What is the salt in the display case used for? Salt solutions are not used as exhibits but as climate protectors for the sustainable adjustment of a constant relative humidity (RH). This is necessary for the long-term preservation of museum objects. This process has long been known, but it has now been mostly replaced by other methods that are assumed to be more advantageous. Only Coburg and a few other museums worldwide continued to use this method. This sustainable method is experiencing a revival to reduce the carbon footprint of museums. Newly discovered: The solutions also absorb harmful air pollutants. Sponsored by the German Federal Environmental Foundation (DBU), the use of these solutions in museums and their properties was tested at the Veste and by more than 60 conservators from Germany and abroad, in cooperation with Saarland University and Stuttgart State Academy of Art and Design.
Fig. 1: Exhibition poster: What is the salt in the display case used for? (Photo: Ertel, graphics: Rößner)
Fig. 2: Saturated salt solutions for climate control are placed in a technical compartment below the display (Photo: Ertel)
DBU Research Project
The project ‘SalzVit’ (DBU Az. 38338/01) systematically investigated the potential of saturated potassium carbonate (RH = 43%) and magnesium carbonate (RH = 53%) solutions for use in museum and archival display cases. The RH of both solutions is nearly independent of temperature variations in museums and remains stable.
Laboratory experiments have shown that
- Potassium carbonate absorbs harmful gases, such as formaldehyde, acetic acid, and formic acid, very efficiently in a short time. The gases then react chemically and are no longer corrosive.
- Magnesium nitrate is useful for materials (e.g. of plant or animal origin) that require a somewhat higher RH.
- Magnesium chloride (RH = 33%) preserved the ‘sick’ glasses in the exhibition of Veste from further decay for more than three decades. The release of potassium and sodium from the glass surface is within the range of stable glass.
Fig. 3: Saturated salt solutions ( green, orange, and red curves) absorb pollutants rapidly (blue: without salt) (Graphics: LMT, Saarland University).
Results of the Practical Tests
The Veste Coburg Art Collections, together with international partners, organised a round-robin test: full annual cycles of temperature and RH were recorded in 2023/4. The results from 80 display cases with saturated salt solutions proved:
- Stable passive control of RH is possible for more than 12 months without the need for electrical current, even if the room climate fluctuates.
- ¾ of the participants were satisfied with the method.
- Less effort: Compared to silica gel products and their need for reconditioning, necessary maintenance intervals are longer.
- No need for expensive equipment.
Fig. 4: The RH in the room (top, blue curve) fluctuates strongly, while it is quite constant (bottom, blue curve) in the display case due to the salt solution.
This proves that salt solutions also function as pollutant absorbers. In the case of potassium carbonate, these substances are also chemically neutralised. They are a reliable, energy-independent, fail-safe, and sustainable method for RH control. The need for maintenance is low, and it can be applied to existing display cases.
The results were collected in a final report (https://doi.org/10.5281/zenodo.15680444). A handbook (see Download) provides useful tips for practical application.
Exhibition ‘Prima clima – the museum gets more eco-friendly’
The research project on RH control in display cases with saturated slat solutions was presented to museum visitors through a compact exhibition at the STUDIO from October 2024 to May 2025. The exhibition provided a unique view behind the museum scene directly into the vitrines.
Modified versions will be shown as a touring exhibition at various places, starting at Castle Friedenstein in Gotha (until 2026).
Conference and Workshops
A joint conference of Veste Coburg Art Collections with Saarland University and Stuttgart State Academy of Art and Design will present exciting results on May 22/23th, 2025, at the Veste. Central to this was the practical everyday experience in museums. These were compared with the use of silica gel and other RH control methods. A workshop the next day trained interested conservators in the application. The course was repeated at the Metal 2025 conference in Cardiff (Wales) and Leipzig (BA course in museology). Further dates are planned for autumn 2026 and spring 2027. Lectures for interested museum staff were given in Bonn, Cologne, Cardiff, Berlin, Tübingen, and Lyon.
Downloads
Handbook (English):
HUMIDITY CONTROL USING SATURATED SALT SOLUTIONS FOR DISPLAY CASES
References
- Brieger/Marschibois/Eggert/Schütze/Bur 2023 Oliver Brieger; My Sa Marschibois; Gerhard Eggert; Andreas Schütze; Christian Bur: MOS Sensors Characterizing Gas Absorption Dynamics for Art Conservation, in: 2023 IEEE SENSORS, Vienna, Austria 2023, S. 1–4. DOI: 10.1109/SENSORS56945.2023.10324921.
