Crystallography and Cultural Heritage: study of caoxite formed after a conservation treatment

In Cultural Heritage field, whewellite (CaC2O4•H2O - COM) and weddellite (CaC2O4•(2+x) H2O - COD) play a relevant role for a twofold reason: they are the main components of a particular kind of film found on the surface of many works of art and they also occur in a conservation treatment for carbonatic substrates [1], due to the reaction between ammonium oxalate solution and calcite of the substrate. In order to obtain a more homogeneous and deeper diffusion of the treatment into decayed materials, we focused our experiments on an alternative method using diethyloxalate [(C2H5)2C2O4] which during the hydrolysis transforms calcium carbonate into calcium oxalate [2]. The reaction between diethyloxalate and calcite produces COM, COD and an unexpected relevant amount of caoxite (CaC2O4•3H2O - COT). The crystallization of COT into the treated carbonatic substrate is the main proof of the reaction between diethyloxalate and the decayed stones, thus its identification is essential to monitor the treatment and define its efficacy. Due to the scarce literature data, an extensive investigation of crystallographic, spectroscopic and microscopic features of COT has been performed, including the development of a new method for its synthesis [3]. The experiments clearly highlight the role played by the H2O molecules linked within the structure by different kinds of hydrogen bonds. The vibrational assignment of the infrared and Raman bands is critically proposed. The fact relevant for the work in biomedicine, cultural heritage and crystallography is that a simple examination of the spectra allows quickly to determine the chemical nature of the material in an unknown sample even in a minute quantity or in awkward experimental conditions. [1] B. Doherty, M. Pamplona, R. Selvaggi, C. Miliani, M. Matteini, A. Sgamellotti, B. Brunetti, Appl. Surf. Sci. 2007, 253, 4477. [2] C. Conti, I. Aliatis, M. Casati, C. Colombo, M. Matteini, R. Negrotti, M. Realini, G. Zerbi, J. Cult. Herit. 2013, 15, 336. [3] C. Conti, M. Casati, C. Colombo, E. Possenti, M. Realini, G.D. Gatta, M. Merlini, L. Brambilla, G. Zerbi, Spectrochim. Acta A, 2015, 150, 721.