HAWK and Marburg University profile substance damage using terahertz scanners
Recently, the German research team has successfully applied the method to the late gothic altar of the Virgin Mary at Isenhagen Monastery.The central problem in art restoration is to determine, whether a cavity defect is located directly beneath the surface and which volume it affects. To date, common restoration methods fail to deliver an objective assessment when examining damaged wooden sculptures and other historically relevant materials. Conservators for instance use the percussion method on wall surfaces, which means tapping on the surface to estimate the extent of the invisible damage. “At the New Museum in Berlin, I tapped plaster surfaces for cavities in the Greek Hall. After a week, I wished for a better method for myself and my knuckles, one that could rather provide me with more information in a non-contact and non-destructive way”, recalls Dr. Dipl. Rest. Kirsti Krügener of HAWK in Göttingen.
This marked the beginning of a collaboration with the Terahertz Group established by Prof. Dr. Martin Koch of Philipps University Marburg. “In very simple terms, terahertz tomography is similar to an ultrasound measurement with invisible light”, explains Prof. Koch. “We use terahertz pulses to penetrate through the material. If we see a pulse reflection at a transition from the material to air, then there is a cavity.”
"Here, we are using a completely new method in art restoration, according to the motto: ‘Technology Meets the Middle Ages.’ At the end of our research, we anticipate to ideally develop a handheld terahertz scanner, which provides reliable data of the sub-surface damage directly at the object. Our aim is to establish a novel monitoring method in the field of art conservation”, says HAWK project leader Prof. Dr. Wolfgang Viöl.
The wooden sculptures in Isenhagen Monastry were scanned twice with terahertz radiation in the course of the project: Before and after restoration (Fig. 3). Before the treatment, defects were visible. After the restoration, the filled cavities and reinforcements could be imaged with terahertz time-domain spectroscopy. Prof. Koch explains that the restoration has now resulted in altered characteristics of the detected pulse sequence, to the point of the absence of pulses, due to the bonding of surfaces. The team compiled the terahertz data into a 3D map using commercial software. This way, the team for the first time interconnects the visible defect mapping of the surface with the non-visible depth defect analysis. “Through the precisely acquired condition of the objects by means of the combined mapping, an appropriate professional restoration is easier to perform and monitoring of the items – tracking of their status – in an objective manner becomes possible”, says Krügener, who took over the scientific conservation consulting and material analytics together with Master Conservator Roksana Jachim.
The figurines of Peter, Simon, and Mary with infant Jesus on her arm which have been selected and removed from the altar not only went through the digital defect mapping and additional 3D mapping by Roksana Jachim, but also through the terahertz tomography of the Marburg PhD candidates Cornelius Mach, Jan Ornik, and Jochen Taiber. To Corinna Lohse, leader of the restoration workshop of the Monastery Chamber Hanover, such set of data offers a very good basis for the currently ongoing restoration and the future monitoring of the Altar of the Virgin Mary at Isenhagen Monastry.
The Klosterkammer Hannover and the German Federal Environmental Foundation (DBU) support the innovative research through financial funding.
