Hydroxide nanoparticles for cultural heritage: Consolidation and protection of wall paintings and carbonate materials

New horizons on advanced nanoscale materials for Cultural Heritage conservation

Nanomaterials have permeated numerous scientific and technological fields, and have gained growing importance over the past decades also in the preservation of Cultural Heritage. After a critical overview of the main nanomaterials adopted in art preservation, we provide new insights into some highly relevant gels, which constitute valuable tools to selectively remove dirt or other unwanted layers from the surface of works of art. In particular, the recent "twin-chain" gels, obtained by phase separation of two different PVAs and freeze-thawing, were considered as the most performing gel systems for the cleaning of Cultural Heritage. Three factors are crucial in determining the final gel properties, i.e., pore size, pore connectivity, and surface roughness, which belong to the micro/nanodomain. The pore size is affected by the molecular weight of the phase-separating PVA polymer, while pore connectivity and tortuosity likely depend on interconnections formed during gelation. Tortuosity greatly impacts on cleaning capability, as the removal of matter at the gel-target interface increases with the uploaded fluid's residence time at the interface (higher tortuosity produces longer residence). The gels' surface roughness, adaptability and stickiness can also be controlled by modulating the porogen amount or adding different polymers to PVA. Finally, PVA can be partially replaced with different biopolymers yielding gels with enhanced sustainability and effective cleaning capability, where the selection of the biopolymer affects the gel porosity and effectiveness. These results shed new light on the effect of micro/nanoscale features on the cleaning performances of "twin-chain" and composite gels, opening new horizons for advanced and "green"/sustainable gel materials that can impact on fields even beyond art preservation, like drug-delivery, detergency, food industry, cosmetics and tissue engineering.

New sustainable polymers and oligomers for Cultural Heritage conservation

The development of "green" chemistry materials with enhanced properties is a central topic in numerous applicative fields, including the design of polymeric systems for the conservation of works of art. Traditional approaches in art restoration comprise polymer thickeners and viscous dispersions to partially control solvents in the removal of soil or aged varnishes/coatings from artifacts. Alternatively, polymeric gel networks can be specifically designed to grant full control of the cleaning action, yielding safe, time- and cost-effective restorations. The selection of polymers and oligomers in gel design is crucial to tune solvent upload, retention, and controlled release over the sensitive artistic surfaces. Starting from an overview of traditional polymer formulations and state-of-the-art gel systems for cleaning works of art, we provide here the design of a new class of gels, focusing on the selection of oligomers to achieve gels with tailored hydrophilicity/hydrophobicity. We evaluated the oligomers Hydrophilic-Lipophilic Balance (HLB) by developing, for the first time, a novel methodology combining SEC and DOSY NMR analysis, which was tested on a library of "green" oligoesters synthesized by polycondensation and poorly explored in the literature. Oligomers with moderate polydispersity were chosen to validate the new protocol as a robust tool for designing polymeric gels even on industrial scale. The methodology is more time-effective than traditional methods, and gives additional insights on the oligomers physico-chemical nature, evaluating their compatibility with different solvents. Then, we used the selected oligoesters with castor oil to obtain a new class of organogels able to upload solvents with varying polarity, which effectively removed different types of unwanted layers typically found in painting restoration. These results validate the oligomers screening approach and the new class of gels as promising chemical processes/materials in art preservation. The methodology can potentially allow evaluation of HLB also for small molecules (e.g., surfactants), opening for the formulation of polymers solutions/gels beyond Cultural Heritage conservation, as in pharmaceutics, cosmetics, food industry, tissue engineering, agriculture, and others.

From Nanoparticles to Gels: A Breakthrough in Art Conservation Science

Cultural heritage is a crucial resource to increase our society's resilience. However, degradation processes, enhanced by environmental and anthropic risks, inevitably affect works of art, hindering their accessibility and socioeconomic value. In response, interfacial and colloidal chemistry has proposed valuable solutions over the past decades, overcoming the limitations of traditional restoration materials and granting cost- and time-effective remedial conservation of the endangered artifacts. Ranging from inorganic nanoparticles to hybrid composites and soft condensed matter (gels, microemulsions), a wide palette of colloidal systems has been made available to conservators worldwide, targeting the consolidation, cleaning, and protection of works of art. The effectiveness and versatility of the proposed solutions allow the safe and effective treatment of masterpieces belonging to different cultural and artistic productions, spanning from classic ages to the Renaissance and modern/contemporary art. Despite these advancements, the formulation of materials for the preservation of cultural heritage is still an open, exciting field, where recent requirements include coping with the imperatives of the Green Deal to foster the production of sustainable, low-toxicity, and environmentally friendly systems. This review gives a critical overview starting from pioneering works up to the latest advancements in colloidal systems for art conservation, a challenging topic where effective solutions can be transversal to multiple sectors even beyond cultural heritage preservation, from the pharmaceutical and food industry, to cosmetics, tissue engineering, and detergency.

