Towards the Definition of Guidelines for the Conservation of Mural Paintings in Hypogea

Abstract

Preventive conservation and restoration of mural paintings in hypogean environments is a significant challenge. These types of settings are particularly difficult to manage due to their peculiar morphological and microclimatic features. Since its foundation in 1939, the Central Institute for Restoration (Istituto Centrale per il Restauro—ICR) within the Italian Ministry of Culture has been engaged in the prevention and safety of these unique cultural heritage assets. Starting from a holistic analysis of vulnerability and hazards specific to hypogean environments, this study examines the main risks and corresponding conservation strategies. Particular attention is given to the impact of residual risk on restoration decision-making, with the aim of defining logistical and operational requirements to carry out sustainable and enduring interventions in these complex settings. The compatibility and durability of restoration materials under hypogean conditions are also examined. Furthermore, thanks to funding provided by the PNRR CHANGES project, a few research directions are proposed to address unresolved issues through the investigation and assessment of innovative products and methodologies. This review aims to lay the foundation for the development of guidelines incorporating protocols for the conservation and restoration of mural paintings in hypogean contexts, with particular attention to the needs and constraints imposed by these specific environments.

1. Introduction

The term “hypogeum” encompasses a variety of underground settings that differ greatly from one another. Hypogea can be either artificial or natural environments, carved out of bedrock or constructed within preexisting subterranean spaces. Certain hypogea originate as natural cavities or anthropogenic excavations [1,2,3,4], whereas others result from traumatic historical events like natural disasters (earthquakes, eruptions, avalanches) [5] or anthropic transformations [6]. These settings can vary widely in size and spatial complexity [7]. However, in terms of conservation issues, they can generally be classified as either confined or unconfined, depending on their level of connection with the external environment [8,9]. The environmental characteristics of hypogea largely depend on the climate of the geographical area in which they are located (T, RH and rainfall values), which in turn influences their internal microclimate [10,11,12,13]. In temperate climates they are usually characterized by high relative humidity and the presence of water, also due to condensation or infiltration phenomena.

These unfavorable environmental conditions make these spaces inherently unsuitable for conservation of mural paintings due to significant conservation challenges. At the turn of the 20th century, limited understanding of degradation phenomena and restricted technological capabilities led to the detachment of subterranean mural paintings from their original contexts being considered the only feasible preservation strategy [14]. This radical approach remains referenced in UNESCO guidelines for the conservation and maintenance of mural paintings in subterranean environments [15], providing evidence that, even today, the conservation of mural paintings in such a unique context presents significant complexities. Despite the growing awareness of the challenges posed by such contexts [15,16,17,18], there is still a lack of operational guidelines specifically dedicated to the restoration of decorative elements and mural paintings in hypogean environments, as well as standardized and widely shared conservation practices and materials. Existing studies present a significant number of case-specific contributions, focused on the study of degradation processes [19,20,21,22,23,24,25]. By contrast, not as many publications focus on the implementation of risk-mitigation measures or restoration interventions in hypogea [26,27,28,29,30,31]. Even fewer focus on evaluating restoration materials’ behavior in such specific contexts [32,33,34,35,36,37,38]. As is frequently observed in the field of restoration, post-intervention monitoring and follow-up studies are lacking, aimed at understanding long-term material performance and identifying potential failures [19,39,40,41,42]. As a result, a disconnect persists between theoretical knowledge of the problem (degradation dynamics) and practical application (indirect and direct methods and materials for restoration). This gap underscores the limited understanding of how the distinctive environment conditions and its peculiar degradation dynamics of hypogea should inform the practical aspects of the intervention and define their limitations, in terms of both restoration materials and logistic aspects.

Moreover, most available studies are always site-specific, and there is a notable lack of contributions addressing the topic from a broader perspective [15]. Establishing general principles and guidelines for the conservation and restoration of hypogean mural paintings through a holistic yet operational approach could serve as valuable operational tool for conservators and restorers, who must consider a wide range of distinct but profoundly interconnected factors influencing the decision-making process.

This review aims to fill these gaps by selecting existing literature with a primarily operational focus to outline the specific features and constraints of conservation interventions in hypogea. The motivation for this systematization effort stems from the long-standing experience of the Central Institute for Restoration (Istituto Centrale per il Restauro—ICR) of the Italian Ministry of Culture, which has been involved for decades in the conservation and restoration of both Italian [4,14,29,32,41,43,44,45,46] and foreign [47,48,49] hypogean mural paintings. This expertise has been fundamental in outlining a methodological and operational framework for addressing such challenges.

Building on the identification of prevailing risks and damage factors affecting these environments, this work aims to provide a brief overview of possible risk mitigation strategies, with particular attention to the persistence of an unavoidable residual risk. Acknowledging this condition represents a necessary premise for tackling both conservation challenges and operational complexities inherent to interventions in these contexts. The analysis of the interaction between residual risk, site logistics, and restoration materials is further supported by comparing past and present solutions. This review aims to provide groundwork for the development of operational guidelines designed to support restorers in making informed decisions about materials and methodologies.

2. Description of the Review’s Structure and Content

This review is organized into four main sections, based on a selection of bibliography sources relevant to the topic under discussion:

(I)

Identification of prevailing damage factors.

A vulnerability analysis for mural paintings in hypogean environments is presented, outlining the specific features and intrinsic characteristics of these contexts. The analysis focuses on risks that lead to cumulative or progressive damage, rather than sudden loss caused by catastrophic events. The identification of degradation factors is based on existing literature and on the expertise of ICR laboratories in the conservation of hypogean mural paintings.

(II)

Examination of possible risk-mitigation strategies and their limitations.

This section briefly discussed potential preventive and indirect measures aimed at counteracting or reducing degradation processes, while addressing their potential side effects and limitations. Special attention is devoted to the concept of unavoidable residual risk, which arises from the limited applicability or effectiveness of indirect risk-mitigation strategies in these environments.

(III)

Analysis of the interaction between residual risk and logistical aspects of restoration-site organization.

This section provides valuable practical insights for conservators involved in the logistical planning of the restoration works. It offers operational guidelines and recommendations to optimize restoration-site management under specific environmental constraints. Given the limited literature available on this topic, the discussion draws on both published studies and practical expertise of ICR professionals in hypogean contexts.

(IV)

Assessment of residual risk’s impact on the selection of restoration materials and methods.

This section examines the specific requirements associated with each category of intervention material, reviews the current state of the art in available solutions and past restoration practices—drawing on both published studies and archival sources—and identifies critical issues identified in current operational procedures and conservation materials. Potential directions for future research and development are also discussed, together with ongoing experimental investigations.

The databases used to source the literature include Google Scholar, PubMed, ResearchGate and Scopus. For topics specifically related to past interventions and materials, AATA Online (managed by the Getty Conservation Institute) and the ICCROM Online Library Catalogue were essential resources, along with ARES (the ICR Online Archive and Library) and PCAS (Pontifical Commission for Sacred Archaeology) restoration archive.

Bibliographic searches were conducted using the keywords hypogeum, subterranean, underground, crypt, and cave. The search results were further refined to include only studies related to the conservation of cultural heritage—particularly mural paintings—and their relevance to the practical objectives of this review was carefully evaluated. Whenever possible, open-access and more recent publications were preferred, except for sections concerning historical methods and materials.

The selected bibliography spans the period from 1950 to the present, with its temporal distribution illustrated in Figure 1.

Figure 1. Ring chart showing the temporal distribution of the selected bibliography.

The distribution of the collected bibliography across the different research themes is shown in Figure 2.

Figure 2. Ring chart illustrating the distribution of selected publications across research themes.

3. Identification of Prevailing Damage Factors in Confined and Unconfined Hypogea

Several key conservation issues in hypogean environments stem directly from their underground location, which makes them particularly vulnerable to damage caused by water infiltration. Such infiltration may result from the proximity to groundwater tables, lakes, sea, rivers or even from drainage of rainwater. Water infiltration could be responsible for the transport of biological contaminants, pollutants and soluble salts, driven inside the porosity of the constitutive materials. The exposure to this damage is deeply influenced by the presence or absence of overlying buildings or protective roofing structures above the hypogeum, as well as by the porosity and permeability of the surrounding bedrock or soil, which determine the site’s susceptibility to infiltration and percolation. The presence of water permeating the rock porosity may reduce its mechanical strength [50] or, conversely, act as a cementing factor for soil masses [51]. The porosity of the stone also influences its bioreceptivity, which depends on both water absorption capacity and surface roughness, facilitating the adhesion of biocontaminants and nutrients [52,53,54].

