# Analytical Evaluation of Laser Cleaning Effectiveness in the Context of Contemporary Muralism
## 1 Introduction  
The preservation of cultural heritage has undergone a significant transformation in recent decades, driven by innovative technologies that have overcome the limitations of traditional restoration methods. Among these advancements, the widespread adoption of laser cleaning technology has emerged as a transformative force, successfully applied to a remarkable array of historical treasures. Over the past two decades, laser cleaning has been widely employed to preserve diverse materials including stones, paintings, and archaeological finds such as metals, textiles, wood, glass, bones, and more [1; 2; 3; 4; 5; 6; 7]. Unlike conventional cleaning methods that often involve the use of solvents, which might be harmful to the environment and operators, laser ablation technology offers a sustainable alternative that overcomes the limitations of chemical processes. Given the immense challenges posed by global climate change, the development and promotion of this technology become crucial options for practical conservation work. Laser ablation is a process that involves the precise removal of material from a surface by exposing it to an intense laser beam. The selectivity of laser ablation arises from the specific interaction between the laser irradiation and the material properties. Different materials absorb and respond to laser beams in unique ways, a phenomenon known as the ablation threshold [8]. Wavelength and pulse duration are the main distinguishing features of laser systems; in the practical working process, average power and frequency can beadjusted, allowing for the fine-tuning of the interaction. Recent studies have demonstrated that the shape of the laser beam and the overlapping rate are crucial for achieving homogeneous ablation and avoiding the edge effect of laser pulses when scanning a surface [9]. The optimization of these parameters in addition to the choice of the instrument are the key points of laser application and have been extensively studied based on specific materials. For instance, the UV region laser irradiation is highly absorbed by most aged varnishes [10] and often used to remove or to thin down aged coatings, while IR region laser irradiation is commonly used to eliminate black crust caused by urban pollution, cleaning spray painting vandalism on stone surfaces [11; 12; 13], and in more recent times, organic materials [14; 15]. In the field of cultural heritage conservation, one of the most diffused issues is the removal of unwanted graffiti from historical monuments and architectural structures. Graffiti vandalism on cultural monuments is causing significant problems both on surfaces' esthetical appearance and on historical material preservation. Recent studies have demonstrated the efficiency, safety, and feasibility of laser cleaning instruments, either used independently or in combination with chemical methods, even for on-site work [12; 16; 17; 18]. Unwanted graffiti also threatens another integral part of our cultural heritage--contemporary muralism. Despite the initial controversy, street art and murals are often considered decorative and didactic elements that reflect and express topics of social concern, deeply integrated into the social life of citizens and local communities. In addition to spontaneous creations, many large-scale urban murals are being executed under the commission of local institutional projects. They are usually created within frameworks of festivals and the regeneration of degraded urban areas, satisfying the needs of communities and attracting tourism interests [19]. Today, there are numerous urban murals that are beginning to receive attention from the heritage safeguarding bodies and local authorities who are in charge of the management of urban decorum, creativity, and culture [20; 21]. However, the degradation phenomenon of outdoor paints is particularly severe since they are constantly exposed directly to various degradation factors, including human vandalism, sunlight, rain, and urban pollution.