SEM-EDS analysis was performed on the blue particles using a JEOL InTouchScope JSM-IT100LA coupled with a JEOL Dry Extra EDS detector in the microscopy facilities of the MPI-SHH Department of Archaeology. Spectra were collected from the archaeological blue particles and the reference pigments at a working distance of 10 mm for optimal data collection (data file S2). EDS analysis was performed qualitatively (that is, nonquantitatively) to compare the spectra produced from each reference pigment and the archaeological particles (Fig. 3and fig. S7). The archaeological particles were each targeted individually, while the reference pigments were each mounted on separate aluminum stubs, and the EDS spectra represent an average of the entire visible area at a magnification of 400×. Spectra were collected for a minimum of 15 min or until the most abundant element reached approximately 18,000 counts. All particles were examined with a 10-kV beam in low vacuum mode to avoid gold coating the samples. The sample navigation system and five-axis motor-driven stage enabled us to locate the archaeological blue particles that had previously been imaged both individually and in the high-resolution map created using the light microscope.

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