Street and Urban Muralism in Public Art: Conservation Between Evolution and Research in the Methods of the Istituto Centrale per Il Restauro

Abstract

This paper presents an overview of a research line developed at the Istituto Centrale per il Restauro within the CHANGES (Cultural Heritage Active Innovation for Next-Gen Sustainable Society) project, funded under the Italian National Recovery and Resilience Plan. The research was developed in different phases: a first one dedicated to the study of the deep background and the state of the art in the ICR background: history, methodologies and research in the field; a second phase was dedicated to the selection of a specific urban art mural, as a key study with conservation problems connected to some of the principal preservation treatments related to the outdoor context; the mural was also identified as a beloved icon in the public space with a profound socio-cultural meaning for the community. Nido di Vespe, created in 2014 by the artist Lucamaleonte is part of a broader artistic project called M.U.Ro-Museum of Urban Art of Rome, an open-air public art museum located in the Quadraro district in Rome, designed by the artist Diavù. A third phase focused on the research in ICR laboratories, specifically addressing: cleaning, reintegration, and protection strategies adapted to dynamic outdoor environments. A multi-step cleaning system based on polyvinyl alcohol-borax semi-interpenetrated hydrogels loaded with nanostructured fluids was developed to selectively remove spray-paint vandalism while preserving the chemically similar original pictorial layers. The reintegration phase investigated acrylic and urea-aldehyde resins as binders to produce compatible, reversible, and UV-traceable retouching and infilling materials. For surface protection, multilayer coating systems incorporating nanoparticles with antimicrobial, photocatalytic, and UV-stabilizing properties were formulated to enhance durability and chromatic stability. Laboratory tests on mock-ups simulating typical street and urban art materials and morphologies showed satisfactory results, while diagnostic investigations on Nido di Vespe provided the reference data to calibrate the experiments with real mural conditions. Cleaning tests demonstrated promising removal efficiency, influenced by the chemical composition, thickness of the overpainted layers, and surface roughness. The reintegration system met the expected performance requirements, as the tested binders provided good results and allowed the development of compatible, reversible, and distinguishable solutions. Protective coatings showed good results in terms of chromatic stability and surface integrity; however, the long-term behavior of both reintegration, cleaning, and protection systems requires further evaluation. The results achieved so far support the development of sustainable and flexible conservation strategies for the conservation of contemporary street and urban murals and will guide the future application of the selected materials and methodologies in pilot conservation interventions on the mural chosen as a meaningful case study within the broader research.

1. Introduction

The conservation of Street Art, Urban Art, and contemporary muralism within public spaces has become a subject of increasing academic attention and professional debate in the international context. While several foundational texts have addressed this topic, it is important to acknowledge that, at the international level, there remains a lack of consensus regarding whether such forms of mural expression should be preserved. This ongoing debate reflects broader tensions between the ephemeral nature of Street Art and the institutional frameworks that seek to conserve it.

Over recent decades, street and urban muralism have undergone significant iconographic and thematic evolution, accompanied by a process of institutionalization. These developments have prompted a re-evaluation of the criteria by which such works are assessed for conservation, particularly in relation to their material characteristics and their embeddedness within the urban fabric [1,2,3,4]. From a technical and technological perspective, the intrinsic properties of wall-based artworks demand specific conservation approaches. However, it is equally essential to adopt broader evaluative frameworks that recognize the cultural, social, and participatory values associated with contemporary public art. These frameworks must consider not only the physical preservation of the artwork but also its role in community engagement and urban identity, as underlined in the Faro and Burra charter [5,6]. In this context, ICCROM’s holistic approach to conservation, although not yet explicitly directed toward Street Art and Urban Art, serves as a significant point of reference. Notably, in 2022, this framework informed the development of a dedicated fellowship focused on the conservation of public art. The approach integrates key dimensions of contemporary conservation practice, including material science, preventive strategies, aesthetic assessment, and active community engagement.

Its integration of material science, preventive conservation strategies, aesthetic analysis, and participatory methodologies aligns with emerging international practices in the preservation of street and urban murals. This convergence suggests the potential for developing inclusive and adaptive conservation models of vulnerability that respect both the artistic intent and the socio-cultural significance of street and urban art.

The CHANGES research project, focused on the conservation of New Contemporary Muralism and Street Art, is grounded in the international academic debate in the field and the historical framework of the Istituto Centrale per il Restauro (ICR), building upon a series of prior key studies concerning the preservation of contemporary public art [7]. Drawing from ICR’s extensive experience and selected international case studies, the project has been structured to establish a sustainable methodology for the conservation of such artworks.

Each section of the research has been conceived and coordinated by ICR professionals-both conservator-restorers and chemists-and carried out by research fellows and junior grant holders. The study is organized into distinct phases, reflected in the structure of this paper, which is divided into main sections, paragraphs, and subparagraphs.

Section 2 explores the concepts of public space and public sphere, examining their respective meanings and the evolving relationship between these domains and the artworks over time. Street art and urban murals continuously redefine their form and significance through interactions with these contexts, including interventions carried out “in the name of conservation.” This dynamic is illustrated through critical international case studies, ranging from forced protection measures to mural erasure or detachment.

This paper aims to investigate how environmental and social factors, as well as the interaction with the urban context, intended as both public space and public sphere, can reshape, alter, or even amplify the original artistic intent [8,9]. Section 3 outlines ICR’s historical background and previous initiatives in the conservation of public art and contemporary muralism, highlighting a trajectory of research, preservation, and training. Subsequent section focuses on the mural Nido di Vespe by Lucamaleonte, analysing its environmental context and the rationale behind its selection for the CHANGES project [10]. The mural was chosen for its specific conservation challenges and its strong figurative and symbolic resonance within the community. Preserving such works entails addressing not only material degradation but also the socio-cultural dynamics of the urban environment that inform their meaning, an aspect exemplified by Nido di Vespe.

Section 5 presents the laboratory research in detail. This part is subdivided into three core areas corresponding to key conservation actions-cleaning, retouching, and protection-which are particularly complex in outdoor settings. Following an introductory overview, the section provides a comprehensive account of the materials and methodologies employed in developing conservation strategies. Each subparagraph describes the preparation of laboratory mock-ups and the selection of materials used in each phase of the research. Subsequently, the discussion, also structured in subparagraphs, clarifies the outcomes that have been defined and achieved, as well as those still under evaluation, with the objective of enabling future in situ application of the tested materials. The paper concludes with a dedicated section summarizing the laboratory findings and a final paragraph outlining ICR’s first recommendations for the conservation of New Contemporary Muralism and Street Art.

2. Public Art Between Public Space and Public Sphere

The definitions of the terms “public artwork” and “public space” have influenced each other and are reflected in the relationship between the artwork, its viewers, and the “public sphere” [9,11,12]. This relationship can involve the viewer through observation or be more physically participatory, both in the act of the creation or in the conservation process of the artwork itself, sometimes also involving inappropriate care methods and actions from communities like superimposing a physical barrier as a “common protection tool” (Figure 1), accelerating instead the degradation process of the artwork: i.e., on Banksy Street art mural in Naples [13,14,15].

Public space is dynamic and open to multiple interpretations and definitions [8,16,17,18]; the artwork itself fosters interaction, participation, and shifts in social relations. Artworks in public space, in fact, can assume a temporary, ephemeral, or transformative nature, even in murals: qualities that can arise from intentional design, environmental or social factors, or interaction with the context. The artwork can change over time, lose materiality, and acquire new symbolic or relational functions, as seen in Blu’s mural in Rome (Figure 2), Porto Fluviale district, and its recent erasure [19,20,21,22].

By the time people had achieved the right to housing and a safe place to live, according to the artist, the mural itself loses its reason for existence. Sometimes the message of a mural in a public space can be so profound in denouncing current and undeniable geopolitical situations that it creates a rift in the public conscience and a growing awakening, such that even its sudden erasure by institutions, a real act of censorship, can amplify its meaning. That is the recent case of Banksy’s mural at the Royal Court of Justice, which stood as a vivid denunciation, a mirror held up to society’s most pressing issues in the very heart of institutional authority (Figure 3).

The mural’s sudden removal by the authorities, a clear act of censorship, did not erase its impact; instead, it magnified it. The attempt to silence such a bold statement only reinforced its urgency, provoking public debate and sparking a wave of collective awareness in the public sphere.

The act of censorship became a catalyst in amplifying both the mural’s message and posing questions, such as: “Who decides what can be seen?” and “What truths are too uncomfortable for public walls?” Nevertheless, the shape of the left shadow of Banksy’s artistic message, still there, becomes a silent monument to the power of artistic dissent and a reminder that, sometimes, erasing a message gives it a louder voice than ever before and that art’s erasure can “amplify its voice” [23,24].

