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Nano-materials for the conservation and preservation of movable and immovable artworks

Final Report Summary - NANOFORART (Nano-materials for the conservation and preservation of movable and immovable artworks)

Executive Summary:
The FP7 project NANOFORART (Nano-materials for the conservation and preservation of movable and immovable artworks) targeted the development of nanomaterials with the following features:

- Physico-chemical compatibility with the components of works of art, i.e. the application of the new systems minimizes or completely avoids any alteration of the original artifacts’ physico- chemical properties.
- The new materials are either non-toxic or they show a reduced toxicity as opposed to traditional restoration materials like toxic solvents.
- Advanced nanomaterials allow for a finer control of the restoration intervention, for instance cleaning can be carried out using microemulsions and chemical hydrogels in a more controlled way than with traditional materials.
- The new methodologies proposed are more reliable than traditional approaches, and in some cases allow for a more gradual and slower (safer) approach. Overall, the new methods grant the stability of the treated works of art also in the long term (as opposed to “quick” traditional interventions that might involve drawbacks, requiring later interventions).

Overall, three main classes of materials were produced, i.e. nanoparticles dispersions (for the consolidation and pH control of artifacts), nanostructured cleaning fluids (for the removal of grime, dirt, aged varnishes, adhesives and detrimental polymer coatings) and chemical gels (for the controlled delivery of solvents and cleaning fluids on sensitive surfaces).
The project covered all the aspects related to the production, assessment and exploitation of the new nanomaterials, namely:

- End-user partners, e.g. the National Museum of Denmark, the National Museum of Anthropology and History of Mexico City, and the Spanish Cultural Heritage Institute, carried extensive assessment and validation of the nanomaterials both on models and real works of art. This also led to the definition of applicative protocols.
- The eco-toxicological impact of the new materials was assessed, and safety rules for their use were defined.
- The large-scale production of the best systems produced by the project has been finalized.
- Extensive dissemination of the project results has taken place through several different channels (publications, seminars/conferences/workshops, internet).
- The exploitation of the new products has been planned thanks to the cooperation and participation of SME partners.

As a result, several formulations of nanomaterials are ready for commercialization, with expected competitive prices on the market. In this way, an alternative and innovative palette will be presented to end-users worldwide.

Project Context and Objectives:
The main objective of the NANOFORART project is the development, validation and upscale of new nano-materials and responsive systems to address the following tasks:

1. Cleaning of immovable works of arts

2. Consolidation of immovable works of arts

3. Cleaning of movable works of arts

4. Deacidification of movable works of arts

In fact, while the progress in material science has generated sophisticated nanostructured materials, conservation of cultural heritage is still mainly based on traditional methods and conventional materials that often lack the necessary compatibility with the original works of art and a durable performance. The following examples summarize the main problems related to traditional restoration materials, which the project addressed:

- Synthetic polymers (polyacrylate, polyvinyl acetate, styrene-based) are traditionally used for the consolidation of mural paintings, but their application strongly alters the surface of the artifacts, depressing permeability to water vapor and in several cases leading to significant damage in the short or long term depending on the environmental conditions.
To address this issue, the project aimed at the development of nano-sized inorganic consolidants to grant stable consolidation of carbonate-based works of art (mural paintings, mortars, stone).

- Aqueous solutions of hydroxides are commonly used for the deacidification of paper, but they can be too aggressive on aged and oxidized documents, and water-soluble components (inks, dyes) can be leached or dissolved. Alternatively, available non-aqueous methods use either solutions of hydroxide precursors or dispersions of micron-sized alkaline particles with stabilizers that remain on the surfaces.
Therefore, the project developed dispersions of hydroxide (or carbonate) particles with reduced size (nano) to avoid the use of stabilizers and achieve better penetration through coated surfaces (e.g. sized paper), as well as high effectiveness in neutralizing acids and adjusting pH.

- The cleaning of artifacts is commonly carried out using neat solvents, which are often toxic and volatile. Moreover, cleaning with solvents is often scarcely controlled, as dissolved materials (e.g. varnishes) can be re-transported through the porous matrix of the artifacts.
The project developed cleaning fluids with low eco-toxicological impact while maintaining a good cleaning efficacy and limiting re-dispersion of the removed matter.

