PHOTOLUMINESCENCE SPECTROSCOPY OF NATURAL RESINS AND ORGANIC BINDING MEDIA OF PAINTINGS
LONDA J. LARSON, KYEONG-SOOK KIM SHIN, & JEFFREY I. ZINK
LUMINESCENCE SPECTROSCOPY is a well-developed and powerful technique for studying molecular properties. In addition to its applications in chemical analysis, the technique can be used to study spatial distribution of microscopic materials in a macroscopic object and molecular changes that occur as a function of time. Luminescence spectroscopy can be used, for example, to measure spectra from samples with dimensions of as small as 1 micron. This spatial resolution makes it possible to measure spectra from very small regions of a large object as well as from very small pieces of an object. Chemical changes that occur in an object can be studied as a function of time. Slow changes that occur over a period of years as well as faster changes that occur within fractions of a second can be measured.
It has long been recognized that ultraviolet light can be used to study painting materials (Rorimer 1931; Radley and Grant 1959). Irradiating a painting with an ultraviolet lamp can cause portions of it to luminesce. Different substances may show different luminescence characteristics, such as different color and intensity. Thus it may be possible to determine the composition of painting layers with this technique. Surprisingly, detailed studies of the luminescence properties of substances used on paintings have been limited, but increasing attention is being given to this technique. E. René de la Rie has made impressive progress in analysis of the luminescence properties of some natural resins, linseed oil, some pure pigments and pigments in the presence of linseed oil (de la Rie 1982a–c). These studies employed a traditional fluorescence spectrometer using a mercury discharge lamp as the excitation source. In further studies, T. Miyoshi and co-workers(1982, 1985) excited samples of poppy and linseed oil as well as various pigments with a pulsed nitrogen laser, and A. Wallert (1986) examined the fluorescence of quinone, lichen, and redwood dyestuffs using a xenon lamp excitation source. More recently, B. Guineau (1989) used the combined techniques of laser-induced Raman and fluorometric microanalysis to study organic dyes and pigments.
The purpose of the studies reported here is to test the feasibility of applying luminescence techniques to the identification of actual painting materials. Specifically, this article surveys a large number of natural resins and organic binding media used on paintings. Both bulk materials and films of selected bulk materials cast from various solvents were studied, including several samples dated from the 1930s. All of the samples surveyed luminesce (tables 1–2). A continuous wave tunable argon-ion laser served as the excitation source, allowing flexibility in the excitation wavelength. Employing a laser as the excitation source allowed spectra from samples or portions of samples as small as approximately 100 microns in diameter to be measured. In addition, the instrumentation permitted temperature effects to be studied. The two experimentally controllable factors of excitation wavelength and sample temperature can be used selectively to obtain an emission profile of each sample. This study thus initiates a database that may be useful in the study of unknown samples.
Table 1 Luminescent Samples Studied Using Laser-Induced Luminescence Spectroscopy
Table 2 Films Cast from Solutions of Slected Bulk Maerials from Table 1 and Studied Using Laser-Induced Luminescence Spectroscopy1
For those unfamiliar with photoluminescence, a simplified description of the theory is given before discussion of the experimental configuration and the results of this survey. The reader is referred to the literature for more detailed information (Turro 1978).