Thermodynamic and Spectroscopic Speciation to Explain the Blackening Process of Hematite Formed by Atmospheric SO₂ Impact: The Case of Marcus Lucretius House (Pompeii)

After many decades exposed to a polluted environment, in some areas of Marcus Lucretius House, there are clear signs that plasters and hematite pigments are suffering deterioration. In the exhaustive analysis of the black layer covering the red pigment hematite it was possible to identify magnetite (Fe₃O₄) as responsible for the black colour, which always appears in combination with gypsum. Thermodynamic modelling stated that the presence of gypsum as well as the transformation of hematite into magnetite is a consequence of the attack of atmospheric SO₂.