Multidisciplinary approach for the identification of the environmental risks and the assessment of the preventive conservation strategy for the Portico de la Gloria, Cathedral of Santiago de Compostela (Spain)

In the framework of the "Santiago de Compostela Cathedral Program" funded by the Fundatión Pedro Barrié de la Maza a multidisciplinary investigation of the Portico de la Gloria was carried out between 2009 and 2011 to identify the main environmental (natural and antropic) risks and to develop a preventive conservation strategy and restoration plan. In order to characterize the materials, investigate their state of conservation and identify the most relevant damage processes, a complete study of the Portico was performed. This study included historic and archivist research, structural studies, mineralogical analyses, biological sampling and cleaning tests. At the same time a microclimatic monitoring was carried out following a specific methodology to understand the interactions between the atmosphere and the stone surfaces of the tympanum. With this approach, the main factors affecting the microclimate of the Portico were identified and consequently those having a greater impact on the observed weathering phenomena. The main deterioration processes related to the environmental conditions were identified and the environmental phenomena playing an important role were prioritized. Nevertheless, the results obtained have shown that the main responsible for the observed damage was the infiltration of rainwater through the roof, due to cracks in the structure of the Cathedral. In fact, water soluble salts (mainly nitrates and sulfates) were detected in high concentrations in the south side of the Portico, causing efflorescence and sub-efflorescence driven by the thermo-hygrometric conditions and by the impact of solar radiation. These phenomena in synergy with biological growth (algae) and thick surface deposits represented a remarkable weathering factor especially for the polycromy. The identification and evaluation of the most important environmental risks affecting the Portico allowed to establish the main conservation and intervention measures to be taken. Besides the repair of the roof to stop external water penetration, other solutions were suggested to improve the microclimatic conditions of the Portico, especially those addressing the atmospheric stability, thus reducing any further damage. The proposed interventions have to be integrated in the general management strategy for the conservation of the Cathedral and pooled with the unavoidable presence of huge quantities of devotes and pilgrims.