Space-conformation features of hypogea determine their degree of connection—or confinement—with respect to the external environment. From a primarily conservation-oriented perspective, two main types of hypogea can be distinguished based on this parameter: confined hypogea (systems closed to air and heat exchange) and unconfined hypogea. These typologies present considerable differences from a conservative point of view. They differ in prevailing damage factors, degradation phenomena and, consequently, in conservation strategies to be applied. The degree of confinement is mainly determined by the depth below ground level and the number, dimensions and orientation of openings, especially with respect to solar exposure and prevailing wind directions. The first mentioned factor plays an important role in determining the site’s thermal insulation [55] and affects the probability of rainwater infiltration, while the latter influences the rate of air exchange with the external environments and, consequently, the ingress of biological contaminants and pollutants.

Confined hypogea are usually characterized by low levels of natural light, high relative humidity, low or absent air-circulation and strong thermal inertia [56,57,58]. These features can lead to surface condensation (particularly in spring/summer) [24,59], and thus to the presence of water on walls (Figure 3).

Figure 3. Water condensation on the painted surface.

Although the environmental conditions of confined hypogea are extreme, their microclimatic parameters are usually characterized by a relevant stability, which leads to slower degradation processes, provided that the delicate balance of these sites remains unaltered. The prevailing risk in this case is usually linked to the high-water content of materials, due to condensation and high RH [21,22,60]. These factors may cause chemical damage through slow cumulative processes, such as dissolution or karst phenomena [61]. In hypogean sites, indeed, the average concentration of CO₂ is typically higher than normal due to the lack of air circulation, the stratification of heavier gases, or the possible release of CO₂ from soil, such as those occurring through fissures in the bedrock [62,63]. In addition, the dissolution rate of carbonate materials in these contexts can be further exacerbated by the ubiquitous presence of water on surfaces, sourced from various origins, whose solubility is enhanced by the low temperatures, and by the eventual presence of salts, such as chlorides, which increase carbonates solubility [64,65]. In confined hypogea, biological risk is particularly high [21] due to persistently high RH and low air circulation, which favor germination and biological colonization (Figure 4). The presence of water due to absorption or condensation phenomena, when combined with the presence of light sources, usually artificial, can promote colonization by autotrophic organisms, while fungal activity could be inhibited if liquid water is present on the wall surfaces.

Figure 4. Biological patinas on the painted surface: (a) Actinomycetota, (b) Algae.

Unconfined hypogea, on the other hand, are not characterized by the same stability due to their connection to the outside (Figure 5). They are typically affected by air circulation, natural light and variation of T and RH parameters.

Figure 5. One-year environmental monitoring showing microclimatic parameters of (a) a confined hypogeum (Saints Peter and Paul Crypt, Matera (MT), Italy) and (b) an unconfined hypogeum (Santa Maria del Piano Crypt in Ausonia, Italy).

Consequently, the most common risks in unconfined hypogea are associated with evaporation phenomena caused by environmental instability of parameters, leading to salt crystallization (Figure 6), a process more disruptive and rapid than chemical dissolution [21].

Figure 6. Salt damage: efflorescence and subflorescence causing flaking of the paint layer.

The risk index related to salt damage in underground environments is particularly critical, considering the rather high average RH values of hypogea (ranging between 70% and 100%). The hydration RH values of some of the most common deliquescent salts found in wall paintings—such as sodium or potassium sulfate and nitrates, which exhibit different degrees of hydration—are between 75% and 99% [66,67]. This considerably increases the risk of salt-induced damage in underground contexts, leading to the disaggregation of the painted plaster. The quantification of salt-related deterioration should also involve the contribution of infiltration or capillary rise, drastically changing the amount of salts transported to the surface, where they can crystallize as efflorescences or subflorescences, the latter being more disruptive and harmful than the former. The occurrence of one degradation pattern over another depends on the porosity of the surface layer and the ion migration capacity relative to the evaporation rate. Efflorescence is favored by slow evaporation processes, typically occurring in environments with consistently high relative humidity and low air circulation, where greater crystal growth occurs [68]. In hypogean systems, damage from the crystallization of calcium sulfate can be extremely relevant as well: this hygroscopic but non-deliquescent salt can crystallize also at very high relative humidity (99%). In addition, calcium sulfate can become harmful in this environment due to the ubiquitous presence of water, supplied to constitutive materials by different sources, such as condensation, capillary rise, or infiltration. While calcium sulfate on its own tends to cause damage only in the presence of water, its combination with sodium chloride is significantly critical, as it increases the solubility of calcium sulfate. In this case, crystallization–dissolution cycles can occur even with normal RH fluctuations between the 75% to 100% range. This makes the system much more reactive and potentially harmful, even without direct wetting [68,69]. Deliquescent salts with low RH equilibrium points, such as chlorides, are less harmful in these contexts, although their high hygroscopicity can increase material moisture content and promote colonization by halotolerant and halophilic organisms even at lower RH levels [70].

In unconfined hypogea, the air exchange rate can also increase the inflow of airborne pollutants, which is usually higher than in confined hypogea and often exacerbated by airflows induced by thermal gradients and stack effects [20,59]. The entry of air pollutants into underground environments is particularly harmful due to the high moisture content of materials, which accelerates chemical degradation processes such as the sulfation of the carbonate material by sulfuric anhydrides. Furthermore, sulfate can react with the silico-aluminates present in constituent materials (bricks and hydraulic mortars), forming ettringite and thaumasite, salts that readily modify their hydration state in humid environments, causing disintegration phenomena [71]. Sulfation processes are particularly harmful in the presence of magnesian lime plasters, forming magnesium sulfate. This salt exhibits three crystallization–dissolution curves, with the hexahydrate and heptahydrate forms occurring at RH values commonly present in hypogean environments (between 80% and 95%) [69]. High humidity and pollutants can also alter some pigments: those based on copper, lead or mercury are affected by chlorides [72,73,74], while arsenic-based pigments may degrade simply through prolonged exposure to moisture [75].

Higher air exchange rates can also increase biocontamination levels due to the entry of airborne contaminants. However, in unconfined hypogea, the higher influx of airborne microorganisms can be counterbalanced by shorter periods of high RH values [56,76]. The drying of wall surfaces can limit the growth of photoautotrophic microorganisms that require a high-water content, while numerous xerophilic or xerotolerant species, characterized by a lower water activity, may be favored [54,77,78].

In addition to spatial conformation and the degree of connection with the external environment, the presence of visitors can also act as a perturbing factor, enhancing the exchange rate with outside and altering the equilibrium of the underground system [79,80]. Variations in light intensity, gradient and duration caused by the presence of visitors in these humid environments, usually leads to uncontrolled growth of photoautotrophic organisms [81,82]. Visitors also act as carriers of airborne particulate contaminants within the site, increasing bioaerosol concentrations [83]. Moreover, temperature and RH variations caused by human presence [84] can sometimes trigger the germination of fungal spores in a quiescent state. Modification of RH and T parameters, caused by human breathing and prolonged doors openings connecting to the outside, can amplify salt damage. Sudden increase of CO₂ levels produced by visitors in enclosed environments can also enhance the acidic dissolution of carbonate materials [61].

Considering all the above, two risk factors are regarded as predominant in hypogean environments: salt-induced deterioration and biodeterioration [21,22]. Their relative predominance largely depends on the degree of confinement of the underground setting, which influences the microclimatic conditions and, consequently, the activation of specific decay processes. Nevertheless, biological risk is a constant feature of hypogean environments, although the composition of biocenoses varies according to water availability and the duration of high-humidity periods. This risk tends to be more significant in confined hypogea. Conversely, risks associated with salt damage are more likely to occur in unconfined hypogea, where environmental fluctuations promote recurrent cycles of salt crystallization and dissolution.

4. Risk Mitigation Strategies and Their Limits: Permanence of Residual Risk

Before undertaking any restoration activity, it is essential to plan and implement risk mitigation strategies through passive intervention measures, ensuring both the durability of the restoration and the improved conservation of mural paintings. The goal is to address not only the existing damage but also its underlying causes. The main concern when planning risk mitigation strategies in such environments lies in the strong interrelationship among risk factors, which makes it difficult to address one degradation factor without inevitably triggering or exacerbating others. Before implementing any risk mitigation measures in hypogean environments, it is therefore essential to anticipate and evaluate potential drawbacks associated with the new scenario, possibly using a predictive tool based on numerical simulation [20,25].

