Effectiveness of granite cleaning procedures in cultural heritage: A review

Microbiome shifts elicited by ornamental lighting of granite facades identified by MinION sequencing

Night-time outdoor illumination in combination with natural sunlight can influence the visible phototrophic colonizers (mainly algae) growing on stone facades; however, the effects on the microbiome (invisible to the naked eye) are not clear. The presence of stone-dwelling microbes, such as bacteria, diatoms, fungi, viruses and archaea, drives further biological colonization, which may exacerbate the biodeterioration of substrates. Considering the microbiome is therefore important for conservation of the built heritage. The impact of the following types of lighting on the relative abundance and diversity of the microbiome on granite ashlars was evaluated in a year-long outdoor pilot study: no lighting; lighting with a metal halide lamp (a traditional lighting system currently used to illuminate monuments); and lighting with a novel LED lamp (an environmentally sound prototype lamp with a biostatic effect, halting biological colonization by phototrophs, currently under trial). Culturable fractions of microbiome and whole-genome sequencing by metabarcoding with Oxford Nanopore Sequencing (MinION) was conducted for bacteria and fungi in order to complement both community characterization strategies. In addition, the possible biodeteriorative profiles of the isolated strains, relative to calcium carbonate precipitation/solubilisation and iron oxidation/reduction, were investigated by plate assays. Alpha and beta diversity indexes were also determined, along with the abundance of biocide and antibiotic resistance genes. Culture-dependent microbiological analysis failed to properly show changes in community composition, for which metagenomic approaches like MinION are better suited. Thus, MinION analysis identified shifts in the granite microbiome elicited by ornamental lighting. The novel LED lamp with the biostatic effect on phototrophs caused an increase in the diversity of bacteria and fungi. In this case, the microbiome was more similar to that in the unlit samples. In the samples illuminated by the metal halide lamp, dominance of bacteria was favoured and the presence of fungi was negligible.

Laser cleaning and Raman analysis of the contamination on the optical window of a rubidium vapor cell

In this work, we present the laser cleaning of a Rubidium vapor cell and the Raman analysis of the contaminant material to be removed. The optical window of the vapor cell had gradually lost transparency due to the development of an opaque layer of unknown composition at the inner side during the normal operation of the cell. Laser cleaning was successfully performed by a frequency-doubled Nd:YAG laser focusing the beam inside the cell, avoiding any possible damage to the window. A single laser pulse was enough to clear away the black discoloration at the focal spot and locally restore the transparency of the window. The Raman spectra of the deposit showed peaks not yet described in the literature. Comparison with known Rubidium germanate spectra and simulation results strongly suggested that the unknown material was Rubidium silicate.

Deterioration of graffiti spray paints applied on granite after a decade of natural environment

Graffiti spray paints are commonly used in contemporaneous mural paintings in public spaces, contributing to the transformation of sites and urban life. These outdoor artworks are now beginning to show different deterioration forms, such as physical-mechanical alteration (loss of material and cohesion, etc.) and chromatic changes. However, the deterioration has not been formally characterized, and the influence of the paint composition and underlying substrate are not known. In this study, three non-metallic (red, blue and black) alkyd graffiti spray paints and one metallic (silver) polyethylene graffiti spray paint were applied to two granite stones with different mineralogy and texture and exposed to a natural urban-marine environment near Vigo (NW Spain) for one decade (2010-2020). Physical changes were evaluated by stereomicroscopy, colour spectrophotometry, measurements of gloss, surface roughness and static contact angle, and peeling test. Mineralogical changes were determined by x-ray diffraction and molecular changes by Fourier transform infrared spectroscopy. Moreover, micromorphological and chemical characterization of the surfaces was conducted by scanning electron microscopy. Physical-mechanical changes, such as craquelure and paint loss, depended on the texture of the granite. More specifically, paint on the granite with the finest grain size showed most intense cracking and loss of material. Chemical changes, which were not related to the granite substrate, were most intense in the red and silver paint coatings. In the red paint, loss of binder was accompanied by an intense fading of the colour (due to titanium dioxide relative enrichment), while in the silver paint coating, chemical changes occurred in both the organic binder and aluminium particles, thus darkening the colour. Fewer chemical changes were observed in the blue and black paints. Physical and chemical changes detected in these paints were not correlated.