By their very nature, even when erased or naturally disappear, artworks in public space are always connected to the context, intended both as urban or natural environment and the social-cultural framework in which they originate or are inserted, taking into consideration the many and varied possible forms of this dialogue and connection to the latter. Sometimes, outside of this relationship, the artwork takes on completely different characteristics and meanings or loses them completely, as in the recent detachment of Banksy’s Migrant Child (Figure 4) [25,26].

It is in the “public sphere”, as delineated by political theory and social philosophy [8,16], that the most profound dynamics of collective life can be activated: it is here that opinions are articulated, meanings are negotiated, dissent is exercised, and the boundaries of the visible and the sayable are redefined. In Castelli’s thought [17], linked to the concept of the “public sphere” as a dynamic and constantly evolving entity, rather than stable or universal, public space promotes the interaction between the plural identities of a society, where social, artistic, cultural and political practices shape, transform and give rise to debates, conflicts and processes of solidarity, also through the dialogue and reciprocity that art can trigger [27]. And even the discipline of conservation, in theory and practice, is contaminated by the dynamism and complexity of public space.

3. ICR Background Experiences in the International Context: History, Evolution, and Research of Public Art in ICR Methods and Practices in the Field Since 1990 to Nowadays

The Istituto Centrale per il Restauro (ICR) experience in the field of contemporary art in public space has well-established roots dating back to the 1990s and has always involved research and study of more professionalism and a multidisciplinary approach to the restoration intervention, in collaboration with other research bodies and universities-such as the National Research Council—Institute of Heritage Science (CNR-ISPC), the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Sapienza University of Rome, Politecnico in Milan, University of Pisa, Ca’ Foscari University of Venice 1. Graffiti writing, graphic vandalism, anti-graffiti, and urban Graffitism: all these words, apparently very similar, have different semantic and practical significance. Since the 1990s, the world of ICR conservation and restoration has known graphic vandalism and urban graffiti in its most negative form, i.e., the graphic vandalism of “defaced” historical monuments and of the facades of historic buildings damaged by spray paints, which are still very difficult to remove. ICR’s experience in addressing graphic vandalism and graffiti on monuments extends to the study of anti-graffiti treatments and protective products, always with an aim toward the conservation of cultural heritage. This technical and scientific expertise, developed over the past decade, encompasses a range of substrates—from natural stone to new contemporary muralism and street art—with recent and ongoing projects in the field. The porous structure, which characterizes the complex natural and artificial stone materials—some more so than others—tends to absorb certain materials, such as dyes, pigments, their binding media, etc., used in acts of vandalism like spray paints or markers, albeit in different ways (i.e., spray paints are more likely to remain on the surface). This damage, caused by anthropic and social factors, including the lack of recognition of artworks, is not only aesthetic but also alters the chemical and physical properties of the mural itself, as foreign materials are absorbed by its original substance [9,28,29,30,31,32,33,34,35,36]. Even in the realm of contemporary mural paintings, particularly those associated with street and urban muralism, the modification of the constituent materials will be closely linked to the type of chemical bond between the original pictorial films, almost always “plastic on wall paintings”, and the material involved in the vandalism act [37]. In accordance with the principle that every act of restoration constitutes a significant intervention on the artwork, the approach of minimal intervention is consistently prioritised. The removal of foreign materials from the original substrate, especially during cleaning processes, represents one of the most delicate phases of conservation. These operations, whether mechanical, chemical, or even physical, such as laser application and hydro-cleaning, or the application of solvents supported in different gelling agents 2. This characteristic often complicates their removal and demands highly specialised and cautious methods to preserve the integrity of the original artwork. Taking into account these particular conservative issues and challenges, during the years 1996–97 and for the following three years ICR, ICCROM and the Municipality of Rome implemented the project “Stop to the graphic vandalism-Propose a prevention campaign” which included actions to raise consciousness in the community in preventive conservation and restoration treatments through an initial information phase aimed at teachers (middle and high school) on the contents of the initiative supported by teaching materials. Subsequently, the teachers had to involve the students in aspects related to the problems of graffiti and their removal from historical artefacts. The program also included conferences and visits to restoration sites. ICR publication of the volume “La conservazione del patrimonio monumentale. Il vandalismo grafico” (The conservation of monumental heritage-Graphic vandalism) was the achievement of the practical and academic results in the field. In the wake of these experiences, study and research projects were subsequently carried out first by Vigliano and then by Sidoti (both official chemists and, in time, ICR Head of Chemistry Laboratory of “Testing on materials”) for the study of the most stable classes of anti-graffiti (2000) and, later, such research also converged in in-depth MA ICR thesis for the removal of graffiti [38,39]. Such research initiatives, including the ongoing investigation into protective materials, are continuously advanced by the ICR through the dedicated efforts of its technical staff and researchers, especially conservators-restorers and chemists. These projects are closely linked to the technical processes designed to safeguard artworks, such as murals, against environmental degradation and anthropic risks. By developing and refining technical tools for preventive conservation, ICR establishes foundational research pathways that remain at the forefront of its activities today. These endeavours are further strengthened through collaborations with external research bodies and institutes and are pursued within the framework of national and international projects—such as Horizon Europe, National Operational Programme (PON), and Regional Operational Programme (POR)—among them, as the Cultural Heritage Active Innovation for Next-Gen Sustainable Society (CHANGES) project, funded under the Italian National Recovery and Resilience Plan (PNRR). But Graffiti is not just this. The multiple facets that characterize this movement make it difficult, and even limiting, to place it in an historical-artistic evolution (or revolution) before in a specific category because graffiti have multiple kind of types and categories (i.e.,: heaven, throw ups, etc.) [10,36,38,39] which can take on different values, depending on the social context and “the public sphere” in which they manifests themselves and on the writers or other professional figures who, at different title, works on them. However, nowadays, the term “graffiti writing” has certainly undergone a sociological and cultural evolution over the years. It was initially strictly linked to the youth revolt and discomfort, even though, over time, it has lost its strength and has also become institutionalized, framing itself in a precise form of street art that has its roots in lettering and which, in the last thirty years, has also found space in galleries and museums [11,40,41]. However, both graffiti writing and street art muralism remain in the collective imagination as an expression of the revolt and existential affirmation in the public space of youthful discomfort, of pure vandalism, of the form of art, of pure fashion, or design. The graffiti revolution, first in America and then in other countries, was the first kick of the so-called street art movement. The latter was surely influenced in its evolution by translating its artistic language, especially on murals, in different perspectives and branches depending on the country itself. Consequently, the introduction of graffiti writing at the national level in Italy immediately became part of the alternative and underground art and fashion market, thanks to the success it had previously achieved in America. Thus, as early as 1979, Claudio Bruni, the enterprising owner of the “La Medusa” Gallery in Rome, was captivated by the explosion of graffiti writing in New York and decided to invite two famous American writers, Lee and Fab 5 Freddy, to present their artworks in the Roman gallery [6,11,29]. In 1983, entrepreneur and patron Elio Fiorucci commissioned Keith Haring—the undisputed king of graffiti and New York street art at the time—to decorate his historic shop in the San Babila district of Milan. The shop, already frequented by young lovers of fashion and art since the 1970s, became a work of art and a symbol of rebellion and creative freedom for the latest expressive artistic movements (even better if illegal!). Fiorucci, well aware of these trends thanks to his trips to America, became one of their most prominent advocates through his collections. The history of graffiti and street art in Italy would materialise further in 1984, thanks to Francesca Alinovi [10], with the exhibition, “Art of the Frontier: New York Graffiti”, realised first at the Municipal Gallery of Modern Art in Bologna and then at the Exhibition Palace in Rome. In summary, the rampant success of importing American graffiti writing from museums and galleries, street art finally arrived on the Italian streets. From there, it would be filtered and reinterpreted through the national context, in all its forms, thanks to an already fertile ground where art on the walls had always been part of the historical and national Cultural Heritage. Finally, the movement, also passing through 1968 and the youth protest, has arrived nowadays at another (and legal) branch called the New Muralism movement. In any case, even in Italy, the phenomenon remains connected and rooted to the jamming culture, hip hop, and crews: those from Bologna, Rome, Milan, and Turin in the first place. Certainly, the spontaneity of the phenomenon is lacking when it comes to its institutionalization, although, fortunately, the artists have given life, over time, to a parallel irreverent production concurrently with their own street artworks playing with the institutionalization in favour of the spontaneity (and vice versa). In conclusion, the arrival of graffiti writing in Italy in the 1980s is distinguished from the American phenomenon by having appeared on the streets from museums and galleries, as already happened in European capitals. These are just some of the definitions adopted to frame this cultural phenomenon, which, precisely because it is still so diverse today, escapes rigid “labelling.” Nevertheless, it is possible to isolate and trace a true historical and artistic trajectory. ICR experience in the field continues with some important mural and urban installations studied in 2004 research related to public art: the monument to Giuseppe di Vittorio in Cerignola and the case study of Ettore de Conciliis in Fiano Romano (Figure 5) [42,43,44,45,46,47].

