Curious Corrosion Compounds Caused by Contact: A Review of Glass-Induced Metal Corrosion on Museum Exhibits (GIMME)

Abstract

Many heritage objects consist of glass in contact with metals. By ion exchange with absorbed water, alkaline aqueous films are formed on the glass surface. They contain sodium and/or potassium, hydroxide, and carbonate (uptake of carbon dioxide) ions. These electrolytes induce corrosion while in contact with metal. Surprisingly, this phenomenon has only been realised by research in Stuttgart in the last two decades. About 350 affected objects were detected in the meantime in a number of heritage collections. Because of the special electrolytes, unusual corrosion products are often formed. The unknown structure and formula of three of them could be determined by modern X-ray powder diffraction data evaluation. One example is the basic potassium lead carbonate, KOH‧2PbCO3, detected on a pewter lid of a glass jug. The sodium analogon of already known structure was found in hollow glass balls mirrored on the inside with molten lead. Chalconatronite, Na2[Cu(CO3)2]‧3H2O, is known as a corrosion product of copper alloys in contact with soda solutions (here: from glass degradation). Exposed to acetic acid emissions (e.g., from wood), it transforms to a sodium copper acetate carbonate of hitherto undetermined structure. The ubiquitous pollutant formaldehyde reacts directly to formate in the alkaline medium provided by glass degradation. On copper alloys in contact with glass, formates are, therefore, frequent: Na4Cu4O(HCOO)8(OH)2‧4H2O in 50% of all cases and in 33% Cu2(HCOO)(OH)3. Zinc (from brass) forms Zn(HCOO)2‧2H2O and Zn4Cu3(Zn1−xCux)6(HCOO)8 (OH)18·6H2O. There are a number of other corrosion products, e.g., containing zinc and carboxylates awaiting further characterisation. Preventive conservation needs to slow down corrosion by dry storage (not lower than 35% rH). Pollutants need to be avoided by careful selection of materials for storage, display, and conservation.

‘Copper and Bronze in Art’ and the Search for Rare Corrosion Products

“…The alkaline surface films formed on the glass during hydrolysis also contained these ions, as they absorb carbon dioxide from the air. In our research on Glass Induced Metal-corrosion on Museum Exhibits (GIMME) [30], in ca 5% of all of the cases investigated, chalconatronite could be detected by Raman microscopy. The objects included baroque reliquaries with set glass gems, enamel on metal (16th century and a modern replica of intentionally unstable composition [31], Christmas tree glass baubles with wires, glass buttons with metal shanks, a glass figure with a wire support, and miners' lamps (Figure 7; [29]).…”

“…Through desiccator experiments, Paterakis [37] showed that it can result from chalconatronite being exposed to acetic acid vapours. The storage of chalconatronite (as corrosion product on combined glass/metal objects) in oak cupboards might, therefore, be the explanation for our few encounters of this compound in the GIMME survey [30,38].…”

“…As the formate:acetate ratio was estimated by the authors to be between 1:2 and 2:1, the general formula might be given as NaCu(HCOO) 1+X (CH 3 COO) 2−X , with 0 ≤ x ≤ 1. With sodium from the glass and copper from the alloy, this was the first time that we understood that there are joint glass/metal corrosion products [30,38], and our research on glass-induced metal corrosion started. Other than expected, carbonates are rare as copper corrosion products and formates dominate [43].…”

Early Stages of Metal Corrosion in Coastal Archaeological Sites: Effects of Chemical Composition in Silver and Copper Alloys

“…Small Cu amounts were also detected and are probably related to copper corrosion products deposited on the crystals, which appear altered in the FE-SEM image. Some occurrences of sodium lead carbonate developed on artistic objects were recently reported [42]. The compound was found in the form of a white pigment and as a corrosion product, exhibiting a hexagonal shape, on a leaded foil that had been exposed to an Na2CO3 solution.…”