## 3 Results  
The OM and FESEM images of the sample A are shown in Figure 2a. The OM image of the thin section shows a preparation layer with an average thickness of ca. 150 \(\upmu\)m. The grains in this layer were identified as minium (Pb3O4) and yellow ochres on the basis of their appearance with transmittedpolarized light. The results of the OM observations were confirmed by the EDX results. On top of this layer, which works as the \"pole\" layer in the typical \"a guazzo\" technique, the gold lamina (with an average thickness below 5 \(\upmu\)m) can been seen. This lamina is visible as a bright strip at one edge of the sample in the FESEM image (Figure 2b). The thickness of the metallic lamina ranges from 2 to 5 \(\upmu\)m. For other samples of this type (i.e., a gold leaf on a preparation layer), FESEM images show that only some residues of the gold leaf are present. The signals of other metals, such as copper or silver, were not detectable above the noise level in the EDX spectra measured on the gold leaf (Figure 2b). Hence, the EDX analysis rules out that the gilding leaf is a copper-gold or copper-silver alloy or a composite lamina of gold and silver foils as recently found in other gilding decorations [6]. The composition of the preparation layer as determined by EDX (Table 1) is similar to that of the preparation layer for the metal decorations in the wall paintings by [PERSON] in the Scrovegni Chapel [2]. In the preparation layer relatively high concentrations of iron and lead were found. These results are consistent with the micro XRD measurements which show the presence of minimum (Pb\({}_{3}\)O\({}_{4}\)) and white lead (2 PbCO\({}_{3}\) Pb(OH)\({}_{2}\)) (Figure 3). The peaks of yellow ochres are too weak to be detectable in the diffractogram. Copper, which was detected by EDX, is probably present as copper resin, a drying agent used in the \"a mission\" technique [1]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Element** & **Atomic \%** \\ \hline Fe & 28 \\ Si & 26 \\ Pb & 16 \\ Al & 13 \\ Ca & 9 \\ Cu & 6 \\ K & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Elemental analysis of the preparation layer of sample A as determined by EDX. The analysis was performed on an area large enough to represent an average composition. Figure 2: (**a**) Optical microscopy (OM) image of the thin section of sample A, reflected light, dark field. (**b**) Field emission scanning electron microscopy (FESEM) image of sample A and energy dispersive X-ray analysis (EDX) spectra measured on the gold leaf (spot analysis) and on the preparation layer (analyzed area \(50\times 50\)\(\upmu\)m\({}^{2}\)). The metallic leaf is visible as bright region on the preparation layer. In the preparation layer grains of various materials appear with different levels of grey. The whole ToF-SIMS spectrum of sample A (not shown), measured on a \(500\times 500\)\(\upmu\)m\({}^{2}\) area, including both the gold leaf and the preparation layer, exhibits peaks of hydrocarbon fragments having _m/z_ values close to those of the elements detected by EDX. For instance, C\({}_{2}\)H\({}_{3}\)\({}^{+}\) (_m/z_ 27.02) and Al\({}^{+}\) (_m/z_ 26.98), C\({}_{3}\)H\({}_{3}\)\({}^{+}\) (_m/z_ 39.02) and K\({}^{+}\) (_m/z_ 38.96), C\({}_{3}\)H\({}_{4}\)\({}^{+}\) (_m/z_ 40.03) and Ca\({}^{+}\) (_m/z_ 39.96). These peaks of hydrocarbon ion fragments are mainly due to surface contamination of the sample during cutting and handling. ToF-SIMS is a surface sensitive technique, thus even relatively small amounts of impurities in the outermost region of a sample are detectable. The peaks of the isotopes of Cu\({}^{+}\) (_m/z_ 62.9 and 64.9) and of Pb\({}^{+}\) (_m/z_ 206.0, 207.0 and 298.0) are observed, in agreement with the EDX results. The peak of Au\({}^{+}\) (_m/z_ 196.9) is also detectable in the spectra (Figure 4a). The location of the gold lamina over the sample cross section was found by acquiring maps of the lateral distribution of the gold signal.