Calcium phosphate coatings for marble conservation: Influence of ethanol and isopropanol addition to the precipitation medium on the coating microstructure and performance

Abstract

The effect of adding ethanol and isopropanol to aqueous solutions of diammonium hydrogen phosphate, used to create a passivating layer of calcium phosphates over marble, was investigated. Thanks to its weakening effect on the hydration sphere of ions in solution, ethanol allowed complete coverage of marble surface by a crackfree and pore-free layer of octacalcium phosphate. Even better results were obtained using isopropanol, because it has lower adsorption affinity to calcite than ethanol. Treatments involving alcohols provided good acid protection and also restored cohesion among calcite grains in weathered marble.

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

“…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]. To identify the most promising treatment condition in the specific case of the specimens resembling paintings in the Magura cave, preliminary tests were carried on the "ST+G" specimens.…”

Electrodeposition of Hydroxyapatite Coatings for Marble Protection: Preliminary Results

“…If a continuous and dense coating of HAP is formed over the marble surface, significant protection against dissolution in rain can be expected [2]. Consequently, in recent years, several studies have investigated the feasibility and the effectiveness of forming a protective layer of HAP over marble [2,[4][5][6][7][8]. HAP can be formed in a few hours in ambient conditions (necessary to treat monuments in the field), by reacting marble with an aqueous solution of a phosphate salt (typically diammonium hydrogen phosphate, DAP, (NH 4 ) 2 HPO 4 ) [8,9].…”

“…HAP can be formed in a few hours in ambient conditions (necessary to treat monuments in the field), by reacting marble with an aqueous solution of a phosphate salt (typically diammonium hydrogen phosphate, DAP, (NH 4 ) 2 HPO 4 ) [8,9]. Although more effective than alternative commercial treatments [7], so-formed HAP coatings have nonetheless been found to provide marble with incomplete protection against acid attack, because some cracks and/or pores are present in the coatings [2,[5][6][7]. With the aim of improving the coverage of the marble surface by HAP and reducing cracks and pores in the HAP layer, the effects of several parameters have been investigated, such as DAP concentration, addition of a calcium source, anionic additions, and organic additions [5,7,8].…”

“…Although more effective than alternative commercial treatments [7], so-formed HAP coatings have nonetheless been found to provide marble with incomplete protection against acid attack, because some cracks and/or pores are present in the coatings [2,[5][6][7]. With the aim of improving the coverage of the marble surface by HAP and reducing cracks and pores in the HAP layer, the effects of several parameters have been investigated, such as DAP concentration, addition of a calcium source, anionic additions, and organic additions [5,7,8]. By tuning such parameters, improved resistance to dissolution in simulated rain has been obtained, but marble dissolution is not completely inhibited yet [7].…”

“…With the aim of improving the coverage of the marble surface by HAP and reducing cracks and pores in the HAP layer, the effects of several parameters have been investigated, such as DAP concentration, addition of a calcium source, anionic additions, and organic additions [5,7,8]. By tuning such parameters, improved resistance to dissolution in simulated rain has been obtained, but marble dissolution is not completely inhibited yet [7]. The main limiting factor is that some calcite grains, likely having unfavorable crystallographic orientation for HAP epitaxial growth, remain bare after treatment, even though all the surrounding grains are completely covered with the new HAP coating [7].…”