Microclimate monitoring by multivariate statistical control: The renaissance frescoes of the Cathedral of Valencia (Spain)

The renaissance frescoes of the metropolitan cathedral of Valencia, located at the vault of the apse, were restored in 2006. We describe a microclimate monitoring system that was implemented for the preventive conservation of the paintings. It is comprised by a set of temperature and relative humidity sensors positioned at different points of the vault. This system is rather unique because some of these sensors were inserted inside the paintings during the restoration process. A principal components analysis was applied to the data of relative humidity recorded in February 2007. The analysis was repeated in three additional months of 2007. The resulting loading plots highlight the most relevant similarities and dissimilarities among sensors. These plots can be considered as some sort of control maps that could be used to detect abnormal conditions in the future. Actually, moisture problems at certain zones of the frescoes are causing the formation of efflorescence, and the sensors located close to these zones are the ones recording the highest values of relative humidity.