A non-destructive, contactless technique for the health monitoring of ancient frescoes.

In this paper an innovative non-destructive, contactless technique applied to the health monitoring of ancient frescoes is presented. The problem of the health monitoring of artistic frescoes without a direct interaction with structures and paintings is of great concern in the field of art restoration and preservation. In artistic frescoes, the partial detachment of plaster portions is a typical and serious problem. Both layer-to-layer detachments and delaminations and surface cracks are usually present in ancient wall paintings. At present, the standard procedure of diagnosis consists of manual inspection, but produces only approximate information. This paper describes an acoustic, non-invasive, experimental technique of diagnosis, based on the acoustic-structural interaction which occurs when a fresco wall is excited by a loudspeaker. The analysis of the acoustic pressure field and of its alterations allows the assessment of detachments, since the acoustic modal parameters are affected by the acoustic system boundary conditions, i.e. the portion of analyzed fresco. The reconstruction of the modal behavior of the analyzed portion of the fresco is made by a scanning laser Doppler which measure the velocity field of the observed surface. It is a non-contact measure technique that provides a great accuracy. Experiments carried out on fresco artificial specimens show the potential of the technique.