Raman investigation of the structural changes during alteration of historic glasses by organic pollutants

Abstract

The combination of an unstable glass composition, fluctuating humidity and a high concentration of organic pollutants is responsible for the widespread alteration of part of the glass collections of the National Museums of Scotland (NMS). The alteration has resulted in the formation of crystalline corrosion at the surface and strong modification of the chemical structure of the glass. The chemical structure before and after alteration of two soda-silicate glasses from the NMS collection was examined by Raman spectroscopy assisted by electron microprobe. Decomposition of the Raman spectra offered an insight into the modified glass structure and the mechanisms of the alteration. The acidic pollutants, acetic and formic acids, were identified as the main cause of alteration, as they provide a source of H + ions that enhanced the ion-exchange reaction. The ion-exchange reaction in the soda glasses causes leaching of sodium ions out of the structure and formation of silanols (Si-OH), but leaves the stabiliser ions such as calcium and lead ions undisturbed. The ion exchange is followed by a polymerisation reaction of the silanols inducing the formation of new Si-O-Si bonds including four-fold silica D 1 rings and the release of molecular water into the structure. The polymerisation reaction is likely to be responsible for the cracking and flaking of the surface through the tensile stresses generated in the glass structure. The alteration process, and in particular the polymerisation reaction, implies that the structural modification of the glass is irreversible. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: historic glass; soda silicate; micro-Raman spectroscopy; alteration; organic pollutants INTRODUCTIONThe deterioration of ancient and historic glass within museum collections results in visual alteration and physical damage to the object, including development of surface iridescence, formation of crystalline corrosion products or liquid droplets depending on humidity, crizzling, cracking and flaking. In extreme cases, the entire object may disintegrate. For this reason, many museums are concerned with the stability of their glass collections and require detailed knowledge of the alteration processes to improve preservation and conservation.A survey of the glass collection of the National Museums of Scotland (NMS) showed higher-than-expected deterioration for the 19th to 20th century British, Islamic and Asian glass collections which had been kept in wooden display and/or storage cabinets under fluctuating relative Ł Correspondence to: Laurianne Robinet, The University of Edinburgh, Centre for Materials Science and Engineering, Edinburgh, EH9 3JL, UK. E-mail: l.robinet@ed.ac.uk humidity.1 The altered (unstable) glass showed significant compositional variety, and in all cases was distinguished from the unaltered (stable) glass by high alkali and low calcium and/or magnesium levels. Scientific studies of the glass collection revealed that the alteration was enhanced by high concentrations of organic pollutants (ac...

A Raman spectroscopic study of the Mapungubwe oblates: glass trade beads excavated at an Iron Age archaeological site in South Africa

“…The I p value has increased to 1.7, an indication of a highly polymerized network, the centre of gravity of the envelope arising from the Si-O bending modes having shifted downwards to 482 cm 1 , which is near that of pure silica glass and similar to that previously observed in highly corroded museum glass. 21 Table 4 shows the depletion of the sodium and potassium ion concentration as expected in corroded glass.…”

Influence of environment on the corrosion of glass–metal connections

“…This is very important and indicates the occurrence of a reaction between formaldehyde and potassium ions leached from the glass. The spectrum shows polymerization of enamel which has confirmed glass aging [20]. Modifications are confirmed by the presence of peaks in the area of 1,500 cm -1 .…”

The Influence of Selected Organic Compounds on the Corrosion of Historical Glass Based on Their State of Preservation

“…• C. Samples were periodically removed from the containers and weighed (R1 has not been corroded in water) [12,[17][18][19][20][21].…”

“…As a result, new compounds are formed on the surface of the silica gel. The first effect looks like iridescence, because of change optical surface feature and gradually the whole glass surface is coated by white deposit [5,6,12].…”