Microclimate and interactions between the atmosphere and Orvieto Cathedral

Some field tests have been carried out to study the weathering of Orvieto Cathedral. Orvieto is a small town in central Italy, sited on the top of a hill, a windy and relatively unpolluted site. The local microclimate, the solar radiation and the induced thermal wave, the main thermohygrometric parameters, the exchanges of heat and moisture and the deposition processes have been carefully taken into consideration with reference to the deterioration of the stone and the fall of the tesseras of mosaic. The thermal pattern of the Cathedral is strongly influenced by solar radiation in that the surface is warmed by the direct radiation which is, however, limited by the surrounding buildings. The different materials and variety of albedos of the coloured tesseras cause thermal gradients and internal stresses. The relative humidity is quite high, especially during the night and, in winter, throughout the whole day. The NNW side of the Cathedral is the coldest and condensation forms in the micropores and on the surface thus favouring the growth of lichens. The diurnal heating of the zones affected by direct solar radiation forces marked condensation-evaporation cycles. The droplets condensing on the mosaic grow until they run off and are partially absorbed between the tesseras, facilitating their detachment. The thermophoretic deposition and the Stefan flow are only seldom active; unlike the inertial deposition which is always present.