Biocleaning to Remove Graffiti: A Real Possibility? Advances towards a Complete Protocol of Action

Effectiveness and durability of anti-graffiti products applied on ETICS: towards a compatible and sustainable graffiti removal protocol

External Thermal Insulation Composite Systems (ETICS) are widely used constructive solutions which aim at enhancing the building thermal performance. Nevertheless, ETICS can often present anomalies (e.g., stains and microcracks) throughout their service life, and vandalism actions, as in the case of graffiti, are rather common in urban areas. The removal of undesired graffiti is generally carried out through invasive chemical-mechanical methods, which may affect the durability of the ETICS. The adoption of anti-graffiti products can be a feasible protection method; however, no comprehensive studies were already addressed on these substrates. This study aims at evaluating the effectiveness, compatibility, and durability of three anti-graffiti products (with permanent, semi-permanent, and sacrificial properties) when applied on different ETICS. The removal of aerosol graffiti paints was carried out with a low-invasive and eco-friendly removal method (i.e., low-pressure steam jet). The water transport properties, as well as color, gloss, and roughness, were evaluated before and after graffiti removal. The durability of the anti-graffiti was also assessed by artificial aging cycles. Results showed that graffiti removal was rather efficient on ETICS with acrylic-based finishing coats and when using (semi) permanent anti-graffiti products (with ΔE*_ab < 5, i.e., not macroscopically visible, when comparing cleaned and reference surfaces), although these products can reduce their effectiveness after aging. Conversely, unsatisfactory graffiti cleaning was observed on ETICS with lime-based or silicate-based finishing coats (with ΔE*_ab > 5), with considerable alteration also of the water transport properties (reducing water absorption and slowing down the drying kinetic).

Bioconservation of Historic Stone Buildings—An Updated Review

Cultural heritage buildings of stone construction require careful restorative actions to maintain them as close to the original condition as possible. This includes consolidation and cleaning of the structure. Traditional consolidants may have poor performance due to structural drawbacks such as low adhesion, poor penetration and flexibility. The requirement for organic consolidants to be dissolved in volatile organic compounds may pose environmental and human health risks. Traditional conservation treatments can be replaced by more environmentally acceptable, biologically-based, measures, including bioconsolidation using whole bacterial cells or cell biomolecules; the latter include plant or microbial biopolymers and bacterial cell walls. Biocleaning can employ microorganisms or their extracted enzymes to remove inorganic and organic surface deposits such as sulfate crusts, animal glues, biofilms and felt tip marker graffiti. This review seeks to provide updated information on the innovative bioconservation treatments that have been or are being developed.

Bioremoval of graffiti using novel commercial strains of bacteria

Previous studies have provided evidence that bioremediation deals a novel approach to graffiti removal, thereby overcoming well-known limitations of current cleaning methods. In the present study eight bacteria aerobic, mesophilic and culturable from the American ATCC and the German DSMZ collections of microorganisms, some isolated from car paint waste, colored deposits in a pulp dryer and wastewater from dye works, were tested in the removal of silver and black graffiti spray paints using immersion strategies with glass slides. Absorbance at 600 nm and live/dead assays were performed to estimate bacterial density and activity in all samples. Also, pH and dissolved organic carbon (DOC) and inorganic carbon (DIC) measurements in the liquid media were made, as well as, thickness, colorimetric and infrared (FTIR) spectroscopy measurements in graffiti paint layers were used to evaluate the presence of the selected bacteria in the samples and the graffiti bioremoval capacity of bacteria. Data demonstrated that of the eight bacteria studied, Enterobacter aerogenes, Comamonas sp. and a mixture of Bacillus sp., Delftia lacustris, Sphingobacterium caeni, and Ochrobactrum anthropi were the most promising for bioremoval of graffiti. According to significant changes in FTIR spectra, indicating an alteration of the paint polymeric structure, coupled with the presence of a consistent quantity of live bacteria in the medium as well as a significant increase of DIC (a measure of metabolic activity) and a change in paint color.

