Fuente: Molecules - Revista científica (MDPI) Molecules, Vol. 31, Pages 776: A Novel Approach to Carbonate Stone Conservation: Induced Calcium Oxalate Formation Through the Application of Ammonium N-Ethyloxamate (AmEtOxam) on White Carrara Marble
Molecules doi: 10.3390/molecules31050776
Authors:
Simone Murgia
M. Carla Aragoni
Gianfranco Carcangiu
Laura Giacopetti
Domingo Gimeno Torrente
Vito Lippolis
Eleonora Loi
Paola Meloni
Antonia Navarro Ezquerra
Enrico Podda
Anna Pintus
Riccardo Serra
Massimiliano Arca

Ammonium N-ethyloxamate (AmEtOxam) was synthesized, fully characterized by microanalytical and spectroscopic means, and assayed as a precursor of calcium oxalate, acting as a protecting agent for white Carrara marble. The monohydrate form of AmEtOxam shows a water solubility of 1.5 mol·L−1 (~23% w/w), significantly higher than that of common calcium oxalate precursors (CaOx), such as ammonium oxalate (0.4 mol·L−1, ~5% w/w). While AmEtOxam is stable in water solution and in the solid state in its monohydrate form, during the application on carbonate stone it undergoes a complete hydrolysis resulting in the formation of a uniform weddellite layer (CaC2O4·2H2O) on carbonate stone surfaces. Application of 5% w/w aqueous solutions by spraying, brushing, and immersion resulted in different effects. Spraying yielded the most balanced performance, improving mechanical strength, reducing water absorption, recovering superficial tension, and limiting chromatic alteration. Brushing achieved significant gain in surface hardness with minimal esthetic impact. Immersion most effectively reduced porosity and increased surface tension. These results, coupled with the negligible chromatic changes induced in all cases, make AmEtOxam a promising candidate for the conservation of stone cultural heritage.