A black paint on the facade of a Renaissance building in Bergamo (Italy)

The first storey of the Casa dell'Arciprete in Bergamo, Italy, built of grey limestone, is today covered with a black paint layer. A detailed investigation of the morphology and composition of this layer was necessary in order to plan conservation work in 1993. Optical and scanning electron microscopy showed that the layer is very thin and closely follows the surface of the stone. Energy-dispersive X-ray (EDX) spectra of polished cross-sections did not reveal significant differences between the elemental composition of the stone and the black layer. Fourier transform infrared (FTIR) spectra of powdered samples provided evidence of a recent conservation treatment with an acrylic resin. Reflectance micro-FTIR analyses of polished cross-sections showed weddellite as the major component of the paint layer. Hydrolysis and gas chromatography-mass spectrometry (GC-MS) analysis of black samples showed the presence of an amino acid residue that has to be ascribed to an original proteinaceous binding medium. X-ray photoelectron spectroscopy (XPS) analysis of some black surface samples characterized the pigment phase as carbon of vegetal origin. Based on this investigation, it is hypothesized that the original paint layer was composed of carbon black applied with a proteinaceous medium, as recommended in ancient recipes. The organic medium is no longer the main component of the layer but present only in trace amounts. An oxidative process may have transformed the original binder into calcium oxalate. As the underlying stone was in good condition, the painted black layer was conserved.