Application of Inorganic Nanomaterials in Cultural Heritage Conservation, Risk of Toxicity, and Preventive Measures

Nanotechnology has allowed for significant progress in architectural, artistic, archaeological, or museum heritage conservation for repairing and preventing damages produced by deterioration agents (weathering, contaminants, or biological actions). This review analyzes the current treatments using nanomaterials, including consolidants, biocides, hydrophobic protectives, mechanical resistance improvers, flame-retardants, and multifunctional nanocomposites. Unfortunately, nanomaterials can affect human and animal health, altering the environment. Right now, it is a priority to stop to analyze its advantages and disadvantages. Therefore, the aims are to raise awareness about the nanotoxicity risks during handling and the subsequent environmental exposure to all those directly or indirectly involved in conservation processes. It reports the human-body interaction mechanisms and provides guidelines for preventing or controlling its toxicity, mentioning the current toxicity research of main compounds and emphasizing the need to provide more information about morphological, structural, and specific features that ultimately contribute to understanding their toxicity. It provides information about the current documents of international organizations (European Commission, NIOSH, OECD, Countries Normative) about worker protection, isolation, laboratory ventilation control, and debris management. Furthermore, it reports the qualitative risk assessment methods, management strategies, dose control, and focus/receptor relationship, besides the latest trends of using nanomaterials in masks and gas emissions control devices, discussing their risk of toxicity.

The Boom in Nanomaterials for Built Heritage Conservation: Why Does Size Matter?

There is no doubt that nanotechnology and nanoscience open new doors to new applications and products that can potentially revolutionize the practice field and how we conserve built heritage materials. However, we are living at the beginning of this era and the potential benefits of nanotechnology to specific conservation practice needs are not always fully understood. This opinion/review paper aims to present reflections and answer a question that we are often asked when working directly with stone field conservators: why should we use a nanomaterial instead of a conventional product? Why does size matter? To answer this question, we revise the basic concepts of nanoscience with implications for the built heritage conservation field.

Current progress on murals: distribution, conservation and utilization

As non-renewable cultural heritages, murals have important implications in historical customs, religions, and philosophy as well as their aesthetic values. Recently, many murals are threatened by natural factors and human activities. During the past decades, there are increasing interest in the investigation of murals. Here we review the current status of murals and provide an up-to-date summary of achievements related to murals. The murals that draw the most attention are distributed in Mexico, Ireland, China, and Spain. The aesthetics, history, cultural, educational, and economic values of murals are comprehensively analyzed. The main research technologies used to detect the chemical compositions and physical structures of murals are also summarized. The restoration of murals includes several procedures such as stabilization, repair, surface cleaning, and pigment reconversion. Emerging technologies such as computer science benefit the research and conservation of murals. We also propose that tourism management and climate change should be incorporated into the conservation of murals in the future.

Advances in Nanolime and Other Nanomaterials for Built Heritage Conservation

This Special Issue examines the synthesis, characterization, and manufacturing of nanoparticles and their potential advantages and applications for the conservation of built cultural heritage materials [...].

Synthesis of Graphene Quantum Dots Enhanced Nano Ca(OH)2 from Ammoniated CaCl2

Ca(OH)₂ nanoparticles are effective materials for cultural heritage restoration, hazardous substance absorption and photocatalyst. However, many methods are complex, and the particle sizes are usually above 80-100 nm, involving mediocre efficacy for application in the stone restoration field. In this work, Nano Ca(OH)₂ with diameters less than 70 nm and composited with Graphene Quantum Dots (GQDs) were successfully synthesized in aqueous media. The morphology and structure of the nanoparticles were investigated with TEM, HRTEM, XRD, Raman and FTIR. The particle size distribution and relative kinetic stability of the Ca(OH)₂ in ethanol were performed using a laser particle size analyzer and spectrophotometer. Firstprinciple calculations based on the spin-polarized density functional theory (DFT) were carried out to study the reaction process and combination model. The nanoparticles, as prepared, are composed of primary hexagonal crystals and high ammoniated precursors, which have a positive effect on reducing the grain size, and interacted with the GQDs hybrid process. According to the First-principle calculations results, the energy variation of the whole reaction process and the bonding mode between Ca(OH)₂ and GQDs can be understood better.

Flame Retardant Nano-Structured Fillers from Huntite/Hydromagnesite Minerals

In the current study, we propose a simple hydrothermal pathway to synthesize nano-structured Mg(OH)₂ after application of thermal decomposition followed by hydration of commercial minerals based on hydromagnesite and huntite. The synthesis of nano-materials is performed without the use of any catalyst. The effect of decomposition temperature on the hydrothermal synthesis of Mg(OH)₂ is extensively studied. It is shown that the morphology of resulting structures consists typically of particles ~200 nm in diameter and ~10 nm in thickness. Study of the structure at the molecular level designates the composition and supports the nano-sized characteristics of the produced materials. The associated thermal properties combined with the corresponding optical properties suggest that the material may be used as a flame retardant filler with enhanced transparency. In this concept, the flame retardancy of composite coatings containing the produced nano-sized Mg(OH)₂ was examined in terms of limiting oxygen index (LOI), i.e., the minimum concentration of oxygen that just supports flaming combustion.

