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

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-μXRD 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 μm 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.M. A., M. C. A., F. I., and V. L. thank the Fondazione di Sardegna (FdS) and Regione Autonoma della Sardegna (RAS) (Progetti Biennali di Ateneo FdS/RAS annualità 2016) for financial support. A. P. acknowledges RAS for the funding in the context of the POR FSE 2014–2020 (CUP F24J17000190009). Comune di Cagliari, Segretariato Regionale per i Beni e le attività culturali per la Sardegna, and Soprintendenza Archeologia, Belle Arti e Paesaggio per la città metropolitana di Cagliari e le province di Oristano e Sud Sardegna are kindly acknowledged for supporting the Cultural Heritage Conservation Laboratory “Colle di Bonaria” in their research and teaching activities.Peer reviewe