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In the National Museums of Scotland (NMS) widespread alterations have been observed in the glass collections of 19th to 20th century, affecting British, Islamic and Asian glasses. It is important for museums to be able to distinguish between stable and unstable glasses, so that particular care can be taken to preserve the most sensitive objects. Elemental analysis by electron microprobe of a selection of stable and unstable British and Islamic glasses indicated that the composition was directly linked to the degree of stability of the glass. Because sampling of glass objects is often difficult or impossible, we investigated the ability of Raman spectroscopy, which can be applied in situ and non-destructively, to distinguish between stable and unstable glasses. The analysis concentrated on the soda-lime silicate glasses, which displayed a mixed stability. We show, by combining the elemental composition and the Raman spectroscopy data, that a correlation can be established between the 550 cm −1 band shift and the SiO 2 content (or degree of polymerisation). A second correlation was established between the 950 cm −1 band and the number of cations charge-coordinated to the silicate with two non-bridging oxygens (NBOs). Finally, a method based on the peak area ratio (A 900 + A 950 + A 990 )/(A 900 -1150 ) from the Raman spectrum is proposed to determine the stability of soda silicate glass.

This article examines the influence of the composition on the Raman spectra of lead silicate glass. Modern and historic lead alkali glasses and high-lead glazed ceramics were analysed complementarily by Raman spectrometry and elemental techniques, either electron microprobe, proton induced X-ray emission (PIXE) or scanning electron microscope with energy dispersive spectrometry (SEM-EDS). The results showed that lead alkali silicate and high-lead silicate glasses can be easily distinguished from their Raman spectra profile. In lead alkali silicate glasses, continuous variations were observed in the spectra with the compositional change. In particular, the position of the intense peak around 1070 cm −1 was linearly correlated to the lead content in the glass. A unique decomposition model was developed for the spectra of lead alkali silicate glasses. From the combination of the Raman and elemental analyses, correlations were established between the spectral components and the composition. These correlations permitted to interpret the spectra and access additional compositional information, such as the lead content from area ratio A 990 /A 900 -1150 , the total alkali + alkaline-earth content from the area ratio A 1100 /A 900 -1150 or the silica content from the area ratio A 1150 /A 900 -1150 . In lead silicate glass containing over 25 mol% PbO, the compositional variation induced no variation in the SiO 4 network region of the Raman spectra [150-1350 cm −1 ], therefore no correlations and compositional information could be gained from the glass spectra in this range of composition. This new development of Raman spectroscopy for the analyses of glass will be very valuable for museums to not only access compositional information non-destructively but also to understand the structural changes involved with their alteration.

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Smalt was commonly used as a pigment by artists between the 16th and 18th centuries. It is a powdered blue potash glass colored by cobalt ions and often degrades causing dramatic changes in the appearance of paintings. The aim of the work presented in this paper was to investigate the changes in the structure and environment around the cobalt ion on deterioration, to further our understanding of the basis of the loss of color. Particles of well-preserved and altered smalt in microsamples from paintings in the National Gallery, London, and the Louvre, Paris, were analyzed using synchrotron micro-X-ray absorption spectroscopy at the Co K-edge. X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements showed that in intense blue particles the cobalt is predominantly present as Co(2+) in tetrahedral coordination, whereas in colorless altered smalt the Co(2+) coordination number in the glass structure is increased and there is a shift from tetrahedral toward octahedral coordination. The extent of this shift correlates clearly with the alkali content, indicating that it is caused by leaching of potassium cations, which act as charge compensators and stabilize the tetrahedral coordination of the cobalt ions that is responsible for the blue color.

This paper presents the correlative imaging of collagen denaturation by nonlinear optical microscopy (NLO) and nanoscale infrared (IR) spectroscopy to obtain morphological and chemical information at different length scales. Such multiscale correlated measurements are applied to the investigation of ancient parchments, which are mainly composed of dermal fibrillar collagen. The main issue is to characterize gelatinization, the ultimate and irreversible alteration corresponding to collagen denaturation to gelatin, which may also occur in biological tissues. Key information about collagen and gelatin signatures is obtained in parchments and assessed by characterizing the denaturation of pure collagen reference samples. A new absorbing band is observed near the amide I band in the IR spectra, correlated to the onset of fluorescence signals in NLO images. Meanwhile, a strong decrease is observed in Second Harmonic signals, which are a structural probe of the fibrillar organization of the collagen at the micrometer scale. NLO microscopy therefore appears as a powerful tool to reveal collagen degradation in a non-invasive way. It should provide a relevant method to assess or monitor the condition of collagen-based materials in museum and archival collections and opens avenues for a broad range of applications regarding this widespread biological material.

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...