Visualization and mapping of literature on the scientific analysis of wall paintings: a bibliometric analysis from 2011 to 2021

As non-renewable cultural heritage, wall paintings play an important role in society. To reveal the trends in the scientific analysis of mural paintings, 845 relevant research articles published from 2011 to 2021 were collected from the Web of Science database and analyzed. The VOSviewer software was adopted to map the network data of scientific publications, so that relationships among authors, countries, institutions can be displayed, and the co-occurrence of keywords and co-citation can be analyzed. The results revealed close and strong interconnections between the top authors, suggesting a considerable strong research link in this field. The cooperation between research institutions was relatively close. The most productive country of relevant publications was Italy. The leading journals for the scientific analysis of wall paintings were Journal of Raman Spectroscopy and Journal of Cultural Heritage. At present, the hotspots of scientific analysis and research on wall painting are revealing the composition, distribution, origin, and deterioration mechanism of pigments, alongside with evaluating the effects and mechanism of conservation materials and techniques. On the one hand, a possible development direction in this field is introducing more cutting-edge analysis and data processing methods. On the other hand, scientific analysis is increasingly adopted to guide the research and development of mural conservation materials.

Polyacrylic Acid-Functionalized Graphene@Ca(OH)2 Nanocomposites for Mural Protection

Murals are one of the precious legacies of our ancestors; however, they face severe damage along with archeological discoveries, which need urgent repair. Nowadays, nanotechnology provides new concepts and materials for the consolidation and protection of murals. In this work, an innovative method for the protection of murals was proposed with graphene-based nanomaterials through strategically synthesizing a polyacrylic acid-functionalized graphene/nano-Ca(OH)₂ material (PAAG@Ca(OH)₂) by a facile and economic aqueous method. As a result, the nanocomposite PAAG@Ca(OH)₂ was demonstrated with high porosity, strong adsorption, appropriate hydrophilicity, and better permeability compared to the commercial AC33 sample according to the simulated tests. As expected, the nanocomposite PAAG@Ca(OH)₂ displayed a promising application for the reinforcement of murals, which opens up a new avenue for the protection of murals.

A novel immersive calcium carbonate coating for conservation of limestone relics with gypsum crust

A novel calcium carbonate coating was prepared to conserve limestone relics with gypsum crust for the first time. In the preparing process, the solution of calcium hydroxy glycolate/urea was introduced...

Colloidal magnesium hydroxide Nanoflake: One-Step Surfactant-Assisted preparation and Paper-Based relics protection with Long-Term Anti-Acidification and Flame-Retardancy

Enhancing the interfacial dispersion and suspension stability is crucial for magnesium hydroxide (Mg(OH)₂) nanomaterials in the long-term deacidification of paper-based cultural relics. However, because of the low specific surface area and the poor solvent compatibility of as-prepared large-sized Mg(OH)₂, it often tends to agglomerate and settle down during the usage and storage, that is harmful for paper protection due to its unevenly deacidification and nonuniformly distribution on paper cellulose. Herein, we propose a feasible preparation of colloidal Mg(OH)₂ ultrathin nanoflakes with high dispersion stability via a simple one-step surfactant-assisted strategy. The surfactant acts as both a structure-direct agent to confine the growth of Mg(OH)₂ with rich active sites and a surface modifier to enhance its solvent adaptability and dispersion stability, avoiding the common fussy procedure with additional steric stabilizer. Owing to the evenly interaction with free acid species therein and the uniformly distribution on the paper fiber as alkaline reserve, the as-obtained Mg(OH)₂ presents the superior paper protection performance characterized by its safer pH of 7.29 for the original aged paper (pH = 5.03) and the excellent long-term anti-acidification effect with competitive pH of 5.47 after accelerated-aging at 105 °C for 5 months. Furthermore, Mg(OH)₂ nanoflakes with surfactant-modified structure also endue them as an improved flame retardant for multifunctional paper protection. The protection with Mg(OH)₂ has little effect on the paper surface properties and cellulose crystallinity, in line with the principle of least intervention. This work will put forward a feasible way toward colloidal Mg(OH)₂ nanoflakes with excellent paper protection performance, shedding light on the development of emerging protection materials for paper-based cultural relics.

The kinetic of calcium silicate hydrate formation from silica and calcium hydroxide nanoparticles

HYPOTHESIS

The mechanism of calcium silicate hydrate (CSH) formation, a relevant component of cement, the largest used material by mankind, is well documented. However, the effects of nano-sized materials on the CSH formation have not yet been evaluated. To this aim, a kinetic study on CSH formation via the "pozzolanic reaction" of nanosilica and calcium hydroxide nanoparticles, and in the presence of hydroxypropyl cellulose (HPC) as hydration regulator, is reported in this paper.

EXPERIMENTS

The reagents were mixed with water and cured at 10, 20, 30 and 40 °C. The reaction kinetics was studied with differential scanning calorimetry (DSC). A Boundary Nucleation and Growth model (BNGM) combined with a diffusion-limited model was used to analyze the data, yielding induction times, reaction rates, activation energies, nucleation and linear growth rates, and the related diffusion coefficients.

FINDINGS

The rate constants k_B and k_G, which are, respectively, the rate at which the nucleated boundary area transforms, and the rate at which the non-nucleated grains between the boundaries transform, increase with temperature. Their different temperature dependence accounts for the prevailing effect of nucleation over nuclei growth at progressively lower temperatures. The nucleation rate, I_B, is strongly enhanced when using nanomaterials, while the linear growth rate, G, is limited by the tightly packed structure of the transforming matrix. HPC influences the kinetics between 10 and 30 °C; at 40 °C the temperature effect becomes predominant. HPC delays induction and acceleration periods, increases E_a(k_B), and enhances the reaction efficiency during the diffusion regime, by retaining and delivering water over the matrix, thus allowing a higher water consumption in the hydration reaction of CSH.

