Scanning Electron Microscopy (SEM)

The scanning electron microscope (SEM) is used to study the materials, technology and manufacturing history of museum objects in great detail at magnifications up to several thousand times, which is much higher than with light microscopes. The SEM has a great depth of focus and high resolution so that sharp images of very fine details are produced. The object is viewed by scanning a beam of electrons onto the surface and collecting the various signals that are produced, which are used to form the image and to analyse the surface. The images appear in shades of grey are very clear and emphasise features of interest.

Electrons provide the magnified image of the surface, while X-rays are produced at the same time by the interaction of the electrons with the atomic structure of the material and the X-ray spectrum gives the composition of the material being examined. The spectrum is quite similar to that produced in X-ray fluorescence analysis, and the technique is called energy dispersive X-ray analysis (EDXA). The distribution of the chemical elements across the sample surface can be mapped and compared to the electron image for greater understanding of the physical and chemical properties of the materials, the methods of manufacture and the condition of the object for conservation purposes.

We have two new SEMs in Scientific Research:  a ‘Variable Pressure’ microscope with a large specimen chamber into which can be put whole objects such as pottery or gold torcs for direct examination. The new technology VP SEM allows it to be used to examine any antiquity material directly, whether it is non-conducting such as glass, ceramics, paper, shell, charcoal and corrosion products etc. or conducting metals.  The second SEM is an ultra-high resolution microscope that is used to examine samples of materials and objects at much greater magnification in much greater detail in order to more fully understand the microstructure of the materials. To examine the internal structure of a material, a polished cross-section may be prepared. The microstructure and composition are locked into the material of an object as a record of the ancient manufacturing processes, which can de unravelled by such study.  

Further reading

Joseph Goldstein et.al. 1981, Scanning Electron Microscopy & X-ray Microanalysis, 2nd edition, Plenum Press.

V.D. Scott &G. Love, 1983,  Quantitative Electron Probe Microanalysis, John Wiley & Sons.