Metal artifacts, analysis

Linke, R. and Schreiner, M. (2000). Energy dispersive X-ray fluorescence analysis and X-ray microanalysis of medieval silver coins - an analytical approach for non-destructive investigation of corroded metallic artifacts. Mikrochimica Acta 133 165-170. 

The analysis of metal artifacts has been used extensively to differentiate materials by sources. X-ray fluorescence and neutron activation analysis have both proved valuable in determining elemental concentrations. Native metals, such as gold, contained impurities that could, in some cases, be used to characterize their sources. However, the smelting of ores to recover the metals often changed the concentrations of impurities. Later, as alloys (e.g., bronze and brass) were produced, the compositions were intentionally altered and controlled. In some cases, the re-use of materials or the lack of quality control made the alloy composition quite variable (especially in terms of the trace components). 

Certain ore deposits may partially overlap in their lead isotope composition. In such a case, it might not be possible to decide on the basis of the lead isotope data alone which of the ore deposits in question provided the ore for metal artifacts that have lead isotope compositions that fall into the overlapping space. Sometimes, one of these ore deposits can be ruled out by using trace element data, particularly from gold and silver analyses (6). Lead isotope analyses by themselves can make a negative statement with absolute certainty in a way which chemical analysis can never hope to do. If the lead isotope composition of an artifact falls well outside the lead isotope field characteristic of a particular ore deposit then it is certain that the metal... 

Last I would like to mention the interesting case of two bronze statues. One, Man with a Beard, a handsome Renaissance bronze statue was owned by the Louvre in Paris, whereas the other, Boy with a Ball, was owned by the National Gallery in Washington, D.C. (8). These two statues were remarkably similar in style, and it was suspected that at some time these may have been part of one artifact. Stylistic analysis, however, was not sufficient to make the final decision. X-ray fluorescence study of the two objects showed identical chemical composition which can happen only if they were poured from the same batch of metal. The analysis also showed that by composition the artifacts were closer to brass than to bronze. The group was reunited and is now known as St. Christopher Carrying the Christ Child with the Globe of the World. The statue illustrates the legend in which St. Christopher carried the Christ Child across a river. 

A nalysis of metal artifacts has long been a major tool in archaeological studies. Since by their very nature metal samples are electrically conducting, they are also well suited for analysis by spark source mass spectroscopy (SSMS). For several years we investigated the use of this technique and have applied it to archaeologically interesting samples. 

Obviously, the bulk composition of these solid standards should ideally be as similar as possible to that of the samples of interest, because the ablation yield, the transport efficiency of the particles produced, and the generation of the signals in the ICPMS instrument will most probably be influenced by the physicochemical characteristics of the matrix. Unfortunately, perfectly matrix-matched standards are not often available, with the possible exception of the analysis of glass and, particularly, of metals, for which a large number of reference materials are accessible. Some research papers on coins and metallic artifacts have used commercially available reference materials for calibration, generally with satisfactory results [26,39,48,49,63,64,72,73]. 

From an instrumental point of view, the research in years to come will be most likely linked to the new femtosecond lasers, which can offer a significantly improved performance for metallic materials, such as coins and metallic artifacts, as confirmed by the first results presented [26]. The development of the so-called direct atmospheric sampling (ablation of sample without the need to use a closed ablation cell) by Kovacs et al. is another significant development that can help in the analysis of large and precious objects [51]. 

Benoit, G. Hunter, K. S. Rozan, T. F. Sources of Trace Metal Contamination Artifacts During Collection, Handling, and Analysis of Freshwaters, Anal. Chem. 1997, 69, 1006-1011. Keith, L. H., ed. Principles of Environmental Sampling, American Chemical Society Washington, DC, 1988. 

If the sample is an insulator there are still methods by which it can be studied in the instrument. The simplest approach is to coat it with a thin (10-nm) conducting film of carbon, gold, or some other metal. In following this approach, care must be taken to avoid artifacts and distortions that could be produced by nonuniform coatings or by agglomeration of the coating material. If an X-ray analysis is to be... 

---