Lead Coupon Reactivity to Organic Acids, Aldehydes, and Esters in an Oddy Test Environment

The Oddy test is an accelerated metal corrosion test used extensively by cultural institutions to determine the suitability of a material for use in museums. Alternatively, the use of gas chromatography-mass spectrometry (GC-MS) to directly identify volatile organic compounds (VOCs) from construction materials is growing in popularity because of its comprehensiveness and speed. Interpreting the reactivity of these potential pollutants, however, relies on ‘chemical intuition’ based on observed functional groups since the reactivity of only a handful of common VOCs has been studied intensively with regard to artworks. While short chain organic acids are known to be deleterious to some metals, polymers, and other culturally relevant materials, the common observation of lower volatility acids as well as their complementary aldehydes and esters in these offgassing experiments do not have clear indicators of their potential for artwork damage. In this work, the lead coupon, known to be a sensitive indicator of damaging organic acids, was exposed to known concentrations of a homologous series of organic acids, aldehydes, and esters from C2 to C18. Analysis of the coupon surface by infrared and Raman spectroscopies, and of the headspace within an Oddy jar by GC-MS, provides insights into the corrosion processes of these potential pollutants. Humidity was identified as a necessary component for corrosion to occur, and very volatile and semi-volatile compounds up to C9 created the corresponding lead carboxylate on the coupon surface in addition to lead carbonate. For higher order acids, and to a far lesser extent the esters and aldehydes, a high concentration of the VOC was necessary to induce small amounts of corrosion. In some instances, the gas phase chemistry of the reactor was particularly complex, suggesting mixtures of pollutants may prove more problematic to artist materials than single offgassed species.