By Gerhard Eggert

Saturated aqueous salt solutions (water containing the maximum amount of a salt soluble) have been used to create a fixed relative humidity (RH) in display cases. Museum handbooks from the ‘70s and ’80s—including those by Thomson or Stolow—devote some pages to their use. The last article on these salt solutions’ practical use appeared in 1991, and then there was nothing more. Why?

One can now hardly find a museum that still uses them. Are there any disadvantages? The earlier literature mentions the risk of spilling liquid, the creeping of salts, the effect of temperature changes, and possible corrosive emissions from solutions. All these problems have been addressed in a recent article in Heritage Science (Open Access: https://doi.org/10.1186/s40494-022-00689-3):

· The risk of spilling can be controlled by placing the solution in the bottom of cases below the exhibits.
· The creeping of salts over container walls can be avoided by using hydrophobic materials (PE, PP), rather than hydrophilic glass or steel which are easily wetted by aqueous solutions.
· By choosing the right salts (e.g., magnesium instead of calcium nitrate), one can avoid larger variations of RH with room temperature.
· Saturated solutions of magnesium nitrate (RH = 53%) and potassium carbonate (43 %) do not emit corrosive gases as was found in thermodynamic calculations and confirmed by the Oddy test.

“Often, good ideas are so simple; they only need to flash through your mind!” That was my thought when I first considered the effect of such solutions on pollutants in the atmosphere of display cases. Could salt solutions also be useful in absorbing pollutants from the air, thus killing two birds with one stone?

This idea first arose during desiccator model experiments at the Stuttgart State Academy of Art and Design, conducted to better understand glass induced metal corrosion (for details on this, see Andrea Fischer’s article in Studies in Conservation 63:6, 342-355). After soaking copper alloy test coupons in alkali carbonate solutions (mimicking contact with unstable glass) and drying them, these metals were then exposed to vapours from a formaldehyde solution. This ubiquitous air pollutant was chosen because we often found formates on metal objects with glass contact (Fig. 1). In the presence of potassium carbonate, it took much longer until corrosion was visible, compared to a control. Potassium carbonate (potash) absorbs humidity and forms a saturated solution maintaining an RH of 43% (as long as there is undissolved potash present).

The obvious question, “Is some of the formaldehyde absorbed into the solution?” can be answered by physical chemistry, which gives a clear answer: yes! However, the next questions of “how much is absorbed?” and “how long does it take?” needed further experimentation using special equipment to measure trace pollutant concentrations. An IIC seed money grant from the Opportunities Fund helped to meet the costs of two experiments at the Fraunhofer Institute in Braunschweig, which yielded exciting results; after 50 minutes, half of the trace concentration of formaldehyde had already been absorbed by a magnesium nitrate salt solution (RH = 53%), and the same absorbance using potassium carbonate salt took only 20 minutes. Potash in aqueous solution is alkaline (pH 11.3). Therefore, it has the advantage of not only being capable of dissolving gases physically but can also react chemically with aldehydes and can neutralise acidic gases.

These encouraging results helped our team to gain a research grant from the German Federal Environmental Foundation (DBU) to measure the effect of salt solutions on relevant corrosive pollutant gases. The list of gases to be tested includes acidic gases from the outdoor air (sulfur dioxide, nitrogen oxides), mineral acids (nitric, hydrochloric), organic acids (formic, acetic), the corresponding aldehydes (formaldehyde, acetaldehyde) and hydrogen sulfide. Experiments at the University of the Saarland (Prof. Andreas Schütze, head of project) (Fig. 2) will use calibrated metal oxide semiconductor (MOS) gas sensors in dynamic mode and methods of deep learning for data evaluation https://www.lmt.uni-saarland.de/index.php/de/forschung. Parallel to this, the practical application of salt solutions in museums will be coordinated by the project partner Art Collections of Veste Coburg https://www.kunstsammlungen-coburg.de/en/home/ (Heiner Grieb). They have 30 years of experience in the climatisation of ‘sick‘ glass using magnesium chloride salt solutions with an equilibrium RH of 33%. (Fig. 3 and 4).

In recent decades, saturated salt solutions were widely replaced by silica gel products. However, Alexandra Schieweck found “the assumption that silica gels … might also act as pollutant adsorbers cannot be confirmed.” Hopefully, our research into the effectiveness of saturated salt solutions will yield better and more solid results. As passive climatisation, salt solutions do not depend on the defect-free performance of active HVAC systems. This is a major advantage in managing such risks in museums, minimising the damage to objects that malfunction of such systems can cause, not to mention the significant energy consumption that could simply be avoided by implementing such passive systems. The use of saturated salt solutions has the potential to lower the carbon footprint for display climatisation considerably making it a more sustainable display system on a global level.

With these promising preliminary results and the great potential for a variety of benefits (from minimizing corrosion to protecting the museum’s pocketbook and our planet) we anticipate this research project will inspire new interest in the application of saturated salt solutions, giving this technique a well-deserved revival.

CALL FOR COLLABORATION

Do you want to take part in the practical tests of salt solutions during the research project? All you need is two or four display cases at hand and RH data loggers. Tests should be able to continuously run over a full-year cycle starting early in 2023. Please join a number of our colleagues who have already agreed to take part, and contact us at: profdreggert@gmail.com.

AUTHOR BIO

Gerhard Eggert, FIIC, professor emeritus at the Institute of Conservation Sciences in Stuttgart. He holds a diploma degree in chemistry and a doctoral degree in natural sciences from the University of Bonn. From 1985 until 1998, he was head of the Conservation Department of Rheinisches Landesmuseum Bonn dealing mainly with archaeological finds from the Rhineland. From 1998 until 2019, he was chair in objects conservation at the Stuttgart State Academy of Art and Design. His research focuses on manufacture, corrosion, and conservation of inorganic objects.

(Read the article in the December-January 2023 "News in Conservation" Issue 93, p. 12-15)