Carbonatic lithotypes passivated with the ammonium oxalate treatment: colorimetric and morphological study of treated surfaces

Since the mid 1980 many researches suggested the use of calcium oxalate on carbonatic lithotypes with protection purposes but only in 1994 a real treatment methodology was proposed. In this paper a new research is reported to deeper investigate the effects induced by the treatment with ammonium oxalate on a number of common carbonatic stone materials used in the Italian and European historical architecture. Some superficial stone properties like colour and morphology are investigated. The colorimetric investigations and the observations with the optical microscope show a lowering of the luminosity, an increase in saturation and in some cases also a light yellowing. This behaviour seems to be related to the formation of the oxalate medium itself that would determines both a smoothing of the surface which causes a decrease of the scattering and a variation of the refractive index which causes an increase of the light absorption