Nanomaterials, optotechnology and image processing for old stones

The aim of the interdisciplinary EU project Nano Cathedral was to develop tailor-made solutions for the conservation of natural stone at the nano level. Now their effect is to be proven in the long term. Restorers, architects, conservation scientists, art historians, but also chemists, material scientists and social and economic scientists are working together in a team to make a decisive contribution to the preservation of cultural heritage.

In addition to strengthening agents that on the basis of nanoparticles that are 1000 times smaller than the diameter of a human hair bring stability to the depths of the stones, the scientists at the 19 partner universities have also developed hydrophobing agents that seal surfaces and allow rainwater to roll off similar to the lotus effect. All the nanomaterials were first tested under laboratory conditions and later applied to the original surfaces of the monuments. Whether the materials will bring the hoped-for benefit in actual use is now to be proven by long-term monitoring using light technology. What is new here is that use of the new materials can be tested without destroying parts of the stone through sample extraction.

The so-called Opto-technical Monitoring, which combines various imaging methods, was developed especially for this project at the Chair of Restoration Sciences in the Preservation of Historic Monuments at the University of Bamberg. In contrast to standard methods such as drilling, which can only cover small areas, the new method can also make comprehensive statements about a test area approximately two square metres in size while at the same time being non-destructive and contact-free. The method of opto-technical monitoring is based on a combination of high-resolution 3D methods, VIS colour photography, ultraviolet photography and infrared photography. These different techniques, in combination, allow the condition of the original rock before and after treatment with the nanomaterials to be determined. While high-resolution 3D images are used for surface measurement and record the condition at the time of measurement with an accuracy of 0.3 millimetres, VIS colour photography is used to highlight colour differences on the surfaces. In UV fluorescence photography and infrared photography, inorganic and organic foreign materials become visible on the surfaces, i.e. preservative coatings or biological fouling such as bacterial films, lichens or mosses.

First, the actual condition of the original rocks was measured and the surface documented. After one year, the scientists carried out new measurements – shortly after the nanomaterials had been applied to the rock. The comparison of the two measurement results makes clear the weak points of the rock, the progressive weathering and the loss of historically-significant surfaces. In order to monitor the long-term effects of the newly-developed nanomaterials and their weather resistance, researchers will carry out further measurements over the next two, five and eight years. The developed nanomaterials are now set to be introduced to the market. The cathedral workshops have already expressed further interest in the monitoring, and even centuries-old murals in Sri Lanka are to be monitored using this method.