The research focus was based on the study of executive techniques, constituent and multi-materials in the experience of the murals and installations of the public and popular art centre of Fiano Romano 1964–1975: first approaches to the problems of degradation of synthetic pictorial films in an external environment on differentiated supports; study of protective products, restoration of pilot areas. In 2009, another important key study, taking into consideration especially natural and artificial stone materials and street and urban murals, was based on the research developed on a Hitnes mural realized, with a significant public scope. In fact, in 2005, the Department of Entomology of the “La Sapienza” University of Rome commissioned the Roman artist Hitnes to create its first artwork on an internal wall of a building inside the complex located between Piazzale Valerio Massimo and Viale delle Provincie. It was a surface of approximately 10 m² on which Hitnes created a composition of enormous insects, studied down to the smallest detail, which extended over the entire wall.

Subsequently, after two years, the artist was also commissioned to paint the entire exterior facade of the perimeter wall of the university complex—an urban mural of approximately 140 m² (Figure 6).

On the outer wall, a long “procession” of gigantic insects is depicted: a sort of open-air museum that illustrates the internal research of the university’s specific department. This work, just two years after its creation, showed very poor conservation conditions, becoming an interesting ICR study opportunity and one of the first cases in which restoration was even taken into consideration for murals of this type (street/urban). In this case, the research focused on studying the constituent materials and the initial systematization of the composition of spray cans, specifically Belton Molotow Premium [48,49]; an initial classification of the specific degradation morphologies of synthetic paint films in relation to the urban context (both environmental and anthropic degradation); and the restoration of a sample area, with particular confirmation of the adhesion of the paint film. The research also included biocidal treatment and the study of various materials for pictorial reintegration in an outdoor environment. Key ICR tools included interviews with the artist to understand the differences in the state of conservation between the 2005 and 2007 murals from a technical perspective and scientific investigations as tools for knowledge (in collaboration with the Diagnostic and Restoration Laboratories of the Vatican Museums and the University of Tuscia). Unfortunately, nowadays, the mural is lost and faded away.

In 2016, ICR was a consultant under the request of the Special Superintendency and the Municipality of Rome for the mural decoration on natural stone designed by the artist William Kentridge, Triumphs and Laments (Figure 7). The site-specific contemporary art project by William Kentridge, proposed by the Tevereterno association was carried out in the spring of 2016 and involved the creation of a frieze composed of 80 figures up to 12 m high which were depicted the triumphs and defeats of the city of Rome; the frieze carried out on the travertine embankment of the river in the area between Ponte Sisto and Ponte Mazzini. The frieze was created through the selective removal of the dark biological and polluted film due to the combined carbon deposits of the atmospheric particles present on the wall. The artist creates an enormous 1:1 scale stencil, realizing the figures, as in a sort of selective skiagraphy, which emerged thanks to a selective cleaning of this film, where the characters of the artwork appeared in a nuanced chiaroscuro negative on the stone wall.

The cleaning carried out in this way should have had a limited duration in time, allowing the artwork, deliberately ephemeral, to disappear when the biological re-colonization of the facade was completed. The subject of temporality, in which not all artworks are designed to be preserved and, indeed, they are designed to be ephemeral and not last over time, is developed by Kentridge, where sometimes murals and artworks are even designed by the artist with a view to their destruction. The ICR consultancy was divided into two main phases: during the creation of the frieze, with the removal of the biological and pollutants-based patina with a selective cleaning of the dark film, and during the removal of a superimposition of graffiti just after its realization. In that case, the graffiti were removed to promote the continuation of the ephemeral artistic message. Nowadays, the frieze has disappeared, completing the ephemeral artistic message desired and planned by the artist; another artist will perform the same technique realized for the Kentridge project with another one dedicated to ecology and migration proposed by Maria Thereza Alves in 2022, Witnesses, and still not realized.

From 2021 to 2024, the pilot conservation project in “Piscina Arte Aperta”, a widespread museum of contemporary art in the province of Turin, has taken place taking into consideration conservation and restoration actions on different kinds of artworks and materials including urban murals by Giorgio Griffa, Antonio Carena, and Paolo Giraudo: some famous artists which worked in the north of Italy. The objectives of the project have been to understand critical methodologies and practical approaches to systematise the main and different categories and classes of artefacts present in the collections and their state of preservation related to the environmental and social context, involving the community in conservation participation processes. Great attention has been devoted to cleaning, reintegration, and protection operations, as well as to the materials applied.

In 2021, another ICR MA thesis on urban art was the basis of the conservation project of Ama il tuo sogno by the Italian urban artist Jorit Agoch (Figure 8) [50,51,52]. The mural, built for “Matera European Capital of Culture” in 2019, portrays Yvan Sagnet, an activist for farm workers’ rights and against illegal hiring. It was painted in an outdoor context, precisely on the surface of the perimeter masonry wall of the “Carlo Levi” Art High School, which is made of blocks of a local bio-calcarenite, set against a filler ground. A few months after its creation, the artwork showed multiple decay processes on the surface. Nowadays, its state of preservation is deeply compromised, as decay extends to both painting layers and mortars.

In this case, the focus of the research was based on the problem of murals created on embankments and/or on so-called “architectural remnants”, intended as those architectural elements or portions which, although load-bearing or functional, have less relevance to the aesthetic qualities of the architectural complex (i.e.,: the pillars of the bridges, the perimeter retaining walls of the gardens, etc.). Tools and methods in the restoration intervention were focused to reconvert soluble salts species with barium and strontium hydroxide, in collaboration with CNR—ISPC of Lecce; the study of the constituent materials and in-depth systematization of the composition of Montana spray cans was carried on, as the classification of the specific degradation morphologies of synthetic pictorial films in relation to the urban context due to environmental and anthropic risks. ICR tools like the interview with the artist, technical sources for the materials, and a participatory comparison of the artist with the intervention choices and scientific investigations are another fundamental means of knowledge.

From 2021 to the present day, another project based on the realization of technical documentation and a specific glossary for this kind of mural was undertaken by Mural Paintings Laboratory, creating a specific technical conservation sheet and structured condition report with the aim of aiding documentation and conservation projects, as a preventive conservation tool. One of the murals taken into consideration was Self-management by Hyuro (Figure 9), in the Ostiense District, created for the exhibition on the artist curated by the Dorothy Circus Gallery in Rome, «Convivencia», in 2016, and now, unfortunately, erased.

Nowadays, ICR has been involved in an important PRIN (Progetti di Ricerca di Interesse Nazionale—Research Projects of National Interest) project named SuPerStar-Sustainable Preservation Strategies for Street Art. The project has seen the participation of five units: University of Pisa, Politecnico in Milan, University of Turin, University of Bologna, CNR-SCITEC (National Research Council—Institute of Chemical Sciences and Technologies) and includes among the partners, collaborations and stakeholders, in addition to the ICR, the Municipality of Milan, the Municipality of Turin, the Center for the Conservation and Restoration Centre La Venaria Reale (CCR), the Opificio delle Pietre Dure (OPD) and a vast network of conservators and stakeholders working in both the public and private fields. ICR and the Superstar consortium organised a theoretical and technical workshop in November 2023, presenting technological innovations in the conservation of urban murals. This initiative aimed to track the developments of technical applications and create an opportunity for the exchange of knowledge on materials and methods among experts and researchers from ICR, universities, and the research centres involved. The principal topics presented were focused on: the development and combination of non-invasive and micro-invasive analytical techniques suitable for investigating the most critical aspects of the chemical-physical degradation processes of modern pictorial materials used in street art and urban art, for identifying risk factors, and for being integrated into the monitoring the state of preservation of the artworks; laboratory research based on case studies and pictorial models; the experimentation of innovative cleaning and protection procedures. Thanks to these previous ICR experiences and background, this research line on preservation of street and urban art muralism has been designed, developed, and structured into the project CHANGES dedicated to this field.

4. Nido di Vespe by Lucamaleonte in the Museum of Urban Art of Rome: Artistic Message, Original Materials, and Degradation Process

In the CHANGES project, a preliminary phase was dedicated to the selection of a specific urban art mural, as a key study with conservation problems connected to some of the principal preservation treatments related to the outdoor context: cleaning, reintegration and protection process; the mural was also identified as a beloved icon in the public space with a profound socio-cultural meaning for the community.

