Cross-section Analysis: SEM-EDS Interpretation

Some scanning electron microscopes have energy dispersive x-ray spectrometers (SEM-EDS), enabling scientists to identify and map the distribution of elements. In the Hogarth samples, investigations focused not only on learning more about the pigments, but also on identifying the composition of the mysterious coating in direct contact with the discontinuous upper paint surface. Here a highly magnified view of the sample shown on the previous page shows a remnant of the unknown coating, which appears light blue under UV illumination (top center). The remaining images are false-color distribution maps of the same area, indicating the location of particular elements. Note that the large black particles shown in the UV image correlate well with the presence of calcium and phosphorous, suggesting that bone black pigment was used. As bone mineral is composed of carbonated hydroxyapatite [Ca10(PO4)6(OH)2], pigment made from charred and ground bone consists in large part of calcium and phosphorous. Note also the increased presence of nitrogen in the area corresponding to the unknown coating; this is a possible indicator that the coating in close contact with the paint surface is protein-based. Finally, the elements present in the brown paint layers visible in the UV image are largely composed of lead, iron, aluminum, and silicon; suggesting that in addition to bone black, the paint layers include a mixture of lead white and iron oxide earth pigments that include aluminum silicates.

Next: Cross-section Analysis: Original Use of a Proteinaceous Coating?
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