Example: A Lost (and Found) Brass Spoon
This spoon was brought to me by a pleasant woman whose husband graduated from the University of Minnesota about 40 years earlier. She found it on the shore of Lake Michigan, protruding from the sand, while out for a stroll and recognized that it was old. The handle was broken, probably near its mid-point, and the bowl was warped. There was some black corrosion across some of the surface. She wondered about the composition of the spoon and was curious to learn more about it, but more information, such as its composition, was necessary. There are some "home test kits" for determining the basic composition of common alloys, but these involve a chemical reaction and dissolving some of the surface. The woman had seen enough Antiques Roadshow to know that she didn't want to clean the spoon, much less dissolve any of it. I offered to analyze the spoon as a courtesy for a U of Mn alum.
The spoon was just under 10 cm in length, so I was able to place it into the electron microprobe without cutting or sampling it. Because the spoon was metal, it was not even necessary to apply a carbon coat. The analysis, therefore, was totally non-destructive. The spoon came out of the instrument in its original condition.
The metal was found to be brass with the following composition (by weight, not volume):
Cu: 64.9%
Zn: 32.9%
Sn: 0.86%
S: 0.59%
Cl: 0.30%
Si: 0.22%
P: 0.19%
Al: 0.10%
A few of the elements (Cl, P, S, etc.) could have been products of corrosion and alteration during burial, but the spoon was clearly a brass with some tin. Tin can inhibit the leeching of zinc in most environments and, therefore, might have been deliberately added. Brass with about 30% zinc and 0.75-1% tin is called admiralty brass.
The black-colored corrosion across some of the spoon surface was also analyzed:
Cu: 56.8%
S: 36.9%
Ag: 2.8%
Cl: 1.3%
Si: 0.90%
Mg: 0.76%
Na: 0.54%
Some of these elements, like silicon, magnesium, and sodium, are geologically common and may have come from the surrounding soil or sand matrix during burial. The bulk of the corrosion is copper and sulfur, and the concentrations of these two elements indicate that the compound is copper sulfide (FeS) or one of the chalcocites.
The other interesting element in the black corrosion is silver, which has an abundance of nearly 3 percent. It may be that the spoon was originally silver-dipped or plated and that just traces remain. The remaining silver was probably bound to some of the sulfur as silver sulfide (Ag2S), which is the black tarnish on silver items.
Examination also revealed remnants of a marker's mark on the handle, right at the break, so it was not complete. As best as I could read it, the maker's mark was: E X I? H? A I? [small, not readable] P I A T H? [break]. It is possible that "EX" stands for Exeter, England, and an expert would be able to tell more from the marks.
This spoon, therefore, was likely admiralty brass and initially silver-dipped or plated, and remnants of a marker's mark was found, suggesting a possible British origin. The owner was happy to know more about her spoon, and she had the information necessary to research more about the spoon's time and place of origin.
6/4/07
Added:
Electron Microprobe Analysis in Archaeology
Electron microprobe analysis (EMPA), also known as electron probe microanalysis (EPMA), is an analytical technique that combines scanning electron microscopy (SEM) and compositional analysis using x-ray spectrometry. The ability to determine structure and chemistry of samples makes EMPA very versatile. This is a dominant analytical technique in geology, but it is not as commonly used in archaeology despite similar materials in studied both fields. Here I will post about topics in EMPA, artifacts I have analyzed, archaeological studies that use EMPA, etc. If there is a topic you'd like to see posted here, please let me know.
Ellery Frahm
Doctoral Candidate, Archaeology
Research Fellow, Geology & Geophysics
University of Minnesota - Twin Cities
