Geophysical investigations of medieval paintings at St. Petri Cathedral Schleswig (Germany) with georadar and thermography

The preservation of culturally significant buildings is challenging due to the variety of historical building materials, the often complex building history and damage patterns. It is usually associated with high financial costs. Non-destructive testing may help to plan, optimize, and monitor conservation measures. Here, we report on non-destructive testing of moisture distribution at the Cathedral St. Petri in Schleswig (Germany) using thermography and georadar measurements.  These methods are standard methods in engineering geology and construction. In the field of heritage conservation, however, the application and especially the combination of several of these methods is not yet established.

The walls of the ‘Schwahl’ (a three-sided cloister) show medieval paintings from the 14th century. In the Schwahl, large-scale alterations occur due to gypsum deposits and a shellac coating.   Active thermography measurements were taken before and after test treatments to evaluate the effectiveness of the use of different solvents to remove the shellac and the gypsum deposits. Passive thermography and georadar measurements indicate increased moisture content in the area of the gypsum deposits likely caused by a permeable horizontal sealing barrier below the paintings. Examples of the measurements are shown and the processing of the thermography and georadar measurements including the attenuation analysis are discussed.