Since water is one of the primary agents of deterioration in hypogean environments, mitigation strategies usually focus on reducing the moisture content within structures, identifying sources of water ingress, and preventing infiltration. Depending on the origin of the water, insulation of the masonry can be achieved by creating a crawl space between the wall and the surrounding soil [43,45] or through various impermeabilization systems. Significant waterproofing interventions on the overlying soil or rock mass have also been carried out to control the diffusion of dispersed rainwater [26,27,46]. However, if not properly conducted—i.e., using unsuitable materials, leaving residual infiltration, and not providing a simultaneous drainage system—such interventions may exacerbate degradation processes by reducing the transpiration of the structure and drastically increasing RH values of the hypogean environment, along with condensation and dissolution phenomena [28]. These interventions are not always feasible, as they are often characterized by engineering complexity and may have a considerable impact on the natural and landscape value of the site [46,85]. Moreover, some building materials (i.e., clay) perform best under constant humidity conditions: excessive drying caused by drainage may reduce their cohesion and stability [51]. In any case, the reduction of the structure’s moisture content must be carefully evaluated through specialist studies and should be followed by a monitoring period to observe the drying process, which should proceed gradually to prevent disruptive salt crystallization on the painted surfaces [29,44,86].

Another key goal of preventive conservation is to stabilize temperature (T) and relative humidity (RH) within the site—especially in unconfined hypogea—by minimizing parameters fluctuations, reducing air exchange through sealing openings and creating buffer zones [29,30,31,87,88]. Some interventions also employ vegetative covers of the overlying soil, with careful root management to avoid mechanical damage to the painted surfaces or to the whole structure [55,89]. While these measures reduce salt damage by stabilizing microclimatic conditions, they can increase average RH values, raising the risk of microbial growth [21,22,30]. Greater site confinement may also lower temperature, which can limit fungal growth but favor autotrophic organisms due to higher moisture in materials. If temperature-lowering strategies are applied, illuminance levels may also need to be reduced, sometimes below those normally recommended for the hypogean context (photosynthetic photon flux Density PPFD < 2 μmol·m⁻²·s⁻¹) [81,82,90], using a diffuse lighting system rather than spotlights and minimizing exposure time. In some case studies, luminance values between 40 and 60 lux have been reported to prevent photoautotrophic colonization [30,90,91,92].

To mitigate the risk of photoautotrophic growth, some experiments have been conducted based on the selection of specific wavelengths of light emission with inhibitory effects, particularly green light [92] or blue light radiation (470–490 nm) [23]. Green light was selected to prevent algae growth by suppressing the spectral emission required for photosynthesis, while blue light aims to inhibit cyanobacterial colonization. The main disadvantage of this preventive approach is the reduction of the aesthetic and chromatic perception of paintings. Furthermore, several species of photoautotrophic microorganisms—particularly cyanobacteria—can adapt and continue to proliferate under selected wavelengths through a process of chromatic adaptation [93].

Another mitigation strategy to control microclimatic fluctuations involves the installation of active air-conditioning systems. This approach is particularly challenging in underground environments, mainly because of the constraints associated with preserving the site’s historical and architectural integrity without altering its appearance. Relying on mechanical systems for environmental control can also introduce additional risk factors in the event of malfunction; therefore, the presence of alarm systems and full redundancy are essential [90]. Moreover, air-conditioning systems must be carefully designed to avoid unwanted airflows caused by thermal gradients or improper air intake positioning. They should also be equipped with appropriate filtration systems to prevent the introduction of bioaerosols and external pollutants [30]. Whenever effective, passive measures should be prioritized due to their greater energy and economic sustainability (i.e., reduced energy consumption and maintenance costs). In practice, the use of integrated systems combining passive and active climate-control strategies are often employed [30,31,42].

Recent research has focused on the use of crystallization inhibitors as a mitigation strategy against salt-induced deterioration [35], particularly when other mitigation measures are not feasible. While these methods have been largely tested on stone materials [94,95], further studies are required for their extensive application to wall paintings [96], especially to evaluate potential interactions with pigments and the long-term effects. For example, the use of phosphonate compounds can interfere with certain copper-based pigments, leading to color alterations or chemical modifications even at very low concentrations (e.g., 0.1%). One of the main limitations of this approach lies in the short-lived effectiveness of these products, which necessitates periodic reapplication and consequently increases the potential risk of cumulative interference over time.

Risk mitigation strategies are often necessary to manage the presence of visitors in hypogean sites. The conflicting objectives of conservation and fruition sometimes make it necessary to adopt drastic measures, such as permanent closure, when preservation priorities prevail [1]. However, compromise solutions are preferable, relying upon mitigation strategies based on a careful assessment of visitors’ impact on the main environmental parameters [61]. Such strategies should include the development of countermeasures, such as defining the maximum number of visitors allowed, regulating visit duration, establishing recovery times from thermally and CO₂ induced alterations (to prevent cumulative effects), and limiting access to specific time of the year [84,97]. This requirement is particularly critical in confined hypogea, where visitor-induced changes can occur more rapidly and intensely due to the lack of air exchange, while the system requires longer recovery periods.

Based on the previous analysis, preventive conservation of wall paintings in hypogean environments is still a challenge, as multiple conflicting requirements must be addressed simultaneously. Compromise solutions are therefore often necessary, representing a balance between contrasting conservation needs. In any case, the implementation of both direct and indirect conservation strategies must always be based on comprehensive and careful studies and be followed by a prolonged monitoring period to assess their effectiveness and the establishment of a new environmental balance [19,44,90]. In most cases, indirect conservation strategies can only mitigate the causes of deterioration, leaving a high level of residual risk unresolved, an aspect that conservation professionals must remain aware of. In this scenario, the accurate identification of restoration materials and procedures plays a pivotal role: it is crucial to ensure the intervention’s durability, physical and chemical compatibility and reversibility, focusing on preventing the activation of new damage factors.

5. Residual Risks and Their Impact on Logistic Organization of Hypogean Worksites

Given the intrinsic features of hypogean environments and the presence of an unavoidable residual risk—even after implementing all feasible mitigation strategies—restoration activities in these contexts must comply with certain limitations, both in terms of worksite logistics and in the selection of restoration procedures and materials.

The major critical issues that must be addressed when planning a hypogean restoration worksite can be grouped into three categories: environmental, biological, and health and safety-related issues for operators, as shown in Table 1.

Table 1. Critical issues in the planning of a hypogean restoration worksite.

Environmental Criticality	Biological Criticality	Health and Safety Criticality
Low air circulation High RH values Perturbative effect of human presence Limited accessibility	High biocontamination	Possible radon values exceeding threshold [98] Possible CO2 values exceeding threshold [99] Possible presence of pathogenic microorganisms [100]

The principles that should guide restoration activities in underground environments are summarized in Table 2, based on the results of the previous risk analysis. Risks arising from restoration activities are considered analogous to the disturbances caused by visitors, and corresponding countermeasures are suggested. The primary logistical and operational requirements for establishing a restoration site in underground environments are outlined, along with preventive measures preparatory to its installation.

Table 2. Main logistical and operational requirements to set up a hypogean restoration worksite.

Type of Risk to Avoid	Logistic Requirements (Relating to Work Organization)	Operative Requirements (Relating to Restoration Procedures)
Health and safety	Assess radon levels and establish work shift duration based on the permitted exposure limits [98] Establish the maximum number of operators at work to avoid exceeding CO2-concentration thresholds that could be dangerous for both operators and paintings [99]	Avoid using products containing volatile compounds (VOCs) due to their toxicity and flammability, exacerbated by their vapor concentration in a low-air-circulation context (the use of portable filtration system is not always feasible, as they can generate undesired airflows and are generally heavy and bulky) Prefer application by brush rather than nebulization or spraying, especially when using biocide or toxic solvents
Environmental Risk	Establish the maximum number of operators to minimize RH/T fluctuations Establish work shifts according to the impact of restorers’ presence on environmental parameters (RH, T, CO2) Monitor environmental parameters during the intervention [39,44] Evaluate the influence of metal scaffolding installation on RH values: a decrease can be observed due to condensation phenomena on the metal surface [44]	Avoid the use of equipment or lamps that produce heat dispersion, as they could promote microbial colonization and the formation of efflorescence [22,92] Use microclimatic monitoring equipment specifically designed for environments that frequently reach RH saturation values [101]
Biological Risk	Assess the effect of scaffolding installation on air circulation: air stagnation can promote biological growth [39,44] Use specific disposable clothing (i.e., footwear, suit, gloves, etc.), preferably made of synthetic tissues, to limit the introduction of spores and nutrients from the outside [52] Monitor wall surface during the intervention to detect any biological growth due to the restorer’s presence [39] Avoid wooden or soft, porous rubber elements (i.e., neoprene) in restoration equipment and in any part of the scaffolding [39,44] Avoid using equipment that may create air circulation or flows, as these could spread biocontaminants or exacerbate salt crystallization Avoid high levels of lighting, in terms of both duration and intensity, as they may promote autotrophic colonization [82,92]	Avoid using products of organic or bioreceptive nature that may leave bioreceptive residues on surface (e.g., wood, paper, tissue paper, powdered cellulose, cotton, organic tissues) [38,102] Avoid products containing volatile organic compounds that could trigger germination of biodeteriogen spores (e.g., short-chain alcohol) [103,104] Periodical disinfection of equipment to minimize contaminants spreading [39]

6. Residual Risks and Their Impact on Restoration Procedures: Approaches and Materials, Past and Present

Knowledge of past intervention materials and strategies has always posed a critical challenge, primarily due to the limited availability of information. Similar obstacles concern the long-term study of these materials’ behavior in situ, owing to the lack of systematic monitoring and follow-up studies. As a result, there is a notable shortage of documentation on long-term performance—whether positive or negative—of both restoration materials and risk mitigation measures employed in relatively recent interventions. To overcome these challenges, archival research has proven essential, as well as the study of historical records documenting past ICR interventions.

