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

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Time for Europe’s statues to put a coat on

From the controversial Parthenon Marbles to Michelangelo’s David, marble is a prominent material in Europe’s rich cultural heritage. EU research has come up with a solution to the material’s sensitivity to exposure to the elements, particularly acid rain.

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Marble is ideal for sculpture and building. Light penetrates white marble and scatters so it looks alive, and it’s easy to carve. However, marble is prone to damage on exposure to the ravages of the outside environment. The HAP4MARBLE project has formulated a solution that literally transforms the marble surface into a more durable material. “By covering the surface with a thin, invisible layer of hydroxyapatite (HAP), the main constituent of bones and teeth, the marble underneath stays intact,” explains Dr Enrico Sassoni, Marie Curie fellow with HAP4MARBLE. It’s all HAP-pening for marble The HAP4MARBLE biomimetic treatment is based on an aqueous phosphate solution that reacts with marble, forming HAP. Not only does the treatment prevent the dissolution of marble in acid conditions, it halts other destructive processes expected to become harsher with the advent of climate change. Temperature fluctuations can cause ‘sugaring’ of the marble, evident in many European monuments, from the Alhambra Palace in Granada to the Père Lachaise cemetery in Paris. The researchers found that HAP growth inside the fissures between loose grains re-establishes their cohesion. This stops the material from weathering and there is no more loss through crumbling. Bowing is another common heat-related phenomenon in thin slabs used as gravestones caused by temperature changes. “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,” reports Dr Elisa Franzoni, project coordinator. Novel self-cleaning treatments – a bright future for marble Beautiful white translucent marble darkens when atmospheric particles interact with it. One of the new solutions developed within HAP4MARBLE is based on a combination of HAP and nano-titanium dioxide (TiO2). Application of TiO2 alone can stop darkening, but addition of HAP enhances its photocatalytic activity and durability. HAP4MARBLE researchers are monitoring the progress of HAP-based treatments on marble exposed outdoors and on real marble artefacts in real environmental conditions. The new multifunctional treatments developed are able to overcome the limitations of currently available commercial treatments. “Indeed, the new treatment has shown better efficacy, compatibility and/or durability,” emphasises Dr Franzoni. Europe’s cultural heritage preserved To maximise the social impact of the project, connections have been established with several authorities in charge of cultural heritage conservation in Europe, including professional restorers working at the Palace of Versailles and the Historical Monuments Research Laboratory in Paris. “This is extremely important, because the loss of our monuments would have dramatic cultural and economic consequences, loss of national/European identity and significant wealth reduction,” points out Dr Sassoni. Also, the Monumental Cemetery in Bologna (Italy) is interested in the new conservation treatment. “At the moment, we are collaborating with professional restorers from Bulgaria, to possibly apply our new treatment to the prehistoric wall paintings in the limestone Magura cave, Bulgaria. Research groups interested in the HAP-based treatment after pilot treatments include groups in Zurich (Switzerland), Granada (Spain) and Milan (Italy).” Extensive outreach activities took HAP4MARBLE to Europe’s cities across Europe, from Bologna to Brussels. Fun met science at Science is Wonder-ful! in Brussels, where one of the many highlights was the exhibition ‘It's Marble-Lous!’. Here, young and the more elderly could learn about HAP as well as the cultural and economic importance of Europe’s monuments and of the challenges connected to their conservation.


HAP4MARBLE, marble, cultural heritage, bowing, artefact, hydroxyapatite, TiO2, protective coat, biomimetic treatment, weathering

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