Chromatic alteration of Roman Heritage in Aosta (Italy).

The conservation study of the Roman Heritage in Aosta (Italy) is aimed at understanding the causes of the superficial chromatic alteration, which has affected the Roman ruins (I sec a.C.) since the last decades. The Roman monuments were built of pudding stone, a quite irregular and inhomogeneous conglomerate of fluvial origin: minerals as quartz, micas, feldspars and fragments of metamorphic rocks are set in an arenaceous matrix, cemented by microcrystalline calcite. The stone in place started showing superficial chromatic alteration as reddish and pinkish areas in the last fifty years only, as deducible from previous studies [1] and photographic records. Previous data discarded the hypothesis that such alterations might be due to biological effects. Working hypothesis is that electromagnetic pollution might lower the activation energy of the dissolution reaction of biotite. From recent analyses it seems instead that also air pollutants (NOx, SO2, CO2) and resulting acidic precipitations increase the weathering rate of biotite, which converts in vermiculite, and induce enhanced loss of Fe from the mineral [2]. Preliminary analyses have been carried out by Scanning Electron Microscope (SEM-EDS), Powder Diffraction (XRPD), μ-Raman Spectroscopy and Electron Paramagnetic Resonance (EPR) on altered and unaltered pudding-stone specimens sampled from the Roman Theatre. First results show that the alteration may be due to the oxidation of ferrous ion, released during the weathering of biotite. So far, the results allow to understand that altered zones are composed by a 50-100 μm thick layer of recrystallized calcite. The iron oxides widespread in the secondary calcite, due to fluid circulation through porous material, and crumble the pudding-stone surface.