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 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 the surface to bulk. Characteristic differences in
the penetration depth of these agents and the distribution of their products of interaction with the substrate are detected and
discussed. 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 to 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.