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Multi-functionalization of hydroxyapatite for restoration and preventive conservation of marble artworks

Periodic Reporting for period 1 - HAP4MARBLE (Multi-functionalization of hydroxyapatite for restoration and preventive conservation of marble artworks)

Reporting period: 2015-09-01 to 2017-08-31

The HAP4MARBLE project is aimed at developing a new multi-functional treatment for conservation of marble artworks in cultural heritage. Marble has been widely used in architecture and sculpture since antiquity, but it is subject to several weathering processes, some of which are predicted to be worsened by future climate change. Unfortunately, all the currently available treatments for conservation of marble exhibit some significant limitations, in terms of efficacy, compatibility and/or durability. Developing new treatments able to overcome the limitations of the available products is fundamental, because cultural heritage plays a fundamental role in defining European identity and substantially contributes to European economic prosperity, to such an extent that cultural heritage conservation is identified as strategic in the Lisbon Treaty on the European Union.
The project is aimed at multi-functionalizing a recently proposed biomimetic treatment based on formation of hydroxyapatite (HAP, the main constituent of human teeth and bones), by reaction of marble with an aqueous phosphate solution. Within the project, the HAP-treatment is expected to be further developed, to achieve the following 4 functionalities:
1) Prevention of marble dissolution. Marble dissolution occurs because of the aqueous solubility of its main mineral component, calcite, which results in marble surface recession and loss of inscriptions and carved elements. By developing a surface coating of HAP (which is much less soluble than calcite), marble dissolution can be prevented. The new formulations of the HAP-treatment are expected to be able to reach complete coverage of marble surface and reduce the porosity of the HAP coating.
2) Repair of sugaring marble. Sugaring (i.e. granular disintegration) originates from temperature excursions, so that marble can be reduced to a sugar-like powder of isolated grains by just the pressure of a finger. By HAP growth inside intergranular fissures, the new treatment is expected to heal marble micro-cracks, re-establish cohesion between loose grains and hence stop material loss. Compared to existing products, the new HAP-treatment is expected to have enhanced effectiveness, compatibility and durability.
3) Arrest of marble bowing. Bowing of thin slabs used as gravestones in historic cemeteries and cladding in modern façades, is originated by cyclic thermal excursions and can lead to slab collapse. By penetration into microcracks formed after bowing, the new HAP-treatment is expected to bond calcite grains more effectively, thus enhancing resistance against further bowing.
4) Development of self-cleaning ability. Marble elements exposed outdoors are affected by darkening induced by deposition and accumulation of atmospheric particles. By combining HAP and nano-TiO2 (which has photocatalytic activity), a new treatment is expected to be developed, having enhanced photocatalytic activity and enhanced durability, compared to application of TiO2 alone.
The achievement of these functionalities requires a multi-disciplinary research activity, combining different competences and laboratory facilities. During the first two years of the project, the Marie Skłodowska-Curie Fellow (Dr. Enrico Sassoni) carried out such multi-disciplinary research at two host institutions: Princeton University (USA, months 1-18) and Göttingen University (Germany, months 19-24). At the two institutions, Dr. Sassoni collaborated with world renowned experts, Prof. George W. Scherer at Princeton University and Prof. Siegfried Siegesmund at Göttingen University. The whole research activity was performed also in collaboration with Dr. Elisa Franzoni (University of Bologna), Coordinator of the project.

For each function described above, the following work was carried out:
1) Prevention of marble dissolution. To prevent formation of microcracks and pores in the HAP coating, the influence of a series of parameters was investigated (concentration of the phosphate precursor, addition of organic solvents, reaction time and pH). The characteristics of the resulting coatings were evaluated by sophisticated techniques. Novel formulations of the treatment were developed, able to form a crack-free and pore-free coating, which protects marble more efficiently than existing commercial alternatives.
2) Repair of sugaring marble. First, a new method was developed to produce samples with properties similar to naturally weathered marble (i.e. near-surface damage). Then, the consolidating efficacy of the new HAP-treatment formulations, developed to prevent marble dissolution, were tested on artificially weathered samples. The new formulations were found to be able to significantly improve the damaged mechanical properties after a single application and to completely restore them after a second application. Compared to commercial alternatives, the new treatment provides a benefit in terms of durability after consolidation. A tailored formulation was also investigated for gypsum-contaminated marble. In addition, a completely new consolidant (not originally planned in the project proposal) was conceived and encouraging results were obtained.
3) Arrest of marble bowing. The ability of the new formulations of the HAP-treatment to mitigate marble thermal damage and to arrest bowing was tested, in comparison with two commercial alternatives. The results suggest that marble bowing can be reduced by some of the HAP-formulations and possibly even prevented. Notably, one of the commercial alternative treatments was found to significantly worsen marble behaviour.
4) Development of self-cleaning ability. Various formulations of the HAP-treatment involving combination with nano-TiO2 were investigated. For each formulation, the effectiveness (i.e. the photocatalytic activity) and the durability (i.e. the ability to maintain the photocatalytic activity after exposure to rain) were evaluated. The treatment formulations combining HAP and TiO2 proved to have a better performance, compared to nano-TiO2 alone.
The obtained results were published in international journals and presented at international conferences. All publications were made freely available for download. A provisional patent application was filed for the new consolidant.
The novel materials and techniques developed within the project have the potential to give a very significant contribution to conservation of cultural heritage in Europe. This is extremely important, because the possible loss of cultural heritage would have dramatic consequences, both in terms of culture (loss of national/European identity) and economy (loss of a significant wealth source, considering cultural heritage-related tourism and activities).
To maximize the social impact of the project, connections have been established and strengthened with several Authorities in charge of cultural heritage conservation in Europe. These Authorities have demonstrated strong interest in testing the new HAP-treatment in some pilot applications to real artworks, in collaboration with Dr. Sassoni and the University of Bologna. This activity of field testing and dissemination to Authorities and professionals will proceed during the final year of the project at the University of Bologna. Here, the possible further improvement of the HAP-treatment by electrochemical methods will be also investigated.
Graphical Abstract HAP4MARBLE