Seite - 139 - in The Vienna Genesis - Material analysis and conservation of a Late Antique illuminated manuscript on purple parchment

Rabitsch, Malissa, Hradil, Erlach, Uhlir, Griesser, Hofmann 139 base46. The quantitative amounts of the phases were analyzed by fitting the diffraction pat- tern using the Rietveld method based on the crystallographic structures within the PANa- lytical B. V. HighScorePlus program47. For the analysis a main issue arose due to the minute size of the micro-samples, re- sulting in a highly-restricted number of crystallites illuminated during the experiment with X-rays, and the analysis is, therefore, influenced by a reduced orientation distribu- tion of crystallites. Consequently, the relative intensity distribution within the diffrac- tion diagram can be changed and the database search of fitted phases can be considerably hampered. In figure 21 a scanning electron microscope (SEM) picture of a typical sample from the Vienna Genesis is presented, which was used for the µ-X-ray diffraction (µ-XRD) phase analysis. The red ellipse indicates the X-ray beam size given by the experimental method: due to the sample extraction by chipping a miniscule fragment from the ink on parchment, it is composed from ink/corrosion product combined with tiny parts of the parchment. This is consistent with the XRF analysis where the bluish marked region shows mainly the elements Ca, K, Si, Al and O (parchment) whereas the greenish marked area displays the main elements Ag and Cl (ink area). In the best case, the standard phase analysis by X-ray diffraction methods uses samples typically in the mm2 range with randomly distributed crystallites. This guarantees a statis- tically balanced orientation distribution of the crystallites present, resulting in an optimal quality of the diffraction pattern, allowing direct comparison with the theoretical diffrac- tion patterns within the database. Typical diffraction patterns are presented in Fig. 22 (sample T1 from folio 4, page 8) and Fig. 23 (sample T9 from folio 18, page 35) taken of the manuscript. Table 1 presents the results for the crystalline phase analysis on the Vienna Genesis samples. 46 Soorya et al., 2002, pp. 333–337. 47 Degen et al., 2014, pp. 13–18. Open-Access-Publikation im Sinne der CC-Lizenz BY 4.0