# Analytical Evaluation of Laser Cleaning Effectiveness in the Context of Contemporary Muralism
## 2 Materials and Methods
### Materials
#### 2.1.2. Cleaning Instrument  
We adopted two types of laser instruments, both produced by the El.En. Group (Firenze, Italy). The instrument features are presented in Table 3.  
All the cleaning tests were conducted using some fixed parameters for a better comparison; these are wavelengths of 1064 nm and a frequency of 5 Hz. Prior research [26] has demonstrated that an Nd:YAG laser system operating at 1064 nm can effectively thin overpainting layers composed of pigments and binders. The choice of a frequency of 5 Hz was based on prior experience [21].  
Before starting the systematic cleaning process, the operational parameters were identified through a series of tests on two samples, Acry-AlkB and AlkB-AcrY. The goal was to determine the ideal combination of energy and spot size for effectively removing this specific top layer. These two factors determine the energy delivered per unit area, known as laser fluence or laser density, which can be obtained by dividing the energy by the beam area. The laser fluence was gradually increased during the test while observing the cleaning effect visually. For the Q-Switch laser system, the laser articulated arm was coupled with a homogenizer hand peace, and the ideal fluence was obtained operating at 266 mJ and a spot size of 10 mm. Instead, the Long Q-Switch laser yielded good results operating at 130 mJ with a spot size of 4 mm. Subsequent tests were based on these parameters.  
Additionally, as expected, we noticed that blue-colored surfaces generally absorb more laser energy than yellow ones. Consequently, under the same laser fluence, the blue surface is consistently better cleaned than the yellow one. To better preserve the bottom layer, cleaning of the yellow surfaces was conducted by repeating two or more cleaning cycles rather than increasing the laser fluence.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Model-Up Name**} & \multicolumn{3}{c}{**Bottom Layer**} & \multicolumn{3}{c}{**Top Layer**} \\ \cline{2-7}  & **Product** & **Color** & **Main Binder** & **Product** & **Color** & **Binder** \\ \hline AcrY–AlkB & Flame Orange & Yellow & Acrylic & Montana 94 & Blue & Alkyl \\ AcrY–AcrB & Flame Orange & Yellow & Acrylic & Flame Orange & Blue & Acrylic \\ AcrY–AcrFB & Flame Orange & Yellow & Acrylic & Urban Fine-Art Neon & Blue Fluorescent & Acrylic \\ AcrB–AlkY & Flame Orange & Blue & Acrylic & Montana 94 & Yellow & Alkyl \\ AcrB–AcrY & Flame Orange & Blue & Acrylic & Flame Orange & Yellow & Acrylic \\ Sky–AlkB & Alpha Acrilmat & Yellow & Styrene Acrylic & Montana 94 & Blue & Alkyl \\ Sky–AcrB & Alpha Acrilmat & Yellow & Styrene Acrylic & Flame Orange & Blue & Acrylic \\ Sky–AcrFB & Alpha Acrilmat & Yellow & Styrene Acrylic & Urban Fine-Art Neon & Blue Fluorescent & Acrylic \\ \hline \hline \end{tabular}
\end{table}
Table 2: _Cont._  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Laser Type**} & **Q-Switch** & **Long Q-Switch** \\ \hline Model & Thunder Art & EOS 1000 \\ \hline System & Nd:YAG & Nd:YAG \\ \hline \multirow{4}{*}{Wavelength} & 1064 nm & 1064 nm \\  & 532 nm & \\  & 355 nm & \\ \hline Pulse duration & 8 ns & 100 ns–200 μs \\ \hline \multirow{2}{*}{Frequency} & \multirow{2}{*}{1–20 Hz} & 1–10 Hz \\  & & 15 Hz \\  & & 20 Hz \\ \hline \multirow{2}{*}{Energy} & \multirow{2}{*}{0–900 mJ} & 130 mJ (1 pulse) \\  & & 250 mJ (2 pulses) \\  & & 380 mJ (3 pulses) \\ \hline Spot size & 4–10 mm & 1–6 mm \\ \hline Beam delivering & 7-mirror articulated arm & Optic fiber \\ \hline \hline \end{tabular}
\end{table}
Table 3: Laser instrument features.