The paper deals with the application of two analytical procedures, based respectively on on-line pyrolysis in the presence of hexamethyldisilazane followed by gas chromatography/mass spectrometric analysis (Py/GC/MS) and on selective extractions and microwave-assisted hydrolysis followed by GC/MS determination. These procedures give reliable data on the organic composition of micro-samples; moreover, when applied to the same micro-sample, they give comparable data on natural substances, and complementary information on synthetic compounds and degradation products. The two methods were used to characterise the organic matter present in several samples taken from scenes of the frescoes 'Giudizio Universale' and the 'Ascensione' by Bonamico Buffalmacco in the Monumental Cemetery of Pisa. The observed hydrophobicity of their surfaces, which hampers traditional means of restoration, is explained by the formation of a polymeric network due to reactions between a synthetic polymer used as adhesive, the proteinaceous material (animal glue and casein), and formaldehyde.
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