AN INVESTIGATION OF THE ORIGIN OF THE COLOUR OF THE LYCURGUS CUP BY ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY

The Lycurgus Cup (Figure 1) is an outstanding example of a late Roman cut glass cage cup. It is decorated in openwork with a frieze showing scenes of the myth of King Lycurgus. The cup, which is 165 mm high, was produced from a blown glass blank about 15 mm thick. The figures were cut and ground and are attached to the wall of the vessel by small glass bridges left by the cutter. It is dated to the fourth century A.D. on stylistic grounds, but is first recorded in 1845 and was acquired by the Rothschild family shortly after this date; its findspot is unknown (Harden and Toynbee 1959; Harden et al. 1987). In addition to the exceptional workmanship demonstrated by the cut decoration. the Lycurgus Cup is also noted for the unusual optical effects displayed by the glass. In transmitted light, the glass appears a deep wine-red, while in reflected light it is an opaque pea-green. Less than ten other examples of Roman glass showing this so-called ‘dichroic’ effect are known, but these are fragments. Furthermore, these other pieces show less intense colours than the Lycurgus Cup, or are brownish or amber in colour (Brill 1965; Harden et al. 1987). Chemical analyses of the glass of the cup were reported by Chirnside and Profitt ( 1 963 and 1965), Chirnside (1965) and Brill (1965). A composite of these analyses (mainly from Brill 1965) is given in Table 1. Brill also measured optical spectra of the glass and had a replica composition prepared. On the basis of these studies, Brill (1965) was able to conclude that the optical properties of the glass were caused by the presence of finely dispersed particles of gold, probably alloyed with silver. At the time of Brill’s study, it was necessary to infer the presence of the metallic particles by indirect means, since particles of the size postulated were then below the resolution limits for techniques such as scanning electron microscopy (SEM). However, modern transmission electron microscopy (TEM) can resolve and determine the structure of metal particles with diameters N 10 nm (e.g. Marks and Smith 1982). Compositional information on slightly larger particles is readily obtained by energy dispersive X-ray (EDX) spectrometry in the TEM. Thus analytical TEM offers a more positive and detailed characterization of the phases responsible for the dichroic properties of the glass of the Lycurgus Cup. In view of the interest generated by this unique piece of glass, it was decided to use about