Читать книгу Monument Future - Siegfried Siegesmund - Страница 237

An alternative approach to estimating the amount of Mn that does not require a calibration standard arises from the overlapping of the Fe and Mn X-ray peaks as illustrated in Fig. 2. A corollary to this relationship is that the two Fe K lines at 6.3 and 7.0 keV bracket the K absorption edge at 6.5 keV in the attenuation factor of Mn. This means that the attenuation of the Fe Kβ is 6 times greater than for the Kα line. This differential attenuation makes it possible to measure very thin layers of Mn on the order of microns on top of the sandstone substrate (Livingston et al. 2020). The Fe X-rays are generated primarily in the bulk of the sandstone, but they are attenuated mainly in the surface layer of Mn. The method requires two pXRF measurements: one on the varnish patch and the other on a nearby area of bare stone. The effective thickness of the Mn layer can be calculated from the decrease of the Fe Kβ/Kα ratio of the varnish point compared to that of the bare stone. This method requires the 164assumption that the Mn is in the form of birnessite. In theory the presence of Fe in the varnish could lead to underestimates of thickness. However, microanalyses of varnish samples have shown that the varnish has very low Fe content (Macholdt et al. 2017b, Sharps et al. 2020). Moreover, sensitivity calculations have shown that the Fe content would have to be greater than 10 % to produce significant error (Livingston et al. 2020).