Читать книгу Monument Future - Siegfried Siegesmund - Страница 234

The simplest approach to quantification consists of calculating the ratio of Mn counts to Fe counts. This takes advantage of the fact that the Fe content of the bulk sandstone dominates over any Fe content in the varnish, at least in the case of red Triassic sandstone. Hence, the Fe can serve as a 163reference value for normalizing the Mn variations and thus confirms that the varnish has enriched Mn. As shown in Fig. 2, the Fe Ka peaks overlap the Mn peaks. Consequently, the attenuation factors are essentially the same for the two elements, and the counts ratio approximates the mass ratio. This avoids the need to convert the counts into masses. In practice, pXRF spectra are acquired for several points in the varnish along with a similar number of points on the adjacent bare stone for comparison. Fig. 3 presents an example of the application of the Mn/Fe ratio method to an area on the Smithsonian Castle, which shows that the points within varnish patches have significantly elevated levels of Mn.

Figure 3: Plot of Mn/Fe ratios for SW corner of the Smithsonian Castle.