Application and Monitoring of Oxidative Alginate–Biocide Hydrogels for Two Case Studies in “The Sassi and the Park of the Rupestrian Churches of Matera”

The removal of biological colonization on building materials of cultural heritage is a difficult challenge, as the treatment must completely eliminate the biological patina without altering the treated substrate and possibly delaying new colonization. With the aim of searching for systems to minimize the biocide impact on the substrate, the environment and the operators, different alginate–oxidizing biocide hydrogels were previously tested and optimized in the laboratory and here selected for application in situ. The churches “San Pietro Barisano” and “Madonna dei Derelitti”, located in the Sassi of Matera (UNESCO World Heritage Site in Basilicata region, Italy), were chosen as case studies. They differ in terms of both the environmental conditions and the microorganisms responsible for colonization. Colorimetric measurements and microscopic investigation proved the efficacy of biocide hydrogels in removing biopatinas and in restoring the original chromaticity of the selected treated surfaces of both sites. After the biocidal treatments, new protective acrylic coatings were applied to prevent recolonization and minimize the loss of material grains. Samples collected, immediately after and two years later, established the absence of biological colonization, demonstrating the long-term efficacy of the proposed restoration protocol.

Current and future chemical treatments to fight biodeterioration of outdoor building materials and associated biofilms: Moving away from ecotoxic and towards efficient, sustainable solutions

All types of building materials are rapidly colonized by microorganisms, initially through an invisible and then later a visible biofilm that leads to their biodeterioration. Over centuries, this natural phenomenon has been managed using mechanical procedures, oils, or even wax. In modern history, many treatments such as high-pressure cleaners, biocides (mainly isothiazolinones and quaternary ammonium compounds) are commercially available, as well as preventive ones, such as the use of water-repellent coatings in the fabrication process. While all these cleaning techniques offer excellent cost-benefit ratios, their limitations are numerous. Indeed, building materials are often quickly recolonized after application, and microorganisms are increasingly reported as resistant to chemical treatments. Furthermore, many antifouling compounds are ecotoxic, harmful to human health and the environment, and new regulations tend to limit their use and constrain their commercialization. The current state-of-the-art highlights an urgent need to develop innovative antifouling strategies and the widespread use of safe and eco-friendly solutions to biodeterioration. Interestingly, innovative approaches and compounds have recently been identified, including the use of photocatalysts or natural compounds such as essential oils or quorum sensing inhibitors. Most of these solutions developed in laboratory settings appear very promising, although their efficiency and ecotoxicological features remain to be further tested before being widely marketed. This review highlights the complexity of choosing the adequate antifouling compounds when fighting biodeterioration and proposes developing case-to-case innovative strategies to raise this challenge, relying on integrative and multidisciplinary approaches.

Geochemical-Microscopical Characterization of the Deterioration of Stone Surfaces in the Cloister of Santa Maria in Vado (Ferrara, Italy)

Santa Maria in Vado is a monument in the rich artistic heritage of the city of Ferrara (north of Italy). In this paper we want to investigate the state of conservation of tombstones, cloister and the entrance to the basilica, in order to keep them in the best possible state for the future generations. From the chemical characterization, the state of conservation was determined focusing on the biodeteriogenic and non-biodeteriogenic factors, which determine a series of unwanted changes in the physical, mechanical and above all aesthetic properties of the material, often closely connected with the environment and conservation conditions. On the macroscopic observation, the state of conservation of the tombstones appeared to be very deteriorated through aesthetic and structural damage. In detail, the stereo microscope observation of samples collected from the tombstones show the presence of efflorescence probably caused by the abundant of water that bring the salts present inside the rock into solution. Relating the columns, μ-XRF analysis confirm the carbonate composition of samples and presence of iron and sulfur. Finally, SEM observation highlighted the presence of black crust on arch samples and the presence of pollen on the black crust and spheroidal particles probably related to atmospheric pollution.