The research has been addressed and focused, then, on an artist deeply rooted and recognized in the field of Public Art, such as Lucamaleonte, with experience in both street art and urban art. His murals convey a social significance closely connected to public space, and conservation would need to interact with issues of degradation as well as socio-cultural challenges. The selected mural, Nido di Vespe, is located in the Museum of Urban Art in Rome (M.U.Ro) 3, in the Quadraro district. The mural, commemorating Quadraro’s Resistance against Nazi-fascism, welcomes visitors with the inscription “You are now entering Free Quadraro”. Therefore, the artwork represents a symbol of socio-political expression in urban and street art muralism, deeply connected to the identity and historical memory of Rome’s Quadraro district.

The mural’s symbolic and historical significance is closely intertwined with the identity of the Quadraro district, where collective memory, resistance, and community engagement converge. In this context, the Museum of Urban Art of Rome, M.U.Ro stands out as a pioneering and innovative project that reestablishes the historic identity and values of the Quadraro district itself. Founded in 2010 from an idea by the artist David “Diavù” Vecchiato, M.U.Ro. is based on a vision that goes beyond the traditional concept of a museum, embracing the city itself as a widespread and open exhibition space. Over time, M.U.Ro. has expanded, involving nationally and internationally renowned artists and transforming various areas of the city into genuine open-air art itineraries. Urban art thus becomes a transversal tool of knowledge: it engages citizens, visitors, artists, and conservation professionals, transforming art into a collective instrument with a critical gaze, celebrating the plurality of community narratives through artistic practices. Lucamaleonte mural Nido di Vespe (Wasp Nest) reveals forgotten or overlooked chapters, highlighting the resistance, triumphs, and contradictions of local, Roman, and universal history. Murals like Nido di Vespe recall founding events and collective tragedies, establishing an immediate and vivid connection with the roots of the territory through the artistic experience. The artist depicts a giant wasp nest, with seven wasps rising above a graphic pattern reminiscent of honeycomb cells (Figure 10), evoking the negative nickname by which Nazis and fascists improperly referred to partisans (the Italian Resistance) while carrying out brutal raids in the neighborhood.

The artwork extends approximately 30 m along the perimeter wall of a sidewalk in a highly trafficked pedestrian and vehicular area. Moreover, the mural is constantly exposed to the sunlight radiation; therefore, this conservation context makes it particularly vulnerable to such an extent that, in just eleven years, the mural has undergone accelerated deterioration. These phenomena have significantly affected the chromatic balance and overall legibility of the composition, making it an exemplary case for testing conservation materials and methodologies designed in this research for the conservation of outdoor contemporary murals.

In this case, in-depth scientific analysis, supported by ICR’s specialized conservation expertise, allowed the identification of the materials used in the mural’s artistic process, supporting conservation knowledge. Indeed, it is a complex artwork made with different binders of vinyl-versatate nature (black, white, and lilac) and acrylic (yellow and violet).

Unfortunately, its placement in a dynamic outdoor urban environment has led to rapid deterioration, including the presence of lacunae and acts of graphic vandalism with blue spray paint overlapped on the original pictorial layers (Figure 11a,b). Analyses revealed that organic dyes in the paint formulation, combined with an acrylic-styrenic resin, penetrated deeply (approximately 150 µm) into the preparatory layer of ochre colour (Figure 11c), posing a significant challenge for cleaning interventions.

However, one of the most evident phenomena is fading, which has particularly affected the yellow paint of the wasps’ bodies, now almost completely disappeared (Figure 12a,b). This phenomenon can be attributed to the photodegradation of organic dyes present in the paint, triggered by prolonged exposure to solar radiation. Indeed, the east-southeast orientation of the mural results in direct irradiation from sunrise to sunset, significantly accelerating the photoinduced degradation processes. Optical microscopy analysis of the stratigraphic sections revealed that fading affected approximately the upper 20 µm of the paint layer (Figure 12c), whereas areas characterized by the presence of an additional overpaint layer, which functioned as a protective barrier against light exposure, preserved the original coloration (Figure 12d).

In view of the complete conservation project of the artwork and the application of the experimental materials identified at the conclusion of the laboratory phase, which will determine the most suitable systems for the cleaning of vandalism, the reintegration of losses, and the application of protective coatings, a pilot area will be carried out. At this stage, only preliminary first emergency measures were performed, including the stabilization of the most vulnerable areas, first aid treatments on some detached fragments (Figure 13), the removal of plants and root systems, and the consolidation of the perimetral areas of the lacunae at risk of detachment.

Direct critical observations carried out by conservators enabled the identification, according to targeted conservation criteria, of the most significant areas for pilot interventions. These treatments will be conducted using the materials and methodologies developed in this research for future on-site applications.

5. Materials and Methods: Study and Application of Innovative Conservation and Eco-Sustainable Procedures and Formulations

This part of the paper outlines the research framework carried out within the CHANGES project, developed at the Istituto Centrale per il Restauro (ICR); it is structured around three main sections based on cleaning, reintegration, and protection treatments and it is aimed at addressing the key conservation challenges affecting street and urban art, which are particularly vulnerable to environmental and anthropic degradation factors (Figure 14).

The three sections of the research were concurrently developed, each with specific objectives and testing protocols, to address the different conservation issues in an integrated model. The design of the experimental phase was guided by a specific case study, which was first selected for its material and conservation characteristics representative of the most recurrent issues in contemporary mural conservation. As described in the previous paragraph, the selected mural underwent a preliminary diagnostic campaign to identify its original materials and main deterioration phenomena.

The results of this phase provided the essential knowledge basis for the development of laboratory experimentation, where conservation materials and operational methods suitable for outdoor conditions were selected, optimized, and tested.

The laboratory phase represented the experimental core of the project, allowing the verification, under controlled conditions, of the behavior, effectiveness, and compatibility of the selected materials with the requirements of contemporary mural conservation. The materials and experimental protocols developed in the laboratory will be applied and validated in situ on the selected case study, to assess their performance under real environmental conditions and to establish guidelines for future conservation interventions on outdoor murals. In order to support the characterization phase, dedicated to the study of the artwork, its constituent materials, and its deterioration phenomena, and to guide the laboratory research, a diagnostic campaign and photographic documentation were conducted.

A detailed ortho-photogrammetric survey of the mural was carried out using Agisoft Metashape Professional software (version 1.7.6). High-resolution overlapping images were aligned and processed to obtain accurate documentation of the mural’s conservation state, used as a reference for mapping deterioration morphologies. The diagnostic campaign included a range of analytical investigations aimed at characterizing the materials and assessing their conservation state. The original executive technique was further investigated through direct observation of the mural, consultation of archival documentation, and interviews with the artist and the curator of M.U.Ro. This provided additional insights into the creative process and the materials used for the realization of the artwork, including commercial paints supplied directly by the manufacturing company (IVAS^® Group Ltd., Brentford, UK).

5.1. Laboratory Research and Testing: Development of Cleaning, Reintegration, and Protection Strategies

The laboratory phase aimed to translate the methodological framework of the CHANGES project into a systematic experimental protocol. The main objective was to develop and validate innovative methodological approaches and sustainable systems suitable for the conservation of outdoor contemporary murals, ensuring both chemical compatibility and long-term stability.

5.2. Preparation of Laboratory Mock-Ups

The preparation of the mock-ups was guided by the need to reproduce the typical material features, texture, and degradation conditions observed in outdoor murals. In particular, the study referred to the characteristics identified in Lucamaleonte’s mural Nido di Vespe, which served as a reference model for the design and calibration of laboratory tests. All specimens were realized in cementitious mortars (KC1; Fassa Bortolo) and coated with synthetic binders: acrylic, vinyl (supplied by IVAS^® Group Ltd.), and alkyd (Montana^®) (Figure 15a).

Specific treatments were applied according to each section of the research:

• Cleaning specimens: the accurate replication of the mural’s rough surface was a key requirement to test the cleaning system under realistic conditions. Each mock-up was painted in two colour variants (white and black) to highlight the cleaning system’s behaviour on both bright and dark substrates, especially for the presence of residual material. They were subsequently treated with blue acrylic-based (Cilvani^®) and pink alkyd-based (Montana^®) spray paints to replicate the act of vandalism (Figure 15b). A controlled application of spray paint was performed at 20 cm from the vertically oriented samples, employing two successive horizontal movements (forward and backward) to ensure a standardized application suitable for comparative testing.
• Reintegration specimens: two types of samples were prepared to support the subsequent testing phase. The first type was painted with a yellow acrylic paint, corresponding to that used by the artist for the yellow areas of the wasps’ bodies, and intentionally abraded to create lacunae of different shapes and sizes (Figure 15c), to be used for testing different reintegration materials and application systems. The second type was entirely prepared with the experimental synthetic mortar and paints formulated with two selected binders, then treated with different protective systems to evaluate their behaviour under artificial ageing.
• Protection specimens: mock-ups were painted in two color variants (white and black). One part was treated with the developed protective coatings, and one was left untreated as a control.

