Phosphor plasters of CaSO_4 : Dy on the courtyard wall Djehuty's tomb (Luxor, Egypt)

Abstract

The X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) analyses of' plasters collected from the courtyard walls of Diehuty'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 tile 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 ^4F_(9/2)→ ^4H_(15/2) transition circa 490 run of Dy^3+ and a 573 nin emission of Dy^3+ masked in a broad emission band centered at 620 nm. The common presence ol' socliUln minerals such as halite or albite together with the probable presence of sodium in waters points to Na^+ ions acting as compensators of Dy^3+ to maintain the electrical neutrality of the CaS0_4: Dy phosphor lattice. Further analyses of more plaster CaSO_4 micro-samples could demonstrate if this observation is just an isolated mineralogical chance or an intentional archaeological handling.