Understanding historical restoration materials is crucial for interpreting ongoing degradation phenomena, particularly those arising from unwanted interaction between original and restoration materials, and for assessing their long-term performance in critical context such as hypogea. The results of the historical investigation are therefore fundamental for informing current research on innovative conservation materials and strategies.

6.1. Historical Overview on Different Approaches to Conservation of Mural Paintings in Hypogean Environments in Italy: ICR’s Expertise

The historical overview of conservation practices employed by ICR laboratories during the 20th century provides a representative example of evolving approaches to the conservation issues posed by hypogean contexts, whose harsh conditions have consistently motivated the development of innovative techniques and materials.

During the first half of the century, the removal of mural paintings from the prohibitive conditions of these underground environments through detachment was considered the only possible intervention to ensure their protection [105]. This drastic approach reflected the limited knowledge of deterioration processes and mechanisms, and the lack of effective indirect preservation methods at the time. The need to carry out restoration activities in hypogean sites led to the evolution of detachment materials: for instance, “white shellac” in alcohol was tested as a replacement for traditional animal glues, which proved unsuitable in humid environments [106]. Using this technique, many Etruscan paintings were removed from the hypogean tombs of Tarquinia in the immediate post-war period by ICR restorers. In the same years, a considerable number of mural paintings were detached and removed from other hypogean sites, often involving highly challenging engineering solutions. This was the case for the wall paintings adorning some of the rock-cut eremitic churches of Southern Italy (1955–1956), which were removed along with their stone support through the massello technique [107].

The 1960s marked a change of approach towards in situ conservation of hypogean paintings, promoting extensive research to identify new materials suitable for humid substrates, through both laboratory and on-site experimental. During these years, Paraloid B72 (an acrylic resin) dissolved in chlorothene was first applied as a fixative in hypogean environments, overcoming the limitations of shellac [108]. An international, interdisciplinary research group was formed under the leadership of chemist Margaret Hey (ICOM), involving ICR technicians and scientists. Their work focused on testing synthetic materials for mural painting conservation under hypogean conditions [109]. In the same period, awareness of the importance of preventive conservation measures began to grow. ICR’s Biology Laboratory conducted pioneering weed-control trials for mounds covering of Etruscan tombs in Tarquinia, aiming to reduce root damage on painting surface through a planned selection of short-rooted species, anticipating approaches now widely adopted [110].

Starting from the 1980s, increasing understanding of the mechanisms and causes of mural paintings of deterioration in hypogea led to adoption of indirect conservation strategies aimed at mitigating deterioration factors. Preventive measures were combined with innovative engineering and architectural solutions, such as the installation of heating coils or radiant floors at San Vincenzo al Volturno (1981–1987) to prevent condensation [27,111]. Engineering interventions—including horizontal cuts along wall and base impermeabilization through resin infiltration—were employed to halt rising damp during the restoration of mural paintings in the underground Basilica of Saint Clement in Rome (1994–2009) [86]. Primitive environmental dehumidification attempts were replaced by environmental containment strategies to stabilize RH and T parameters, such as those used in the Hall of the Masks and Corridor 131 of the Domus Aurea (1984–1990 and 2004–2013) [29]. There, low-impact lighting systems were designed to prevent biocolonization of photoautotrophic microorganisms [92], whereas thermo-heated, anti-condensation airtight doors were installed for environmental containment. During the restoration of the Tomb of the Ogre in Tarquinia (1996–2005) and in the Crypt of Anagni (1991–1994) [44], studies were conducted on the impact of the restoration works on the delicate balance of underground environments, as well as on the development of protocols to mitigate consequent harmful effects.

Alongside developing indirect conservation strategies, experimental studies addressed specific conservation issues related to restoration materials for hypogean wall paintings. In the 1980s, several ICR interventions revealed that emulsions or solutions of acrylic resins had become standard material for pigment consolidation, while calcium caseinate was gradually replaced by hydraulic mortars for filling plaster detachments, due to their higher resistance to microbial colonization in humid environments [112]. In the early years of this century, some of the ICR’s hypogean restoration sites served as testing grounds for pioneering trials, involving both procedures and materials. These included the use of crystallization inhibitors to prevent salt damage on the mural paintings of the Basilica of Saint Clement [113] and one of the earliest applications of laser cleaning in a hypogean environment on the Etruscan paintings in the Tomb of the Orcus in Tarquinia.

In continuity with its decades-long tradition, ICR’s latest studies have been developed through integrated research and in field activity addressed hypogean conservation topics. Currently, a national research partnership (CHANGES Project-Cultural Heritage Active Innovation for Sustainable Society) supports three lines of investigation aimed at drafting operational guidelines for the conservation of mural paintings in hypogean contexts, addressing the considerable operational complexity of such work.

6.2. Materials for Restoration in Hypogean Environment: Issues and Requirements

In hypogean sites, risk mitigation is often particularly challenging because many risk factors are inherent to the site and thus unavoidable. In these cases, conservation strategies must recognize residual risk as an inherent component and develop protocols aimed at controlling degradation through efficient maintenance programs and the use of suitable materials and procedures. This often requires a reassessment of certain traditional restoration practices, as many materials conventionally employed in mural painting restoration prove unsuitable in these specific contexts, failing to meet all the essential criteria. In settings where degradation processes advance rapidly, restoration materials frequently exacerbate existing conservation problems or induce new deterioration phenomena. Testing innovative materials and assessing their suitability for specific conservation practices is therefore essential to identify new solutions to otherwise appear unsolvable.

This section focuses on issues related to principal restoration activities, particularly those involving the permanent application of materials directly onto the painted surface. Such materials remain in situ over time and interact directly with the original substrates under the site’s specific environmental conditions.

As discussed in Section 3, the predominant deterioration factors in hypogean environments are salt-induced decay and biodeterioration. While the degree of confinement of the underground setting may influence the relative predominance of these processes, it does not exclude their concurrent occurrence. Consequently, the materials employed in these contexts—whether confined or unconfined—should possess properties capable of preventing both types of deterioration.

6.2.1. Injection Grouting

Injection grouting aims to re-establish adhesion between the detached plaster and the underlying layer. The procedure involves injecting a fluid grout—usually with a syringe—into the cavity created by the detachment.

Injection mortars should meet specific requirements based on the characteristics of the original plaster and its degradation assessment. Their evaluation should consider both the working properties of the liquid grouts (Injectability EN 1771:2005, Wet density ASTM C 185-02, Expansion and bleeding ASTM C 940-03, Drying shrinkage ASTM C 474-05) and performance characteristics once hardened (Splitting tensile strength EN 1771:2004; Soluble salt content by ion chromatography ASTM D 4327-03, Capillary water absorption NORMAL 11/85 or RILEM test no. II.6 EN 1771 (2005); Water vapor transmission by the wet cup method NORMAL 21/85 or ASTM E96-05) [114].

To promote proper diffusion of the liquid mortar within the cavity, pre-wetting of the cavity’s inner surfaces is necessary. This improves wettability and prevents excessive absorption of water from the injection grout into the porosity of the original plaster and wall. Pre-wetting is commonly performed using a water-ethyl alcohol mixture, which raises concerns in hypogean environments. However, due to the lack of studies on how different pre-wetting treatments affect adhesion properties of the injection mortar, no substitutes for ethyl alcohol can currently be recommended. Although pure water may be used, its application should be avoided in the presence of salt-rich plasters, plasters with poor mechanical properties, or very thin layers, as the reduction in mechanical strength—caused by wetting—may lead to further detachment and loss of plaster [115].

Historically, common materials for injection grouting included gypsum [39,44], calcium caseinate [116] and cement [11]. Each was ultimately abandoned due to specific limitations: inadequate mechanical properties and high salt content of cement, high hygroscopicity for gypsum, and susceptibility to biodegradation for calcium caseinate.

