Beyond the constraints set by the wavelength limit of light, the world of electron microscopy offers almost unlimited resolution, current research in aberration correction lenses in transmission electron microscopes giving spatial resolution down to 0.08 nm. The most commonly used form of electron metallography is secondary electron imaging (SEI) in the SEM. Increasingly, instruments with large specimen chambers and low vacuum or variable pressure mean that large objects can be examined without preparation so that the maximum of surface detail and attached organic and other material can be imaged. For smaller objects the technique has been exceptionally useful in the investigation of jewelry, gold in particular giving excellent contrast and allowing even the smallest features of granulation and soldering to be explored. New software now means the three-dimensional potential of SEM images can be fully realized with the same possibilities of contour mapping, depth profiling, and surface roughness measurement as with optical techniques, as well as creating anaglyphs for a true three-dimensional view.
SEI is not the only imaging technique, backscattered electron imaging (BEI) in topographic and atomic number contrast, EBSD, and cathodoluminescence (CL) all having uses. EBSD has great potential for the future because, at the expense of stringent sample preparation requirements, it gives important crystallographic data. When metals sections are formed, the methods used can influence the preferred orientation of grains and the resulting texture can be quantified by EBSD. Applications might include differentiating between rolled and hammered sheet for making coin blanks, or exploring how the making of sheet bronze for cauldrons and shields might have changed over time, or perhaps even providing convincing evidence of hot working of silver and bronze. EBSD has also been used to explore how microstructure might change with time through phenomena such as discontinuous precipitation of an alloying element from supersaturated solid solution or diffusion induced grain boundary migration. This has been most studied in silver but has a wider significance.
These grain boundary changes could be studied in greater detail in the TEM and the mechanisms probably fully understood. This has not happened for a number of reasons: specimen preparation is challenging, access for archaeology to instruments and trained microscopists has not been easy but, probably most importantly, the relevance of the technique to metallurgical questions in archaeology has often not been recognized. Hence any review of the technique will be little more than a wish list but it could have a considerable impact. First and foremost, many microstructural features could be correctly identified for the first time, such as the composition and structure of carbo-nitride precipitates in ancient iron, or the interpretation of cold worked microstructures in iron and steel. Questions long discussed, such as the impact of certain conservation techniques which use moderately elevated temperatures, or the origins of the extreme ductility of certain alloys such as some arsenical coppers could at last be resolved. Nanotechnology is a modern coinage but its products have been in use for two milleniums or more: the purple surface of shakudo, or the techniques of luster ware are two examples and they will not be fully appreciated until observed with sufficient resolution and, equally significantly, quantified (see Conservation and Stabilization of Materials).
World History
