Design, synthesis, and quantum-mechanical modeling of inorganic salts as protection and consolidation agents of stone substrates

Natural stones have been used since ancient times to produce manufacts, to build monuments and to create works of art. Among lithic materials, white marble is widely used in sculpture and architecture. Unfortunately, this metamorphic natural stone, composed by mainly four kinds of carbonatic minerals (calcite, dolomite, aragonite, and vaterite), is subject to deterioration due to both natural and anthropological phenomena, such as climatic conditions and exposure to pollutant agents, which are the main causes of monuments degradation. In this context, chemistry can be of great help. During the past two decades, different organic and inorganic compounds have been tested as new consolidants for the protection and restoration of marble manufacts. Ammonium oxalate is one of the most promising salts in virtue of its protective properties, but it is not able to deeply penetrate the carbonatic matrix, and the attainment of more effective treatments for weathered marbles is still a goal to reach. The synthesis, characterization and determination of the solubility in water or different solvent mixtures are reported for the ammonium salts of the oxamate and mono-methyl oxalate anions. The two compounds have been tested for the restoration and conservation of marble manufacts coming from the Cimitero Monumentale di Bonaria in Cagliari (Italy). In addition, preliminary results on the structural and computational aspects deriving from the DFT modeling of the protective agents involving the carbonate matrix are presented.