When Glass and Metal Corrode Together, VI: Chalconatronite

ABSTRACT

Chalconatronite, Na₂[Cu(CO₃)]₂·3H₂O, is formed as a corrosion product when copper alloys are exposed to sodium carbonate solutions in the air. This also happens when metals come into contact with corroding soda glass which forms alkaline surface films in reaction with humidity from clean air. More often, substantial amounts of formaldehyde are present indoors which react to formate via the Cannizzarro reaction and accumulate over time in the films. Twenty cases of chalconatronite (including two mentioned in the literature) are reported as occurring on heritage objects with glass in contact with copper alloys: Baroque reliquaries with set glass gems, enamel on metal (sixteenth century and a modern replica of intentionally unstable composition), Christmas tree glass baubles with wires, glass buttons with metal shanks, a glass figure with a wire support, miners’ lamps, and a glass framed daguerreotype. These confirmed identifications might help conservators in investigating similar cases to shed more light on the formation conditions.

KEYWORDS:

• Chalconatronite
• copper alloys
• formaldehyde
• glass corrosion
• indoor pollutants
• Raman spectroscopy
• sodium copper carbonate

Acknowledgements

The authors are grateful to Rachel Sabino (Art Institute of Chicago), Hans-Jörg Ranz (Bavarian National Museum Munich), Monika Schwarzenberger-Wurster (Tegernsee/Dresden), Teresa Lamers (University of Applied Arts Vienna), Rainer Richter (Staatliche Kunstsammlungen Dresden), Isabel Keller, Miriam Braun, and Alexandra Schorpp (all at the time at State Academy of Art and Design Stuttgart), and Steffi Wirsing (University of Applied Sciences Erfurt) for information on corrosion studies cases and access to samples. Robert J. G. Sobott (University of Leipzig) shared his measurements on the Leipzig glass figure with us. Christoph Krekel (State Academy of Art and Design Stuttgart) provided access to µ-Raman spectroscopy and SEM-EDX in his lab, and Robert E. Dinnebier (Max Planck Institute for Solid State Research Stuttgart) to XRD measurements performed by Christine Stefani.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Andrea Fischer http://orcid.org/0000-0001-9496-5097

Gerhard Eggert http://orcid.org/0000-0003-2443-1094

Jörg Stelzner http://orcid.org/0000-0003-1295-5894

Notes

1 Strictly speaking, the term chalconatronite should be restricted to occurrences in nature as a mineral without human involvement. For sake of brevity, it is here used as short synonym for the longer chemical name disodium dicarbonatocuprate(II) trihydrate.

2 The compound has now been identified as a basic zinc copper formate, see Fischer et al. (2019).

Additional information

Funding

Our research on glass-induced metal corrosion has been funded from 2012 to 2015 by Friede Springer Stiftung. Deutsche Bundesstiftung Umwelt (DBU, 33255/01) supports the model project in the DBM from 2016 to 2019.