The stucco decorations in the church of San Ambrogio, Cantù (Como, Italy) date to the baroque and represent an important example of decorative schemes in Lombardy. Gold leaf was originally applied to a 'finishing mortar', composed essentially of calcic lime and marble fragments. Examination of gilded fragments by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and X-ray analysis (SEM-EDX), Fourier transform infrared spectroscopy (FTIR) and micro-Raman spectroscopy established that under the gold leaf (which was up to 5μm thick), a layer composed of lead oxides (PbO, Pb3O4) and lead sulphate (PbSO4) was present. Gas-chromatographic/mass-spectrometric analysis (GC/MS) revealed that linseed oil was used in the mordant for the gilding. It seems that the gold leaf was applied using linseed oil as a mordant (fixing agent) and litharge as a dryer and/or inert pigment. Where the gold leaf had become detached, the orange-red surface was covered with a lime painting, scialbo, which contains a proteinaceous binder (most probably egg) together with gypsum and calcium oxalate. Owing to extensive sulphation, this white layer is now almost completely decayed, leaving the stuccoes exposed to further degradation.