# Analytical Evaluation of Laser Cleaning Effectiveness in the Context of Contemporary Muralism
## 3 Results
### Analytical Results
#### 3.3.3 Residues and Effectiveness: ATR-FTIR and Py-GC/MS  
We employed micro-ATR-FTIR and Py-GC/MS analyses to evaluate residues and cleaning effectiveness, utilizing non-invasive methods. Micro-ATR-FTIR analyses were carried out directly on the uncleaned and cleaned areas without sampling or sample preparation, while Py-GC/MS analysis involved sampling with the quartz tissue (Figure 4). Micro-ATR-FTIR yielded rapid and indicative results, generally aligning with our empirical evaluation. Figure 5a shows the spectrum of sample AcrB-AcY cleaned with the QS laser under unaged conditions, along with two reference spectra of AcrB and AcrY. The spectrum after the cleaning process closely resembled that of AcrB, indicating the presence of the blue pigment at 1508, 1120, 1090, and 900 cm\({}^{-1}\), with no significant peaks suggesting the presence of yellow pigment. Py-GC/MS analysis also indicated a satisfactory cleaning effect. Figure 5b shows the chromatogram of the same sample, revealing three principal components: peaks 1 and 2 correspond to MMA (methyl methacrylate) and nBMA (butyl methacrylate), respectively, associated with the acrylic binder, while peak 3 is 1,2-benzenedicarbonitrile, a marker of phthalocyanines used as blue pigments [32; 33]. This result indicates that no residue remains on the cleaned surface, in line with our empirical evaluation, where this cleaning case received the highest score (nine) for the \"residue\" parameter among all samples. Figure 4: (a) Quartz tissue combined with a micro aspirator during the cleaning process; (b) photo of quartz tissue after the cleaning process; (c) quartz tissue under microscope. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Condition** & **Unaged** & **Aged** \\ \hline AcrY-AcrB & & & \\ Laser type & QS & LQS & QS & LQS \\ Score & 6 & 5 & 6 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Raking light photography of sample AcrY–AcrB with roughness evaluation score. However, in some critical cases, micro-ATR-FTIR may lack sensitivity and produce false-positive results, especially when the first paint layer and the unwanted one share the same binder and poorly infrared-absorbing pigments. Figure 6a shows the effect of the two-cycle cleaning of the QS laser on the aged sample AlkY-AlkB by micro-ATR-FTIR analysis, where the spectrum matches well with the aged AlkY reference. Peaks at 1674, 1516, 1463, 1337, 1224, 1202, and 1178 cm\({}^{-1}\) are related to the presence of pigment PY74, with no significant peaks contributing to the blue pigment. However, with significantly higher sensitivity, the pyrogram in Figure 6b of the same sample reveals peaks related to the blue pigment in the aged AlkY-AlkB. Peak 1 (benzoic acid), peak 4 (phthalic anhydride), peak 8 (hexadecenoic acid), and peak 9 (octadecanoic acid) are related to the alkyl binder; peak 2 (2-methoxy-benzenamine), peak 3 (1-isocyanato-2-methoxy-benzene), and peak 7 (2-methoxy-4-nitro-benzenamine) are markers of PY74 [34]. Furthermore, we observed the presence of peak 5 and peak 6, tentatively assigned to 3-benzoylpropionic acid methyl ester (m/z = 105, 77, 68) and 2-ethylpropyl benzoate (m/z = 105, 77, 123), both of which were also identified in the pyrogram of spray AlkB (Figure 6c) used as a reference. Although their chemical compositions remain uncertain, their presence could be linked to the production process of synthetic blue pigments. This result highlights the capability of Py-GC/MS to provide a more detailed and accurate analysis of residues. Figure 5: (**a**) Micro–ATR–FTIR spectrum of unaged AcrB, AcrY, and AcrB–AcrY cleaned with QS laser (asterisks indicate PB15 signals); (**b**) pyrogram of unaged AcrB–AcrY cleaned with QS laser. An in-depth study of the selectivity of the laser cleaning method revealed intriguing details. As illustrated in Figure 7, the spectra of the aged sample AlkY-AlkB, after undergoing the cleaning treatment with the removal of the blue layer, match better to the unaged reference of AlkY rather than the aged reference.