A literature survey reveals that injection grouts currently in use include both hydrated and hydraulic lime-based mortar [117,118]. Formulations are particularly complex, involving various additives with specific functions, including plasticizers, fluidizers, expansive agents, surfactants, retardants and fillers. For this reason, many restorers prefer premixed commercial products, as they are easy to handle and have been widely adopted in practice, although not always thoroughly tested [38,117,119,120,121,122]. However, the uncertain composition and chemical nature of these premixed products raises concerns, especially when applied in delicate environments such as hypogea. Many of the additives employed are considered bioreceptive or biodegradable substances, yet studies on the resistance of injection grouts to biodegradation remain limited [38,123]. Thermal behavior of grouting mortar relative to the original plaster is also underexplored. This information is crucial in environments prone to frequent condensation phenomena, to avoid the onset of preferential degradation pathways.

Given the wet operating conditions, hydraulic grouts are generally preferred in hypogean environments to ensure proper setting and hardening. However, when working on particularly degraded air-lime mortars, it can be challenging to match the mechanical characteristics of the original material using hydraulic binders or pozzolanic aggregates [124,125]. Some research has been conducted to improve the setting properties of air-lime injection grouts by adding CO₂-generating substances [124,126,127]. However, CO₂-producing reaction also often generates ammonia or ethanol, which may increase the risk of biological colonization in particularly humid environments.

In underground settings, where water and moisture movements are frequent, the porosity and water vapor permeability of injection grouts are crucial properties. Grouts should be compatible with the original materials and specially adapted to the conservation requirements of the painting under treatment. Each injection grout formulation must be tailored to a particular case. Mortars with low porosity may impede capillary suction, conveying water and soluble salts to painted areas and exacerbating damage [128]. Conversely, grouts with excessively high porosity may produce a similar effect, as large pores reduce capillarity suction efficiency.

Some interesting research [129,130] has addressed the issues relating to the injection grouting of salt-containing plaster, a common and complex operation in hypogean mural paintings. These studies aimed to minimize water content supplied to the original plaster to prevent salt solubilization and recrystallization. Methods included partial replacement of water with alcohol and adding ovalbumin as water-reducing and air-entraining agent. Although promising, these products may present drawbacks in hypogean environments mainly due to the biodegradability of natural organics materials such as albumin and the bio-enhancing properties of ethyl alcohol.

When injection grouting is required for very narrow detachments (microgrouting), the diffusion of materials within the voids often becomes problematic when using lime-based injection mortars. The limitations of both hydrated and hydraulic lime injection grouts have led conservators to adopt synthetic resin emulsions, even in hypogean environments. Recently, promising research has explored alternatives to synthetic resin for microgrouting, testing nanolime and newly formulated products based on diethyl oxalate [131,132], the latter evaluated on both dry and wet substrates.

These approaches hold potential for re-adhesion of thin delaminated plaster and narrow voids in hypogean mural paintings, although further long-term evaluation is needed to assess performance and durability.

6.2.2. Disinfection

The disinfection of wall paintings involves the application of various substances or processes aimed at devitalizing microorganisms responsible for biodeterioration phenomena such as patinas or other aesthetic alterations [133].

Devitalization can be carried out by applying biocidal substances on the alteration of the painted surface, typically before patina removal, to prevent the spread of viable spores or contaminants during removal. The rinsing of the biocidal substance is generally performed according to the condition and fragility of the original substrate and is therefore not always possible [90]. For this reason, this operation requires special attention, as it presents a potential risk of product accumulation on the surface over time. Moreover, in environments such as hypogea—where maintenance interventions involving the application of biocides could be frequent and extensive—one of the main critical issues to address is the potential harmful interaction between biocides and original materials, resulting from the cumulative effect of repeated treatments [134].

In subterranean environments, characterized by poor air circulation and the impossibility of promoting air exchange, the toxicity of the applied products influences both product selection and application methods. A biocidal formulation generally consists of an active substance—which is always toxic—and a dispersing agent or solvent, whose toxicity may vary. Some biocides are applied in aqueous solutions, while others are formulated with organic solvents of moderate toxicity. Based on this premise, in such environments and when the condition of the substrate allows (owing to a low salt content), water-based biocides should be preferred over solvent-based formulations, whose dispersion medium is inherently volatile and toxic. However, in some cases, the application of large amounts of water to the painted surface may cause the migration of solubilized carbonates and other salts toward the surface, leading to whitening phenomena [37]. When possible, brush application should be preferred to spray application in order to avoid air spraying of the biocidal substance, which generates a hazardous aerosol potentially harmful to both personnel and visitors. The use of poultice compresses based on cellulosic and thus organic materials, such as paper powder, should be avoided: if not thoroughly removed, they can promote colonization. Even the dispersion of small paper fibers into the environment during the preparation and application may serve as nutrients for biodeteriogenic microorganisms [102].

A limited number of biocides are suitable for application in cultural heritage conservation, as they must satisfy a range of stringent criteria. These include efficacy at low concentrations against target organisms, non-interference with the constituent materials of the object, and minimal risk to human health and the environment. Traditional biocides employed and currently available for such purposes are based on various formulations of quaternary ammonium salts, containing chlorine and isothiazolinones, dilutable in water and/or organic solvents [135,136,137]. Despite their widespread use, the cumulative effects of these applications on wall paintings have been little studied [138]. In particular, the possibility of long-term interactions between chlorine and copper-, lead- or mercury-based pigments cannot be excluded, due to cumulative exposure.

In recent years, green biocides based on essential oils have been proposed for application in cultural heritage preservation, particularly on stone materials [139,140,141,142,143]. Despite their environmental sustainability, some concerns have recently arisen regarding their toxicity to human health [144,145]. Although this feature, combined with their volatile nature, may make them difficult to apply in hypogean environments, several studies have been made [37,83,146]. Furthermore, many essential oils are applied in hydroalcoholic solutions. The presence of alcohol in poorly ventilated underground environments raises some concerns about operator health and safety. In addition, short-chain alcohols may act as a germination activator for fungal spores in untreated areas [103,104]. Emulsified products have also been tested in hypogean environments, revealing, in some cases, a significant drawback that requires careful evaluation: certain emulsifiers within the formulation exhibit bioreceptivity, acting as carbon sources and promoting tertiary colonization [37]. Documented interactions with original materials, including color alterations [147], underscore the need for further studies focused on painted surfaces prior to any consideration of large-scale application [138,147,148]. Post-treatment monitoring of recolonization dynamics on disinfected surfaces is essential to evaluate potential biocide accumulation resulting from repeated applications over time. The monitoring protocol should combine visual inspection by conservators, digital microscopy observation, and, when necessary, microbiological analyses (e.g., ATP testing or plate assays). The monitoring frequency and duration must be tailored to the specific environmental and structural characteristics of each hypogeum. For instance, confined hypogea exhibiting high and sustained relative humidity levels, which promote biological growth, will require more frequent monitoring.

To prevent potential issues related to toxicity and sustainability, studies have been focused on physical disinfection methods. UV-C irradiation of surfaces has been applied in underground sites for both biocidal and inhibitory purposes [122,149]. Although efficient, this method has potential drawbacks, such as the limited penetration power of UV-C radiation, whose effectiveness depends on the thickness of the microbial biofilm, and the need to remove dead organic matter from the surface after irradiation to prevent secondary colonization in these favorable environments [150]. Furthermore, few studies have evaluated the interaction of certain pigments with UV-C radiation in humid environments [151], while the use of this method is highly discouraged when organic pigments or binders are present on the surface.

6.2.3. Consolidation

The aim of consolidation is to improve the physical and mechanical properties of the artifact, which are compromised by defects of adhesion (flaking) and cohesion (powdering) in the preparatory and paint layers. A suitable consolidant must demonstrate maximum compatibility with the original materials, strengthening the matrix without altering the chromatic appearance of the surface, and preserving permeability to both water vapor and liquid water [152].

In hypogean environments, however, the performance requirements for consolidants become more stringent and must be critically addressed, considering the severity of the conservation conditions (Table 3). To be truly effective, a consolidant must remain stable under high relative humidity levels and be capable of penetrating substrates that are partially or fully water saturated. In the presence of free solution or dispersion, penetration occurs easily by convection; however, water within the substrate can impede the diffusion of consolidants through the pore system, significantly affecting their penetration [153]. This behavior depends on the type of solvent used in the formulation [154,155,156,157]. Given the range of vulnerability factors of hypogean environments, an ideal consolidant should be non-bioreceptive and free from solvents hazardous to operators, as interventions are often performed in poorly ventilated conditions.

