Ammonium monoethyloxalate (AmEtOx): a new agent for the conservation of carbonate stone substrates

M. Carla Aragoni

First

Member of the Collaboration Group

;Laura Giacopetti

Second

Member of the Collaboration Group

;Massimiliano Arca

;Gianfranco Carcangiu;Stefano Columbu;Domingo Gimeno;Francesco Isaia;Vito Lippolis

Member of the Collaboration Group

;Paola Meloni

Member of the Collaboration Group

;Antonia Navarro Ezquerra;Enrico Podda;Jordi Rius;Oriol Vallcorba;Anna Pintus

Last

Member of the Collaboration Group

2021-01-01

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Abstract

The ammonium salt of monoethyloxalate (AmEtOx) was investigated as a novel precursor for the conservation of carbonate stone substrates, such as biomicritic limestone and marble. Full characterization of treated and untreated authentic stone samples was carried out by means of SEM, X-ray powder diffraction, synchrotron tts-mXRD measurements, mercury intrusion porosimetry, determination of water transport properties, and pull-off tests. The improved solubility (1.49 M, 20.1% w/w) of AmEtOx as compared to that of ammonium oxalate (AmOx; 0.4 M, 5% w/w) results in the formation of microcrystalline phases 30–50 and 200–500 mm thick of calcium oxalate mono-(whewellite) or dihydrate (weddellite), on marble and biomicrite samples, respectively, after treatment with AmEtOx 5% and 12% w/w aqueous solutions. As a result, a reduction in the porosity of the stone samples and an enhancement of their cohesion are observed. DFT calculations, carried out to investigate the hydrolysis reaction leading from AmEtOx to AmOx, showed that the localization of the Lowest Unoccupied Molecular Orbital (LUMO) and the natural charge distribution account nicely for the tendency to hydrolyse observed experimentally, eventually leading to the formation of whewellite and weddellite on the stone surface.