Investigation of the Thermal Behavior and Determination of the Progressive Residual Expansion at Low Humidity of Untreated and Hydrophobized Baumberger Lime Sandstone

Changing climatic conditions are one of the most important factors for the weathering of natural stone objects in outdoor areas. Changes in temperature and humidity, either individually or in combination, are decisive for the course of certain weathering processes. The minerals that make up a stone respond to thermal stress with expansion and contraction. The same applies in connection with changing humidity. These changes in volume lead to stresses within the rock and thus to a successive wearing down of the microstructure. For this reason, thermo-hygric processes are largely responsible for the weathering of natural stone objects. Accordingly, it is all the more important to understand these processes more precisely in order to develop appropriate conservation and protection measures for art and cultural objects made of natural stone. To date, there have been isolated, unsystematic and sometimes contradictory measurements of Baumberger lime sandstone. The studies have rarely considered or mentioned stone varieties and sample orientations. The effects of repeated heating and possible residual expansion in this stone are also not considered. None of these studies consider the influence of moisture on the measurement. The combination of thermal and hygric expansion of Baumberger lime sandstone samples before and after hydrophobization has also not yet been investigated. The study as part of a DBU project “Dealing with old hydrophobization: Model conservation of objects made of Baumberger lime sandstone deteriorated by old hydrophobization and development of a practice-oriented guideline” (DBU 2024) aimed to fulfil this urgent desideratum by including these correlations. The aim of the study was to gain new insights into the weathering processes of objects made of this material by comparing the expansion values before and after hydrophobization.