Physical weathering of stones

This paper reviews the main physical agents responsible for stone weathering. Particular emphasis is placed on the effects of wind and water. Aeolian erosion occurs in windy regions with sandy terrain, but in urban areas it is extremely rare and is often confused with other forms of deterioration. The problem of condensation on surfaces and in pores is extensively discussed, in relation to pore shape and size. Physical effects dominate in the smallest pores, and solution effects in the largest ones. The Kelvin law and thermodynamic considerations are used to explain condensation-evaporation cycles and adsorption isotherms. Capillary rise, increase in pressure in air pockets, freezing-thawing cycles and micro meteorological conditions favorable for the soaking of monuments are discussed. A final section is devoted to the dissolution of stones and the formation of black crusts as a consequence of the way the stone is washed out or simply wetted by meteoric water. The intensity of rainfall is important in activating or removing the dry pollutant deposits that form on monuments, especially during the dry periods characteristic of the Mediterranean climate. Condensation plays a minor role compared with rainwater. However, the porosity, geometrical shape and exposition of the stone are also important factors which can characterize different local balances and, consequently, the form of weathering.