Materials are measured using sophisticated methods in geophysics. Specialists in the field of restoration, however, rely on their eyes, ears and experience as well. When the two disciplines come together, the result can be remarkable.
Irrespective of whether it is a matter of an unadorned functional construction like the Rader Hochbrücke bridge, medieval wall paintings in the Schleswig Cathedral or ancient paintings in a Pompeian villa: the structural condition of these objects must undergo frequent examination in order ensure they retain the wide variety of functions they serve. A bridge should not collapse and wall paintings should bear witness to earlier cultures for as long as possible. And for this reason alone, it is important to do as little damage as possible to the substance when examining it.
“How thick are the walls? Do they have formwork, fillings or other special features?” These are the questions that are particularly relevant to historical buildings, says Professor Thomas Meier from the Institute of Geosciences at Kiel University. And he adds: “The surface properties are important because they are not only crucial for the appearance of old buildings, but also for their preservation”.
The geophysics department in Kiel has now for the first time explored a new method for the scientific analysis of murals. The 14th century fresco “murder of the innocents in Bethlehem” in Schleswig and the more than 2,000-year-old murals of the Villa dei Misteri in Pompeii, Italy, were subjected to ultrasound surface measurements. In describing an essential result, Professor Meier says, “The weakness of the ultrasonic surface measurement proves to be its strength at the same time.” With this ultrasonic measurement, it is only possible to penetrate a few centimetres into the surface, but close to the surface it provides very precise data.
The method is also relatively simple and, more importantly, non-invasive. A pulse generator and receiver are briefly affixed to the wall on either side and the signal path between them can be measured at almost any chosen interval. Depending on the appearance of the resulting waves, conclusions can be drawn about the properties of the material. If it is a very solid medium with a simple structure, the waves are fast and exhibit a simple shape. If, on the other hand, the surface is rather porous, the waves are clearly dispersed.
The geophysicist Sandra Hintz, who evaluated the data from Pompeii and Schleswig in her master's thesis and was also involved in some of the measurements in Schleswig, has investigated this extensively. According to her, the method proved to be very suitable for assessing the condition of an object – and equally suitable for assessing the effectiveness of efforts to restore it.
The science of geophysics, however, cannot achieve this with its own expertise alone. “We need the experience that restorers have,” says Professor Meier. His own discipline can, for example, marvellously determine whether and to what extent a painting is on an elastic background, but what this means for the painting itself and its preservation is a matter for the restoration experts.
As this soon became apparent after the first experiments, the geophysics department sought and established contact with the restoration centre Kiel. There, restorer Henrieke Drengemann, who recently completed her studies at the University of Applied Sciences and Arts in Hildesheim, wrote her master's thesis on ultrasonic surface measurement. “The method works, even if some of the waveforms are somewhat surprising for us,” says Drengemann, who also spent some time in Pompeii. Her colleague Julika Heller points out that scientific methods are playing an increasing role in her field and that the method now being tested is a useful addition: “Precisely because it is non-invasive and can show whether our work has been effective or not.”
This has been successful in the case of Schleswig. “In the end, we were able to demonstrate the effectiveness of the restoration,” says Thomas Meier. He adds that it is very likely that such methods will be used even more routinely in future. According to the geophysicist, this should ideally always be done in cooperation with the restoration experts from the very outset.
Author: Martin Geist