Dynamic water vapor sorption: a helpful tool for preventive conservation of salt contaminated built heritage

Abstract The presence of salts and related salt-induced damage represent one of the major threats to the preservation of our built heritage. Identifying critical relative humidity values that facilitate crystallization cycles is essential for understanding damage risks and extents. This knowledge helps in developing recommendations for favorable, damage-avoiding climates, particularly in controllable indoor environments. While for single salts their deliquescence humidity is known, for multi-ion mixtures relevant for the built heritage multiple transitions happen over a range of relative humidity. Modeling of equilibrium crystallization pathways is possible, e.g. using the Pitzer formalism. However, for complex mixtures, only predictions can be given, which need to be validated through experimental results. This work focuses on the use of dynamic water vapor sorption measurements to investigate phase transitions in salt mixtures, demonstrating its applicability, scrutinizing different influencing factors and an appropriate interpretation of results. Additionally, presenting an experimental design that delivers reliable results for the conservation of cultural heritage is crucial. In addition to single salts, mixtures from the common hygroscopic system Na + –K + –Mg 2+ –Ca 2+ –Cl – –NO 3 – –H 2 O are investigated, including their behavior in a stone material. The identified transitions are compared to the calculated behavior using the ECOS–Runsalt model. The presented results are accurate and reproducible. They show the ability to determine the critical relative humidity ranges (in bulk and in porous materials) and validate thermodynamic models.