Detection of Concealed, Graphite Containing Frescos, Using Microwave Enhanced Infrared Thermography

Active infrared thermography is one of the most popular NDT techniques. In this method an external energy source is used to produce the temperature difference between the feature of interest in examined specimen and the background. Thermovision cameras are used do observe obtained temperature differences. The variety of external energy sources can be used in IR thermography. Conventional sources (such as powerful halogen lamps or infrared heaters), apply heat externally to the surface of the material. The microwave irradiation, unlike to mentioned previously sources, penetrates and simultaneously heat the bulk of the specimen. For that reason, microwave heating can effectively minimize the heating times. This paper presents the method of detection of concealed graphite containing frescoes, using active infrared thermography with microwave irradiation as a heat source. It has be determined, that most of mural paintings, like frescoes, consist some amount of coal (in the form of carbon black used as a paint or charcoal used by the artist for sketching). Analysis of dielectric properties of coal shows that generally it can be considered as a good microwave absorber, therefore the high temperature difference between coal containing frescoes and the background is produced during the microwave heating of painting.