Toxic or Otherwise Harmful Algae and the Built Environment

This article gives a comprehensive overview on potentially harmful algae occurring in the built environment. Man-made structures provide diverse habitats where algae can grow, mainly aerophytic in nature. Literature reveals that algae that is potentially harmful to humans do occur in the anthropogenic environment in the air, on surfaces or in water bodies. Algae may negatively affect humans in different ways: they may be toxic, allergenic and pathogenic to humans or attack human structures. Toxin-producing alga are represented in the built environment mainly by blue green algae (Cyanoprokaryota). In special occasions, other toxic algae may also be involved. Green algae (Chlorophyta) found airborne or growing on manmade surfaces may be allergenic whereas Cyanoprokaryota and other forms may not only be toxic but also allergenic. Pathogenicity is found only in a special group of algae, especially in the genus Prototheca. In addition, rare cases with infections due to algae with green chloroplasts are reported. Algal action may be involved in the biodeterioration of buildings and works of art, which is still discussed controversially. Whereas in many cases the disfigurement of surfaces and even the corrosion of materials is encountered, in other cases a protective effect on the materials is reported. A comprehensive list of 79 taxa of potentially harmful, airborne algae supplemented with their counterparts occurring in the built environment, is given. Due to global climate change, it is not unlikely that the built environment will suffer from more and higher amounts of harmful algal species in the future. Therefore, intensified research in composition, ecophysiology and development of algal growth in the built environment is indicated.

Development of a novel method for the in-situ dechlorination of immovable iron elements: optimization of Cl- extraction yield through experimental design

The conservation of iron objects exposed to marine aerosol is threatened by the formation of akaganeite, a highly unstable Cl-bearing corrosion phase. As akaganeite formation is responsible of the exfoliation of the rust layer, chlorides trigger a cyclic alteration phenomenon that often ends with the total consumption of the iron core. To prevent this degradation process, movable iron elements (e.g. archaeometallurgical artefacts) are generally immersed in alkaline dechlorination baths. Aiming to transfer this successful method to the treatment of immovable iron objects, we propose the in-situ application of alkaline solutions through the use of highly absorbent wraps. As first step of this novel research line, the present work defines the best desalination solution to be used and optimizes its extraction yield. After literature review, a screening experimental design was performed to understand the single and synergic effects of common additives used for NaOH baths. Once the most effective variables were selected, an optimization design was carried out to determine the optimal conditions to be set during treatment. According to the experimental work here presented, the use of 0.7 M NaOH solutions applied at high temperatures (above 50 °C) is recommended. Indeed, these conditions enhance chloride extraction and iron leaching inhibition, while promoting corrosion stabilization.

Moving toward Smart Cities: Evaluation of the Self-Cleaning Properties of Si-Based Consolidants Containing Nanocrystalline TiO2 Activated by either UV-A or UV-B Radiation

This study evaluated the self-cleaning ability and durability of Si-based consolidants (an ethyl silicate consolidant and a consolidant based on nanosized silica) spiked with nanocrystalline TiO₂ activated by either UV-A radiation (spectral region between 340 and 400 nm, and main peak at 365 nm) or UV-B radiation (spectral region between 270 and 420 nm, and main peak at 310 nm). Granite samples were coated with consolidant, to which nanocrystalline TiO₂ was added at different concentrations (0.5, 1, and 3%, by wt.). Diesel soot was then applied to the coated surfaces, and the samples were exposed to UV-A or UV-B radiation for 1650 h. The surface color changes, relative to the color of untreated granite, were determined every 330 h by color spectrophotometry. Slight color changes indicated a recovery of the reference color due to the degradation of the soot. The final surfaces of both the untreated and treated surfaces were compared by stereomicroscopy and scanning electron microscopy. The main findings were that: (1) In general, the consolidant containing nanosized silica induced the most intense photocatalytic activity. In the more compact xerogel coating formed by the nanosized silica, more TiO₂ nanoparticles were available to interact with the radiation. (2) For all consolidant mixtures, soot degradation remained constant or decreased over time, except with ethyl silicate with 0.5 wt % TiO₂ (no self-cleaning capacity). (3) Soot degradation increased with the concentration of TiO₂. (4) The UV-B radiation was the most effective in terms of soot degradation, except for the surface coated with the ethyl silicate and 3% wt. TiO₂.

