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.
