Hydroxyapatite and Other Calcium Phosphates for the Conservation of Cultural Heritage: A Review

Abstract

The present paper reviews the methods and the performance of in situ formation of calcium phosphates (CaP) for the conservation of materials belonging to cultural heritage. The core idea is to form CaP (ideally hydroxyapatite, HAP, the most stable CaP at pH > 4) by reaction between the substrate and an aqueous solution of a phosphate salt. Initially proposed for the conservation of marble and limestone, the treatment has been explored for a variety of different substrates, including sandstones, sulphated stones, gypsum stuccoes, concrete, wall paintings, archaeological bones and paper. First, the studies aimed at identifying the best treatment conditions (e.g., nature and concentration of the phosphate precursor, solution pH, treatment duration, ionic and organic additions to the phosphate solution, mineralogical composition of the new CaP phases) are summarized. Then, the treatment performance on marble and limestone is reviewed, in terms of protective and consolidating effectiveness, compatibility (aesthetic, microstructural and physical) and durability. Some pilot applications in real case studies are also reported. Recent research aimed at extending the phosphate treatment to other substrates is then illustrated. Finally, the strengths of the phosphate treatment are summarized, in comparison with alternative products, and some aspects needing future research are outlined.

Comparative Study Between Ammonium Phosphate and Ethyl Silicate Towards Conservation of Prehistoric Paintings in the Magura Cave (Bulgaria)

“…After treatment with "DAP", small clusters of the new calcium phosphates were visible on the stone surface, which, also in this case, was not continuously coated with the new phases. Generally, new calcium phosphates formed in carbonate stones from the reaction with ammonium phosphate solutions show a flower-like morphology [25], but, in the case of dilute solutions (such as that adopted in the present case), the formation of small clusters has been reported before [59,60].…”

“…For the phosphate treatment, diammonium hydrogen phosphate (DAP, (NH 4 ) 2 HPO 4 ), calcium chloride (CaCl 2 ·2H 2 O), and ethanol (EtOH), all purchased from Sigma-Aldrich (Milan, Italy, assay > 99%) were used. Based on results obtained in previous studies [22,25,51], two formulations of the phosphate treatment were initially considered for the tests: (i) 1 M DAP + 1 mM CaCl 2 and (ii) 0.1 M DAP + 0.1 mM CaCl 2 in 30 vol.% EtOH. Calcium chloride has the beneficial effect of accelerating the formation of calcium phosphates [51], while ethanol has been found to promote densification of the new calcium phosphates [52].…”

“…This reaction occurs in 24-48 h during which the stone is kept impregnated with the ammonium phosphate solution; therefore, the environmental conditions of the cave are not expected to negatively affect the treatment success. Thanks to the low viscosity of the phosphate solution and to the absence of nanoparticles, a good penetration depth is usually achieved (>5 cm in highly porous limestone) [25]. So far, the treatment has provided good results on a variety of substrates [25], including many types of natural stones [26][27][28][29][30][31][32], mortars [33,34], and stuccoes [35].…”

“…Thanks to the low viscosity of the phosphate solution and to the absence of nanoparticles, a good penetration depth is usually achieved (>5 cm in highly porous limestone) [25]. So far, the treatment has provided good results on a variety of substrates [25], including many types of natural stones [26][27][28][29][30][31][32], mortars [33,34], and stuccoes [35]. Recently, the interaction between ammonium phosphate solutions and pigments used in wall paintings has been investigated, and, in most of the cases (but not in all), the treatment has been found to cause negligible color changes [36].…”

Selected Aspects Regarding the Restoration/Conservation of Traditional Wood and Masonry Building Materials: A Short Overview of the Last Decade Findings

“…It is achieved by the reaction of a phosphate salt and the substrate (marble, limestone, sandstone, sulfated stone, gypsum stucco, concrete, etc.). The topic was recently reviewed by Sassoni [109], so a very thorough presentation of the method would be redundant. We have chosen to discuss only a modification of the method, proposed by Pesce et al [110].…”