The House of Marcus Lucretius preserves the remains of paintings of the so-called Pompeian second, third and fourth styles. In this work, samples from the second style painting (architectural style AD 40-80), buried in the ground and not exposed to open air, recovered from recent excavations (2004)(2005)(2006), were analyzed. Moreover, wall paintings and wall fragments from the fourth style (baroque ornate style, before Vesuvius eruption), excavated about 150 years ago and exposed since then to outdoors, were also analyzed. Raman spectroscopy was used to characterize the original composition and decay products in the mortars and pigment layers. These spectroscopic results were complemented with quantitative analysis (ionic chromatography) of soluble salts and chemometric and chemical equilibrium calculations. Probable decay pathways are proposed to explain the formation of some decay products of red pigments and for the original components of the mortars. Moreover, a final diagnosis as well as comparison of the conservation state of the mortars and pigments exposed to two different kinds of environments (outdoors and under a burial) is discussed emphasizing the importance of SO x impacts on the open air artworks.

This work presents the results of field Raman analyses, assisted by a hand-held energy dispersive-X-ray fluorescence spectrometer, to experimentally determine the composition of compounds present in the walls and wall paintings of two Pompeian houses, one with many luxurious decorative elements (the House of Marcus Lucretius, Regio IX, Insula 3, House 5/24) and a more modest building (Regio IX, Insula 3, House 1-2). These houses were excavated 150 years ago, and the majority of the rooms have been exposed outdoors. The chemical attacks of the acid gases and the biological colonisation can be considered the most serious problems of the archaeological remains from Insula IX 3 of Pompeii. The walls and wall paintings exposed to the rain-wash are the worst preserved ones, probably due to a continuous cycle of SO 2 attack to the original materials, involving loss of plaster. This severe decay was not observed in the rooms covered by roofs; in these last rooms, the most noticeable pathologies are the presence of high humidity in the walls and the elevated amount of efflorescences.

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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 (Fe(3)O(4)) 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 SO(2).