Table 3. Consolidants’ requirements related to hypogean conservation issues and testing methods.

Issue to Be Addressed	Product Requirement	Testing Method
Powdering and flaking of the paint layer	Cohesive and adhesive power	Tape test [158,159]
Water saturated substrate	Appropriate penetration	Histochemical and SEM analysis
Condensation	Appropriate porosity	Water absorption by contact sponge [160]
Water vapor flow	Appropriate permeability	Water vapor permeability [161]
Optical properties	No chromatic alteration	Spectrocolorimetry [162]
Biodeterioration	Absence of organic compounds	Bioreceptivity assessment

Organic resins, introduced in the 1960s and appreciated for their easy handling and rapid setting, have proven damaging over time. They may form impermeable films that hinder vapor and water exchange, promote the salts crystallization beneath the pictorial surface, and generate mechanical stress, leading to cracking and disintegration of the original carbonate substrate [163]. In addition, organic compounds provide a favorable substrate for the growth of biodeteriogenic agents, especially due to the presence of rheological modifiers such as cellulose derivatives, emulsifying agent or surfactant in emulsion and dispersion resins [90,164,165].

Concerns related to the use of organic products on porous substrates, as well as the search for greater compatibility with original materials, led, from the 1960s onwards, to the introduction of inorganic consolidants such as ethyl silicate, barium hydroxide, ammonium oxalate and diammonium phosphate (DAP). Although ethyl silicate is resistant to biodeterioration, it shows poor chemical and mechanical compatibility with carbonate matrices. Its toxicity, flammability, limited penetration capacity on substrates with high capillary saturation and tendency to form rigid and unaesthetic surface crusts strongly restrict its application in hypogean environments [166]. Barium hydroxide and ammonium oxalate generally ensure durable results [167,168], but their use in biologically vulnerable contexts is limited by their reliance on cellulose pulp poultices, which are prone to microbial colonization [102]. Under conditions of high capillary saturation and presence of soluble salts, barium hydroxide can lead to a non-uniform distribution within the substrate, causing superficial precipitation of insoluble compounds [168]. Ammonium oxalate, which acts mainly as a passivating agent, may result in superficial consolidation likely due to an excessive reaction rate [169]. Additionally, this treatment can produce slight color variations, with a moderate increase in saturation [167]. The use of DAP for mural paintings consolidation remains under investigation in relation to its interactions with pigments [170]. However, its application in hypogean contexts could be critical due to its phosphorus and nitrogen content, which may promote biological growth [171].

Since the 2000s, the application of nanotechnologies in conservation [172,173] has led to the development of aqueous nanosilica, less toxic than ethyl silicate but only partially compatible with carbonate substrates. Also, nanolimes, i.e., calcium hydroxide nanoparticles dispersed in alcoholic suspension [174,175], were applied, demonstrating performances more suited to hypogean contexts. These dispersions do not significantly affect the porosity of treated surfaces [153,176,177] and are suitable for high-humidity conditions, promoting full carbonation [153,176,178,179].

Although these characteristics have made nanolimes among the most widely used products [180], including for consolidating plasters and paint films in hypogean environments [14,181,182,183], recent studies have highlighted some drawbacks. Under high-humidity conditions, and because of the carbonation process, alcoholic nanolimes have shown the formation of different mineralogical forms of calcium carbonate, including calcite but also aragonite [184,185]. Furthermore, the short-chain alcohols in which they are dispersed may promote fungal spore germination [103,104] and pose toxicity risks to operators and visitors, due to the release of volatile organic compounds (VOCs) in poorly ventilated spaces.

On the basis of the considerations outlined, the consolidants used so far have often shown some limitations in meeting these criteria (Table 4).

Table 4. Comparative evaluation of consolidants’ categories based on requirements for hypogean environments.

Products	Cohesive Power	Penetration	Porosity	Permeability	Optical Properties	Bio- Resistance	VOCs Absence
Acrylic resins	+/−	+/−	−	−	−	+	−
Nanoacrylic emulsions	+	+	+/−	+/−	+	−	+
Ethyl silicate	+	+/−	+/−	−	−	+	−
Barium hydroxide	+	+/−	+	+	+	+/− *	+ ***
AMOX	+	+/−	+	+	+/−	+/− *	+ ***
DAP	+	+/−	+	+	+	+/−	+ ***
Aqueous nanosilica	+	+	+	+	+/−	+	+
Water-based nanolimes	+	+	+	+	+	+	+
Alcohol-based nanolimes	+	+	+	+	+	+/− **	−

* due to the necessity of application by poulticing. ** possible solvent-induced activation of spore germination. *** ammonia release.

In the pursuit of solutions specific to hypogean contexts—which demand stringent requirements for sustainability and operator safety—a new nanolime formulation has been proposed [186,187,188,189]. This formulation consists solely of calcium hydroxide nanoparticles dispersed in water, obtained through an innovative and eco-friendly patented process [190,191,192]. This new aqueous nanolime exhibits characteristics suitable for hypogean applications [32,193]: it is fully compatible with carbonate substrates, thanks to its complete transformation into pure calcite during the carbonation process; it does not significantly affect the porosity of treated surfaces [187,188,189,193]; it maintains the permeability to vapor and liquid water unaltered [187,189]; it does not trigger the germination of biodeteriogenic agents, since water is used as the dispersing medium avoiding the use of organic additives. Another advantage of aqueous nanolime is its ability to be formulated at high concentrations of Ca(OH)₂ (up to 150/200 g/L), which allows the treatment of both powdering and flaking paint layer (Figure 7), avoiding the use of traditional acrylic resins in the treatment of adhesion defects [32].

Figure 7. Paint flaking treatment by aqueous nanolime injection.

Considering the experimental results obtained to date, and although further investigation is required, the product emerges as a promising solution for addressing one of the most complex challenges in the conservation of hypogean cultural heritage.

Given the shortage of comparative studies—almost exclusively conducted on limestone or mortar specimens [194]—and the lack of research specifically addressing the pictorial layer, a research program is currently in progress. It includes experimental trials on mock-ups and in situ applications, set out to investigate the effectiveness of new water-based nanolime in comparison with traditional nanolime dispersions in alcoholic solvents. Although numerous studies investigate alcoholic nanolimes applied to specimens later exposed to controlled high-humidity conditions [153,176,178,179,195], the behavior of consolidants on damp substrates with partially saturated capillary porosity [182,183,196]—common in hypogean mural paintings—remains largely unexplored. To assess the suitability of this product for such conditions, tests were conducted on specially prepared specimens. Fresco mock-ups were prepared to simulate the paint layer decohesion by applying pigments onto plaster surfaces at the nearly dry stage. The selected products were applied under different experimental conditions simulating humid and wet substrates. In the first case, specimens were conditioned at a temperature between 14–16 °C and relative humidity of approximately 98% prior to treatment; in the second, the substrates were partially saturated through water imbibition (ca. 8% water content) and stored in semi-sealed containers during testing to prevent drying.

During the preliminary application tests, special attention was given to the pre-wetting phase [197,198,199], aimed at improving the well-known issues of nanolime penetration [156,180] and addressing the challenges posed by partial pore saturation by water.

To evaluate the effectiveness of the treatments, several control tests were performed, including spectrocolorimetric measurements, Scotch tape tests, water absorption tests, and water vapor permeability assessments. Phenolphthalein testing and SEM analyses were also conducted to examine the penetration depth of the consolidants.

The experimental results obtained so far have allowed a comparative evaluation of alcoholic and water-based nanolimes applied to moist and wet supports, highlighting the influence of variables such as the substrate water content, the dispersing medium and the chemical nature of the paint layer. These preliminary findings have made it possible to identify promising materials with respect to the performance requirements of environments characterized by high humidity and biological risk. Ongoing work will focus on refining the definition of optimal application procedures (e.g., product concentration and methodology) to establish operational criteria for effective interventions in hypogean contexts.

6.2.4. Infilling

The infilling of plaster in mural painting aims to stabilize the edges of lacunae and fill losses, addressing both conservation and aesthetic targets [118].

Restoration mortars intended for hypogean environments must satisfy specific requirements, including not only the environmental factors but also the material and structural characteristics of the artifacts themselves. This implies that the evaluation of repair mortar properties and requirements should be carried out on a case-by-case basis, according to the characteristics of constituent materials of each painting (Table 5).

