Micro-Raman analysis on the combined use of ammonium oxalate and ammonium phosphate for the consolidation and protection of carbonate stone artifacts

Ammonium oxalate ((NH4)2C2O4, AmOx) and more recently di-ammonium phosphate ((NH4)2HPO4, DAP) are used as inorganic agents in the conservation of cultural heritage for protection and consolidation of carbonate stone artifacts. In this work, we carry out a Raman investigation on the extent of penetration provided by a combined use of them. In particular, AmOx followed by DAP, DAP followed by AmOx, and a DAP + AmOx mixture are applied on tablets of pure CaCO3 as well as on degraded marble samples. Then, cross-sections of samples are analyzed in depth from surface to bulk. Characteristic differences in penetration depth of these agents and distribution of their products of interaction with the substrate are detected and discussed. A homogeneous distribution of whewellite inside the substrates down to a depth of ~1 mm was detected, which became larger in highly degraded regions of marble substrate. Ca-phosphates in the form of hydroxyapatite were detected at greater depth (down to 2.5 mm), confirming better consolidating properties of DAP with respect of AmOx. Among the application methods tested in our investigation, the DAP followed by AmOx treatment appears the most effective. The discussion of results takes into consideration several aspects including solubility and interaction dynamics between reaction products as well as the peculiar morphological features of the artifact, which are evidenced to play a significant role in treatment choice.