Overview of recent knowledge of patinas on stone monuments: The Spanish experience

The historic treatment of stonework has often been linked to the artificial application of patinas, mainly for aesthetic and protective reasons. Increasingly, however, researchers have identified a possible combined origin for patinas that has linked natural, biological processes to those associated with an artificial, man-made origin. This suggests that, although coatings may have been initially applied on purpose, they transform over time with the aid of micro-organisms and other chemical interactions. The original mixture applied to create a patina could include lime and/or gypsum, water, natural pigments and organic additives. However, their present-day mineralogy is varied and includes a wide range of minerals from calcium carbonates to calcium sulphates, calcium oxalates, calcium phosphates, silicates (quartz, feldspar, clay minerals) and iron oxides/hydroxides. Patinas have been studied in detail in Greece and Italy, but rarely in Spain. In this paper, existing knowledge on Spanish patinas is co-ordinated and previous and current research summarized. Emphasis is placed on artificial patinas initially applied to protect stone. These both appear to effectively protect the stone substrates on which they were applied and provide an insight into historical techniques of stone conservation. Because of this their preservation should be a strong consideration in restoration projects. Ongoing research focuses on the challenges of reproducing patinas, based on historical references.