5.3. The Three Sections of the Research Line on Conservation of Street Art and New Contemporary Muralism in the CHANGES Project

(I) Cleaning systems, focused on the development of a combined and controlled cleaning protocol, exploring various traditional green approaches-chemical, mechanical and physical- while concentrating on a final refining action using polyvinyl alcohol-borax semi-interpenetrated hydrogels (PVA-Borax gels) [53,54,55,56,57,58,59,60,61,62,63,64] integrated with protein or polysaccharide materials [59,60,61,62,63,64] and loaded nanostructured fluids (NSFs) [60,63,65,66,67,68,69,70,71,72,73]. Their water-based, low-toxicity formulations enable controlled and retained action adaptable to the specific texture and characteristics of the artefact, combining sustainability with accuracy.

The research initially involved a bibliographic study and information gathering, followed by the drafting of a state of the art, and then the execution of practical and preliminary tests in the laboratory. In the initial stage, a previously tested PVA-Borax double-network gel integrated with agarose was selected as a reference system, due to its optimal performance in earlier studies [59,60,61,62,63]. Its excellent properties made it a suitable starting point for the development of new variants, with systematic adjustment of component ratios to optimize performance and enhance compatibility with the application context. The experimental objectives to achieve were the following:

• Targeted, selective, and controlled cleaning action
• Effectiveness on overlaid materials with solubility similar to the original
• Adaptability to rough textures and complex surfaces
• Long-term stability on vertical surfaces
• No residual material releases
• Self-healing behaviour.

Several materials were screened in preliminary tests, as Agarose (Antares^®), Konjac Gum, Locus Bean Gum, Sodium Alginate (Special Ingredients^® Ltd., Chesterfield, UK), Gelatine powder-300 Bloom (Antichità Belsito^®), to evaluate key properties, such as viscoelasticity, transparency, liquid phase retention, self-healing behaviour, stability on vertical surfaces, and residue release. These first trials, conducted through direct observation and practical handling, allowed for the identification of the most suitable formulations, while excluding those with instability or unsatisfactory performance. Less suitable options were discarded, while promising ones, such as agarose and konjac gum [74], were selected for further gel-like system optimization. The next phase focused on loading the PVA-borax double-network gel with solvents and nanostructured fluids to evaluate its liquid phase retention, structural stability, and define its operational limits.

Concurrently, solubility tests were performed to identify the most suitable solvents and NSFs to be incorporated into the gel system, based on their removal potential on the acrylic and alkyd-based spray paints used in the study. Solubility tests were carried out with two complementary strategies. On one side, small fragments of paint layers (0.015 g) were fully immersed in vials containing 3 mL of each selected solvent and NSFs, to evaluate solubility under conditions of prolonged exposure [70], visually monitoring the dissolution degree and liquid-phase coloration. On the other side, solvents and NSFs were applied via cotton swab onto alkyd and acrylic-based spray paint layers previously coated with acrylic, vinyl, and alkyd paints, to simulate a more localized and mechanical cleaning action of the overlapped layer. In the final phase, the chosen materials were applied to mock-ups to evaluate their effectiveness and to identify the most appropriate cleaning strategy for each specific combination of original paint and overpaint layer. Comparative tests were also performed with commercially available and widely used gel-based formulations (Nanorestore^® Peggy gels) along with Nanorestore^® Polar Coating nanostructured fluids, and complemented by traditional cleaning techniques, to validate and optimize the proposed system.

(II) Reintegration process, aimed at the selection and evaluation of synthetic binders suitable for the chromatic and material retouching process of reconstructible lacunae, through reversible and stable compounds capable of visually integrating with the original surface while maintaining clear distinguishability. The research began with a critical review of the scientific literature and an analysis of relevant case studies, with the aim of evaluating the performance criteria required for specific materials intended for particularly challenging outdoor conditions.

Synthetic binders–intended for both infilling mortars and pictorial reintegration– were selected based on their compatibility with the original materials and with the protective coatings developed in the following section of this research, as well as on their reversibility, workability, long-term stability, and commercial availability. The final selection focused on two synthetic resins widely used in the conservation field as binders: the urea-aldehyde resin Laropal^® A81 and the acrylic resin Paraloid^® B67 [75,76,77]. These resins were chosen for their good stability under thermo-hygrometric stress, low yellowing tendency, resistance to biological colonization, and high glass transition temperature (Tg). Their solubility in aliphatic solvents, which also ensures reversibility and selective removability, minimizes the risk of interference with the original paint layers.

• Formulation of the infilling material

Laropal^® A81 was selected for the formulation of the infilling material used in treating lacunae, given its proven effectiveness in previous ICR projects on contemporary outdoor wall paintings and earlier research (e.g., Canadian Conservation Institute for the realization of Gamblin^® colors). Based on these previous experiences [77], the synthetic mortars developed in those contexts were refined and optimized through preliminary tests, adjusting resin concentration and the ratio between binder and inorganic fillers. Various combinations of inorganic aggregates and additives were tested by changing their particle size, type, and proportions to identify the most suitable formulation. White-coloured fillers (micronized calcium carbonate, talcum powder, and Carrara marble powder) were used to obtain a neutral base suitable for the subsequent mimetic pictorial reintegration. Zinc oxide was also added as an inert component as a UV marker to ensure the recognizability of the intervention. The final mortar was selected based on specific performance parameters: ease of application by spatula, filling capacity, drying time to avoid the shrinkage of the resin, aesthetic texture compatibility with the original materials, and appropriate opacity consistent with the original paint.

• Pictorial Reintegration

In continuity with the formulation of the infilling material, an experimental phase was conducted to define the most suitable systems for pictorial reintegration using the two selected resins. These were mixed with pigments and additives to create standardized colour tablets [78]. The choice to produce tablets rather than mix the colours directly on the palette allowed better control of the additives and pigments ratio, ensuring a standardized and reproducible process. Preliminary solvent tests were performed to determine the most appropriate solvent mixture for dissolving each resin, evaluating solubility, workability, and opacity prior to application on the mock-ups. Eight colour tablets were then produced for each binder, using cadmium yellow and red, alizarin, viridian green, raw sienna, burnt sienna, yellow ochre, and vermilion red. Talc, micronized calcium carbonate, and zinc oxide were added to each mixture in calibrated quantities to provide consistency to the paste, improve handling properties, application, and UV fluorescence response, for the recognizability of the intervention [75,79,80]. For particularly photosensitive pigments such as alizarin and vermilion red, Tinuvin^® 292 (UV stabilizer) and Tinuvin^® 479 (UV absorber) were directly incorporated into the binder matrix during tablet formulation, to enhance chromatic durability over time and protect the pigments from sunlight UV radiation, especially considering the continuous outdoor exposure of the treated artworks. Several laboratory tests were carried out to evaluate the performance and effectiveness of the developed reintegration systems:

✓

Application tests for pictorial reintegration on infilling material: before application, various solvent mixtures were tested in mixing and diluting the colour tablets. The aim was to identify the most suitable solution for achieving good palette workability and satisfactory aesthetic qualities. After selecting the most appropriate solvent for each binder, reintegration trials were performed directly on the infilling substrates, simulating a real reintegration process on mock-ups. This test evaluated the brushability, chromatic performance, and hiding power of the colour tablets in relation to the different binders.

✓

UV fluorescence tests: since zinc oxide acted as a UV marker and UV stabilizers and absorbers were present both in the pigment formulations and in the experimental protective coating, their potential interference was evaluated. Mock-ups with complete reintegration systems were coated with protective treatments and examined under a Wood’s lamp to verify the persistence of zinc oxide fluorescence.

✓

Reversibility tests: mock-ups painted with an acrylic resin, on which lacunae of varying shape and size were made and subsequently reintegrated with the mortar and colour formulations developed during the research, were used to assess the selective removability of the reintegration system in relation to the original paint layers.

✓

Accelerated ageing tests: the long-term chromatic stability of the developed reintegration systems was assessed through accelerated ageing cycles performed under the same environmental conditions described in Section 3. Mock-ups painted with alizarin and cadmium yellow were used, due to their known low lightfastness. To simulate a realistic application, some areas of the specimens were additionally coated with the experimental protective coatings, allowing a direct comparison with untreated areas.

(III) Protective coatings, dedicated to developing a multilayer protective system based on functionalized materials designed to enhance the resistance of outdoor murals to UV light, pollutants, and water exposure.

A preliminary research phase, based on an in-depth bibliographic and documentary analysis, which allowed for the selection of more than ninety products previously tested in national and European projects, such as Nanocathedral, InnovaConcrete, Graffitage, and Superstar [14,81,82,83,84]. This selection enabled the identification of commercially available inorganic nanomaterials and nanocomposite polymers, chosen for their compatibility with the synthetic paint films most used in contemporary muralism.

