Polychromy on stone bas-reliefs. The case of the basilica of Saint Ambrogio in Milan

Remnants of colour recently discovered on the central entrance portal of the Romanesque Basilica of S. Ambrogio in Milano (XI century)
were investigated by means of cross-sections, optical microscopy, scanning electron microscopy with energy dispersive X-ray analysis (SEMEDS),
Fourier-Transform Infrared (FTIR) spectroscopy and gas chromatography-mass spectrometry (GC/MS). The bas-reliefs showed the
presence of ancient paint (probably original) laid directly onto the marble's surface. The peculiar painting technique employing paint layers
made of gypsum, egg, and pigments (the palette consisting in different ochres, green earth, and vermilion), was pointed out. In particular, on
the lintel and capital, the presence of glue was detected, which could be interpreted as an original pre-treatment of the marble substrate.
Evidence for a later maintenance operation of repainting was also found, consisting of layers made with azurite, barite, gypsum, lead white,
and nut oil. The analytical findings are put in context with published results on coeval Cathedrals, looking at the obtained data from the point
of view of a general survey of painting techniques applied on stone materials.