- “Gel-like” thickeners (e.g. based on cellulose derivatives or polyacrylic acid) are traditionally used to control the action of solvents, but these systems often lack the retentiveness necessary to control the delivery of loaded fluids on highly sensitive surfaces. Moreover, these thickeners tend to leave solid residues on the surfaces.
Therefore, the project produced highly retentive chemical gels, with good mechanical properties to grant the controlled release of loaded cleaning fluids and easy handling/removal of the gel without leaving solid residues after the cleaning intervention.

Overall, NANOFORART developed manageable methodologies, based on materials with a low environmental impact, namely nanoparticles dispersions, micellar solutions, microemulsions and chemical gels.

In the second part of the project great importance has been given to the evaluation of the eco-toxicity of nano-materials, and to the technology transfer to SMEs that played an important role in the standardization of applicative protocols, the up-scale and the exploitation of the developed technology.
A fundamental part of the project was also related to the role of end-users. Important museums, such as the National Museum of Denmark, the National Museum of Anthropology and History of Mexico City and the Spanish Cultural Heritage Institute, have validated the technology and the methods developed in the project, and have provided training activities and dissemination of the developed techniques.

Project Results:
The hard- and soft-matter systems developed within NANOFORART constitute a wide palette of new reliable tools to restore and preserve works of art.
The main systems developed by the project comprise:

- Dispersions of nanoparticles in solvents (for the consolidation and pH control of artifacts).
In particular, nanoparticles of hydroxides (Ca, Mg, Ba), carbonates (Ca, Mg, Sr), oxide (Mg) and lactates (Ca, Al) have been prepared and stably dispersed in solvents such as short chain alcohols (ethanol, propanol), cyclohexane or water.

- Nanostructured fluids (for the cleaning of artifacts).
Both oil-in-water systems (water content ranging ca. from 70 to 90%) with one or more organic solvents, and water-in-oil systems (water content ranging ca. from 5 to 10%) have been developed.

- Chemical gels (for the cleaning of water- and solvent-sensitive surfaces).
Both hydrogels and organogels have been developed, for the confinement and controlled release of aqueous systems or organic solvents.

The following paragraphs resume the main results regarding the preparation, assessment and eco-toxicological impact of these three classes of materials.

1. Dispersions of nanoparticles
The following project partners have carried out the production of nanoparticles dispersions, thanks to their expertise in this area:

- CSGI (Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase – Center for Colloid and Surface Science), Project Coordinating unit, Italy
- ZFB (Zentrum Fur Bucherhaltung GMBH), Germany
- MBN Nanomaterialia S.p.a Italy

Different methodologies have been adopted to produce materials with different properties in terms of particles properties (morphology, size distribution, dispersity, cristallinity, dispersing medium), to address different conservation tasks. Throughout the project, the feedback from end-users and assessment partners was helpful to refine the formulations.

The main systems produced comprise dispersions of nanoparticles of:

- Ca(OH)2 in ethanol, 1-propanol or 2-propanol (consolidation, pH control)
- Ca(OH)2 in 2-propanol diluted with polar/non-polar solvents (pH control)
- Ca(OH)2 in cyclohexane (pH control of modern documents)
- Ba(OH)2 in 2-propanol (consolidation of sulfate-polluted substrates)
- Ca(OH)2 and Ba(OH)2 in 2-propanol (consolidation of sulfate-polluted substrates)
- Mg(OH)2 in 2-propanol (consolidation of dolomite, pH control)
- CaCO3 in 2-propanol, water or cyclohexane (pH control of historical and modern documents)
- MgCO3 in 2-propanol (pH control)
- CaMg(CO3)2 in water (pH control)
- SrCO3 in water (pH control)
- MgO in water (pH control)
- Ca lactate in 2-propanol (pH control of leather)
- Al lactate in 2-propanol (pH control of leather)

The concentration can range from 1 g/L to 50 g/L (or higher), even if for many applications concentrations in the range of 1-10 g/L are used.
The particles size can range from 30-70 nm to ca. 350 nm, while polydisperse formulations can also exhibit particles with larger size (ca. 800 nm). Having different possibilities is advantageous because it is possible to match the porosity of different kinds of substrates that need to be consolidated or deacidified, such as degraded painted layers, mortars, sized paper etc.
For instance, heavily degraded porous matrices may exhibit a porosity gradient; therefore mono-disperse nanoparticles might prove partially effective in consolidating such substrates. In this case, nanoparticles with multi-modal size distribution exhibit an increased effectiveness.