How Science Can Contribute to the Remedial Conservation of Cultural Heritage

Colloid science is contributing solutions to counteract the degradation of artifacts, favoring their transfer to future generations. Advanced materials such as nanoparticles, coatings, gels and microemulsions have been assessed in conservation, spanning from archeological sites to modern and contemporary art. We give an overview of the fundamental milestones and latest innovations in conservation science, targeting solutions and tools for remedial conservation based on green nanomaterials and hybrid systems. Future perspectives and outstanding challenges in this exciting field are then outlined.

Advanced Materials in Cultural Heritage Conservation

Cultural Heritage is a crucial socioeconomic resource; yet, recurring degradation processes endanger its preservation. Serendipitous approaches in restoration practice need to be replaced by systematically addressing conservation issues through the development of advanced materials for the preservation of the artifacts. In the last few decades, materials and colloid science have provided valid solutions to counteract degradation, and we report here the main highlights in the formulation and application of materials and methodologies for the cleaning, protection and consolidation of works of art. Several types of artifacts are addressed, from murals to canvas paintings, metal objects, and paper artworks, comprising both classic and modern/contemporary art. Systems, such as nanoparticles, gels, nanostructured cleaning fluids, composites, and other functional materials, are reviewed. Future perspectives are also commented, outlining open issues and trends in this challenging and exciting field.

Comparison of Latest and Innovative Silica-Based Consolidants for Volcanic Stones

This research explores the new perspectives in conservation and protection of two macroporous tuff stones, widely employed in the architectural heritage of Campania region, characterized by highly heterogeneous rock fabric and texture and a variable mineralogical composition that represent crucial factors responsible for their weak durability. The consolidation treatments were performed with a recently and widely used suspension of nano-silica crystals in water and with a lithium silicate solution that has received up to now scarce attention as a consolidant agent. Physical investigations (open porosity, Hg porosimetry, water absorption), morphological observations (SEM analyses) and visual appearance test (colorimetric measurements), along with assessments of performance indicators such as ultrasonic pulse velocity, surface cohesion test (peeling test) and durability test (salt crystallization), were carried out to investigate the consolidation effectiveness. Overall, lithium silicate consolidant showed a better behavior in terms of superficial cohesion, a most successful strengthening action and a considerable enhancement of salt resistance.

Ammonium monoethyloxalate (AmEtOx): a new agent for the conservation of carbonate stone substrates

Ammonium monoethyloxalate (AmEtOx) is proposed as a consolidanting agent for carbonate stones such as biomicritic limestone and marble, resulting in the formation of a microcrystalline passivating phase of calcium oxalate (whewellite and weddellite).

Bionanocomposite Films Containing Halloysite Nanotubes and Natural Antioxidants with Enhanced Performance and Durability as Promising Materials for Cultural Heritage Protection

In the last decade, the interest toward the formulation of polymer films for cultural heritage protection continuously grew, and these films must be imperatively transparent, removable, and should not react/interact with surface of the artworks. In this research, bionanocomposite films, based on chitosan (Ch) and pectin (P) and containing naturally occurring fillers and antioxidants, were formulated by solvent casting methods and were accurately characterized. The natural halloysite nanotubes (HNT) have a two-fold role, specifically, physical compatibilizer and antioxidant carrier. Therefore, the theoretical solubility between Ch and P was estimated considering Hoy’s method for solubility of polymers, while the optimum ratio between biopolymer constituents was assessed by ζ-potential measurements. The transparency, wettability, and mechanical behavior of Ch:P films, also in presence of HNT without and with antioxidants, were investigated. The beneficial effects of natural antioxidants, such as vanillic acid (VA) and quercetin (Q), on Ch:P/HNT durability were found.

Facile Synthesis of Calcium Hydroxide Nanoparticles onto TEMPO-Oxidized Cellulose Nanofibers for Heritage Conservation

Calcium hydroxide is used in diverse applications including heritage conservation where supplying it in the form of nanoparticles allows easy carbonation with atmospheric air contacts. The effects of cellulose nanofibers on the precipitation of calcium hydroxide nanoparticles were investigated by varying the reaction time, concentration, and carboxylation content of cellulose nanofibers. Cellulose nanofibers were very effective in producing calcium hydroxide nanoparticles with less than 50 nm sizes out of calcium nitrate-sodium hydroxide precipitation reactions. The formation of smaller-size calcium hydroxide nanoparticles is believed to be the result of heterogeneous nucleation and growth of calcium hydroxide particles on cellulose nanofibers. The liquid-phase nucleated and grown calcium hydroxide nanoparticles were also deposited onto cellulose nanofibers. The resulting calcium hydroxide nanoparticles were carbonized and generated calcite under atmospheric carbon dioxide in an efficient way.

Effectiveness and Compatibility of a Novel Sustainable Method for Stone Consolidation Based on Di-Ammonium Phosphate and Calcium-Based Nanomaterials

External surfaces of stones used in historic buildings often carry high artistic value and need to be preserved from the damages of time, especially from the detrimental effects of the weathering. This study aimed to test the effectiveness and compatibility of some new environmentally-friendly materials for stone consolidation, as the use thereof has been so far poorly investigated. The treatments were based on combinations of an aqueous solution of di-ammonium phosphate (DAP) and two calcium-based nanomaterials, namely a commercial nanosuspension of Ca(OH)₂ and a novel nanosuspension of calcite. The treatments were applied to samples of two porous stones: a limestone and a sandstone. The effectiveness of the treatments was assessed using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, ultrasound pulse velocity test, colour measurements, and capillary water absorption test. The results suggest that the combined use of DAP and Ca-based nanosuspensions can be advantageous over other commonly used consolidants in terms of retreatability and physical-chemical compatibility with the stone. Some limitations are also highlighted, such as the uneven distribution and low penetration of the consolidants.