Therefore, a stratified protective system was designed and developed, consisting of a protective layer named “interface” with UV-absorbing and stabilizing additives— applied directly over the pictorial film—and a topcoat enriched with titanium dioxide (TiO₂), silver (Ag), and zinc oxide (ZnO) nanoparticles, which should exhibit photocatalytic and biocidal properties [85,86,87,88,89,90,91,92]. For the interface, two protective products were selected and tested: one based on polydimethylsiloxane and one fluorinated, both dissolved in White Spirit, as most of these specific paint films are sensitive to ketones, alcohols, and esters, being based on vinyl, acrylic, and alkyd binders. Both products were further modified with UV stabilizers and absorbers (Tinunvin^® 292 and Tinuvin^® 479), to improve their stability, to limit photo-oxidation phenomena, and direct the photocatalytic processes—activated by the nanoparticles—exclusively towards the outer surface of the artwork, thereby preserving the integrity of the original painting.

Formulation tests were conducted to determine the maximum concentrations of active ingredients compatible with the pictorial substrates. This phase was fundamental for optimizing the protective system, balancing the loading of UV absorber and stabilizer, nanoparticles, and, in the case of silver, a surfactant. The nanoparticle concentrations were carefully calibrated to ensure material compatibility and to avoid visible alterations of the surface.

The protective coatings were applied by airbrush nebulization to ensure uniform and controlled deposition. The mock-ups were then subjected to artificial ageing using a QUV accelerated weathering tester (QUV/se), designed to replicate real environmental conditions. The ageing protocol simulated an exposure equivalent to approximately three years of natural ageing, reproducing daytime solar radiation and night-time condensation (8 h of UV exposure at 60 °C and 4 h of condensation at 50 °C, under UVA-340 lamps for 1000 h, with an irradiation intensity of 1.00 W/m²).

The performance of the protective system was subsequently assessed through a series of laboratory control tests performed on treated and untreated specimens, both before and after accelerated ageing, including:

• Colorimetric measurements: carried out using a Konica Minolta CM-700d spectrophotometer in the CIE Lab* space, to quantify colour changes (ΔE) induced by the protective treatments.
• Static contact angle measurements: performed with a Mobile Surface Analyzer (MSA) to evaluate the hydrophobicity of treated surfaces, indicating the protective system’s ability to repel moisture and limit the absorption of external agents.
• Rhodamine B (RhB) test: used to assess the photocatalytic activity of the protective materials by monitoring the degradation of the organic dye (RhB) under UV light exposure (30 h of UV exposure at 60 °C under UVA-340 lamps for 1000 h, with an irradiation intensity of 1.00 W/m²). The dye fading was determined through colorimetric measurements, recording variations in reflectance over time.
• ATP bioluminometer analysis: employed to estimate the residual microbial load on surfaces, serving as an indicator of the biocidal effectiveness of the treatments.

The following section presents the main results derived from these methodological phases, highlighting the experimental validation of the proposed conservation strategies.

6. Discussion and Ongoing Results

6.1. Integrated Cleaning Methods for Sustainable Preservation Actions Against Graphic Vandalism

One of the most complex and significant challenges in conserving urban and street art is the presence of acts of vandalism. The complexity of their removal is strongly influenced by the timing of the intervention and the morphology of the substrate: prolonged exposure to sunlight promotes the ageing and interpenetration of both original and overpainted materials, while the high surface roughness enables spray paints to penetrate within surface irregularities to a depth of several microns. The effects of time and long sun exposure make the original and overlying materials age and interpenetrate to such a degree that their removal becomes extremely complicated. Furthermore, the chemical similarity between the original paint layers and the overlapped materials used in vandalism often complicates the cleaning process more than ever. From these considerations, it became necessary to conduct cleaning tests on mock-ups designed to faithfully reproduce the features of real cases, in order to combine effectiveness with respect for the original materials.

To meet the specific conservation needs of contemporary wall paintings, an initial experimental phase was dedicated to the optimization of the gel-based supporting system, ensuring that it met the previously defined requirements, rheological and viscoelastic properties (Figure 16). Among the various polysaccharides tested (Figure 17), agarose and konjac gum proved to be the most suitable candidates with an overall compatibility with the application context. Agarose was already a well-known and established material in conservation applications. Its incorporation into the PVA-borax gel system contributes to improving structural stability, residue release, and viscoelastic behaviour, while minimizing liquid-phase release and ensuring controlled cleaning. Konjac gum emerged as a promising alternative: it exhibited reduced liquid-phase release and excellent stability on vertical surfaces (Figure 18), though it showed slightly higher rigidity and less self-healing behaviour than agarose when tested at the same concentrations. Given these results, agarose and konjac gum were selected as the most suitable polysaccharides and added to the PVA-borax gel to develop supporting systems for solvent and nanostructured fluid loading tests.

Solubility tests revealed the sensitivity of the acrylic and alkyd-based paints to different solvents (Figure 19), providing the basis for selecting the most suitable ones to be incorporated into the cleaning systems and allowing further considerations about their compatibility and potential effects on the original layers.

Acrylic paints showed high solubility in some solvents used for conservation purposes, especially in EA (ethyl acetate), MEK (methyl ethyl ketone), GVL (γ-valerolactone), and Green Varnish Rescue. In some cases, the samples even completely dissolved while being immersed in the vials. The nanostructured fluids, including both commercial Polar Coating^® products from Nanorestore and lab-made formulations based on literature protocols [60,68], exhibited a much lighter coloration of the liquid phase containing small, dispersed pigment particles. Alkyd samples, on the other hand, exhibited only minimal pigment release, evidenced by a slight coloration of the liquid phase, with a more intense saturation observed for EL (ethyl lactate) and GVL (γ-valerolactone). In the case of the nanostructured fluids, the liquid phase showed a slight orange tint of very low intensity. Swab cleaning (Figure 20) generally showed poor selectivity. While certain solvents were effective in removing the overpaint, they also affected underlying layers of similar composition, although the overlapped layer was effectively removed in the case of acrylic paint. In particular, the removal of the alkyd-based paint required excessive mechanical action, which led to highly irregular cleaning results and compromised the integrity of the original paint layer.

Three principal criteria were highlighted in the removal efficiency, which depends primarily on the chemical interaction with the solvent, then on the combined mechanical action, and finally on the duration of contact.

The selected solvents, NSFs, and cleaning systems were tested on mock-ups specifically designed for this purpose, to observe and evaluate their behavior on the samples. Generally, it was found that the action of PVA-Borax semi-interpenetrated hydrogels and Nanorestore^® Peggy gels [70] loaded with NSFs was effective for refining cleaning-treatments, due to their high retentive behaviour and the thickness of the applied spray-paint layers that simulate the graphic vandalism. As an initial step, traditional cleaning systems such as fat emulsions and Carbopol^® Ultrez 21 gels prepared with the selected solvents were applied to gradually soften the overpaint layer. In the second phase, gentler and more retentive methods employing hydrogels loaded with NSFs were applied to further refine the cleaning process. The residues were then mechanically removed using cotton swabs and synthetic sponges slightly wetted in the same nanostructured fluid. PVA-Borax semi-interpenetrated gels loaded with NSFs, previously optimized within this research, were preferred over commercial Peggy^® gels due to their greater selectivity, ease of application, and excellent adherence to the mock-up surface. The tested PVA-Borax gel-based formulations contained both 25% and 59% (w/w) of nanostructured fluids to evaluate if the NSF’s increased content improves cleaning efficiency. It was actually observed that repeated and brief applications of the gel-like systems were found to be more effective, regardless of the nanostructured fluid concentration. Despite PVA-Borax semi-interpenetrated gels loaded with NSFs helping regulate swelling and penetration, the subsequent removal process with rolling cotton swabs on the overpaint was still particularly challenging due to the rough surfaces. Comparable literature approaches were performed on smooth substrates, where the mechanical action was easier to control.

• Acryl spray paint on Alkyd/Acryl/Vinyl paint layer

The cleaning actions were found to be more effective on alkyd-based samples, whose solvent-resistant properties allowed the use of a broader spectrum of cleaning systems, without compromising the integrity of the original pictorial layer (Figure 21).

Cleaning the acrylic and vinyl layers turned out to be more challenging. Both surfaces exhibited colour migration and intermixing between the superimposed blue spray paint layer and the underlying black and white layer. This phenomenon is particularly visible on the white layer, necessitating a slow and meticulous approach for the removal. Furthermore, the mechanical action of cotton swabs was found to be especially damaging for the paint, requiring heightened caution during treatment (Figure 22).