As shown above, different formulations can be used either for consolidation or pH control purposes.

Regarding the consolidation of murals, plasters, mortars and stone, the main rationale for the production of these nanomaterials is their physico-chemical compatibility with the substrates that need to be consolidated. As a matter of fact, lime has been the main construction material of all civilizations since early ages, and carbonate-based stones have been widely used for the realization of architectural and artistic works of art.
In the last 60 years, polymeric commercial products such as acrylic and vinyl co-polymers have been widely used by conservators to consolidate wall-paintings, since they provide protection and hydrophobicity to the artistic surface. However, these materials lack the necessary physico-chemical compatibility with the treated substrates and, in the case of porous surfaces, they produce alterations that may strongly enhance several degradation phenomena, e.g. salt crystallization and the detachment of the painted layer (for wall-paintings).
On the other hand, dispersions of alkaline earth hydroxides’ nanoparticles in short chain alcohols are effective materials for consolidation purposes, and their full physico-chemical compatibility with carbonate-based substrates grant the long-term stability of the treatment. The dispersions can be applied by spraying or brushing, and after evaporation of the solvent the particles are left within the pores of the artifact. Due to the carbonation process (that typically occurs in 2-3 weeks) these compounds are able to turn into the same materials that compose the original artworks. Therefore, following the application of the particles, a network of carbonate crystals is built within the artifacts’ pores, providing mechanical consolidation without altering the original physico-chemical properties. Crystalline hydroxide particles are normally considered for consolidation purposes.
Magnesium hydroxide can be complementary to calcium hydroxide especially when the consolidation of mixed calcium-magnesium carbonate substrates is considered, e.g. dolomitic limestone and several stone that are characteristic both of Central Europe and some Mesoamerican areas (e.g. the Yucatan peninsula). In such cases, in fact, the presence of magnesium hydroxide increases the compatibility of the nanomaterials with the treated artifacts.
The rationale for the application of Ba(OH)2 together with Ca(OH)2 for the preservation of wall-paintings, is due to the possibility of consolidating substrates where large amounts of soluble sulfates (e.g. NaSO4) are present. In general, sulfates should be extracted as much as possible previous to the consolidation treatment, however in some cases the extraction cannot be carried out completely. In this case, the application of barium hydroxide particles allows the transformation of soluble sulfates into insoluble barium sulfate, preventing the (partial) conversion of calcium hydroxide particles into gypsum, which would hinder the consolidation effect of the treatment.

Besides consolidation of works of art, some of the nanoparticles dispersions developed by the project were aimed at the deacidification and pH control of movable artifacts, such as paper and parchment documents, canvas paintings and leather objects (covers, bookbindings). In fact, acid-catalyzed hydrolysis and oxidation are two of the main degradation mechanisms for both cellulose- and collagen-based artworks.
Alkaline nanoparticles can be stably dispersed in alcohols and less polar solvents without using stabilizers, and the dispersions can be applied onto the artifacts by brushing, spraying (or nebullization), dripping or immersion. After the evaporation of solvents the particles adhere to the fibers, neutralizing acids and leaving a buffer against recurring acidity. Any excess of hydroxide particles will eventually transform into carbonate by reacting with atmospheric CO2; alternatively carbonate particles can be applied directly as a milder alkaline buffer. Dispersions in apolar solvents (hydrocarbons) are particularly useful for the treatment of paper documents featuring modern inks that are sensitive to short chain alcohols. Lactate particles were used as combined to calcium carbonate or hydroxide for the pH adjustment of strongly acidic leather objects (the “natural” pH of leather is ca. 4.0-4.5).

