# Analytical Evaluation of Laser Cleaning Effectiveness in the Context of Contemporary Muralism
## 1 Introduction
The conservation of contemporary murals remains a relatively new and evolving topic. While some progress has been made in recent years, there is still not a standard protocol or shared guidelines for the general approach to the conservation and maintenance of these artworks. A significant challenge lies in cleaning unwanted graffiti without compromising the integrity of the original paint layer. Equally significant is the need to define widely useful criteria for proper methodologies and scientific evaluation. As mentioned previously, the efficacy of laser irradiation relies on the different ablation thresholds of the materials. However, graffiti itself is often created by materials similar to those used to make the mural, reducing the laser ablation threshold and creating a complex interplay of materials. Murals and graffiti are typically made with commercially available spray cans, rollers, or brushes, and they often employ synthetic paints primarily composed of alkyld, acrylic, styrene resins, or various combinations thereof [21; 22; 23; 24; 25]. This complexity adds layers of difficulty to optimizing laser operating parameters and establishing the criteria for evaluating effectiveness. In this study, we explore and present a pilot protocol specially designed to evaluate the effectiveness and safety of laser cleaning on contemporary murals. The protocol provides for the empirical evaluation of some specific criteria and their validation through a set of instrumental chemical analyses, intrinsically non-invasive or involving passive sampling. We employed two types of lasers, Q-Switch (QS) and Long Q-Switch (LQS), to target both fresh and artificially aged mock-ups, reflecting the diverse conditions that urban art encounters over time. Three types of paints were carefully selected from commercially available products: acrylic spray, alkyld spray, and acrylic-styrene water-based paint applied by brush. Additionally, to facilitate the clear observation of cleaning results, we selected three contrasting paint colors, namely yellow, blue, and fluorescent blue. In particular, we created 13 different combinations of paints to simulate hypothetical vandalism. We established five criteria that can be applied in real time to evaluate the effectiveness and safety of this cleaning methodology by constructing radar diagrams, representing empirical evaluations of significant parameters for cleaning monitoring. In a subsequent phase, these parameters were analytically assessed using specific techniques, including Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), optical microscopy, colorimetric measurements, and Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS). These instrumental responses enabled us to validate or reject the initial empirical evaluations. In particular, for the latter analysis, we adopted an inert and thermally stable material, a quartz tissue, to sample the laser-ablated material during the cleaning process. This sampling method provides the possibility for the rigorous analysis of the laser-ablated material by Py-GC/MS, which is crucial for evaluating possible thermal damage to the original paint layer without having to take another sample from the clean surface. We believe this method can be adopted in a broader range of scientific activities involving laser ablation technology. This study contributes to advancing the conservation of contemporary murals and street art by offering a simple, scientifically backed protocol for laser cleaning evaluation. By bridging the gap between empirical assessment and analytical techniques, we aim to provide conservators and cultural heritage experts with a reliable tool to ensure the preservation of these vibrant expressions of our modern culture. Furthermore, as laser technology continues to evolve, its applications in cultural heritage conservation are expected to expand, offering new avenues for safeguarding the art that enriches our urban landscapes.