Influence of the weathering rate on the response of granite to nanosecond UV laser irradiation

Laser ablation is an accepted cleaning technology for cultural heritage stonework. Optimized laser ablation may, however, have different side effects depending on the mineralogical composition of the stone. In the case of granitic rocks, observed side effects - colour changes, fracturing or/and mineral melting - have been attributed to laser parameter interactions with the patina to be cleaned from this complex rock with a grained texture and polymineral composition. We describe the influence of the weathering degree of a granite on its response to laser irradiation, confirming that the existence of secondary minerals in the rock influences the intensity and typology of side effects. Knowledge of this influence is of special relevance to the conservation of heritage built with Western European Variscan granites, which are already slightly weathered in quarry. Two specimens of the same Variscan granite taken from the same quarry but with different colours (suggesting different weathering rates) were subjected to nanosecond (ns) Nd:YVO₄ laser irradiation, working at 355 nm (UV irradiation) under different fluences. The specimens responded very differently to laser irradiation in terms of colour changes and resistance of the main minerals to laser radiation. The existence on one of the specimens of kaolinite deposits covering feldspar grains and Fe oxyhydroxides filling fissures seemed to be the reason for the different response to laser radiation. Our findings would suggest the need, during laser interventions, to take into account - in addition to texture and porosity - the degree of weathering of this particular kind of granite, widely used in the architectural and archaeological heritage of Western Europe.

Influence of the Laser Wavelength on Harmful Effects on Granite Due to Biofilm Removal

The colonization of stone-built monuments by different organisms (algae, fungi, lichens, bacteria, and cyanobacteria) can lead to biodeterioration of the stone, negatively affecting the artistic value of the heritage. To address this issue, laser cleaning has been widely investigated in recent years, due to the advantages it offers over traditional mechanical and chemical methods: it is gradual, selective, contactless, and environmentally friendly. That said, the laser parameters should be optimized in order to avoid any by-effects on the surface as a result of overcleaning. However, as the adjustment of each parameter to clean polymineralic stones is a difficult task, it would be useful to know the effect of overcleaning on the different forming minerals depending on the wavelength used. In this paper, three different wavelengths (355 nm, 532 nm, and 1064 nm) of a Q-Switch neodymium-doped yttrium aluminum garnet (Nd:Y3Al5O12) laser, commonly known as QS Nd:YAG laser were applied to extract a naturally developed sub-aerial biofilm from Vilachán granite, commonly used in monuments in the Northwest (NW)Iberian Peninsula. In addition to the removal rate of the biofilm, the by-effects induced for fluences higher than the damage threshold of the stone were evaluated using stereomicroscopy, color spectrophotometry, and scanning electron microscopy with energy-dispersive x-ray spectroscopy. The results showed that different removal rates were obtained depending on the wavelength used and 532 nm obtained the highest removal level. In terms of by-effects, biotite melting was registered on all surfaces regardless of the wavelength. In addition, 532 nm seemed to be the most aggressive laser system, inducing the greatest change in appearance as a result of extracting the kaolinite crackled coating and the segregations rich in Fe, which are a result of natural weathering. These changes were translated into colorimetric changes visible to the human eye. The surfaces treated with 355 nm and 1064 nm showed lower surface changes.

Selected Aspects Regarding the Restoration/Conservation of Traditional Wood and Masonry Building Materials: A Short Overview of the Last Decade Findings

Vernacular buildings are usually constructed using materials at hand, including wood, natural stone and bricks (either clay or mud bricks). All those materials are exposed to a series of environmental factors, affecting their structure and integrity. The literature review was conducted using different databases (Scopus, Web of Science, ScienceDirect, SpringerLink) using as keywords the historical material, “heritage” and the terms regarding the desired effect, within the envisaged time period (2010–2019). The assessment of the results was performed by manual inspection (reading the entire article) and the selection of the works to be inserted in the current review was made by evaluating the contribution to the field. This review summarizes different aspects related to the restoration and conservation of wooden and masonry elements of traditional buildings, including materials used for biocidal interventions, protection against abiotic factors, cleaning and consolidation agents. Finally, a critical discussion regarding the current limitations and future perspectives concludes the review work, envisaging the role of researchers specialized in materials science in the context of cultural heritage conservation.

