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DYNAMIC STUDIES OF MATERIALS USING THE ENVIRONMENTAL SCANNING ELECTRON MICROSCOPE Eric Doehne and Dusan Stulik The Getty Conservation Institute 4503 Glencoe Avenue Marina del Rey, CA 90292

ABSTRACT Dynamic studies allow the observation of microscopical changes of materials over time as various factors alter an object. Using this methodology, processes important in art conservation and archaeology such as the wetting and drying of consolidated and unconsolidated building materials or the corrosion of metals from air pollutants can be studied in situ and in tempora. The development of the Environmental Scanning Electron Microscope has made it possible to videotape these dynamic processes at nearly the same resolution limits as traditional SEM technologies without elaborate sample preparation. Experiments examining salt crystallization, shrinkage in adobe, and lead corrosion illustrate the value and applicability of the new E-SEM technology.

INTRODUCTION The study of natural processes at high magnification (100-100,000 times) has taken a substantial step forward with the introduction of the Environmental Scanning Electron Microscope (E-SEM). Light microscopy combined with video cameras and recording equipment provided the basis for traditional video microscopy [1]. With the superior imaging and analysis capabilities of the E-SEM it is now possible to extend the analysis of dynamic processes much further than before. The kinds of questions we hope to explore with this new instrument include: 1) better understanding the corrosion of art objects by indoor air pollutants, 2) the fine scale response of materials exposed to cyclic environmental conditions, and 3) the relation of physical and chemical changes in inhomogeneous materials during aging experiments. As a test of the Environmental SEM's ability to examine conservation-related problems we performed three dynamic experiments: dissolution and crystallization of NaCl, humidity cycling of adobe, and corrosion of lead by formic acid.

Mat. Res. Soc. Symp. Proc. Vol. 185. c 1991 Materials Research Society

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EXPERIMENTAL The normal sample chamber pressure in a scanning electron microscope is 10-5 Pa. The Environmental SEM increases the range of pressure in the sample chamber to 900 Pa, allowing the examination of wet samples at room temperature (Fig. 1). This is made possible by a differential pumping system that keeps the electron source at high vacuum and a new type of detector that uses the chamber gas to amplify the secondary electron signal called the ESD or Environmental Secondary Detector [2-5]. Conventional high vacuum SEM can also be performed with the E-SEM. Dynamic processes often occur at rates both faster and slower than the human eye can register. In order to capture these events they must be recorded in a form that may be transformed into an easily observable rate. The results of this paper are presented as still photographs taken from VHS and S-VHS video recordings of the video signal from the Environmental SEM. Video tape has a typical horizontal re