XPS INVESTIGATION ON CALCIUM OXALATE MONOHYDRATE FOR USE IN STUDIES OF CULTURAL HERITAGE

In preparation for its use on samples obtained from historic marbles1, an accurate XPS study was here performed to characterize the standard compound, calcium oxalate monohydrate, chemically synthesized as reported in literature2. Oxalate films are widely found on stone monuments of various natures and surroundings, as a result of either chemical (oxidation of pre-existing organic molecules) and biological (micro-organisms metabolites) degradation. Both, often mutual, chemical and biological processes produce oxalic acid which reacts with the calcareous substrate to give calcium oxalate. The results obtained have allowed us to identify the spectral features of calcium oxalate and to compare them with those of other standard such as calcium carbonates and sulphates (XPS data set already available3), knowing the simultaneous presence of these compounds in oxalate films, together with variable amounts of silicates and other minerals4. Importantly, repeat XPS acquisitions have definitely shown spectral changes of calcium oxalate monohydrate with time, Figure 1, that could be compensated for in deducing the original signals by considering X-ray power, UHV (ultra- high vacuum) and thermal effects suffered by the sample during the overall time of the analysis.