Terrestrial eutrophication of building materials and buildings: An emerging topic in environmental studies

Eutrophication has been analyzed mostly in aquatic or soil environments to date. Direct terrestrial eutrophication of building materials and buildings, contrary e.g. to their biodeterioration or biodegradation, was studied so rarely that even its exact definition does not exist yet. In this paper, eutrophication of building materials and buildings as an emerging topic in environmental studies is analyzed in detail and future developments in the field are contemplated. The analysis includes a survey of directly and indirectly related research studies, identification of basic mechanisms and principal factors, and a critical assessment of current methodologies potentially applicable for recognition and classification of eutrophication of building materials and buildings. A definition of direct terrestrial eutrophication of building materials and buildings is proposed afterwards and an alternative method for the calculation of their eutrophication potential is suggested. Finally, recommendations for solving the most urgent problems in future research are formulated.

IR irradiation to remove a sub-aerial biofilm from granitic stones using two different laser systems: An Nd: YAG (1064 nm) and an Er:YAG (2940 nm)

A sub-aerial biofilm (SAB) developed on a granite commonly found in the built cultural heritage of the NW Iberian Peninsula was extracted with 2 different IR irradiations using an Nd:YAG laser at 1064 nm and an Er:YAG laser at 2940 nm. The methodology was based on the application of only one scan in order to evaluate the effect of the laser cleaning operated by applying different consecutive laser scanning and the suitability of these lasers as quick tools. The aim of this comparative study was twofold. The first goal was to find the most satisfactory level of extraction by comparing the results obtained by the different laser sources (IR wavelengths). The other aim was to investigate the by-effects induced by both lasers on each granite-forming mineral. Evaluations were made using stereomicroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy with energy-dispersive x-ray spectroscopy. The results were interpreted in terms of SAB extraction and damage induced on the granite. The results showed that the Nd:YAG laser achieved the most successful level of cleaning, because it extracted the most SAB, while causing the least amount of damage to the surfaces. Regardless of the fluence applied, the Er:YAG laser did not completely extract the SAB in only one scan; in addition, a more intense melting of biotite grains was found, producing amorphous fusion crusts and losing the distinction of the cleavage planes.

A Study on the Suitability of Mechanical Soft-Abrasive Blasting Methods to Extract Graffiti Paints on Ornamental Stones

Mechanical methods to extract undesired graffiti paints on ornamental stones are efficient cleaning methods from an economical point of view. However, effort on the optimization of mechanical cleaning procedures to avoid any damage to the substrate is required for large areas. In this study, two ornamental stones with different composition and texture, and which are commonly used in Spain and Portugal were selected: Granite Vilachán and Limestone Lioz. Moreover, the most common surface finishes were selected-disc-cutting and bush-hammering to simulate the stones found in buildings. Two graffiti spray paints were selected: Blue Ultramarine and Silver Chrome. As cleaning methods, three soft-abrasive blasting procedures: Hydrogommage (mixture of air–water–micro grained silicon abrasive), IBIX (mixture of air–micro grained silicon abrasive), and dry-ice procedure (carbon dioxide ice pellets), were tested at pressure below 0.4 MPa. The methodology for evaluating the effectiveness and harmfulness of each cleaning method was based on stereomicroscopy, scanning electron microscopy, color spectrophotometry, and confocal microscopy. As result, IBIX achieved the highest level of graffiti paint extraction although this method increased the surface roughness. Conversely, cleaning based on dry-ice projection did not achieve a satisfactory extraction of the graffiti, mainly of the blue paint. Dry-ice blasting can induce acid environments and IBIX causes dust emission during the projection. Hydrogommage was the most efficient cleaning method amongst the tested procedures, because it induced the lowest roughness change and although the graffiti extraction was not complete, it achieved the highest removal level. Therefore, the most satisfactory cleaning method was that achieving a satisfactory extraction level, minimal modifications of the surface roughness, an economic suitability, an environmental integration, and lower human health risks.

