Detective work using terahertz radiation

Damaging biocides can be detected on old wooden sculptures, hidden wall paintings can be made visible again and the layered structures of pieces of art analysed. Using terahertz scanners restorers will soon be able to identify quickly, and completely non-destructively, what is happening with an object of art. Fraunhofer scientists will introduce a new generation of such scanners at the denkmal trade fair from 22 - 24 November in Leipzig, Germany.

It was a special moment for Michael Panzner of the Fraunhofer Institute for Material and Beam Technology IWS in Dresden, Germany and his partners in the Dresden Hygiene Museum, where the scientists were examining a wall picture by Gerhard Richter that had been believed lost long ago.

Shortly before leaving the German Democratic Republic the artist had left the picture behind as a journeyman's project. Then, in the 1960s, it was unceremoniously painted over. However, instead of being interested in the picture, Panzer was far more interested in the new detector which was being used for the first time here.

Using it, the scientists gained important information about the layered structure of the wall and the structure of the picture area being examined. The joint project was sponsored by IWS, the Academy of Fine Arts Dresden, the Research Institute for Monument Conservation and Archeometry ) and the Technical University Dresden through the German Federal Ministry of Education and Research.

According to Panzer, the special thing about the terahertz (THz) scanner is that, in comparison with traditional processes, such as X-ray scanners, it works without causing any damage whatsoever. In addition, it  does not require a special permit, as in the case of harmful X-rays. This is because the scanner only generates a radiated power of less than 1 µW.

For comparison: under less than ideal conditions, cell phones emit up to 2 watts. Furthermore the process,  provides concrete data on the structure of the individual layers or of potential hollow areas. In this way the device also indicated in the Hygiene Museum that in one area the plaster on the wall had evidently been repaired - a valuable clue for the restorer.