In such contexts especially, where the presence of water is pervasive, appropriate hygric performance, including vapor permeability, water absorption and capillary capacity, is essential for the effective management of both water and vapor. Under these conditions, the frequent presence of soluble salts transported by water can lead to the disintegration and detachment of the original plaster, significantly compromising its mechanical strength, especially in unconfined hypogea with significant temperature and relative humidity fluctuations. To mitigate this risk, high porosity in repair mortars is crucial, enabling them to function as a ‘sacrificial surface’ in which soluble salts preferentially accumulate or crystallize within the pore structure, thereby avoiding damages to the painting [200].

Porosity significantly influences the mechanical properties of mortars [201,202]: generally, higher porosity corresponds to lower mechanical strength. To allow for the safe removal of repair material without damaging the original plaster, the mechanical resistance of the repair mortar must always be lower than that of the original substrate. This ensures the reversibility of the intervention. Finally, the selection of infilling materials must consider the hypogean environment itself, avoiding organic additives that may promote the growth of biodeteriogenic microorganisms.

Table 5. Repair mortars’ requirements related to hypogean conservation issues and testing methods.

Issue to Be Addressed	Related Mortar Requirement	Testing Method
Water vapor flow	Appropriate permeability	Water vapor permeability [161]
Condensation	Appropriate porosity	Water absorption by contact sponge [160]
High water content of materials	Appropriate porosity	Water absorption by capillarity [203] Mercury Intrusion Porosimetry(MIP) [204]
Salt crystallization	Appropriate porosity	Salt Crystallization Resistance [205] (adapted for specific mortar testing conditions)
Biodeterioration	Appropriate drying rate	Drying properties [206] Bioreceptivity assessment
	Absence of organic additives	/
Disaggregation of historic mortars	Appropriate mechanical strength	Flexural and compressive strength of hardened mortar [207]
		Adhesive strength on substrates [208]

Table 5. Repair mortars’ requirements related to hypogean conservation issues and testing methods.

Issue to Be Addressed	Related Mortar Requirement	Testing Method
Water vapor flow	Appropriate permeability	Water vapor permeability [161]
Condensation	Appropriate porosity	Water absorption by contact sponge [160]
High water content of materials	Appropriate porosity	Water absorption by capillarity [203] Mercury Intrusion Porosimetry(MIP) [204]
Salt crystallization	Appropriate porosity	Salt Crystallization Resistance [205] (adapted for specific mortar testing conditions)
Biodeterioration	Appropriate drying rate	Drying properties [206] Bioreceptivity assessment
	Absence of organic additives	/
Disaggregation of historic mortars	Appropriate mechanical strength	Flexural and compressive strength of hardened mortar [207]
		Adhesive strength on substrates [208]

Historically, hydraulic binders or aggregates have often been preferred for infilling mural paintings in a hypogean environment due to their ability to set under conditions of high relative humidity [38,209,210], whereas aerial lime requires significantly longer carbonation times. Such extended setting periods present operational disadvantages, as inpainting over non-carbonated restoration mortar frequently undergoes color changes after mortar hardening [210]. However, while the use of hydraulic mortars for repairs raises no concerns when the original plaster has the same composition, their application in the presence of aerial plaster may be more critical. Hydraulic binders typically exhibit greater mechanical strength compared to aerial binders [125,211,212,213]. In addition, mechanical performance of historical aerial plaster is furthermore reduced by increased porosity over time due to weathering.

A previous study investigated the use of traditional lime-based mortars for the restoration of mural paintings in hypogean environments, based on the assumption that introducing a material as close as possible to the original in terms of composition constitutes the best conservation practice [36]. This study aimed to achieve a satisfactory porosity based on how the binder-to-aggregate ratio, chemical composition, grain size distribution, and aggregate morphology influence the physical, mechanical, and hygric properties of the restoration mortar [212,214,215,216,217].

However, despite compositional similarity, newly repair aerial-lime mortar typically exhibits reduced porosity and increased hardness compared to historic materials, as they lack the secondary porosity developed over time through natural aging and material degradation. In hypogean settings, where water movements and condensation phenomena are frequent and likely, this discrepancy may promote additional degradation mechanisms affecting the original substrate, more rapidly than in other contexts. Consequently, condensation phenomena are often observed on repair mortars, potentially promoting biological growth. In addition, salts are forced to crystallize on the original painted surface rather than on the restoration mortar [39,40], causing efflorescence and irreversible damage to the pictorial layer (Figure 8).

Figure 8. Preferential salt crystallization on the original paint layer caused by reduced porosity of the repair mortar.

Very few studies have examined the influence of mortar surface finishing (in terms of compaction and smoothing) on the hygric behavior of infill materials [200,215]. Generally, a less compact surface may enhance vapor permeability and facilitates salt efflorescence; however, it also increases susceptibility to biological colonization due to a greater deposition of airborne contaminants on the rough surface [39,54,78]. Conversely, a smoother surface obtained through compaction may reduce water absorption and increase the risk of surface condensation [210]. Localized condensation on restoration infills can exacerbate water stagnation, promoting biological growth [39,54,78] and potentially causing water runoff that may abrade the original pictorial layer.

To address these critical issues, ongoing research is focused on studying the influence of porosity enhancers in traditional lime mortars, evaluating their application on hypogean mural paintings. Research in the field of building materials has focused their attention on high-porosity mortar, specifically to avoid problems due to salt damage, capillary rise and thermal insulation [218,219,220,221]. These mortars can be categorized based on the nature of the additives used as porosity enhancers. Among these, two types have been considered potentially suitable for use in hypogean environments: mortars with porous aggregates (e.g., perlite, expanded glass) [222,223] and mortars with expansive additives [224,225]. In the former, increased porosity is conferred by the nature of the aggregate, while in the latter, it results from the gas-producing reaction occurring during mortar mixing and until its setting. Given the shortage of studies specifically addressing the use of these products as repair mortars for mural paintings in hypogean environments, and considering that both materials have demonstrated promising performance in meeting established requirements [226], an experimental study has been set up to evaluate mechanical properties, hygric behavior, and resistance to salt aging of specifically tailored formulation of macroporous mortar compared with traditional air-lime mortars. Furthermore, the experimental setting is designed to study how the surface finish of the plaster influences its hygric properties and drying kinetics. This latter characteristic is often overlooked in research on restoration mortars, yet it is critically important [54,78,220], as it emphasizes the importance of rapid water release in hypogean conditions, where moisture retention can foster biological colonization (Table 6).

Table 6. Comparative evaluation of mortars’ categories based on requirements for hypogean environments.

		Permeability	Porosity	Mechanical Strength	Bio-Resistance
Type of repair mortar	Traditional mortars (hydraulic and aerial)	+/− *	+/− *	+	+
	Experimental macroporous mortars	+	+	−	+/−
Surface finish	Rough surface	+	+	/	−
	Smooth surface	+/−	+/−	/	+

* varies with the binder-to-aggregate ratio and the specific nature of both the binder and the aggregate.

6.2.5. Retouching

Deterioration processes in hypogean environments often lead to the progressive loss of the pictorial material, manifesting macroscopically as abrasions and lacunae of the paint layer, which, in more severe cases, extend into the preparatory plaster layers.

These forms of degradation interrupt the continuity of the pictorial surface and, depending on their severity, prevent correct and immediate reading of the painted image. Consequently, retouching operations are often necessary, all aimed at enhancing the visual intelligibility of the artwork.

To be appropriate for application in hypogean settings, retouching materials must fulfill a range of specific performance criteria [34] (Table 7). Primarily, the retouching paint must demonstrate resistance to moisture and water exposure. Under conditions of high relative humidity or condensation, painting materials should exhibit good stability and resistance to solubilization. Concurrently, high degree of vapor permeability is essential: the paint layer, composed of both pigment and binder, must allow for adequate vapor exchange to preserve the natural thermo-hygrometric balance between the mural surface and the surrounding environment. Inadequate vapor permeability may lead to localized moisture accumulation, promoting salt migration toward adjacent untreated areas and exacerbating the deterioration of the original materials. A further essential requirement is the low bioreceptivity of the materials, which must not support or facilitate microbial colonization. Formulations should also exclude toxic solvents, due to the health risks posed to conservators working in poorly ventilated hypogea. Moreover, binders dissolved in solvents known to promote microbial activity, such as short-chain alcohols, should be avoided due to their potential in stimulating microbial spore germination [104]. In terms of aesthetic compatibility, the paint film should possess a matte, non-glossy yet bright appearance, consistent with the optical properties of traditional fresco surfaces. Furthermore, the degree of penetration into the substrate is essential to minimize deep absorption into the porous plaster, which facilitates future removal. Indeed, reversibility remains a fundamental principle, particularly when retouching is applied directly onto original plaster layers: materials should be designed to allow a complete and safe removal over time without compromising the integrity of the underlying historic materials.

Table 7. Retouching colors requirements related to hypogean conservation issues and testing methods.