Exposure to artificial daylight or UV irradiation (A, B or C) prior to chemical cleaning: an effective combination for removing phototrophs from granite

This study evaluated whether exposing samples of granite colonized by a natural biofilm to artificial daylight or UV-A/B/C irradiation for 48 h enhanced removal of the biofilm with a chemical product previously approved for conservation of monuments by the European Biocide Directive. Rodas granite, which is commonly found in stone-built heritage monuments in Galicia (NW Spain), was naturally colonized by a sub-aerial biofilm. The efficacy of the cleaning method was evaluated relative to uncolonized surfaces and colonized control samples without previous irradiation, treated by dry-brushing or with benzalkonium chloride. The effect of UV irradiation in the combined treatment was evident, as comparable cleaning levels were not reached in the controls. Although the biofilm was not totally removed by any of the treatments, UV-B irradiation followed by benzalkonium chloride was potentially useful for cleaning stone, with results comparable to those achieved by UV-C irradiation, which is known to have germicidal effects.

The influence of the SO2 ageing on the graffiti cleaning effectiveness with chemical procedures on a granite substrate

Graffiti are one of the most severe threats to Stone Cultural Heritage and are most of the times removed after long periods of environmental exposure. This research intends to evaluate the influence of the ageing of the graffitis on the effectiveness of their cleaning. So, comparative studies on unaged and on artificially SO₂ aged samples were conducted. Four graffiti spray colours were applied on a granite stone and cleaned with two chemical commercial cleaners: a solution of KOH and a solution of n-butyl acetate, xylene and alcohol isobutyl. The spray paints (unaged and aged) and cleaning effectiveness were characterized by stereomicroscopy, colour spectrophotometry, adhesion tests, SEM, μEDXRF, XRD and FTIR. The cleaning effectiveness was also evaluated through surface roughness and static contact angle measurements. The alkyd graffiti paints presented greatest resistance under SO₂ rich environments than the polyethylene paint. The aged polyethylene paint showed chemical modifications that resulted in graffiti losses and neo formed mineralogical phases in the surface of the paint. After ageing, the paints became more difficult to clean, showed higher global colour changes and higher residue percentages. No significant roughness variations were detected after chemical cleaning. After the cleaning procedures aged surfaces became more water repellent comparatively to unaged and reference samples. The best cleaning effectiveness was mainly achieved with the potassium hydroxide solution.

Assessing the effects of UVA photocatalysis on soot-coated TiO2-containing mortars

The deposition of soot on building surfaces darkens their colour and leads to undesirable black crusts, which are one of the most serious problems on the conservation of built cultural heritage. As a preventive strategy, self-cleaning systems based on the use of titanium dioxide (TiO₂) coatings have been employed on building materials for degrading organic compounds deposited on building surfaces, improving their durability and performance. In this study, the self-cleaning effect of TiO₂-containing mortars coated with diesel soot has been appraised under laboratory conditions. The mortar samples were manufactured using lime putty and two different doses of TiO₂ (2.5% and 5%). The lime mortars were then coated with diesel engine soot and irradiated with ultraviolet A (UVA) illumination for 30days. The photocatalytic efficiency was evaluated by visual inspection, field emission scanning electron microscopy (FESEM) and colour spectrophotometry. Changes in the chemical composition of the soot particles (including persistent organic pollutants) were assessed by analytical pyrolysis (Py-GC/MS) and solid state ¹³C NMR spectroscopy. The FESEM and colour spectrophotometry revealed that the soot-coated TiO₂-containing mortars promoted a self-cleaning effect after UVA irradiation. The combination of analytical pyrolysis and ¹³C solid state NMR showed that the UVA irradiation caused the cracking of polycyclic aromatic structures and n-alkyl compounds of the diesel soot and its transformation into methyl polymers. Our findings also revealed that the inclusion of TiO₂ in the lime mortar formulations catalysed these transformations promoting the self-cleaning of the soot-stained mortars. The combined action of TiO₂ and UVA irradiation is a promising proxy to clean lime mortars affected by soot deposition.

