Weathering of soapstone in a historical perspective

Soapstone is a main material used in Norwegian medieval stone architecture. It is a metamorphic, dense and durable stone, but like other stones, it is occasionally subject to severe weathering. This paper gives an overview of the core issues related to outdoor soapstone weathering in Norway and shows how the weathering has changed over time. First, objects that evidently weathered quite strongly during the Little Ice Age (14th–19th century), especially in the countryside, have now been relatively stable for a hundred years or more. Although a multitude of weathering processes might have been active, this could point to a higher frequency of damaging frost events in the Little Ice Age than at present. In the case of former ruins, the present stable condition might also be related to good roofing and less moisture in the masonry. Second, the weathering was in complex ways, often governed by changes in architectural design, influenced by air pollution in the cities from the late 19th century until the recent drastic reductions in SO2 emissions. Third, alkaline salts from Portland cement, sulfate from air pollution and some stones, as well as chloride from acid cleaning, greatly enhanced the salt load at many monuments in the 19th and 20th centuries, giving rise to severe weathering, especially at places where water leaks and damaging run-off have prevailed. This situation obviously continues to be problematic.