The vinyl pictorial layer was even more problematic, due to its water-sensitive nature: the predominance of water in gel formulations, as well as in the NSFs used in the refinement stage (>60%), led to softening the original layer too, underneath the graphic blue vandalism on the top (Figure 23). While certain limitations persist and the trials are still ongoing, the combination of a gradual, highly controlled chemical and mechanical approach has already made it possible to obtain a balanced cleaning result.

• Alkyd spray paint on Alkyd/Acryl/Vinyl paint layer:

The removal of the alkyd spray paint is widely known in the literature as a particularly demanding challenge due to the oil fraction in the polyester resin, which, in time, tends to reticulate with the lower layer, especially if similar to the alkyd resin itself. The layer thickness can be reduced, even though a complete and uniform cleaning cannot be achieved (Figure 24). The procedure also demands extensive mechanical action that compromises the original substrate, particularly in the case of such rough surfaces. Current trials are still in the process of being refined.

In conclusion, the removal of spray paint from chemically similar paint layers is still a challenging task. However, a gradual and controlled approach-integrating both traditional and innovative gel-based systems along with physical methods- has proven to be capable of achieving effective results with a two-step process. This study is still ongoing, but the findings so far already provide a solid basis for developing more practically applicable conservation protocols.

6.2. Towards a Protocol for the Outdoor Reintegration of New Contemporary Muralism and Street Art

This part of the research required a critical reflection on the issues involved in the reintegration of contemporary murals, closely related to the need for theoretical and technical practice on the materials and methods to be adopted in relation to the figurative message. In fact, when the artwork itself does not allow the application of traditional recognizable reintegration methodologies, due to its intrinsic essence, materiality, or aesthetic features, alternative practical solutions must be sought that recall Brandi’s principles and, when necessary, reinterpret or extend them while maintaining a coherent [75,79,80]. Monochrome-painted or repetitive incised murals, for instance, often require a mimetic reintegration to regain the visual and conceptual unity of the artwork, thus preserving what Brandi defined as its “potential unity”. In these cases, the challenge lies in achieving a balance between visual coherence and recognizability, ensuring that the reintegration remains both perceptively integrated and technically distinct [12,75,76,77,79,80].

The present study, therefore, focused on developing operational solutions for the reintegration of contemporary murals, defining materials and methods capable of reconstructing lacunae while respecting the fundamental principles of compatibility, reversibility, and recognizability, as well as ensuring durability under outdoor exposure. To meet these criteria, the experimental phase focused on the evaluation of synthetic binders suitable for both infilling mortars and pictorial reintegration, and on their formulation with different additives that allowed the desired properties to be achieved.

Binders soluble in alcohols or ketones were excluded from the outset to prevent potential interactions with original paint layers sensitive to these solvents. Similarly, synthetic resins with limited commercial availability were discarded, in accordance with a sustainable and reproducible experimental approach.

Laropal A81 and Paraloid B67 were selected for their suitability in outdoor applications, as well as for their solubility in aliphatic solvents, a property that allowed the formulation of intervention materials compatible and reversible with respect to the original synthetic paint surfaces. Following preliminary tests on the solubility of the resins and the workability of the mixtures (for both the synthetic mortar and the colour tablets), a solvent blend composed of 70% ligroin and 30% ethanol was chosen for the dissolution of both resins. The resin-to-solvent ratios were adjusted differently for the formulation of the synthetic mortar and the colour tablets, according to specific operational requirements (Figure 25). These variations proved to significantly influence the optical properties of the reintegration layer (hue, saturation, gloss, and hiding power), which were strongly dependent on the type and concentration of the binder, as well as on the binder-to-aggregate ratio [93].

• Application, reversibility, and aesthetic qualities of the reintegration system

Both resins tested (Laropal A81 and Paraloid B67) showed good performance as binders for pictorial reintegration systems. In both cases, the formulations allowed for the achievement of uniform and chromatically balanced paint layers, suitable for reaching the desired tone and ensuring a mimetic integration of the lacunae (Figure 26).

However, some differences were observed using the same aliphatic-base solvent mixture for colour dilution and application. Colours prepared with Paraloid B67 mixtures required more applications to achieve full coverage, whereas Laropal A81 showed greater covering power and reached the desired tone with fewer layers, thereby reducing the risk of affecting the underlying synthetic infill during the reintegration process.

This behaviour is partly related to the aliphatic sensitivity of the intervention mortar, which was intentionally designed to ensure the complete removability of the reintegration system. In the preliminary tests, the removal process was carried out by applying aliphatic solvents with cotton swabs, allowing the entire reintegration system to be removed without altering the original acrylic surface. However, achieving the selective removal of the pictorial layer alone, without affecting the intervention mortar, proved to be more complex. The reversibility tests highlighted a specific challenge inherent in these systems: since both the infill and the pictorial layers are made with resins sensitive to the same solvent families, further refinement of the formulations may help improve selective reversibility in future developments.

Beyond material performance and reversibility, aesthetic consistency and optical behaviour emerged as equally critical aspects of the reintegration process. Among these, the phenomenon of metamerism represents the main critical issue identified, posing a particularly complex challenge in monochrome areas [94,95,96,97,98].

Metamerism occurs when two colours appear to match under certain lighting conditions but diverge, sometimes considerably, under others. This issue is critical in the conservation of wall paintings and mural art, where reintegration work must strive to blend seamlessly with the original surface, and the perception of colour must remain stable under both natural and artificial light.

In monochrome areas, the lack of polychromatic cues accentuates even the smallest discrepancies. A retouched patch may look identical to the original in the controlled lighting of the laboratory, but once exposed to daylight or exhibition lighting, subtle differences in the spectral reflectance of the materials used can become glaringly obvious. Factors such as the choice of pigments, binders, and additives interact with the spectral composition of the light source, exacerbating metameric mismatch.

Therefore, preliminary tests should focus on the physical and chemical stability of the materials, but also on their optical behaviour under a variety of lighting scenarios (e.g., D65, incandescent, LED, but especially sunlight related to the outdoor context). The use of UV markers, matting powders, and stabilizers is evaluated not only for their technical function but also for their influence on the overall colour appearance and recognizability of the intervention.

Ultimately, achieving a truly reversible and compatible protocol for the reintegration of monochrome zones requires a nuanced approach. It calls for the selection of pigments with spectral curves similar to the original materials, the careful calibration of binders and fillers, and, ideally, the use of reference samples and repeated visual assessments under diverse lighting. Only in this way can the risk of metameric failure be minimized, ensuring that conservation interventions remain optically stable and respectful of the original work’s visual integrity.

• Recognizability of the reintegration system

All mixtures, prepared both for infilling and pictorial reintegration, were additivated with zinc oxide (ZnO) as a UV marker to ensure recognizability under ultraviolet light, in accordance with the fundamental principles of conservation (Figure 27). The tests carried out under UV light using a Wood’s lamp yielded positive results, as the fluorescence of the reintegration materials remained clear and easily detectable, even in the presence of UV stabilizers and absorbers incorporated into the colour formulations to improve the lightfastness of photosensitive pigments.

Therefore, the reintegration system can be considered fully recognizable under UV light, while maintaining its protective and optical stability properties. However, the use of zinc-based markers raises further considerations. One issue concerns the recognizability of reintegration treatments in mural paintings whose original layers already contain zinc-based paints, which naturally exhibit UV fluorescence.

In such cases, it may be advisable to employ experimental mixtures with alternative UV markers or formulations without zinc oxide to generate a contrasting fluorescent response with respect to the original surface.

• Chromatic Stability of the Pictorial Reintegration (Ongoing)

Although both Laropal A81 and Paraloid B67 are known for their chemical stability and resistance to yellowing, the pictorial mixtures were further additivated with Tinuvin^® 292 (UV stabilizer) and Tinuvin^® 479 (UV absorber) to improve the lightfastness of the pigments exposed to outdoor conditions.

To evaluate the effectiveness of these additives and the long-term chromatic stability of the pictorial reintegration materials, accelerated ageing tests were initiated under controlled conditions simulating outdoor exposure. Some specimens were also treated with protective coatings to compare the chromatic evolution of treated and untreated surfaces.

At present, the tests are still ongoing, and post-ageing colorimetric analyses will be carried out to quantify possible chromatic variations and assess the optical stability of the materials over time. Preliminary results, however, confirm that the addition of UV stabilizers and absorbers represents an effective strategy to mitigate photodegradation phenomena affecting photosensitive pigments, such as alizarin and cadmium yellow, which are particularly vulnerable to sunlight exposure.

It is nevertheless important to emphasize that, even with the inclusion of protective additives, all materials used outdoors are subject to progressive degradation. Therefore, regular maintenance, continuous monitoring, and the use of protective coatings compatible with both the pictorial reintegration layers and the original materials of the artwork remain essential to ensure the long-term chromatic and material stability of the intervention.

