[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The so 3 in the air combines with the lime caco 3 to

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ically changes in its composition. The SO 3 in the air combines with the lime (CaCO 3 ) to form Gypsum (CaSO 4 ). Gypsum is softer than lime and more easily damaged by mechanical influences. Therefore, different techniques for restoration are under review. One of the conservation procedures is carried out with a Barium-two-step-technique [27]. The fresco is initially treated with a compress of cellulose pulp saturated with (NH 4 ) 2 CO 3 in water. This removes the sulphate and transforms the gypsum (CaSO 4 ) back into lime (CaCO 3 ) according to (NH 4 ) 2 CO 3 + CaSO 4 CaCO 3 + (NH 4 ) 2 SO 4 . In a second step, a compress of a cellulose pulp with a barium-hydroxide Ba(OH) 2 solution is used to stabilise the lime by substituting Ca by Ba. This change is described by the following processes: Ba(OH) 2 + CaCO 3 BaSO 4 +Ca(OH) 2 Ba(OH) 2 + (NH 4 ) 2 SO 4 BaSO 4 +NH 3 + H 2 O Ba(OH) 2 + CO 2 BaCO 3 +H 2 O . The barium salts are much more stable against the influence of SO 3 pollu- tants than the Ca-compounds. The frescoes will not only be restored but also protected against further damage from similar environmental influences.
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Methodological Developments and Applications 457 Optical image BaK distribution S K distribution Fig. 7.10. Distribution of Ba and S in a fresco cut The arising analytical problem is to determine the penetration depth of the barium into the fresco and to investigate the de-enrichment of sulphur on the surface. This task can be carried out with micro-XRF. A piece of a fresco was cut perpendicular to the surface and the distribution of Ba and S were measured over this area. The results are displayed in Fig. 7.10. The surface of the fresco is at the right-hand side. The figure shows both the op- tical image and the elemental distributions of Ba and S. It can be seen that Ba shows a high intensity close to the surface with a penetration depth of around 1.5 mm. On the other hand, the distribution of S clearly shows that S has a very low intensity at the surface. Only beyond a depth of around 1.5 mm the S concentration does become higher being similar to that nor- mally found in lime. The area analysed was around 20 × 16 mm 2 and the matrix size was 64 × 50, that is, the step size was approx. 300 µ m with a spot size of 300 µ m. The measurement conditions were: 40 kV, 1000 µ A, 1 s per pixel. If the mapping is performed as a spectral mapping, that is, for all pixels analysed, the complete spectrum is saved, the data can be reprocessed and results may be displayed in various forms. The data can be evaluated as a line scan using the saved spectral mapping data. For any given line defined within the mapping area the data stored within the spectra are associated to the pixels closest to this line. They are used to calculate the desired intensity distribution. If more than one line is drawn, an averaging of the distribution in this direction is possible. The intensities of the elements of interest for the defined line scan are calculated in this way and displayed in Fig. 7.11.
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