Seite - 228 - in The Vienna Genesis - Material analysis and conservation of a Late Antique illuminated manuscript on purple parchment

228 The miniatures of the Vienna Genesis: colour identification and painters’ palettes raphy, exploiting a physical mechanism known as Visible-induced Infrared Luminescence (VIL)18. The typical and extremely selective luminescence of Egyptian blue at 940 nm is induced with a visible LED light and collected with a camera especially modified for being sensitive to IR light. When taking pictures in proper conditions, i. e. inside a dark room, only the areas painted with Egyptian blue will generate a response that can be recorded with the modified camera. The fluorescence of Egyptian blue can be seen on IR images from folios 15 and 16. Figure 31 shows the miniature on folio 16, page 32, figure 32 the same miniature on the VIL image. The fluorescence is lower than expected for pure Egyptian blue, which confirms the use of a mixture with ultramarine. In the areas with fluorescence in VIL, copper was detected by XRF. A detail of blue paint on folio 15, page 30, shows dark blue and light blue particles under the microscope at 32x magnification (Fig. 33). The dark blue particles seem to be ultramarine and the light blue particles Egyptian blue. The size of Egyptian blue particles is in the range of 30–40 µm. A SEM/EDX analysis carried out selectively on a grain of Egyptian blue inside a loose particle showed that the composition was 55.0 % O, 28.4 % Si, 7.4 % Ca, and 6.9 % Cu, which leads to the expected stoichiom- etry of CaCuSi4O10. 18 VIL images were obtained with a modified Nikon D70 camera equipped with an Infrarex BLACK filter (cut-off 840 nm); excitation was obtained with a LED ring light set on the camera lens. Expo- sure parameters were as follows: sensitivity 100 ASA, time 1/30 sec., F-stop f/5, focal length 22 mm. Fig. 30: XRF spectrum of a blue area on folio 16, page 32 (MP 16_11). A spectrum of the parchment of folio 16 (MP16_3) shows the background signal detected. Open-Access-Publikation im Sinne der CC-Lizenz BY 4.0