Issue to Be Addressed	Colors Requirement	Testing Method
Water vapor flow	Appropriate vapor permeability	Water vapor permeability [161]
High water content/Condensation	Water resistance/durability	Contact angle test [227], Tape test [158,159]
Salt crystallization	Appropriate water permeability	Salt Crystallization Resistance [205] (adapted for color testing conditions) Water absorption by contact sponge [160]
Biodeterioration	Bio-resistance	Bioreceptivity assessment ATP test
Low air circulation	Low VOCs content	/
Optical properties	Matt finish and chromatic stability	Spectrocolorimetry [162]
Reversibility	Easy removal in solvents not harmful for original materials	Solubility test after artificial ageing

Watercolors have traditionally been used for the pictorial reintegration of mural paintings [228,229]. However, in hypogean contexts, characterized by high relative humidity, watercolors prove to be unsuitable due to their hygroscopic nature, which undermines stability and durability [40], and to the binder’s organic composition (gum arabic), particularly susceptible to microbial growth [38,122] (Figure 9).

Figure 9. Biological alteration on watercolors retouching.

The inadequacy of watercolors became widely acknowledged in the early 2000s through studies confirming their instability in terms of color fading or chromatic alterations [229,230,231] and through multiple restoration reports [44,232]. Archival research, primarily based on the analysis of restoration reports and interviews with conservators specialized in hypogean environments [233], has demonstrated that various possible substitutes for watercolor have been tested over time.

From the 1990s onwards, alternative binders have been used, including synthetic resins such as Paraloid B72 and Primal AC33. These compounds were chosen for their adhesive properties and proven chemical stability [209]. Primal was also selected for its formulation, which contained a biocidal component [234]. However, by the early 2000s these materials fell out of use due to certain drawbacks: bioreceptivity [90,160,235,236], difficulties in application and undesired visual effects such as streaking or glossy patches [232].

Nowadays, one of the most common practices for retouching in hypogean environments is the application of powdered pigments mixed only with distilled water, relying on capillary cohesion for adhesion to the painted surface [34,122,237,238]. To enhance microbial resistance and surface wettability, some interventions incorporated biocidal additives into the water/pigment dispersion. Despite the absence of a binder, this formulation still allows tonal reductions, underpainting, and hatching techniques, thanks to the cohesive action of capillarity and the improved wettability of the substrate enabled by the surfactants in the biocides. This method has been used selectively, particularly in the archaeological context of the painted tombs in Tarquinia [14]. Although considered biologically safe, it presents limitations in terms of reversibility and retreatability. In fact, any post-reintegration or maintenance intervention, which requires the application of a product dispersed in aqueous or solvent-based solutions (such as the application of a biocide), carries the risk of washing away the binder-free pigment [33]. This condition greatly complicates maintenance practices and imposes severe limitations. Moreover, washing away incoherent pigment particles can also be caused simply as a result of condensate water runoff.

To address the lack of a suitable binder and improve durability of retouching intervention, starting in 2017, some conservators began adding to the pigment-water mixture limewater or, more recently, nanolime [210,239,240,241], nanosilica or silica sol–gel [242]. While these entirely inorganic binders offer improved stability, they raise significant concerns regarding the reversibility of the retouching materials: processes such as carbonation—and even more critically, condensation—may irreversibly fix the pigments to the original materials.

At present, no single binder available fully satisfies all the requirements needed for retouching in hypogean environments (Table 8).

Table 8. Comparative evaluation of binders based on requirements for retouching wall paintings in hypogean environments.

Binders	Permeability	Bio- Resistance	Water Resistance/ Durability	Low Toxicity	Reversibility	Optical Properties
arabic gum (watercolors)	+	−	−	+	+	+
acrylic resins	+/−	+/−	+	+/−	+	+/−
inorganic materials (nanolime, silica sol–gel)	+	+ *	+	+ *	−	+
water	+	+	−	+	+	+
water + biocide	+	+	−	+/−	+	+
varnishes	+/−	+/−	+	+/−	+	+/−

* may vary depending on solvents used for dispersion.

Despite decades of experimentation, no systematic study or structured experimental validation has been carried out yet to identify an ideal alternative to watercolors for retouching mural paintings in hypogean contexts. In some cases, conservators decided not to carry out any reintegration at all, aiming at a “realistic presentation” of the original layers or citing the lack of a suitable binder for this specific application as the primary reason [243,244,245].

To address this gap, research is being conducted in laboratories, focusing on the evaluation of synthetic binders, some of which have already been noted in restoration reports for their favorable stability [232,246,247].

The binders were selected based on a previous study [34], which identified several synthetic resins that, although promising in terms of performance, exhibited moderate bioreceptivity. The innovative aspect of this research consists of testing metal and metal-oxide nanoparticles with bioinhibitory activity, to be incorporated into resins matrices enhancing their resistance to microbial colonization [248,249,250,251].

A further element of novelty consists of testing these products on substrates designed to simulate plasters pre-moistened prior to color application, thereby approximating real-case conditions—namely, the high moisture content characteristic of hypogean building materials. Application on damp substrates required an extensive adaptation phase of the color formulations to ensure acceptable levels of film applicability and uniformity, as well as sufficient improved adhesion.

Laboratory experimentation focuses on the interaction between colors and moist substrates, analyzing optical alterations, permeability and the cohesion of colored films. Additionally, the painted films will be subjected to artificial thermo-hygrometric aging under conditions of persistently high relative humidity and elevated CO₂ concentrations, simulating real hypogean environmental conditions.

The research will concurrently evaluate the inhibitory efficacy and long-term stability of the nanoparticles, first in laboratory conditions and later in situ, where only formulations that proved most effective in the preliminary laboratory phase will be applied and monitored over time.

7. Conclusions

This review represents a first attempt to collect and systematize both literary and archival sources concerning the conservation of wall paintings in hypogean sites, with specific attention to operational, practical and logistical aspects. The reviewed literature highlights the critical challenges faced by restorers working in these environments.

These challenges are mainly related to the presence of unavoidable residual risk, resulting from the difficulty of improving environmental parameters to levels that are suitable for the conservation of wall paintings.

This contribution identifies a lack of specific studies on hypogean mural painting conservation, especially those addressing risk mitigation interventions and passive remedies, as well as post-intervention long-term monitoring and follow-up. This gap limits the understanding of the effectiveness of restoration methods and the long-term performance of materials over time. Furthermore, the review highlights an evolving approach to the conservation of mural paintings in hypogea over time while also noting the absence of widely shared conservation practices.

Drawing on risk analyses specific to underground environments, this review proposes a set of technical guidelines for conservation work in hypogea, based on a comprehensive examination of the relevant literature and ICR’s established expertise in the field. These focus on three main aspects:

• the logistical organization of the restoration worksite,
• procedural requirements during conservation operations,
• the selection of materials used in restoration treatments.

From a worksite logistics perspective, preventive measures should be implemented to minimize the impact of the installation activities on microclimatic stability. Critical parameters include the influence of scaffolding on air circulation, the selection of appropriate equipment materials, the choice of suitable lighting systems and the regulation of both the number of operators and the duration of work shifts. Such measures help maintain stable microclimatic conditions while ensuring compliance with health and safety regulations.

Regarding operational procedures, restorers should adopt specific practices such as periodic disinfection of tools, daily removal of the equipment from the worksite, the use of disposable clothing to prevent the introduction of outdoor contaminants and avoidance of spray applications of potentially toxic products whenever possible.

In terms of material selection, general requirements have been outlined, such as the exclusion of products containing VOCs or bioreceptive components, the use of materials resistant to humidity and water, and the avoidance of short-chain alcohols. Particular attention should also be given to properties such as porosity and water vapor permeability of materials used in conservation processes.

By deepening the analysis of this topic through a detailed assessment of available products for each restoration treatment, this study reveals a critical need for the development of materials specifically designed for underground environments, demonstrating long-term durability and compatibility with their unique microclimatic conditions. The current lack of targeted studies indicates the importance of broader and coordinated research programs, as well as the establishment of an experimental setup for testing materials intended for hypogean restoration under representative environmental conditions.

As shown in this contribution, ICR, supported by funding from the PNRR CHANGES Project, has developed specific research lines aimed at identifying innovative materials and methods tailored to suit the conservation requirements of these contexts. Special attention has been devoted to the consolidation of paint layers, the formulation of infilling mortars, and the selection of suitable binders for retouching mural paintings. Further research is still required to develop appropriate products for additional treatments, in accordance with the requirements identified in this study as essential for interventions in such environments. The outcomes of the current research will contribute to the development of operational guidelines for the conservation of hypogean mural paintings, outlining key issues and best practices for their restoration.