Phosphor plasters of CaSO4:Dy on the courtyard wall of Djehuty's tomb (Luxor, Egypt)

The X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) analyses of plasters collected from the courtyard walls of Djehuty's tomb show anhydrite, calcite, dolomite, quartz, alkali feldspars and accessorial amounts of halite and illite. The external outer bed is mainly composed by anhydrite, since the original hydrous phases of gypsum plaster were desiccated during thirty centuries in the dry land environment of the Luxor area, under low relative humidity and high temperatures. The luminescence analyses by thermoluminescence (TL) and cathodoluminescence (CL) demonstrate as one plaster sample (m8), i.e., 95% anhydrite, displays a gigantic TL emission of 33 555 a.u. and a SEM/CL emission of 2319 a.u. maxima peak. The spectra CL also exhibits a 484 nm peak attributable to the classic 4F9/2→6H15/2 transition circa 490 nm of Dy3+ and a 573 nm emission of Dy3+ masked in a broad emission band centered at 620 nm. The common presence of sodium minerals such as halite or albite together with the probable presence of sodium in waters points to Na+ ions acting as compensators of Dy3+ to maintain the electrical neutrality of the CaSO4:Dy phosphor lattice. Further analyses of more plaster CaSO4 micro-samples could demonstrate if this observation is just an isolated mineralogical chance or an intentional archaeological handling