Conservation and restoration of St. George's church (Nördlingen, Germany), a 15th century Gothic church built using suevite from the Ries impact crater

An asteroid impact in southwest Germany ca. 15 Ma left a > 20 km-diameter blanket of poorly sorted clastic rock, called suevite. In the 15th century, the suevite was used to construct a Gothic church (St. George's church, Nördlingen). Visual inspection of the church highlights its strong weathering susceptibility. We investigate whether the suevite used to construct the church and the rocks used in its restoration (suevite and sandstone) are weaker when saturated with water, important for conservation strategies. We additionally assess, by comparing their physical properties, the suitability of the replacement stones. We show that the strength of the suevite used to build the church is reduced by half when water-saturated, considered here a consequence of its high smectite content. This helps explain the prevalent weathering of the stones forming the outer walls of the church. The replacement suevite has a higher wet/dry strength ratio than the original suevite and is therefore more resistant to weathering. Not only does the replacement sandstone have a low ratio of wet/dry strength, but also its physical properties are different from the original suevite, which has almost identical physical properties to those of the replacement suevite. Based on these results, and the appearance of the stones, we recommend that, where possible, natural suevite stones be used to replace the weathered blocks of suevite at St. George's church and at buildings built using suevite within the region.