Fog and gypsum crystals on building materials

Samples of fog water collected near Modena (Po Valley) are found to deposit numerous minerals after evaporation in the laboratory. The main minerals are nitrammite, mascagnite, koktaite, salammoniac, nitratine, niter, hannebachite, while many others are present, either sporadically and/or in trace form. Conversely, when fog water evaporation occurs in the field, such minerals, which are all hygroscopic and deliquescent, either do not form at all or have a very brief life span. The only one to survive for a few hours (alongside traces of nitrammite and mascagnite) is koktaite, which tends to transform rapidly into gypsum. Gypsum (CaS04 • 2H20) is therefore the only mineral remaining on the surfaces interacting with fog: it is possible that through this mechanism, considerable quantities of S042− (formed by the oxidation/ hydration of atmospheric SO2) and Ca2+ are removed from the atmosphere-hydrosphere cycle to form a relatively stable mineral. In particular, concerning the gypsum encountered today on all materials in polluted towns, this mechanism assumes a crucial role in the case of artefacts located in areas sheltered from rainwater wetting but prone to fogs, especially those containing little or no calcium.