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

Periodic Reporting for period 2 - HAP4MARBLE (Multi-functionalization of hydroxyapatite for restoration and preventive conservation of marble artworks)

Reporting period: 2017-09-01 to 2018-08-31

The HAP4MARBLE project was aimed at developing a new multi-functional treatment for conservation of marble artworks in cultural heritage and at validating it in the field. Marble has been widely used in architecture and sculpture since antiquity, but it is subject to several deterioration 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.

The project was 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 was further developed, to achieve the following objectives:
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 covering the marble surface with a continuous and dense layer of HAP (which is much less soluble than calcite), marble dissolution can be prevented.
2) Repair of sugaring marble. Granular disintegration (the co-called “sugaring”) 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 re-establish cohesion among loose grains and hence stop material loss.
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 micro-cracks, the new HAP-treatment is expected to bond calcite grains more effectively, thus enhancing resistance against further bowing and even prevention of bowing in fresh marble.
4) Development of self-cleaning ability. Marble elements exposed outdoors are affected by darkening induced by deposition 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.
5) Validation in the field. By pilot application onto marble samples exposed outdoors and onto real marble artefacts, followed by periodical monitoring, the performance of the newly developed formulations of the HAP treatment are expected to validated in real environmental conditions.
The achievement of these objectives was made possible by a multi-disciplinary research activity, combining different competences and laboratory facilities. To this aim, the Marie Skłodowska-Curie Fellow, Dr. Enrico Sassoni, spent 18 months at Princeton University (USA) working with Prof. George W. Scherer, 6 months at Göttingen University (Germany) with Prof. Siegfried Siegesmund and 12 months at the University of Bologna (Italy), with Dr. Elisa Franzoni.

For each objective described above, the following work was carried out:
1) Prevention of marble dissolution. To obtain continuous and dense coatings, the influence of several parameters (concentration of the phosphate precursor, addition of organic solvents, reaction time and pH) was investigated by sophisticated techniques. Novel formulations were developed, able to form a crack-free and pore-free coating, which protects marble more efficiently than existing commercial alternatives. To favour formation of a continuous layer of HAP over marble surface, the possible benefit of using electrochemical methods (i.e. by electrodepositing HAP) was also investigated.
2) Repair of sugaring marble. First, a new method was developed to produce artificially aged samples in a more realistic way, compared to available methods. Then, the consolidating efficacy of the new formulations of the HAP-treatment was tested, with very encouraging results. Compared to commercial alternatives, the new HAP-treatment offers an advantage in terms of durability after consolidation. A tailored formulation was also developed for gypsum-contaminated marble.
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 and possibly even prevented by some of the HAP-formulations. Notably, one of the commercial alternative treatments was found to worsen significantly the marble thermal behaviour.
4) Development of self-cleaning ability. Various combinations of HAP with nano-TiO2 were investigated. The self-cleaning ability and the durability (i.e. the ability to maintain the self-cleaning ability after prolonged exposure to rain) were evaluated. The treatment formulations combining HAP and TiO2 proved to have a better performance, compared to nano-TiO2 alone.
5) Validation in the field. The most promising formulations of the HAP treatment were applied onto marble samples, which were exposed in the Park of the Royal Palace in Versailles, France. One formulation was also applied onto a marble sculpture in the Versailles Park. The samples and the sculpture have shown a good conservation state so far and periodical monitoring is planned in the future.

The obtained results were published in 14 scientific publications with open access, 7 articles in high-level journals and 7 contributions in international conference proceedings. One additional journal paper is also currently in preparation.
The novel materials and techniques developed within the project have the potential to contribute significantly to the conservation of European cultural heritage, because they are able to overcome many limitations of the available conservation treatments. This is extremely important, because the loss of our monuments would have dramatic cultural and economic consequences (loss of national/European identity and loss of a significant wealth source).

To maximize the social impact of the project, connections have been established with several Authorities in charge of cultural heritage conservation in Europe, which demonstrated strong interest in testing the new HAP-treatment in some pilot applications. This is an important step towards the final use of the HAP-treatment in the conservation practice. Moreover, the project also contributed to make European citizens, including little school children, more aware of the cultural and economic importance of our monuments and of the challenges connected to their conservation, thanks to the extensive outreach activities that Dr Sassoni has carried out during the project.

Thanks to the HAP4MARBLE project, the Marie Skłodowska-Curie Fellow, Dr Enrico Sassoni, has developed a considerable expertise on the topic, which led him to receive several scientific acknowledgements and prizes and to establish new collaborations with international research groups. This will significantly help the further development of his career.
Graphical Abstract HAP4MARBLE