- Brieger/Pfeifer/Schütze/Bur 2024 Oliver Brieger; Robin Alexander Pfeifer; Andreas Schütze; Christian Bur: Charakterisierung der Absorption von Luftschadstoffen durch gesättigte Salzlösungen zum Schutz wertvoller Kulturgüter, in: 17. Dresdner Sensor-Symposium 2024. Wunstorf; AMA Service, S. 148–153. DOI: 10.5162/17dss2024/P24.
- Eggert 2022 Gerhard Eggert: Saturated salt solutions in showcases: humidity control and pollutant absorption, in: Heritage Science 10/54 (2022), S. 1–6. DOI: 10.1186/s40494-022-00689-3.
- Eggert/Brieger/Marschibois/Becker/Bur/Siebel/Grieb 2024 Gerhard Eggert; Oliver Brieger; My Sa Marschibois; Daniel Becker; Christian Bur; Katja Franziska Siebel; Heiner Grieb: Sustainable and Beneficial: Pollutant Absorption and Relative Humidity Control in Display Cases by Saturated Salt Solutions, in: Studies in Conservation 69/sup 1 (2024), S. 72–80. DOI: 10.1080/00393630.2024.2339727.
- Eggert/Grieb/Brather 2022 Gerhard Eggert; Heiner Grieb; Stephan Brather: „Salz in der Vitrine“: Neues Forschungsprojekt startet, in: RESTAURO 7 (2022). S. 36–39.
- Eggert/Schütze 2023 Gerhard Eggert; Andreas Schütze: Zwei auf einen Streich: Klimatisierung und Schadstoffabsorption in Vitrinen durch gesättigte Salzlösungen, in: METALLA Sonderheft 12 (2023), S. 112-114. https://doi.org/10.46586/metalla.v.2023.i12
- Grieb/Siebel/Eggert 2026 Heiner Grieb; Katja Franziska Siebel; Gerhard Eggert: Das Coburger Modell als Inspiration für neue Forschungen zur Vitrinenklimatisierung, in: Burg, Schloss, Fränkische Krone: 800 Jahre Veste Coburg. (Jahrbuch der Coburger Landesstiftung 2024), Coburg 2026, S. 406-422.
- Schütze 2025 Andreas Schütze: Schutz national wertvoller Kulturgüter durch Einsatz gesättigter Salzlösungen in Vitrinen zur Absorption anthropogener Luftschadstoffe (DBU-Abschlussbericht) https://doi.org/10.5281/zenodo.15680444
- Siebel/Grieb/Brieger/Pfeifer/Bur/Eggert 2025 Katja Franziska Siebel; Heiner Grieb; Oliver Brieger; Robin Pfeifer; Christian Bur; Gerhard Eggert: Dicke Luft? Pottasche! Ein wiederbelebter Ansatz zur Klimatisierung von Museumsvitrinen, in: METALLA Sonderheft 13 (2025), S. 56-58. https://doi.org/10.46586/metalla.v.2025.i13
- Siebel/Grieb/Brieger/Pfeifer/Bur/Eggert 2025 Katja Franziska Siebel; Heiner Grieb; Oliver Brieger; Robin Pfeifer; Christian Bur; Gerhard Eggert: Pollutants? Potassium carbonate! A new solution for a healthy atmosphere in metal display cases, in: Metal 2025. Proceedings of the International Conference on Metals Conservation, hg. von N. Emmerson, J. Thunberg und D. Watkinson, Cardiff (Wales), 1–5 September 2025, S. 181–189. https://www.researchgate.net/publication/395384454
- Siebel/Grieb/Brieger/Pfeifer/Bur/Eggert 2025 Katja Franziska Siebel; Heiner Grieb; Oliver Brieger; Robin Pfeifer; Christian Bur; Gerhard Eggert: Magnesium nitrate as sustainable solution for constant RH (53%) and pollutant absorption in display cases, in: npj Heritage Science 13/537 (2025), S. 1–6. https://doi.org/10.1038/s40494-025-02111-0.
- Siebel/Grieb/Brieger/Pfeifer/Bur/Thickett/Eggert 2026 Katja Franziska Siebel; Heiner Grieb; Oliver Brieger; Robin Pfeifer; Christian Bur; David Thickett; Gerhard Eggert: Drying the tears of ‘weeping’ glass – The Coburg magnesium chloride experience, zur Veröffentlichung eingereicht.
- Ulitzka 1992 Stanislav Ulitzka: Schädigung von museal aufbewahrten Gläsern durch die Glaskrankheit und Abhilfemaßnahmen. Kurzfassung des Abschlussberichts zum Forschungsvorhaben S188, gefördert durch die Stiftung Industrieforschung, Erlangen: Institut für Werkstoffwissenschaften III, 1992.