The producer partners have worked together with end-user partners to assess the effectiveness of the developed nanomaterials both on model and real samples belonging to different cultural heritage assets. In particular:

- The National Museum of Denmark focused on Northern Europe heritage (consolidation of wall paintings)
- The University of Pardubice and Gema Art Group focused on Eastern Europe heritage (consolidation of wall paintings, plasters, mortars, stone)
- The National Institute of Anthropology and History of Mexico focused on Mesoamerican heritage (consolidation of wall paintings, pH control of ancient and modern documents)
- The Spanish Cultural Heritage Institute focused on pH control of paper and parchment documents and leather bookbindings.
- ZFB (Zentrum Fur Bucherhaltung GMBH), Germany, assessed the pH control of paper documents and leather covers.
- Morana RTD DOO (Slovenia) assessed the pH control of paper documents.
- The University College of London provided extensive physico-chemical characterization of the nanoparticles and of the treated works of art.
- Birkbeck College (University of London) provided extensive physico-chemical characterization of the treated works of art (in particular paper, canvas, parchment and leather substrates).
- The University Ca’ Foscari (Venice) assessed the eco-toxicological impact of the best nanoparticles dispersions developed by the project, and defined the safety rules for their use (essentially: standard goggles and gloves for eyes/hands protection, and the presence of good ventilation/no flames/no sparks when solvents are used).

2. Nanostructured cleaning fluids and chemical gels
The following systems were developed and characterized by CSGI, thanks to its expertise in the colloid and nanoscience fields:

- Oil-in-water (o/w) nanostructured cleaning fluids, i.e. micellar solutions and microemulsions (water content ranging ca. from 70 to 90%) with one or more organic solvents (for the removal of unwanted synthetic polymers, adhesives, natural varnishes, wax, grime, dirt).
- Water-in-oil (w/o) nanostructured cleaning fluids (water content ranging ca. from 5 to 10%) (for the removal of unwanted layers on water-sensitive surfaces).
- pHEMA/PVP semi-Interpenetrating hydrogels (for the confinement of aqueous systems and polar solvents).
- pMMA organogels (for the confinement of organic solvents).

The main advantage in using o/w micellar solutions and microemulsions lies in the low eco-toxicological impact (the systems are mainly water-based) while maintaining a good cleaning efficacy and limiting re-dispersion of the removed matter, as opposed to traditional solvents used in conservation.
Whenever the advantageous o/w systems need to be used on water-sensitive surfaces, they can be confined within chemical hydrogels that allow the controlled delivery of the fluids onto the substrate. Therefore, controlled and selective removal of unwanted layers (e.g. grime/dirt, aged natural and synthetic polymers, varnishes, adhesives and coatings) is carried out, and the chemical gels can be removed feasibly without leaving residues on the treated work of art.
Chemical hydrogels can also be used to confine other aqueous systems and polar solvents, and the range of confined organic solvents expands further when organic chemical gels are used.
Chemical gels proved essential for the controlled cleaning of sensitive substrates, and a valid alternative to traditional “gel-like” thickeners based on cellulose derivatives or polyacrylic acid.
Overall, the project produced a wide palette of cleaning systems, as microemulsions were prepared dispersing different solvents in water for the removal of different unwanted layers and materials. Similar safety rules were defined as for the nanoparticles dispersions.
End-user partners carried out extensive assessment of the new materials both on models and real works of art:

- The National Museum of Denmark focused on Northern Europe heritage (cleaning of wall paintings)
- The University of Pardubice and Gema Art Group focused on Eastern Europe heritage (cleaning of wall paintings and stone)
- The National Institute of Anthropology and History of Mexico focused on Mesoamerican heritage (cleaning of wall paintings and documents)
- Aurelia Chevalier (SME, painting conservator) focused on the cleaning of canvas and easel paintings
- The Spanish Cultural Heritage Institute focused on the cleaning of paper and parchment documents, and leather bookbindings.
- Birkbeck College and the University College of London (University of London) provided extensive physico-chemical characterization of the treated works of art (in particular paper, canvas, parchment and leather substrates).

3. Upscale, dissemination, exploitation
The constant information exchange with end-user partners allowed the optimization of the project products for future exploitation and commercialization: a survey (market research) was prepared and carried out by partners MBN, SIT (Spain) and CSGI with the cooperation of all the end-user partners, in order to investigate the market needs and the most promising systems produced by the project as compared to competitor products.
The indications of the Advisory Committee, a board of external conservation experts and scientists that provided recommendations throughout the project, complemented the information gathered by the survey.
Applicative protocols were developed for the materials produced by the project, dealing with the main issue related to the high variety of different real-life conservation problems (an issue common to all the conservation projects).
As a result of the activities described above, the best formulations were upscaled by production partners (CSGI, MBN, ZFB) up to medium- and large-scale production, for future commercialization.
At the same time, the project results were widely disseminated:

