Characterization of patinas by means of microscopic techniques

Many stone-made historic buildings have a yellowish layer called 'patina' on their external surface. In some cases, it is due to the natural ageing of the stone caused by chemical-physical reactions between the surface of the stone and the environment, and in other cases it is the result of biological activity. The origin of these patinas can be also be due to ancient protective treatments. The use of organic additives, such as protein-based compounds, in lime or gypsum-based patinas is a traditional technique, which has been used in past centuries for the conservation and protection of stone materials. The thinness of the patinas ensures that microscopic techniques are irreplaceable for their analysis. Optical Microscopy, Fluorescence Microscopy, Scanning Electron Microscopy together with an Energy Dispersive X-ray Spectrometer, and Electron Microprobe are the microscopic techniques used for the characterization of these coverings, providing very useful information on their composition, texture and structure.