Applications of Environmental Scanning Electron Microscopy in Art Conservation and Archaeology
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APPLICATIONS OF ENVIRONMENTAL SCANNING ELECTRON MICROSCOPY IN ART CONSERVATION AND ARCHAEOLOGY Dusan Stulik and Eric Doehne The Getty Conservation Institute 4503 Glencoe Avenue Marina del Rey, CA 90292 ABSTRACT The principles of Environmental Scanning Electron Microscopy (E-SEM) are explained and discussed. The performance of the E-SEM compares favorably with the performance of traditional SEM instruments. This new technology has sig-. nificant advantages in art conservation and archaeology. In this paper we describe several pilot studies which explored potential uses of the E-SEM. Electron micro-graphs recorded from moist, outgassing, and difficult to coat samples are presented, together with X-ray spectra recorded from uncoated samples of electrically nonconductive materials.
INTRODUCTION Scanning electron microscopy (SEM) has found a broad range of application in the art conservation field. Conservation science is truly a multidisciplinary field which applies the latest developments in experimental chemistry, physics, biology and materials science to the study of art objects, to providing knowledge about ancient materials and technologies, and to the development of new conservation methods. SEM is used to reach beyond the capabilities of widely used optical microscopes. The application of SEM brings higher resolution, exceptional depth of focus, and the compatibility with X-ray analytical methods. These advantages are sometime balanced by serious limitations based on the high vacuum requirements of classic SEM technology. The conservation scientist often works with materials which are moist or which may outgas in a vacuum. Extensive sample preparation is needed to dry samples and make them vacuum compatible. A number of dehydration techniques have been developed, including air drying, critical point drying and freeze drying from water or nonaqueous solutions [1]. The majority of drying procedures do not prevent dimensional distortion during dehydration. To study moist or hydrated samples under SEM, two techniques have been developed. The first uses a liquid nitrogen cold stage to lower the vapor pressure of water. The second technique examines wet or hydrated specimens in a special environmental cell, which isolates the high water pressure from the high vacuum Mat. Res. Soc. Symp. Proc. Vol. 185. c 1991 Materials Research Society
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The Environmental Scanning Electron Microscopy (E-SEM) is a new technology introduced recently to the art conservation field which brings a solution to the experimental problems of moist and nonconductive samples [2-5]. A very sophisticated differential pumping system and a series of pressure limiting apertures are used in the E-SEM microscope to create a pressure gradient between the sample chamber (900 Pa) and the electron gun compartment (10-5 Pa). At relatively high vapor pressure in the sample compartment, the wet or hydrated samples remain stable during examination, and most liquids may be observed. To maintain the high resolution of a SEM and minimize scattering of the pri
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