Sampling

While, as will be described later, there is much to be learned from the macro - and microscopic examination of the surfaces of artifacts, the majority of their history is contained within them and so the removal of one or more samples is in general essential. It must be said, though, that as with compositional data, to recover microstructural information from a metal artifact sampling or other intervention may not be necessary. For example, developments in radiography can reveal structure on a variety of scales, even down to fine microstructural detail such as deformation twins, especially where the detail has been highlighted by corrosion. Where the removal of samples is necessary, their size and location should be determined by the information sought from an object but this will be constrained by its condition and geometry - it is much simpler to cut a section from a sharp cutting edge than a thick circular section bracelet. The microscopy technique used also has an influence. For transmission electron microscopy (TEM) which is not used as often as it should be in archaeology, sufficient material has to be removed to be able to prepare a thin foil 3 mm in diameter. For optical microscopy (OM) and scanning electron microscopy (SEM) the sample may be as small as a single fragment of gold thread a few micrometers thick, or as large as a complete crosssection of a pattern-welded sword or, indeed, a complete object. In some circumstances metallography can be carried out in situ on an object, with a small area perhaps 2-3 mm in diameter being polished and etched. As an example, this technique has been used with success in a major study of medieval and renaissance armours.

Where a sample has been taken for study of a crosssection it is rarely so large that it can be prepared without being mounted. (Samples for scanning electron microscopy are discussed briefly in a later section). Mounting can be hot or cold. Cold mounting in epoxy, acrylic, or polyester resins has the advantage that delicate or brittle specimens can be mounted without being crushed by the hydrostatic pressure in a hot mounting press, and also low viscosity resins can be used to impregnate porous samples such as bronze massively corroded by fuel ash. Tin and lead require particular care as even some cold resins reach a temperature during curing which will significantly alter their microstructure. This is even more true of hot mounting, but hot mounting is undoubtedly an efficient process and some resins have useful properties such as conductivity or good edge retention.

Once samples are mounted for optical or other microscopy they need to be ground and polished, using successively finer grades of abrasive. Modern industrial methods of metallographic preparation are designed to provide flat, damage free surfaces of a wide variety of materials using a minimal number of process steps. Developments such as diamond grinding disks and polycrystalline diamond suspensions undoubtedly produce good results but removal rates are often too rapid for small samples and a more traditional multistep approach is safer. The final polish is aimed at producing a visually scratch-free and undeformed surface. Each grinding and polishing stage, besides removing metal, deforms the surface layers of the sample and this deformation will become visible once the sample is etched and obscure a lot of detail. A deformed surface also precludes important new techniques such as electron backscattered diffraction (EBSD) in the scanning electron microscope. Care also has to be taken in preparing soft metals such as tin, lead or pure gold and silver to ensure that grinding and polishing debris is not left embedded in the soft metal. In all these latter circumstances the final polish could require specialized compounds such as colloidal silica, or be substituted by chemical or electrolytic methods (see Metals: Primary Production Studies of).

Finally, for the optical microscope, or for accurate microanalysis in a scanning electron microscope or electron microprobe the sample surface must be placed normal to the incident beam.