Crack assessment in marble sculptures using ultrasonic measurements: Laboratory tests and application on the statue of David by Michelangelo

The large marble statues can suffer serious fractures, due to the stress states originated by the weight and the shape, often thin and articulated. Fractures are often triggered by surface cracking. For this reason, it is important to assess the severity of the apparent cracks, by performing periodic nondestructive surveys. The ultrasonic method is well suited for this purpose. This paper presents a research activity finalized at improving the application of the ultrasound method to the detection of crack depth in marble elements. Two different techniques are presented. These, after having been validated in the laboratory by operating on marble specimens, have been applied to a diagnostic investigation of the Michelangelo's David, one of the most famous masterpieces of Western art. The results obtained have allowed us to provide useful information about the severity of the damage. The depth was estimated with good reliability for some of the more evident cracks present in the left leg and in “broncone”, the false tree trunk on which the left leg rests. The maximum depth is evaluated at approximately 20 mm. In other areas, diffuse cracking or surface deterioration of the marble have been detected.