3 ENVIRONMENTAL SCANNING ELECTRON MICROSCOPE (E-SEM)

Scanning election microscopy (SEM), with its high resolution, exceptional depth of focus, and compatibility with x-ray analytical techniques, has been used for a broad range of applications in the art conservation field. Recently, a new type of SEM, the environmental scanning electron microscope (E-SEM, Electroscan Corp.), has been developed, which provides the added capability of working at near atmospheric pressure. The use of near atmospheric pressures provides three significant advantages over conventional SEM analysis. First, vacuum sensitive materials, such as moist, liquid, or outgassing samples may be analyzed. Second, analysis and imaging of noncoated, non-conductive materials can be done without local surface charging. And third, the atmosphere within the analysis chamber is now a variable that can be modified to facilitate the examination of dynamic processes, such as wetting and drying, at high magnifications.

A 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) (fig. 4). Thus the electron gun and beam column remain in high vacuum for good focusing and low beam diffusion, while the sample is near a pressure exerted by liquid water at ambient temperature. With this relatively high pressure in the sample compartment, wet or hydrated samples remain stable during examination, and most liquids may be observed.

Fig. 4. The electron optical column and differential pumping system of the environmental scanning electron microscope.

The image is generated by a secondary electron detector developed specifically for this instrument. It works on the principle that secondary electrons emitted from the sample collide and ionize with neutral molecules in the air, creating a cascade effect, producing more electrons, thereby effectively multiplying the secondary electron signal. To maintain the high resolution of a SEM and minimize scattering of the primary electrons by gas molecules, the working distance between the sample and the final lens is kept as short as possible. Any surface charge generated on the sample by the primary electron beam is neutralized by a number of slow-moving positive ions that are formed with the interaction of the gas in the chamber and the secondary electrons emitted from the sample.

The advantages of E-SEM technology simplify the methodology of current SEM experiments and open avenues for application that could previously not be explored due to difficulties in sample preparation. Several tests have examined potential use of E-SEM in art conservation (Doehne and Stulik 1990; Stulik and Doehne 1991; Doehne and Stulik 1991), and a few examples are presented below.

Conventional SEM images are obtained by coating nonconductive samples with a thin gold or palladium layer to prevent charge buildup and arcing. However, this metal coating layer can interfere with the x-ray analysis of the sample. An alternate procedure of carbon coating the sample does not allow for as high a resolution imaging. Use of the E-SEM eliminates the need for sample coating and allows high resolution imaging and simultaneous x-ray analysis. Another advantage is that a small artifact, such as a semiprecious stone, may be analyzed without the potential risk of damage from a surface coating. In this example, the surface morphology and chemical composition of an uncoated gold pendant with a dark red stone was imaged and analyzed using the E-SEM equipped with an EDS spectrometer. The cut of stone facets and polishing marks are clearly visible in figure 5. The x-ray spectrum (fig. 6) and its qualitative interpretation identify the stone as an almandine garnet (Fe3Al2(SiO4)3), with substitution of Mg and Ca for iron in the crystal lattice.

Fig. 5. Electron micrograph of uncoated garnet pendant

Fig. 6. Energy dispersive x-ray spectrum of uncoated garnet pendant showing qualitative composition

Dynamic studies allow the observation of microscopical changes of materials over time as various factors alter an object. Since processes often occur at rates both faster and slower than the human eye can register, the events are recorded on videotape for later transformation into an easily observable rate. Still images also can be produced at different points in the process for side-by-side comparisons.

One test case was the examination of dissolution and crystallization processes. Using a heating and cooling stage, crystals of NaCl were observed at 20�C using water vapor as the imaging gas in the E-SEM sample chamber (fig. 7a, 7b). Upon cooling the stage to 9�C, at a constant pressure, water condensed on the surface of the salt, rapidly surrounding and dissolving the crystals into a droplet of salt solution (fig. 7c, 7d). The temperature was then raised back to 20�C, and the salt recrystallized as the water evaporated (fig. 7e, 7f). Further work has characterized the crystallization of additional types of salts, and future studies will examine the characteristics of salt crystallization in stone during thermal and wet-dry cycling. Other applications have shown the potential use of the E-SEM for dynamic studies of corrosion by air pollutants and the swelling and shrinking of adobe.

Fig. 7. A series of electron micrographs showing the dissolution (a–c) and reprecipitation (d–f) of NaCl as water is condensed on the surface of the salt crystals and then evaporated. White bar at the lower center part of each micrograph = 20μm

The E-SEM has several advantages over a conventional SEM that give it great potential for use in the field of conservation. The potential for dynamic examinations opens doors for several types of new studies, such as dynamic measurements of material deterioration mechanisms and conservation treatment failure modes. The disadvantages of the E-SEM compared to a SEM is that it has slightly lower resolution at high magnifications as well as limited working distance and low magnification capability.