Non-Destructive Testing Techniques to Help the Restoration of Frescoes

Among the several issues to be considered during fresco's restoration, the understanding of the effectiveness of the intervention, the identification of the main chemical elements used in previous restorations and the attention to weak areas of the building structure, adjacent to frescoes, are of paramount importance. This work describes an integrated, non-destructive testing approach focusing on these three main issues. In particular, two frescoes of Giacomo Farelli are analyzed herein. These artworks were affected by a strong earthquake in 2009, which had a heavy impact on several cultural heritage objects in L'Aquila (Italy), including on the Santa Maria della Croce di Roio Church (1625), where these two frescoes are located. One of the frescoes, which underwent a restoring before the quake, was previously tested by electronic speckle pattern interferometry (ESPI) before and after the restoration. These previous results are compared with new measurements carried out after the 2009 earthquake using infrared thermography (IRT). The combined approach, ESPI-IRT, clearly highlighted that the structure of the frescoes was significantly affected by the earthquake, since the old subsurface cracks, restored before 2009, were once again evident after the earthquake. In addition, the presence of a subsurface niche containing an ancient statue, also detected by means of IRT, might contribute to increase the severity of the damages. Finally, the joint examination of these frescoes using near-infrared reflectography and X-ray diffractometry was crucial to confirm the presence of a radioactive chemical element in the wall painting.