The Halberstadt Cathedral is one of the few Gothic cathedrals in the French architectural style in Germany, and its medieval furnishings are unusually complete. But air pollutants have severely affected the 500-year-old sculptures and ornaments. Now the most modern digital and laser technology should save the art treasures. To do this, restorers use drone-supported analyzes and lasers for contact-free removal of the layers of dirt.
The cathedral in Halberstadt, Saxony-Anhalt, was built between 1236 and 1486 on the model of French cathedrals. Similar to these, the three-aisled Gothic church building is stabilized by open flying buttresses and ribbed vaulting on the inside. In contrast to many other church buildings from this period, the medieval furnishings have been almost completely preserved. This includes the early Gothic portal depicting the Last Judgment, flanked by angels and evangelists. The facades and the interior are decorated with numerous sculptures. The Halbstadt Cathedral is one of the few examples of a German Gothic cathedral that has been preserved almost completely.
Affected by pollutants
But this historical treasure is under threat: air pollutants and other environmental influences have turned the stones black and are causing increasing corrosion of the reliefs and sculptures. One cause: Sulfur dioxide, which is mainly produced when burning sulphurous fuels such as coal, starts damage processes to stone materials. This also affected the colored choir sculptures from the 15th century. “The plaster used in the Middle Ages as part of the paintwork could not withstand the permanent climatic and microbiologically unfavorable environment,” explains the restorer Corinna Grimm-Remus. “That’s why the color versions came off.” In addition, the air containing sulfur dioxide and lack of care had a negative effect.
There is another aspect to the exterior facades: “The increasing humidity as a result of the climate crisis is worsening the damage already caused by sulfur dioxide,” says Sybille Weigelt-Röseler from the Saxony-Anhalt Cultural Foundation. Because the moisture dissolves the existing sulphurous and nitrogenous salts and thereby causes additional structural damage through the resulting acids. “The so-called acid rain leads, for example, to the defective conversion of lime into gypsum,” explains Weigelt-Röseler. “This is the case for many buildings in the region.”
Help from laser technology and drones
In order to save the medieval ornaments and sculptures in Halberstadt Cathedral and other cathedrals, restorers will now systematically test new technologies for the preservation and restoration of the works of art and components as part of a new project. “It is our responsibility to restore and preserve nationally important cultural assets,” says Alexander Bonde, Secretary General of the German Federal Environmental Foundation (DBU), which is funding this project. “Digital methods play a special role, the application of which must be made easier in practice.”
One of these technologies is the use of lasers to clean surfaces. Laser technology is recognized as a high-quality cleaning method in restoration science, particularly because of the non-contact handling, says Constanze Fuhrmann, head of the DBU’s Environment and Cultural Property Protection department. However, the application is not very simple: the setting parameters of the devices must be continuously adapted to the various existing harmful phenomena and materials. “This is where the Halberstadt Cathedral comes into play,” says Fuhrmann. “It is so interesting because it shows many different damage patterns on the stone, on which various laser applications can be tested as models.”
In the project, an application and setting catalog for the operation of laser technology in the monument area is to be developed for the first time. This is supported, among other things, by a drone-based analysis and documentation of the damage to the facade and sculptures in the interior. “We also want to carry out the laser cleaning on the Gothic cathedrals in Halle and Magdeburg, which are also nationally important, in order to gain comprehensive experience with different material substrates and soiling,” says Grimm-Remus.
Source: German Federal Foundation for the Environment (DBU)