El deterioro de las piedras graníticas de la fachada de la catedral de Vila Real (N de Portugal)

Abstract

The cathedral of Vila Real (North of Portugal) is located in the city centre and at least four regional two-mica granites were used in its construction. A petrographic study of these granites concluded that the stones had different degrees of alterations. The identification of some physical properties of the granites -open porosity, real and apparent density, free water absorption, porometry and capillary transmission-confirms their susceptibility to degrees of weathering. A recent cleaning intervention was made and several types of deterioration were visible in the monuments stones, namely: granular disintegration, some thin black layers, plates, flakes and black crusts, especially in the main front exposed to SW. Granular disintegration is the most important deterioration process and is responsible for the loss of material in the most-affected stones; it is also related to the presence of small amounts of highly soluble salts such as halite and nitrates and to solar exposure.