Glossary

Electron microscopy Uses both wave and particle properties of electrons to form images and generate other signals. The principal division is between transmission and scanning electron microscopes. In the transmission electron microscope (TEM), as the name implies, a focused beam of electrons passes through the sample and the contrast in the projected image is formed by the interaction of the beam with microstructural features in the sample. In high-resolution electron microscopes (HREMs) the spatial resolution achieved can be as small as 0.08 nm. In the scanning electron microscope (SEM) a focused beam of electrons is scanned in a raster over the sample surface. The interaction of the beam with a small volume of material at the sample surface produces a number of signals. Secondary electrons are low-energy electrons ejected from the surface and the contrast of the image depends on the number reaching the detector, which is a function of the surface topography. Images can also be formed by ‘backscattered electrons’ where the contrast additionally contains compositional information as backscattering is related to atomic number. With a specially prepared sample with a strain-free surface, backscattered electrons can also give important crystallographic information (‘electron backscatter diffraction’ (EBSD)). Also ejected are ‘Auger’ electrons whose energy depends on the atomic species present and, often, the state of their chemical bonding. The electron beam may also cause light to be emitted (‘cathodoluminescence’) or generate an X-ray spectrum characteristic of the elements present. The combination of a rastered beam and the X-ray analysis allows ‘elemental mapping’. light or optical microscopy (OM) Most generally used in metallography using reflected light in ‘bright field’, with a maximum spatial resolution of approximately 250nm. The use of low magnifications or macroscopy is also of importance in studying large and complex structures. Variations that can be useful in identifying phases, including corrosion products, are dark field, where annular illumination means that the illuminating light is not collected but only light scattered from surface features, polarized light, and ‘differential interference contrast’ which in reflected light can emphasize shallow surface relief. A different form of optical microscopy is confocal microscopy. This is an imaging technique usefully used in archaeological metallography to reconstruct three-dimensional images by using a spatial pinhole to eliminate out-of-focus light or flare in specimens that are thicker than the focal plane. The information collected can be used to produce contour or relief maps of surfaces and measure surface roughness, for example, in quantifying grinding and polishing traces. ‘Raman’ and ‘infrared microscopy’ are more specialized optical techniques. Raman microscopy depends on the scattering of laser light by the vibration of chemical bonds which can give very precise information about the molecular species present. ‘Infrared microscopy’ utilizes the absorption of infrared radiation by chemical bonds.

Microhardness testing In combination with optical microscopy to aid interpretation measures the resistance of a material to indentation. A diamond indenter is pressed into the sample under a known load for a known time, and the size of the impression measured. This gives a hardness number which is a function of yield stress and work hardening rate. For samples of ancient metals it may be the only means of measuring mechanical properties that can be applied.

Proton microprobes Have primarily been used in archaeology as a microanalytical tool but they can be used in a scanning mode to produce elemental maps using all the signals available. These include X-rays (PIXE or particle induced X-ray emission), gamma rays, and backscattered protons. (‘Rutherford backscattering’ (RBS)). The energy of the backscattered particles depends on the species of atom they are interacting with and the depth at which the scattering takes place. Because of the sensitivity of this technique for some light elements it permits the three-dimensional reconstruction of the corrosion and patination of metal artifacts.

X-ray photoelectron spectroscopy Can give a measure of the chemical state of atoms in the surface of a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the energy and number of electrons that escape from the surface one to ten. It is potentially a very useful technique for assessing the surface state of some artifacts, for example, the patination of gold, in relation to their appearance.