Hist Metall 39 (2)

Report on work reproducing both traditional cementation brass production methods and the high-zinc production technique reported by Zwicker et al (1992) to explore the true limit of zinc content and how this relates to historic brass compositions. Metallography and advanced scanning electron X-ray composition mapping showed that it is possible to produce brasses with over 40wt% zinc by cementation. However, to achieve these compositions the process must be performed by very small quantities and at very short furnace times, which is contrary to historical written accounts of the cementation brass production techniques. It is the opinion of the authors that while it is possible to produce a >40wt% brass via cementation, the commonly reported zinc content limit of ~30wt% is valid for historic artefacts.

The report questions the assumption that plasma treatment of corroding metals has little significant effect on the microstructure of the metals. It is argued that, 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 by a series of test pieces of iron and silver metallographically examined before and after plasma treatment. The tests are described and some of the changes in structure that are likely to occur in metals in the temperature range 100--300ºC are outlined. The authors contend that these changes are significant and are often the result of deliberate heat treatments during the original manufacture of the artefacts. Once they have been modified by subsequent heating much of the fabrication history will have been lost. It is also argued that other structural changes occur over long periods of time and these are also vulnerable to quite modest heating, thereby compromising future authentication examinations.