Biocleaning on Cultural Heritage: new frontiers of microbial biotechnologies

Over the last two decades, the biotechnologies applied to Cultural heritage (CH) have become a successful novel alternative to the traditional approaches in the CH conservation and preservation. From these new perspectives, microorganisms and their metabolisms can be used for the safeguarding of artworks. Biocleaning is a field with a growing interest, based on eco-friendly processes and safe procedures, where biological reactions occurring in natural habitats are optimized in artificial conditions with the aim of CH conservation. This represents a new tool and opportunity for the development and improvement of the sector, with a great advantage for the CH conservation-restoration, in terms of safety, effectiveness, costs and environmental sustainability. This review focuses on the use of microbes and enzymes involved in biocleaning of CH artworks. The aim is to provide a comprehensive, critical and chronological view of the scientific works published until now where 'virtuous' microorganisms are applied on different CH materials, pointing out strength and drawback of the biocleaning treatments.

The Bad and the Good-Microorganisms in Cultural Heritage Environments-An Update on Biodeterioration and Biotreatment Approaches

Cultural heritage objects constitute a very diverse environment, inhabited by various bacteria and fungi. The impact of these microorganisms on the degradation of artworks is undeniable, but at the same time, some of them may be applied for the efficient biotreatment of cultural heritage assets. Interventions with microorganisms have been proven to be useful in restoration of artworks, when classical chemical and mechanical methods fail or produce poor or short-term effects. The path to understanding the impact of microbes on historical objects relies mostly on multidisciplinary approaches, combining novel meta-omic technologies with classical cultivation experiments, and physico-chemical characterization of artworks. In particular, the development of metabolomic- and metatranscriptomic-based analyses associated with metagenomic studies may significantly increase our understanding of the microbial processes occurring on different materials and under various environmental conditions. Moreover, the progress in environmental microbiology and biotechnology may enable more effective application of microorganisms in the biotreatment of historical objects, creating an alternative to highly invasive chemical and mechanical methods.

Impact of wildfire on granite outcrops in archaeological sites surrounded by different types of vegetation

The lack of scientific information about the effects of wildfire on prehistoric structures and rock art, such as dolmens and petroglyphs, impedes the development of conservation guidelines. In this study, the impact of a recent wildfire (late 2017) on granite outcrops in the San Salvador de Coruxo archaeological site (Vigo, SW Galicia) was evaluated. Samples of the same type of granite were obtained from three sites characterised by different types of vegetation (natural scrub, native deciduous oak and non-native pine-eucalypt forest) in order to determine how the vegetation influences the fire-caused damage to the rock. Three subsamples were taken from each of the granite samples at depths of 1 cm-3 cm to study how fire affects the rock at depth. In all sites, the temperature reached at the granite surface was below 380 °C. No mineralogical changes due to fire exposure were detected, and no physical changes that could be attributed to the effect of the fire on the fissure system of the granite were identified. However, aesthetic colour changes due to the deposition of organic and charred matter, which even penetrated the fissures, were detected. The existence of lignin-derived compounds, lipids and carbohydrates in the samples from the oakwood site indicates greater resistance to fire effects in this type of vegetation than in the other two types. Although preliminary, these findings suggest that oakwoods could act as protective belts around archaeological sites by reducing the wildfire severity, because of their greater resistance to being burnt, and that they could buffer the damaging effects of fire in natural areas where parietal art is found.

Overcoming Challenges in Street Art Murals Conservation: A Comparative Study on Cleaning Approach and Methodology

Street art, and its preservation, has become one of the most discussed and still open topics in the field of conservation. The absence of a sharable conservation protocol in terms of cleaning and protection is a delicate subject to deal with. The cleaning of contemporary murals and urban artworks stained by graphical vandalism is a real challenge, due to the similar chemical composition of the materials involved. In this work two different approaches to the cleaning of street art murals, namely chemical methods and laser cleaning, are reported and compared with the aim of identifying a suitable conservation practice. Ad hoc concrete mock-ups coated with a paint layer and a simulated vandalism on top have been prepared and used for the testing and analytical assessment of the cleaning procedures.

