Can you tell the difference between brass and bronze? How about 1000 year old brass and recycled artillery shells? Proper identification may be the key to detecting a forgery. Electron probe microanalysis (EPMA) and X-ray fluorescence (XRF) analysis can determine the composition of a metal sample. Used in conjunction with metallographic examination it may be possible to find evidence of antiquity, or the lack of it, in metal objects.
In addition to surface study and metallographic examination, the composition of a metal may provide tremendous information. The average composition of a metal can be determined by numerous methods. Databases exist which provide guidelines for the composition of Chinese bronzes of various ages. If an analyzed sample falls outside the normal parameters, it is not necessarily a forgery. Conversely, one within normal parameters is not necessarily authentic. For this reason, compositional analysis is not used alone, but is combined with metallographic and surface examination to detect evidence of antiquity, or the lack thereof.
In one of the analytical techniques most commonly employed by TK, small solid samples of the metal, generally only a few cubic millimeters, are mounted and polished, then analyzed by electron microprobe (EPMA), with wavelength dispersive spectrometry (WDS).1,2 On samples analyzed for TK, the technique tests areas of approximately 30 X 50 microns (0.03 X 0.05 mm), detecting certain elements, depending on the set-up of the equipment. Most of the samples analyzed for TK are Chinese bronze or other copper alloys, and sixteen elements are generally sought. Detection limits are typically 100-200 parts per million (ppm) for any given element, though the limit is 400 ppm for gold. Several areas are analyzed on a single sample, and then the results are averaged and normalized to 100%, giving the average composition of the metal.
X-ray fluorescence (XRF) can also be used to determine the average composition of a metal. Depending on the equipment, this technique can be performed on a clean metal surface, without removing a sample. XRF equipment comes in several varieties and the detection limits of each can vary greatly. Some models are totally unsuited for use on certain metals, especially bronze. Before choosing an analytical facility it is of critical importance to know their equipment and its limitations. The same facility may use equipment suited to one of your needs, but not all. While XRF results can be fairly accurate, and the technique less invasive, the more exact EPMA is generally used on TK's artifacts.
Test Facilities and Sampling Procedure
Depending on their location and hardness, samples may be cut with a jeweler's saw, diamond coated burr, or an emery cutting wheel. The sample is sealed in a plastic vial and labeled. Accompanying documentation includes a photo of the whole object, photo and description of the sample site, object description, dimensions and proposed age. Oxford Materials Characterization Services (Oxford, England) is equipped for EPMA and XRF analysis, as well as numerous other analytical techniques. A metallurgical examination is also performed on all samples analyzed by OMCS for TK. The work is generally supervised by Dr. Peter Northover.
Metal samples removed for testing can be quite small. Figure 6·1B is shown with a millimeter rule for scale. This particular sample is actually larger than required for microprobe analysis. At TK, the sample size is generally similar to that shown here. When necessary, smaller samples can be taken. Figure 6·1B Sample site
Conclusion
Electron probe microanalysis (EPMA) and X-ray fluorescence (XRF) are both commonly used to determine the composition of archaeological materials. The data collected is seldom sufficient, in and of itself, to categorize a metallic object as antique or modern, and is generally used in conjunction with other forms of analysis and examination when studying purported artifacts.
Notes
1 "Electron microprobe", "electron microprobe analyzer" and "electron probe" are interchangeable terms referring to the physical microprobe equipment. The abbreviations EPMA, EMP and EMPA are used interchangeably.
2 Electron microprobes use wavelength dispersive and/or energy dispersive X-ray spectrometry (WDS and EDS respectively) to analyze the composition of a given sample. The microprobe ionizes the surface of a solid sample, resulting in the emission of X-rays characteristic to the elements present. The spectrometers measure the X-ray wavelengths and energies, and then compare them to standards to identify each element in the sample. The number of characteristic X-rays emitted (intensity) is measured to determine the quantity of a given element. EDS is a much faster analytical method than WDS, but the detection limits of WDS are greater, making it a more sensitive form of analysis. Light elements from hydrogen to boron (atomic numbers 1-5) cannot be analyzed by most WDS or EDS systems.
3 The chart was recreated from the chart of results in Dr. Northover's report, Analysis and metallography of three garlic-mouthed hu, #R2086-88. [2002].
4 Samples of polished metal may be exposed to chemicals in a process known as etching. Etching has varying effects on different features in a sample. Some features not visible in an unetched metal may become visible after etching.
