Minute cross-sections were taken from two Lekythoi via a technique perfected for sampling materials from sensitive museum quality objects. The composition and morphology of the purplish-red discolouration was investigated first by electron probe microanalysis (EPMA) and scanning electron microscopy (SEM). Fig 2A, shows a representative cross-section taken from a purplish-red area with overlying Egyptian blue (EB) pigment. There is a thin glass layer sandwiched between the stacked lathes of EB and the ceramic, but significant features in this image are the small, spherical, high Z-contrast nanoparticles lying at and just below the glass/ceramic slip interface, determined by EPMA to be copper-rich. Fig. 2B shows the concentrations of Ca, Al and Cu, the elements that demonstrate the most significant variations, as a function of position. The concentrations of both Cu and Ca are highest in the EB and gradually decrease across the glass layer. The bump in Cu content near the glass-ceramic slip interface corresponds to the location of the Cu-rich particle. By contrast, the Al profile exhibits the opposite trend: the lowest concentration is found in the EB, with a gradual increase across the glass layer, with the highest concentration found in the ceramic slip. These data indicate a bi-directional diffusion: diffusion of Al out of the ceramic layer and counter-diffusion of Ca and Cu from the EB particle into the ceramic matrix underneath. These counter diffusion profiles indicate that the Al rich white-ground vessels overpainted with EB and other pigments were subjected to prolonged exposure to high temperature. The Micro x-ray absorption near edge structure (XANES) , shown in Fig 3A, spectroscopy along a similar depth profile indicates that the oxidation state of Cu reduces from +2 to metallic as it diffuse out from EB into Cu rich precipitates just below the surface of the Al rich ceramic. The XANES depth profile, therefore, indicates that not only these vessels were cooked at high temperatures after overpainting, but they were cooked in reducing atmosphere. Extended x-ray absorption fine structure (EXAFS) spectra taken from the Cu-rich nanoparticles, further indicate that these reduced Cu particles are metallic Cu in nature. Light absorption at the surface Plasmon excitation in these metallic Cu nanoparticles gives rise to the purplish discoloration.

Fig. 3 (A) Cu K-edge XANES spectra of the Cu foil standard, Cu-rich particles, glass layer, and EB from the cross-section shown in Fig. 2. (B) EXAFS spectrum of Cu particle (open circles) compared to Cu foil demonstrating that copper particles have a bulk metallic structure.