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Plasma treatment became an established method for the treatment of corroding metals, particularly iron, in the 1990s. There seemed to be a general assumption that the treatment had little significant effect on the microstructure of the metals and this has never been seriously challenged. Although under the less extreme conditions now adopted in some laboratories gross phase changes may not happen, more subtle changes are very likely to be taking place, as exemplified here by a series of test pieces of iron and silver metallographically examined before and after plasma treatment. These tests are described and some of the changes in structure thatare likely to occur in metals in the temperature range 100–300°C are outlined. These changes are significant and are often the result of deliberate heat treatments during the original manufacture of the artefacts. Once they have been modifi ed by subsequent heating much of the fabrication history will have been lost. Other structural changes occur over long periods of time and these are also vulnerable to quite modest heating, thereby compromising future authentication examinations.
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