The analytical and diagnostic techniques applicable to artworks are usually divided into two groups: - Invasive: that is, examinations that require the taking of a sample, for example, the removal of a minimal quantity of material from the work that is then used in various examinations (chromatography, staining, etc.); - Non-invasive: those examinations such as x-ray radiography or infrared reflectography that can be carried out directly on the work by direct interaction with the surface using various types of energy. Invasive methods are further broken down into destructive (for a long time the only method used), that brings about the modification or the destruction of the sample, and non-destructive, for example x-ray fluorescence analysis (XRF) or infrared-attenuated total reflectance (IR-ATR). The examinations permit the study of the nature and the structure of the sample without it being altered or modified.