Combining digital speckle pattern interferometry with shearography in a new instrument to characterize surface delamination in museum artefacts

Digital speckle-shearing-pattern interferometry (DSSPI) combined with digital speckle pattern interferometry (DSPI) were used to document the state of preservation of 8th to 14th century wall paintings of Christian Nubia. The original plasters of the paintings were multilayer inhomogeneous structures cracked in some parts. They were removed from the cathedral in Faras, Sudan, between 1961 and 1964 as part of efforts to rescue archaeological works threatened by flooding and are now held in the collection of the National Museum in Warsaw. Delaminated areas in surfaces of the paintings were detected by inducing surface vibrations with a sonic wave of varying frequency emitted from a loudspeaker. DSPI allowed the size of the areas and vibration resonant frequencies to be characterized, whereas DSSPI was successful in obtaining the spatial distribution of the vibration amplitude with sub-micrometre accuracy as a function of the sound frequency used. The procedure provided precise, detailed and reproducible information on the character of each damaged area in spite of the considerable size of the paintings and the unstable out-of-lab conditions of a conservation studio in which the measurements were carried out. The obtained results will be used as a reference for future surveys of the paintings in order to trace the possible development of surface detachments, even at an incipient stage, which cannot be easily detected by the naked eye or manual inspection.