Investigation of the Optical, Physical, and Chemical Interactions between Diammonium Hydrogen Phosphate (DAP) and Pigments

This research investigates and evaluates the optical, physical, and chemical interactions between a diammonium hydrogen phosphate (DAP) solution and seven pigments commonly encountered in archaeological and historic fresco and secco wall paintings and polychrome monuments. The pigments include cinnabar, French ochre, chalk, lapis lazuli, raw sienna, burnt umber, and red lead. The raw pigments were analyzed before and after the interaction with the DAP solution, and the reaction products resulting from the contact of the pigments with the DAP solution were evaluated to obtain a comprehensive understanding of the effects of diammonium phosphate on the color, morphology, and chemical composition of the pigments. The results indicated no significant changes of the color or of the chemistry of cinnabar, French ochre, and lapis lazuli. Carbonate-containing primary and secondary (found as impurities in earth pigments) pigments, such as chalk and calcium carbonate, were transformed into calcium phosphate, though without a significant change in color. Phase and strong color changes occurred only for the red lead pigment, associated with the transformation of red lead into hydroxypyromorphite. These data established the parameters and identified the risks of the direct application of DAP solutions on pigments. Further research will be undertaken to assess the potential use of DAP as a consolidant of wall paintings and other polychrome surfaces through testing on wall painting/polychromy mockups and on-site archaeological/historic painted surfaces.

Synthesis of Nano-Calcium Oxide from Waste Eggshell by Sol-Gel Method

The sol-gel technique has many advantages over the other mechanism for synthesizing metal oxide nanoparticles such as being simple, cheap and having low temperature and pressure. Utilization of waste materials as a precursor for synthesis makes the whole process cheaper, green and sustainable. Calcium Oxide nanoparticles have been synthesized from eggshell through the sol-gel method. Raw eggshell was dissolved by HCl to form CaCl2 solution, adding NaOH to the solution dropwise to agitate Ca (OH)2 gel and finally drying the gel at 900 °C for 1 h. The synthesized nanoparticle was characterized by scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), X-Ray fluorescence (XRF) and X-ray diffraction (XRD). The FTIR and XRD results have clearly depicted the synthesis of calcium oxide from eggshell, which is mainly composed of calcium carbonate. The FE-SEM images of calcium oxide nanoparticles showed that the particles were almost spherical in morphology. The particle size of the nanoparticles was in the range 50 nm–198 nm. Therefore, waste eggshell can be considered as a promising resource of calcium for application of versatile fields.

The carbonation kinetics of calcium hydroxide nanoparticles: A Boundary Nucleation and Growth description

HYPOTHESIS

The reaction of Ca(OH)₂ with CO₂ to form CaCO₃ (carbonation process) is of high interest for construction materials, environmental applications and art preservation. Here, the "Boundary Nucleation and Growth" model (BNGM) was adopted for the first time to consider the effect of the surface area of Ca(OH)₂ nanoparticles on the carbonation kinetics.

EXPERIMENTS

The carbonation of commercial and laboratory-prepared particles' dispersions was monitored by Fourier Transform Infrared Spectroscopy, and the BNGM was used to analyze the data. The contributions of nucleation and growth of CaCO₃ were evaluated separately.

FINDINGS

During carbonation the boundary regions of the Ca(OH)₂ particles are densely populated with CaCO₃ nuclei, and transform early with subsequent thickening of slab-like regions centered on the original boundaries. A BNGM limiting case equation was thus used to fit the kinetics, where the transformation rate decreases exponentially with time. The carbonation rate constants, activation energies, and linear growth rate were calculated. Particles with larger size and lower surface area show a decrease of the rate at which the non-nucleated grains between the boundaries transform, and an increase of the ending time of Ca(OH)₂ transformation. The effect of temperature on the carbonation kinetics and on the CaCO₃ polymorphs formation was evaluated.

Hybrid nano-composites for the consolidation of earthen masonry

HYPOTHESIS

Earth is one of the oldest silicate-based materials in stone heritage, still largely used in architecture worldwide. Earthen materials are highly susceptible to wind and water erosion, leading to loss of cohesion and crumbling. Conventional consolidants (alkoxysilanes, synthetic or natural polymers) lack physico-chemical compatibility or effectiveness, and can promote degradation. We propose for the first time nano-composites for the surface consolidation of adobe, i.e. unbaked earth bricks often containing organic fibers and lime.

EXPERIMENTS

We investigated, mimicking the setting of portland cement, the formation of calcium silicate hydrate (CSH) within adobe porosities, owing to the pozzolanic reaction between nanoparticles of silica and calcium hydroxide, to consolidate a powdery substrate. Different formulations were characterized by Fourier Transform Infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM), dynamic light scattering (DLS), and turbidimetry (UV-Vis spectroscopy).