6.3. Formulating Protective Systems: Balancing Between Photocatalytic and Biocidal Nanoparticles, UV Stabilizers, and Aesthetic Qualities

Despite the use of protective systems representing significant progress in the field of Street and Urban Art conservation, their long-term effectiveness remains an open area of research, as it is strongly influenced by external environmental conditions and by the composite nature of painted surfaces. In particular, the specific conservation requirements of these artworks—combined with the degradation phenomena typical of outdoor settings, such as the combined action of atmospheric agents (UV radiation, thermal fluctuations, relative humidity, precipitation etc.) and pollutants (NOx, SO₂, O₃, fine particulate matter)-pose considerable challenges in terms of protection, stability, and long-term compatibility with original materials [99,100,101,102]. Considering these complex issues, special attention in the research was devoted to the development of eco-compatible materials with self-cleaning and biocidal properties, potentially useful for mitigating the accumulation of atmospheric pollutants and the biological colonization of the mural surfaces. This approach promotes a form of preventive conservation that aims to reduce the frequency and invasiveness of conservation and restoration interventions, favouring long-term maintenance strategies. The preliminary experimental results, following application by spray nebulization (Figure 28), confirmed that the selected protective formulations, optimized for a minimal aesthetic interference, provided a generally satisfactory performance immediately after application. However, the incorporation of Ag nanoparticles led to early darkening and yellowing phenomena even at very low concentrations (0.05% w/w), demonstrating their unsuitability for the defined objectives. Following artificial ageing cycles, the protective systems exhibited variable chromatic behaviour depending on their composition. In coatings consisting of a silicon-based interface and a topcoat enriched with TiO₂ and ZnO nanoparticles, the recorded colorimetric variations remained below the threshold of visual perception, which were therefore considered acceptable. However, coatings with a fluorinated interface and a ZnO-based topcoat exhibited more pronounced chromatic alterations, which were incompatible with the established aesthetic requirements.

Regarding functional performance, the photocatalytic and biocidal activities of the developed protective systems were confirmed but were less intense than expected, most likely due to the low concentrations of active ingredients added. Nevertheless, these concentrations were defined with the aim of preserving the visual integrity of the surfaces, as previously discussed, and represent a necessary compromise between functional performance and aesthetic conservation. On the contrary, the innermost protective layer, composed of interfaces enriched with UV absorber and stabilizers, exhibited good resistance to artificial ageing, both in terms of chromatic stability and retention of hydrophobic properties. Although the fluorinated interface demonstrated higher hydrophobic performance compared to the silicon-based one, it showed more significant colorimetric variations, particularly on alkyd-painted surfaces. These changes can be attributed, at least in part, to an uneven distribution of protective coating during application, as already observed prior to ageing.

Ongoing investigations are currently underway to evaluate the protective efficacy of these interfaces, both with and without UV stabilizers, on particularly light-sensitive pigments. This work aims to further extend the applicability of the system to artworks characterized by greater vulnerability to photodegradation and outdoor exposure.

7. Conclusions to the Laboratory Research Activities

The conclusion of the three complementary research sections related to cleaning, retouching, and protection was addressed to an integrated approach, combining diagnostic, laboratory, and field studies, enabling a thorough understanding of materials and the optimization of suitable and sustainable conservation methods for outdoor application. The cleaning research highlighted the importance of adapting operational protocols not only to the chemical composition of the superimposed paint layers but also to their thickness and substrate morphology, requiring case-specific strategies and a multiple-step process. In particular, the development of sustainable semi-interpenetrated gel systems loaded with nanostructured fluids proved to optimize liquid phase release and cleaning selectivity to preserve the integrity of original pictorial layers. These water-based, low-toxicity formulations ensure controlled and accurate action, combining efficacy with sustainability.

The reintegration systems developed with synthetic resins proved to be effective, showing good compatibility with the original materials, full reversibility, and clear recognizability, all consistent with the fundamental principles of conservation.

Preliminary laboratory tests confirmed that the formulated materials met the required aesthetic, mechanical, and applicative standards, displaying good workability, covering power, and a distinct UV fluorescent response due to the inclusion of zinc oxide as a marker. Currently, artificial ageing tests are still ongoing to evaluate the long-term chromatic and UV stability of the reintegration systems, particularly those enhanced with stabilizers and absorbers.

The research laboratory on the multilayered protective coatings formulated with photocatalytic and biocidal nanoparticles, combined with carefully calibrated UV stabilizers, has been focused on setting the best type of blend of the materials to minimize chromatic alterations whilst maintaining surface aesthetic qualities. Preliminary results confirmed a generally satisfactory performance of the designed protective systems immediately after application; the protective layer, consisting of interfaces enriched with UV absorber and stabilizers, exhibited good resistance to artificial ageing, both in terms of chromatic stability and retention of hydrophobic properties. However, further controls need to be carried out to enhance knowledge in these innovative materials in order to achieve the final application in situ.

8. First ICR Recommendations for the Conservation of New Contemporary Muralism and Street Art

The development of the first ICR recommendations for the conservation of Street art and New Contemporary muralism underlines the importance of protecting what can be described as “a double ephemeral dimension” strictly linked to Public Art. The first dimension concerns the outdoor context, where the artwork is influenced not only by physical and chemical factors but also by the social and cultural environment in which it exists. This ongoing interaction reveals the fragility of its materials, which are constantly exposed to the outdoor context. The second dimension is rooted in the “public sphere”, where each passing moment adds new layers of meaning shaped by society, culture, and history. Preserving this evolving significance is as important as safeguarding the artwork’s physical integrity; this represents one of the greatest challenges faced by conservation science. While every street or urban mural is initially created with an explicit figurative and iconographic intent set by the artist, it is equally true that these works inevitably undergo transformation and evolution within the public sphere as the surrounding communities influence them. Therefore, the unpredictable axiom “Artwork/Context/Communities” which generally governs Public Art, demands flexible conservation protocols that cross conservation, science, and innovation, as well as ethical and social choices. Indeed, a society that recognises its own cultural heritage, from the very moment of its creation, wishes to take care of it and to participate actively in the processes concerning its preservation. The relationship between the artwork and the community becomes fundamental for the emergence of that social recognition which, subsequently, determines one of the most important mechanisms, namely, the initial phase related to preventive conservation: that of shared “care”. Therefore, starting from the necessity to preserve the dual nature of the ephemerality inherent in street and urban murals-governed by the axiom Artwork/Context/Communities-the act of restoration and the materials used in such interventions must adapt to these principles and seek to slow a particularly rapid and natural deterioration characteristic of this type of murals, encouraging scheduled maintenance plans through the repeated application of innovative and multifunctional products. The CHANGES project has underscored the critical role of ICR interdisciplinary collaboration, involving experts such as conservators, chemists, conservation scientists, artists, art historians, architects, and community stakeholders to enrich the research process and foster solutions that are both scientifically valuable and ethically shareable. The involvement of artists like Lucamaleonte and Diavù for M.U.Ro. has been crucial in facilitating a deeper understanding of the social and artistic dimensions of the mural, as well as the social recognition of the artwork from the Quadraro community. Therefore, the three different research sections developed in the CHANGES project, related to cleaning, retouching, and protection, were addressed to the specific needs of outdoor Street and Urban muralism and their double ephemeral dimensions. The development of eco-sustainable cleaning systems based on a multiple-step process, also employing integrated hydrogels and nanostructured fluids, that balance efficacy and the application of materials specifically targeted for the rough texture of these murals, will surely contribute to gaining a conservative protocol approach. The importance of using UV markers and synthetic resins with targeted properties for the outdoor context highlights the need to balance the recognisability criteria in a mimetic reintegration process while preserving Brandi’s potential unity of the artwork. Nevertheless, metamerism phenomena require critical study, as the accurate restoration and perception of colour and shape in monochromatic paint strictly depend on them. Protective coatings formulated with photocatalytic and biocidal nanoparticles, combined with carefully calibrated UV stabilizers, to minimize chromatic alterations whilst maintaining surface aesthetic qualities, could probably represent a significant evolution in materials, as in the reduction of fading phenomena of Street and Urban New Muralism. However, further controls need to be carried out to enhance knowledge in these innovative materials in order to achieve the final application in situ. Throughout the research, the selection of materials and methods has been guided by a commitment to eco-sustainability and ethical responsibility. Low-toxicity formulations, prioritization of commercially available and reversible resins, and avoidance of environmentally harmful substances reflect an awareness of the broader impacts of conservation practice. These principles will be the basis for the first recommendations proposed for the ongoing development of best practices in the conservation of outdoor New contemporary muralism and Street Art.