Non-Destructive Testing (NDT) methods used for the study of stone frescoes

The value of cultural heritage objects is very high, requiring an observation technique that is safe, highly efficient, and sensitive enough to detect defects that occur in these objects. A combination of non-destructive testing (NDT) methods that meet these requirements increases the probability of detection (POD) and reduces the chance of false detection (false positives or false negatives). In this work, the applicability of optical, infrared and ultrasonic techniques, originally developed for industry, in the architectural restoration field is examined. As an example, we investigate a brick basis covered with two plaster layers containing artificial defects (crack and detachment) hidden in the sample. Squares of different colours have been painted with the last layer comparable to a real fresco, constituting a challenge for thermographic measurements (e.g., in pulse thermography, data is affected by non-uniform heating and local variation of thermal emission). The advantages and disadvantages of Holographic Interferometry (Double- Exposure – DE –), Digital Speckle Photography (DSP), Shearography, Transient Thermal Thermography and Ultrasonic Testing are discussed with respect to their ability to detect the previously described defects.