinaceous materials were surveyed. A continuous wave tunable argon ion laser served as the excitation source, thus allowing flexibility in the excitation wavelength. By employing a laser as the excitation source, samples or portions of samples as small as approximately 100 microns in diameter were analyzed. In addition, the instrumentation allowed temperature effects to be studied. The two experimentally controllable factors of excitation wavelength and sample temperature may be selectively used to obtain an emission profile of each sample. This study thus initiates a data base which may be useful in the study of unknown samples. RESULTS AND DISCUSSION The materials studied are listed in Table I. The bulk materials were either in the form of lumps or powders. Several samples dated from the 1930's obtained fro
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