Anti-Graffiti Behavior of Oleo/Hydrophobic Nano-Filled Coatings Applied on Natural Stone Materials

In recent years, graffiti writings are increasingly regarded as a form of art. However, their presence on historic building remains a vandalism and different strategies have been developed to clean or, preferably, protect the surfaces. In this study, an experimental nano-filled coating, based on fluorine resin containing SiO2 nano-particles, and two commercial products have been applied on compact and porous calcareous stones, representative of building materials used in the Mediterranean basin, and their anti-graffiti ability has been analyzed. All the tested experimental and commercial coatings exhibited high hydrophobicity and oleophobicity, thus meeting one of the basic requirements for anti-graffiti systems. The effects of staining by acrylic blu-colored spray paint and felt-tip marker were, then, assessed; the properties of the treated stone surfaces after cleaning by acetone were also investigated. Visual observations, contact angle measurements and color evaluations were performed to this aim. It was found that the protective coatings facilitated the spray paint removal; however high oleophobicity or paint repellence did not guarantee a complete cleaning. The stain from the felt-tip marker was confirmed to be extremely difficult to remove. The cleaning with a neat unconfined solvent promoted the movement of the applied polymers (and likely of the paint, as well) in the porous structure of the stone substrate.

The Potential Use of Microorganisms as Restorative Agents: An Update

The biodeterioration process involves every type of Cultural Heritage item, including monuments, stoneworks, frescoes, and easel paintings. The accurate study of the microbial and fungal communities dwelling on artworks, and involved in their deterioration, is essential for the adoption of optimal prevention and conservation strategies. Conventional restorative methods, that usually involve chemical and physical technologies, present some disadvantages, including short-term and unsatisfactory effects, potential damage to the treated works, human toxicity, and environmental hazards. Research in the field of restoration has paved the way for innovative biological approaches, or ‘biorestoration’, in which microorganisms are not only considered as an eventual danger for artworks, but rather as potential tools for restoration. The present review describes the main aspects of the biodeterioration process and highlights the most relevant biorestoration approaches: bioconsolidation, biocleaning, biological control, and new promising bio-decontaminating compounds.

Onsite advanced biocleaning system for historical wall paintings using new agar-gauze bacteria gel

AIMS

This study reports the results of the application of a new agar-gauze biogel system activated with viable bacterial cells to altered wall paintings.

METHODS AND RESULTS

Biocleaning using agar biogel and agar-gauze biogel systems was performed onsite by direct application to altered wall painting surfaces (25-1000 cm² ). The treatments were performed for the restoration of two original Italian sites: (i) at the Vatican Museums, Cristo che salva Pietro dalle acque-La Navicella, a wall painting by Giovanni Lanfranco (1627-1628) and (ii) at Pisa Cathedral Cupola, Incarnato, a wall painting by Orazio Riminaldi (1593-1630) and his brother Girolamo Riminaldi. The novelty of this study is the use of viable Pseudomonas stutzeri A29 cells in an advanced agar-gauze biogel system and the short bio-application contact times of between 3 and 12 h. The historical artworks were altered by lipid and protein residues from past restoration, as confirmed by Py-gas chromatography-mass spectrometry and FT-IR data. The effectiveness of the biological treatment was assessed, and general considerations were discussed.

CONCLUSIONS

The short bio-application contact time of advanced agar-gauze gel activated with viable P. stutzeri cells makes this biotechnology promising as an alternative method to the traditional onsite cleaning techniques currently in use for altered historical wall paintings.

SIGNIFICANCE AND IMPACT OF THE STUDY

In this study, we report for the first time the biocleaning of altered materials located in vertical and vaulted areas using agar-gauze biogel with short application times. These findings are of great significance for future restoration activities and are crucial for determining the best preservation strategies in this field.