Detection of calcium phosphates in calcium oxalate patinas

Abstract

Calcium oxalate patinas have been studied on historical monuments for years. In some cases they are associated with calcium phosphate, the presence of which has received contrasted attention and may have been overlooked with the characterisation techniques used. This study addresses the state of the art, the most suitable characterisation techniques for identifying calcium phosphate – the most effective and reliable being SEM-EDS of thin sections – and the shortcomings of other methods such as XRD, FT-IR or optical microscopy. It also describes examples of the presence of calcium phosphate analysed in patinas on Spanish monuments. Calcium phosphates were found to appear in these patinas in different ways and shapes: in small aggregates, in a layer mixed with other patina components, as a uniform layer, or constituting a layer with filaments, or forming a porous layer with a sponge-like texture. These findings add new data for the debate about the origin of calcium oxalate patina. Moreover, a comparison of these with other findings is the basis for a proposal of a new denomination for such patinas.