Understanding Glass Deterioration in Museum Collections through Raman Spectroscopy and SIMS analysis
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Understanding Glass Deterioration in Museum Collections through Raman Spectroscopy and SIMS analysis Laurianne Robinet1, 2, Katherine Eremin3, Sarah Fearn4, Colin Pulham1, Christopher Hall1 1 The University of Edinburgh, Centre for Materials Science and Engineering, Edinburgh, EH9 3JL, UK; 2 Laboratoire de Dynamique Interactions et Réactivité, CNRS, 94320 Thiais, France; 3 Harvard University Art Museums, 32 Quincy Street, Cambridge, MA 02138, USA; 4 Department of Materials, Imperial College, London, SW7 2AZ, UK. ABSTRACT The combination of Raman spectroscopy and Secondary Ion Mass Spectrometry can improve understanding of the chemistry of the glass alteration process. Formic and acetic acids play an important role in the alteration of museum glass objects placed in a humid atmosphere. Raman spectroscopy indicates that the soda-rich glass structure is modified differently when exposed to a humid versus a humid and polluted atmosphere at 60˚C. Formic acid was not formed from soda-rich glass in the presence of carbon dioxide, high humidity and light. INTRODUCTION Deterioration of soda and potash glass is known from the study of both modern and historic glass [1, 2, 3]. This is normally attributed to reaction between the glass surface and moisture in a two-stage process [1]. The first stage is an ion exchange process between penetrating H+ and/or H3O+ ions from the moisture and an alkali metal ion (typically Na+), which is removed from the glass. The rate of alkali extraction is diffusion-controlled and depends on the square root of time [3, 4]. At this stage, the silica network remains unchanged but an alkaline film containing NaOH and H2O forms on the glass surface. This film becomes increasingly alkaline and above pH 9 the second stage occurs with decomposition of the silica network. The effects of the two reactions cannot be separated and occur simultaneously, normally with one dominating, resulting in both chemical and structural changes to the glass. Study of glass collections within the National Museums of Scotland (NMS) showed that a higher proportion of the historical glass collections had deteriorated than expected and that alteration was enhanced by high concentrations of organic pollutants (acetic acid, formic acid and formaldehyde) in the display and storage cabinets [5, 6]. These pollutants influence the corrosion products formed at the glass surface [5, 6], but their effect on the glass structure is unknown. In the current work, artificial corrosion experiments were used to increase understanding of the processes involved in the alteration of soda glass by organic pollutants, and investigate the effect of acetic and formic acids at different humidities. The formation of formic acid from reaction of CO2 with silicate rocks in the presence of water and light, possibly catalysed by metal ions, has been described [7] and the reaction of silicate glass in a similar environment was examined. Raman spectroscopy and Secondary Ion Mass Spectrometry (SIMS) have been used to follow the chemical and structur
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