Micro-invasive techniques will damage the artefact either by taking a sample or by altering a small area of the artefact, such as an edge, in situ, to facilitate inspection. For metallic artefacts, in this case the helm and sword, this commonly involves micro-polishing, etching and examination under a reflected-light metallurgical microscope to ascertain what the artefact is made of and to determine the method of manufacture, which can sometimes help with dating. If a sample has actually been removed, then further analyses such as micro-hardness tests or chemical analysis using combustion are possible. However, taking even a small specimen sample damages the artefact and can understandably be unpopular. Apart from potential damage there is the question of where to take the specimen from and whether it is a representative sample of the entire object. If it is judged necessary to take a sample then ideally it should be small, if possible about 1mm in size. The place of the sample’s removal should be as near as possible invisible to the naked eye. In the past, some metallurgists were keen to take as large a sample to maximise their chances of getting reliable results, but while a large specimen or a section can arguably be more representative, such sampling can often result in unnecessary visual damage.

Despite the fact that non-invasive scientific analytical methods such as neutron diffraction continue to become more widely used, the examination of ferrous metals by metallography is still the most practical and informative technique because it readily and inexpensively enables iron and steel to be distinguished, methods of manufacture to be ascertained, and heat-treatment techniques for hardening to be identified. It is important that metallurgists investigating historic arms and armour have a sound knowledge of historical ferrous metal production and working, because medieval ferrous-metal technology differs in a number of important ways from modern iron and steel technology.

One of the most important differences between modern and medieval iron (ferrite) is that during production of the latter the metal was never actually melted and therefore always contains impurities such as slag (fayalite), left behind from the smelting process. The semi-molten bloom had to be consolidated with hammers while still red-hot, to solidify it and simultaneously squeeze out most of the liquid slag still trapped within it.