FINDINGS

A ternary composite made of SiO₂ nanoparticles, Ca(OH)₂ nanoparticles, and hydroxypropyl cellulose, dispersed in a (4:1) ethanol:water blend, was formulated. Each component is compatible with adobe, and plays a role in its consolidation. The treatment of adobe samples with the composite leads to the in situ formation of CSH, providing resistance to peeling, abrasion, and wet-dry cycles, with no aesthetic alteration. This opens new perpectives in the preservation of one of the most widely used construction materials.

Inorganic Nanomaterials for Restoration of Cultural Heritage: Synthesis Approaches towards Nanoconsolidants for Stone and Wall Paintings

The synthesis of inorganic nanostructured materials for the consolidation of stone and wall paintings is reviewed. To begin, a description of the methods most commonly used to prepare nanoconsolidants is provided, particularly in the frame of colloid chemistry. Some concepts of the carbonation mechanism as well as the transport properties of some of these materials are addressed. An overview of the synthesis methods together with some of the application particularities of the distinct consolidants are presented thereafter. Furthermore, the requisites for efficient consolidants and some drawbacks of the nanoconsolidants are discussed.

Smart Portable Devices Suitable for Cultural Heritage: A Review

This article reviews recent portable sensor technologies to apply in the Cultural Heritage (CH) fields. The review has been prepared in the form of a retrospective description of the sensor's history and technological evolution, having: new nanomaterials for transducers, miniaturized, portable and integrated sensors, the wireless transmission of the analytical signals, ICT_Information Communication Technology and IoT_Internet of Things to apply to the cultural heritage field. In addition, a new trend of movable tattoo sensors devices is discussed, referred to in situ analysis, which is especially important when scientists are in the presence of un-movable and un-tangible Cultural Heritage and Art Work objects. The new proposed portable contact sensors (directly applied to art work objects and surfaces) are non-invasive and non-destructive to the different materials and surfaces of which cultural heritage is composed.

Preparation of Water Suspensions of Nanocalcite for Cultural Heritage Applications

The consolidation of degraded carbonate stone used in ancient monuments is an important topic for European cultural heritage conservation. The products most frequently used as consolidants are based on tetraalkoxy- or alkylalkoxy-silanes (in particular tetraethyl-orthosilicate, TEOS), resulting in the formation of relatively stable amorphous silica or alkylated (hydrophobic) silica inside the stone pores. However, silica is not chemically compatible with carbonate stones; in this respect, nanocalcite may be a suitable alternative. The present work concerns the preparation of water suspensions of calcite nanoparticles (CCNPs) by controlled carbonation of slaked lime using a pilot-scale reactor. A simplified design of experiment was adopted for product optimization. Calcite nanoparticles of narrow size distribution averaging about 30 nm were successfully obtained, the concentration of the interfacial agent and the size of CaO being the most critical parameters. Primary nanoparticle aggregation causing flocculation could be substantially prevented by the addition of polymeric dispersants. Copolymer-based dispersants were produced in situ by controlled heterophase polymerisation mediated by an amphiphilic macro-RAFT (reversible addition-fragmentation transfer) agent. The stabilized CCNP aqueous dispersions were then applied on carbonate and silicate substrates; Scanning Electron Microscopy (SEM)analysis of cross-sections allowed the evaluation of pore penetration, interfacial binding, and bridging (gap-filling) properties of these novel consolidants.

Application Protocol for the Consolidation of Calcareous Substrates by the Use of Nanolimes: From Laboratory Research to Practice

Calcareous materials such as limestone and lime-based mortars, widely used in the Built Heritage, are often subjected to degradation processes that can lead to loss of cohesion and material loss. Consolidation of these materials with liquid products via the surface is a common practice; however, the most used consolidation products (e. g. TEOS-based) show a poor physical-chemical compatibility with calcareous substrates. For application on calcareous materials, the so-called nanolimes, i. e. dispersions of lime nanoparticles in alcohols, are an alternative to TEOS-based products, thanks to their chemical compatibility with lime-based substrates. Nanolimes can help to recover a superficial loss of cohesion. However, their in-depth consolidation effect is not always satisfactory. Previous work has shown that a better deposition of lime nanoparticles in depth can be achieved by adapting the properties of the nanolime dispersion (kinetic stability and evaporation rate) to the moisture transport properties of the substrate, through optimization of the solvent. In this paper, freshly synthetized nanolimes were dispersed in pure ethanol and/or in binary mixture of ethanol (95 %) and water (5 %). These nanolimes were applied on Maastricht limestone and on a lime-based mortar by capillary absorption (method commonly used for laboratory tests) and by nebulization (method widely used in situ). The aim of this research is to fill the gap between laboratory tests and on site application, providing an application protocol for restorers and professionals in the field. The research shows that results obtained by application by capillary absorption do not always correspond to those obtained by nebulization. This fact should be considered when deciding on the use of a consolidation surface treatment in practice.

Correlation between microstructure and cathodoluminescence properties of Mg(OH)2 (brucite) nanoparticles: effect of synthesis method

The study shows a correlation between microstructure, atomic-scale observations and intrinsic cathodoluminescence from two types of nano-brucite (Mg(OH)₂) synthesized by hydrothermal and sol–gel methods, and a third one that was commercially acquired.

