## 3 EM Techniques for CH Diagnosis
### X-Ray Fluorescence (XRF) in CH Applications  
The technique is based on the emission of secondary fluorescent X-rays. If an atom is exposed to high-energy radiation, it may be ionized by the ejection of electrons. When an inner-shell electron is ejected, the vacancy is filled by an electron from an outer shell, and other bound electrons cascade to fill the vacancies. X-rays and gamma rays have enough energy to induce electron ejection. The advantages and limitations of the XRF technique are reported in Table 12.  
The system must be calibrated. Portable XRF devices allow in situ analysis but are usually difficult to manage. Results are presented as plots of intensity (the amount of radioactivity detected per second) against energy, where peaks represent specific X-ray emissions. An example is shown in Figure 9.  
Practical Application of XRF to CH  
X-ray fluorescence permits the study of the elemental composition and is particularly useful when studying inorganic materials and polished areas. To be detected, the elements must have an atomic number equal to or greater than that of sodium (11). It is a useful technique, especially when comparing colors with similar chromatic yields. In fact, in the aforementioned work of [PERSON]. [PERSON] et al. [28], the analysis of similar chromatic pigments (yellow in particular) has been crucial to reinforce the hypothesis that the third wall of \begin{table}
\begin{tabular}{c c} \hline \hline
**Advantages** & **Limits** \\ \hline Nondestructive & Penetration is to a few ten of \(\mu\)m below surfaces of analysis. \\ High sensitivity (p.p.m) 2 & The detection rate depends on the abundance of specific \\ High spatial resolution 2 & \\ Rapidity of analysis & \\ \hline \hline \end{tabular}
\end{table} 
Table 12: XRF advantages and limits for CH applications 1. Figure 9: [PERSON] family before [PERSON], Fresco by [PERSON], known as [PERSON], 1519. Villa Farnesina, Rome. Credit [PERSON] et al. [28]. Distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license [[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)) (accessed on 20 November 2024). Changes were made to figure and to capture. [PERSON] and [PERSON]'s Wedding Room in Villa Farnesina was painted by someone who knew very well [PERSON]'s technique and was aware of the materials he used. Energy dispersive X-ray fluorescence (EDXRF) has been used by [PERSON] et al. [22] to analyze paper engravings. The technique was chosen because of the possibility of in situ investigation and because of its non-destructive nature, an essential aspect for artifacts as fragile as engraved papers. In [38], micro X-ray fluorescence (\(\upmu\)-XRF) has been used to distinguish three groups of the 111 copper-based alloy coins analyzed. The authors define \(\upmu\)-XRF as an analysis technique that \"allows rapid, non-destructive, and sensitive in situ elemental analysis of major, minor, and trace elements in archaeological and historical metals\" [38] (p. 5). In another work by [PERSON] [68], this time on copper alloy pins, \(\upmu\)-XRF was used together with stereo microscopy to gain insight into the metalworking practices employed. In [127], XRF was used to evaluate and compare the restoration interventions conducted in 1919 with those of the second half of the 20 th century. XRF techniques are numerous, and their coupling with other techniques is a common routine nowadays. In [21], a comparison is made between total reflection X-ray fluorescence (TRXRF) and EDXRF spectrometers. TRXRF turned out to be the best for very sensitive trace analysis, while EDXRF is particularly indicated for paper, as also stated in [22]. EDXRF is also used in [26] to study overglaze and underglaze cobalt decorations on ceramics and in [25] to study Chinese pigments and the preparation layers of a polychrome sculpture and mural paintings in Spain.