Corrosion on Metallic Tokens Stored in Polyurethane Foam

Abstract

Metallic tokens had been stored in direct contact with polyurethane foam since the mid 1970s. Upon examination in 2004 the foam was found to have badly deteriorated and some of the tokens showed signs of corrosion. The surfaces of copper and brass tokens were speckled with dark stains, green stains were visible on pewter tokens and a white powder had formed on zinc tokens. The polyurethane foam was analysed and found to be of the ester type. Polyurethane foam of the ester type has been reported to release adipic acid upon ageing. It was suspected that adipic acid had been released by the foam and had reacted with the tokens to form metal adipates. Copper and zinc adipates were synthesized and characterized using Fourier transform infrared and Raman spectroscopy. By comparing these to the spectra obtained for the corrosion products, the green stains on the pewter tokens were identified as copper adipate and the white powder on the zinc tokens as zinc adipate. Copper adipate was detected on the copper and brass tokens and the dark stains were found to be cuprite. This study highlights the risk of storing artefacts in direct contact with polyurethane foam, which should be replaced with a more stable storage medium such as polyethylene foam.