- The project activities resulted in the publication of 15 articles on peer-reviewed journals (American Chemical Society, Royal Society of Chemistry, etc.), 3 articles on the proceedings of the ICOM-CC 17th Triennal Conference (Melbourne 2014), 1 chapter of a book (“Science and Technology for the Conservation of Cultural Heritage”, CRC Press, 2013).
- Two books were authored and edited by the Project Coordinator, dealing with the theoretical and practical aspects of nanotechnology applied to cultural heritage preservation (including some of the materials developed by the project, which was duly acknowledged):
• “Nanoscience for the Conservation of Works of Art” (2013), eds. Baglioni P and Chelazzi D, RSC Nanoscience & Nanotechnology, The Royal Society of Chemistry, Cambridge, UK.
• “Nanotechnologies in the Conservation of Cultural Heritage” (2014) – A compendium of materials and techniques”, Baglioni P, Chelazzi D, Giorgi R, Springer Science + Business Media Dordrecht.
- The partners actively participated in (and organized) national and international workshops and seminars (more than 100 dissemination events).
- The project website is up and running and constantly updated with news on the different project’s activities. It also features a newsletter that is periodically sent to subscribers (mainly end users and conservators also external to the project). The site includes a multimedia section featuring videos and photo-galleries.
- Training of internal and external personnel took place in the framework of PhD, master and bachelor theses, internships, restoration workshops.

Potential Impact:
The project produced a wide palette of nanomaterials that are specifically tailored to the conservation of cultural heritage, and take advantage of concepts and solutions provided by advanced materials and colloid sciences (and more in general nanosciences), as opposed to many traditional and current restoration methods. Therefore the new systems are specifically tailored to solve the degradation issues while respecting the physico-chemical properties of the original artifacts. This is the only way to grant long-term stability of the treated artifacts, avoiding the drawbacks already observed on works of art treated with traditional materials. In fact, only the durability and long-term stability of the treatments can grant the transfer of cultural heritage to future generations.

The production of nanomaterials has been supported by the development of applicative protocols to maximize the effectiveness of the applications and to ease the technology transfer to end-users.
The technology developed by the project is expected to impact socio-economic aspects in the following ways:
a) The developed nanomaterials are compatible with works of art, meaning that they exhibit as much as possible the same physico-chemical characteristics of the treated substrates, or minimize any possible alteration of the substrate as opposed to traditional restoration materials. For instance, the consolidation of a fresco painting with calcium hydroxide nanoparticles produces a calcium carbonate network in the painting’s pores, which re-adheres flaking pigments and strengthens the porous matrix, without altering the original composition of the artefact. On the other hand, it is well known that the use of scarcely compatible materials such as traditional adhesives and synthetic polymer coatings results in the alteration of the painting’s characteristics, which produces significant damage in the short or long term, depending on the environmental conditions. Therefore, the use of compatible nanomaterials is expected to minimize drawbacks and grant long-term stability of the treated works of art, reducing the number (and hence the cost) of the needed restoration interventions. In this way, the sustainability of cultural heritage is increased, with economical and societal advantages.
b) The removal of undesired varnish and coatings from the surface of works of art is traditionally carried out using solvents and solvents’ blends. Alternatively, the project has developed several nanostructured cleaning fluids (oil-in-water microemulsions, micellar solutions) that are substantially based on water, with a drastically reduced solvent content. The environmental impact is therefore reduced.
Moreover, solvents (and the aforementioned cleaning fluids) can be confined in chemical gels that allow the control of the cleaning action while depressing the liquids’ volatility, so that the safety of the cleaning systems is further increased.
c) The developed nanomaterials have been refined and selected through extensive assessment on real works of art concerning all the main foreseen applications (consolidation, cleaning, pH control). The results obtained are good and assessment activities have led to the selection of the most promising materials. The best nanomaterials are ready for commercialization and exploitation, with expected competitive prices on the market. In this way, an alternative and innovative palette will be presented to end-users worldwide.
Obviously, innovative and feasible solutions to cultural heritage conservation issues are advantageous because they provide a crucial contribution to the valorization of works of art, with positive effects on society (favoring social inclusion and the expression of cultural identities) and economy (for instance tourism-related industry).

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