Surface studies

Metalwork from archaeological contexts, with the exception of some precious metal finds can be expected to be significantly corroded so that the traces of forming, finishing, use, and deposition might be expected to be visible only on a relatively coarse scale. This is not necessarily so since the preservation of detail, such as use wear or combat damage, depends very much on the mode of corrosion. An extreme example of preservation can be seen in the Late Bronze Age hoard from Isleham, Cambridgeshire where some parts of a large mass of scrap metal were effectively protected for the environment for over 3000 years. As a result some of the bronze has a surface layer of corrosion products less than 1 mm thick and it has been possible to use a confocal surface profiler to provide a quantitative measurement of grinding polishing traces as a step toward assessing the type and grade of abrasives used for producing a polished surface. Other surface techniques, for example X-ray photo-electron spectroscopy (XPS) can be used to characterize the corrosion product or, in say a Renaissance bronze, to provide analysis of a patina and relate it to the metal substrate. Where an object is more corroded it may well be possible that within the stratified corrosion products is a layer which preserves the original surface detail and which may be revealed by suitable cleaning (usually mechanical). It may even be possible to detect a chemically treated or patinated surface within such corrosion layers by a technique such as Rutherford backscattering spectroscopy (RBS) in a proton microprobe. Because of its high energy (typically 3 MeV) the proton beam has a much greater penetration than other microbeams and an analysis of the energies of backscattered protons can provide compositional and structural data over depth. Identifying an unusual species in a confined layer could well highlight an ancient patination process that might be difficult to observe in any other way. Even so, to calibrate any surface technique properly, and to relate the surface to the underlying metal structure, some form of sampling and microscopic study is necessary, supported as appropriate by microanalysis.