Metallurgical Examination

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.