Application of ultrasonic pulse velocity for the condition assessment of plaster sculptures: a preliminary study

Abstract

Ultrasonic pulse velocity (UPV) is generally used in Civil Engineering as an in-situ and non-destructive methodology to assess the condition of construction materials. Given the fact that non-contemporary sculpture was traditionally made with similar materials – stone, wood, clay or plaster – a hypothesis arose regarding the possible contribution of UPV for the conservation assessment of sculptures with stone-based materials. Plaster is a fragile material, very susceptible to internal and external fractures, alterations and losses. Museum sculptures cannot be moved to a laboratory, due to conservation protocols and therefore portable UPV could, in theory, play an important role in assessing these artistic structures in-situ. There are scarce references to such a methodology, and its implementation implied a partnership with Soares dos Reis National Museum (Porto, Portugal) for an experimental approach to three plaster sculptures made by A. Soares dos Reis: “St. Joseph”, “St. Joachim” (both made in 1880) and “Narcissus” (1881). The aim of this study was to assess if portable UPV could help to detect superficial and internal damages and differences between the original plaster and added materials. UPV uses two transducers: a very high frequency pulse emitter (54 kHz or 150 kHz, depending on the type of transducer) and a receiver which measures the transit time, or the Time-of-Flight (ToF), between the transducers. Depending on the relative orientation of the transducers, the measurements are said to be direct, indirect or semi-direct. The distance between the transducers (the shortest path) divided by the ToF gives the velocity of propagation of longitudinal mechanical waves in the material, for the frequency used. If the shortest effective path does not match the measurement of the distance between the transducers, the time of flight is longer than it should be. Therefore, obtaining a lower velocity than the expected for a given material allows to identify an internal defect, a different material or alteration. An excessively high attenuation can also occur, depending on the extension of the wave path, or an interruption of the signal transmission due to the existence of a fault or due to the signal propagation in a material whose acoustic impedance is very different from the original material. When this happens, the signal may be lost. Before using UPV in situ, laboratory tests were carried out with plaster prototypes and different types of measurements. A fundamental part of the testing process involved the experimentation of flexible materials that could play the role of interface between the transducers and the sculptures. This is of the most importance in order to guarantee that the methodology is entirely safe concerning conservation standards.