Thermographic inspection in the service of art science: theory, methods and considerations

Since the beginning of the last century, the number of methods available to art analysts has significantly increased. In addition to methods based on personal experience, new approaches have appeared that rely on the use of contemporary technologies, including lasers, radiation of various wavelengths, acoustics and any combination thereof for the extraction of a full range of information. The information of interest is often related to the composition and the conservation state of an artwork, as well as to the materials used and their age. Quite recently, thermographic defectoscopy has been introduced as a new method for artwork diagnostics. After being proven effective for the analysis of wood and plasters, this method has been applied to the inspection of canvas-based paintings. Thermography of pictorial art objects usually utilises a short heat pulse in order to bring the object out of thermal equilibrium. The following evolution of the temperature is recorded by a thermal imager. Since the temperature distribution on the surface is affected by the internal structure of the object, it becomes possible to draw conclusions on the condition of the subsurface layers. The readings of the thermal imager can sometimes be used without any post-processing (for example in the detection of large detachments), but may require an additional mathematical algorithm to reveal minute internal defects. There are several ways of post-processing the raw thermographic measurements for the extraction of information. This article outlines some existing methods, as well as their theoretical backgrounds, and discusses their applicability. Several examples of thermographic analyses of real paintings are given and discussed.