Nanolimes: from synthesis to application

Cultural heritage objects and structures are subjected to a range of weathering processes that result in their decay and destruction. To slow weathering rates and/or mitigate their effects, several protective and consolidant materials have been used during conservation interventions. Treatments based on organic polymers and alkoxysilanes, as well as some traditional inorganic treatments such as lime water, are in many cases either incompatible and/or show limited efficacy. In recent years nanolimes, that is, dispersions of Ca(OH)2 nanoparticles in alcohol (as well as alcohol dispersions of other alkaline-earth metal hydroxide nanoparticles), have emerged as an effective and compatible conservation material. Here we review recent advances in the synthesis and application of nanolimes in the field of heritage conservation. First, we present an overview of lime-based conservation materials, with an emphasis on the earliest reports on the use of nanolimes. Subsequently, we present the different methods used to synthesize nanolimes. Afterwards, we describe their carbonation and its consolidation effects. Practical application of nanolimes in heritage conservation are summarized, including consolidation of stone, ceramics, lime mortars and mural painting, as well as deacidification of paper, canvas, and wood. The advantages and limitations of this novel nanotechnology for cultural heritage conservation are outlined. Finally, some conclusions and areas for future research are presented.

Evaluation of the antibacterial power and biocompatibility of zinc oxide nanorods decorated graphene nanoplatelets: new perspectives for antibiodeteriorative approaches

BACKGROUND

Nanotechnologies are currently revolutionizing the world around us, improving the quality of our lives thanks to a multitude of applications in several areas including the environmental preservation, with the biodeterioration phenomenon representing one of the major concerns.

RESULTS

In this study, an innovative nanomaterial consisting of graphene nanoplatelets decorated by zinc oxide nanorods (ZNGs) was tested for the ability to inhibit two different pathogens belonging to bacterial genera frequently associated with nosocomial infections as well as biodeterioration phenomenon: the Gram-positive Staphylococcus aureus and the Gram-negative Pseudomonas aeruginosa. A time- and dose-dependent bactericidal effect in cell viability was highlighted against both bacteria, demonstrating a strong antimicrobial potential of ZNGs. Furthermore, the analysis of bacterial surfaces through Field emission scanning electron microscopy (FESEM) revealed ZNGs mechanical interaction at cell wall level. ZNGs induced in those bacteria deep physical damages not compatible with life as a result of nanoneedle-like action of this nanomaterial together with its nanoblade effect. Cell injuries were confirmed by Fourier transform infrared spectroscopy, revealing that ZNGs antimicrobial effect was related to protein and phospholipid changes as well as a decrease in extracellular polymeric substances; this was also supported by a reduction in biofilm formation of both bacteria. The antibacterial properties of ZNGs applied on building-related materials make them a promising tool for the conservation of indoor/outdoor surfaces. Finally, ZNGs nanotoxicity was assessed in vivo by exploiting the soil free living nematode Caenorhabditis elegans. Notably, no harmful effects of ZNGs on larval development, lifespan, fertility as well as neuromuscular functionality were highlighted in this excellent model for environmental nanotoxicology.

CONCLUSIONS

Overall, ZNGs represent a promising candidate for developing biocompatible materials that can be exploitable in antimicrobial applications without releasing toxic compounds, harmful to the environment.

Durability assessment to environmental impact of nano-structured consolidants on Carrara marble by field exposure tests

The EU policy of reducing the emissions of combustion generated pollutants entails climate induced deterioration to become more important. Moreover, products applied to preserve outdoor built heritage and their preliminary performance tests often turn out to be improper. In such context, the paper reports the outcomes of the methodology adopted to assess the durability and efficiency of nano-based consolidating products utilized for the conservation of carbonate artworks, performing field exposure tests on Carrara marble model samples in different sites in the framework of the EC Project NANOMATCH. Surface properties and cohesion, extent and penetration of the conservative products and their interactions with marble substrates and environmental conditions are here examined after outdoor exposure for eleven months in four different European cities and compared with the features of undamaged and of untreated damaged specimens undergoing the same exposure settings.

Kinetics and Mechanism of Calcium Hydroxide Conversion into Calcium Alkoxides: Implications in Heritage Conservation Using Nanolimes

Nanolimes are alcohol dispersions of Ca(OH)2 nanoparticles used in the conservation of cultural heritage. Although it was believed that Ca(OH)2 particles were inert when dispersed in short-chain alcohols, it has been recently shown that they can undergo transformation into calcium alkoxides. Little is known, however, about the mechanism and kinetics of such a phase transformation as well as its effect on the performance of nanolimes. Here we show that Ca(OH)2 particles formed after lime slaking react with ethanol and isopropanol and partially transform (fractional conversion, α up to 0.08) into calcium ethoxide and isopropoxide, respectively. The transformation shows Arrhenius behavior, with apparent activation energy Ea of 29 ± 4 and 37 ± 6 kJ mol(-1) for Ca-ethoxide and Ca-isopropoxide conversion, respectively. High resolution transmission electron microscopy analyses of reactant and product phases show that the alkoxides replace the crystalline structure of Ca(OH)2 along specific [hkl] directions, preserving the external hexagonal (platelike) morphology of the parent phase. Textural and kinetic results reveal that this pseudomorphic replacement involves a 3D diffusion-controlled deceleratory advancement of the reaction front. The results are consistent with an interface-coupled dissolution-precipitation replacement mechanism. Analysis of the carbonation of Ca(OH)2 particles with different degree of conversion into Ca-ethoxide (α up to 0.08) and Ca-isopropoxide (α up to 0.04) exposed to air (20 °C, 80% relative humidity) reveals that Ca-alkoxides significantly reduce the rate of transformation into cementing CaCO3 and induce the formation of metastable vaterite, as opposed to stable calcite which forms in untransformed Ca(OH)2 samples. Similar effects are obtained when a commercial nanolime partially transformed into Ca-ethoxide is subjected to carbonation. Such effects may hamper/delay the strengthening or consolidation effects of nanolimes, thus having important implications in the conservation of cultural heritage.

