Examination of metallic samples or surfaces of artefacts under a metallurgical microscope can reveal much about the material, method of construction and the mechanical properties of an historic object. Microscopy can indicate even small changes in composition, heat treatment, and mechanical working through the major changes it produces in the visible crystal structure.
The method allows the gathering of a very wide range of compositional, microstructural, and even cultural information which allows higher levels of interpretation. How effective was the artefact? What access did the metal worker have to particular alloys? How did the metalworker’s skill enable him to exploit the materials to the full?
The underlying basis of the technique is that all solid metals are crystalline and may adopt a wide range of microstructures dependent on composition, mechanical working, or thermal treatments. Specimens are etched using appropriate reagents, making the crystalline structure visible under an optical or electron microscope.
The technique is dependent on the experience of the metallographer and interpretations may differ slightly between practitioners. In a museum, where any sampling must be kept to a minimum, it is possible that the area selected is not representative of the whole. Caution must therefore be exercised in order not to over-interpret the evidence from small samples.
Metallographic specimens can be further investigated by microhardness testing, which measures the resistance to indentation by a pyramidal-cut diamond. The resultant hardness value may provide an indication of the effectiveness of heat-treatments, presence of impurity elements, and degree of work hardening (case study 9).
Prepared specimens are also suitable for examination using a range of microanalysis techniques, such as scanning electron microscope (SEM) based microanalysis.
Uses of Optical Microscopy
- Basic identification of the metal or alloy used. The method is particularly suitable for ferrous alloys such as iron, phosphoric iron, steel and cast iron: Case study 5.
- Provides evidence of heat treatments including quenching and tempering used to enhance the physical properties of an alloy.
- Mechanical working, such as cold working, used to harden a metal or alloy, can be identified.
- Composite construction, such as the steeling of the edges of cutting weapons, can be seen and described: Case study 6.
- An assessment of the homogeneity of an alloy and its non-metallic inclusion content can be made
- Metallic surface coatings can be examined and their means of attachment determined.