Synthesis, Photocatalytic, and Antifungal Properties of MgO, ZnO and Zn/Mg Oxide Nanoparticles for the Protection of Calcareous Stone Heritage

More recently, the biological colonization of stone heritage and consequently its biodeterioration has become the focus of numerous studies. Among all microorganisms, fungi are considered to be one of the most important colonizers and biodegraders on stone materials. This is why the development of new antifungal materials requires immediate action. ZnMgO nanoparticles (NPs) have several exciting applications in different areas, highlighting as an efficient antimicrobial agent for medical application. In this research, the application of Zn-doped MgO (Mg_1-xZn_xO, x = 0.096) NPs obtained by sol-gel method as antifungal coatings on dolomitic and calcitic stones has been explored as a means to develop effective protective coatings for stone heritage. Moreover, the photocatalytic and antifungal activity of Mg_1-xZn_xO NPs were comparatively studied with single ZnO and MgO NPs. Thus, compared to the MgO and ZnO nanomaterials, the Mg_1-xZn_xO NPs exhibited an enhanced photocatalytic activity. After UV irradiation for 60 min, 87% methylene blue was degraded over Zn-doped MgO NPs, whereas only 58% and 38% of MB was degraded over ZnO and MgO NPs, respectively. These nanoparticles also displayed a better antifungal activity than that of single pure MgO or ZnO NPs, inhibiting the growth of fungi Aspergillus niger, Penicillium oxalicum, Paraconiothyrium sp., and Pestalotiopsis maculans, which are especially active in the bioweathering of stone. The improved photocatalytic and antifungal properties detected in the Mg_1-xZn_xO NPs was attributed to the formation of crystal defects by the incorporation of Zn into MgO. The application of the MgO- and Zn-doped MgO NPs as protective coatings on calcareous stones showed important antifungal properties, inhibiting successfully the epilithic and endolithic colonization of A. niger and P. oxalicum in both lithotypes, and indicating a greater antifungal effectiveness on Zn-doped MgO NPs. The use of Zn-doped MgO NPs may thus represent a highly efficient antifungal protection for calcareous stone heritage.

Microscopic characterisation of black crusts on different substrates

Black crusts on different substrates (carbonate and silicate stones, cement-based mortar and carbonate detrital deposits) in urban environments were characterized microscopically by analysing their morphologies and compositions. The objective of this article is to study the interaction between the substrate and the crust and the influence of the environmental conditions on the crust development. On the one hand, the internal structure and morphology of each sample were evaluated with stereo and scanning electron microscopies. On the other hand, the black crust composition was accessed by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy in backscattered electron mode combined with energy dispersive x-ray spectroscopy. The results of these analytical techniques provided interesting information about the composition and the mineralogical phases present in the collected black crusts. In most of the cases, gypsum was detected as the main component exhibiting different habits. Calcite was also detected in all the evaluated gypsum black crusts; its presence was attributed to different origins. The substrate-crust interaction was also evaluated, contributing to distinguish different crust development processes in relation to the substrate. In carbonate substrates (limestones, mortar and carbonate detrital deposits), it was detected a continuous diffuse boundary related to the replacement of Ca-carbonate by Ca-sulphate, while this boundary was significantly more defined for the granitic stone. This study shows that the substrate, the presence of different construction materials, (e.g. mortars), the motor exhaust particulate substances and the concentrations of atmospheric pollutants, the marine influence as well as biological or other anthropogenic compounds are decisive factors in the development of the black crust. Some ideas about the establishment of conservation strategies are also shown.

Nanofluids and chemical highly retentive hydrogels for controlled and selective removal of overpaintings and undesired graffiti from street art

One of the main problems connected to the conservation of street art is the selective removal of overlying undesired graffiti, i.e., drawings and tags. Unfortunately, selective and controlled removal of graffiti and overpaintings from street art is almost unachievable using traditional methodologies. Recently, the use of nanofluids confined in highly retentive pHEMA/PVP semi-interpenetrated polymer networks was proposed. Here, we report on the selective removal of acrylic overpaintings from a layer of acrylic paint on mortar mockups in laboratory tests. The results of the cleaning tests were characterized by visual and photographic observation, optical microscopy, and FT-IR microreflectance investigation. It was shown that this methodology represents a major advancement with respect to the use of nonconfined neat solvents.