Calcium hydroxide nanoparticles from solvothermal reaction for the deacidification of degraded waterlogged wood

HYPOTHESIS

A combination of acid and iron ions inside the wood has been corroding the cellulose matrix of the Swedish warship Vasa, imposing its deacidification. Past deacidification treatments displayed poor penetration inside the wood matrix with limited efficacy. A vacuum assisted treatment of wood using newly developed calcium hydroxide nanoparticle dispersions represents a possible candidate for the treatment of acidic waterlogged wood objects such as sculptures and decorative artifacts.

EXPERIMENTS

A solvothermal process was used for the synthesis of calcium hydroxide nanoparticle dispersions. Before the application on waterlogged wood, the physico-chemical characterization of these systems was carried out using several techniques. The efficacy of the deacidification treatment of wood samples from the Vasa was assessed by determination of pH and Differential Thermal Gravimetric (DTG) measurements.

FINDINGS

The proposed solvothermal reactions can be used to produce stable and highly concentrated calcium hydroxide nanoparticle dispersions in alcohols, needing no further purification before the application. This process has also the advantage to be upscalable to industrial level. Both pH and DTG measurements showed that the newly developed dispersions can homogenously penetrate inside the wood up to 20cm, neutralizing acidity and creating an alkaline buffer inside the wooden matrix, to hinder the degradation of residual cellulose.

Amorphous and crystalline calcium carbonate phases during carbonation of nanolimes: implications in heritage conservation

Alcohol dispersions of Ca(OH)₂nanoparticles, the so-called nanolimes, carbonate in air following first order kinetics,viaa multistep, non-classical crystallization process involving amorphous and crystalline CaCO₃phases.

Highly transparent poly(2-ethyl-2-oxazoline)-TiO2 nanocomposite coatings for the conservation of matte painted artworks

A nanocomposite coating based on TiO₂ nanoparticles and poly(2-ethyl-2-oxazoline) is used as consolidant of matte paintings (such as temperas), by exploiting bulk scattering phenomenon to mimic the surface diffusive properties of the painted surface.

An amine-oxide surfactant-based microemulsion for the cleaning of works of art

Surfactant-based aqueous fluids, such as micellar solutions and microemulsions, are effective, safe and selective media for cleaning operations in conservation of cultural heritage. The search for better-performing systems and eco-friendly cleaning systems is currently a major goal in conservation science. We report here on a ternary o/w microemulsion, composed of diethyl carbonate (DC) as the oil phase and N,N-Dimethyldodecan-1-amine oxide (DDAO) as the surfactant. DDAO is a well known and widely used detergent and solubilizing agent, selected here for its degradability and eco-compatibility. Due to its nonionic/cationic nature, it can be used also when nonionic-based formulations become ineffective because of clouding and phase separation. Moreover, DDAO is insensitive to the presence of divalent metal ions, usually abundant in wall paintings substrates. Small-Angle Neutron Scattering (SANS) provided detailed information about the nanostructure of the surfactant aggregates. Finally, the cleaning effectiveness of the nanofluid was assessed both on fresco mock-ups and on real wall paintings conserved in the archeological site of Tulum, Mexico. Here, conservators successfully used the microemulsion to remove naturally aged films of complex polymer mixtures from the works of art surface.

Consolidation of archaeological gypsum plaster by bacterial biomineralization of calcium carbonate

Gypsum plasterworks and decorative surfaces are easily degraded, especially when exposed to humidity, and thus they require protection and/or consolidation. However, the conservation of historical gypsum-based structural and decorative materials by conventional organic and inorganic consolidants shows limited efficacy. Here, a new method based on the bioconsolidation capacity of carbonatogenic bacteria inhabiting the material was assayed on historical gypsum plasters and compared with conventional consolidation treatments (ethyl silicate; methylacrylate-ethylmethacrylate copolymer and polyvinyl butyral). Conventional products do not reach in-depth consolidation, typically forming a thin impervious surface layer which blocks pores. In contrast, the bacterial treatment produces vaterite (CaCO3) biocement, which does not block pores and produces a good level of consolidation, both at the surface and in-depth, as shown by drilling resistance measurement system analyses. Transmission electron microscopy analyses show that bacterial vaterite cement formed via oriented aggregation of CaCO3 nanoparticles (∼20nm in size), resulting in mesocrystals which incorporate bacterial biopolymers. Such a biocomposite has superior mechanical properties, thus explaining the fact that drilling resistance of bioconsolidated gypsum plasters is within the range of inorganic calcite materials of equivalent porosity, despite the fact that the bacterial vaterite cement accounts for only a 0.02 solid volume fraction. Bacterial bioconsolidation is proposed for the effective consolidation of this type of material. The potential applications of bacterial calcium carbonate consolidation of gypsum biomaterials used as bone graft substitutes are discussed.