3 ANALYTICAL TECHNIQUES FOR DISTINGUISHING METHODS OF FABRICATION

METHODS OF EXAMINATION other than visual include the measurement of dimensions, specific gravity measurement, radiography, X-ray fluorescence, scanning electron microscopy, and X-ray diffraction.11 Surmoulages, or reproductions cast from earlier medals, are smaller than their originals because of the shrinkage of cooling metal. Thus, simple dimensional measurement may indicate that one cast is a later edition than another. Since electrotypes were frequently filled or soldered with lead, specific gravity measurement or radiography could aid in distinguishing them from cast or struck medals. If X-ray fluorescence or scanning electron microscopy indicates a pure metal, such as copper, the medal is quite likely an electrotype.

The characteristic differences in microstructure of cast, struck and electroformed metal also make metallography an ideal, albeit destructive method of analysis. Although it has previously received little attention in the analysis of works of art, Laue back reflection X-ray diffraction offers a non-destructive method to obtain similar information on metal microstructure.12 Before discussing its application, it must be understood that the grains in electroformed metal are significantly smaller than those of cast and struck metal.

The photomicrographs in Figure 4 illustrate the difference in size between typical cast, struck, and electroformed grains. Although the grain size will vary according to fabrication conditions, the grains of cast and struck metal will normally be much larger in scale.

Fig. 4. Photomicrographs illustrating the difference in grain size between a. Lost wax cast bronze. b. Struck silver. c. Electroformed copper. Magnification 80X. The arrows indicate grain boundaries.