Magnesium nitrate as sustainable solution for constant RH (53%) and pollutant absorption in display cases

The Viking Ship Museum Oslo started in 1957 using saturated salt solutions (here calcium nitrate) to stabilise the relative humidity (RH) in display cases. Three decades later, excellent results with low effort and cost were reported. However, many museums have since replaced these solutions with preconditioned silica gels. Their much lower water capacity causing more frequent need for maintenance, shifting equilibrium RH, unreliability as pollutant absorbers, and possible contamination were not considered or unknown at the time. The absorption capacity of saturated magnesium nitrate solutions for corrosive pollutant traces was studied using metal-oxide-semiconductor (MOS) gas sensors. Corrosive carbonyl pollutants (formic and acetic acid, formaldehyde, and acetaldehyde), often emitted from wood, are indeed highly absorbed. Some absorption was also demonstrated for hydrogen sulfide and sulfur dioxide. Practical application tests worldwide in museums have confirmed the advantages of this sustainable and failsafe passive method.