Vessel Glass Deterioration in the Museum Environment: A Quantitative Study by Surface Analysis

The deterioration of vessel glass is a phenomenon familiar to all those involved with its care. To curators and collectors it is a frustrating problem for which there is no satisfactory solution. The decay which takes place over the course of time by interaction of the glass surface with its environment cannot be artificially reversed and so, once affected, the glass cannot be returned to its original condition. At present the conservator can only temporarily halt or slow down the process where it has already begun, or take steps to prevent it starting by controlling the environmental conditions if vulnerable objects can be identified sufficiently early.

Fig 1. Venetian wine glass, 16th-17th century, from the collection at the V&A Museum (click image for larger version)

Although almost all glass is to a greater or lesser extent chemically unstable to its environment, in practice some groups of glass are particularly vulnerable. For example, a recent survey of the condition of 6,500 objects in the collections at the Victoria and Albert Museum¹ highlighted some four hundred objects which had suffered surface deterioration. These ranged from vases and goblets from seventeenth century Spain and Italy to twentieth century 'designer' glass from Scandinavia. The rate of decay depends primarily on two factors: the composition of the glass and the surrounding environment. Conservators and glass technologists have given much consideration to these factors (see for example the review by Newton and Davison2), but there is still much to be done to develop an adequate scientific understanding. When this is achieved, the craftsman will be able to adjust the formulation of the glass composition to optimize durability and the conservator can work towards controlling the rate of deterioration of objects which are known to be vulnerable.

The active participation of Imperial College in the new postgraduate course in Conservation which is run jointly by the Royal College of Art and the V&A has established many new contacts across academic disciplines. In this case it was clear that the equipment, experimental techniques and experience at Imperial College complemented those at the Museum, and that collaboration should achieve much in approaching a quantitative understanding of the processes of surface deterioration and in being able to recommend the best possible procedures to circumvent it.

The symptoms of glass decay

The composition of glass consists essentially of oxides which contribute to the formation of an amorphous network and those which break up or modify this network. In the former group, silica, SiO2, is the principal network-former, while the latter group includes oxides of the alkali metals sodium and potassium, Na20 and K20, and of the alkaline earth metals calcium and magnesium, CaO and MgO. When a glass surface is exposed to an aqueous solution or to a humid atmosphere reactions take place at and below the glass surface. These reactions result in chemical and structural change and are initiated by an exchange of alkali metal ions from within the glass with hydrogen ions from outside. In the worst cases these ion exchange processes leave a silica-rich layer below the surface containing hydrated micropores. The composition changes in the near-surface layer, which involve the removal of alkali metal cations and their replacement by the smaller hydrogen ion, result in the build-up of tensile stress within the layer since it is constrained by the unaffected glass beneath the surface. This stress can result in crack formation in the near-surface layer.

Fig 2(a). Scanning electron micrographs of surface of a Venetian glass goblet: Shows crizzles, debris and the site of a detached flake. Field of view 290um. (click image for larger version)

If alkalis are able to build up at the surface as a result of the ion exchange, for example if the surface is not immersed in an aqueous solution or is not washed, then moisture from the humid environment and the alkalis react with the network-forming silica and produce further deterioration.

The early stages of the decay are visible as a dulling of the surface of the object, resulting from the accumulation of products of the ion exchange processes. These deposits are hygroscopic and in high humidities produce a slippery 'weeping' surface. Later on, as the deterioration progresses and the extraction of the alkali continues, a fine network of crazing or 'crizzles' forms3. A sixteenth/seventeenth century Venetian wine glass which has deteriorated in this way is shown in Fig 1. As the crizzling develops and deepens, small flakes of glass spall away, imparting an opaque, sugary appearance; in the worst cases the glass eventually loses its mechanical strength and the object will collapse and disintegrate.

Experimental programme

The full research programme is in two parts; it has been designed to investigate the nature of selected Museum objects which have crizzled surfaces and to monitor with sensitive surface-analysis equipment the onset of near-surface reactions which lead to the deterioration of precious glass objects using synthetic glasses. For this second purpose chosen glass compositions are being melted and cast in the laboratory, and samples are undergoing exposure to different storage conditions (humidity, temperature) for various periods of time. The surfaces will be examined and analysed by scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS) to determine the extent of the exchange reactions which are taking place. The application of the SIMS instruments to this problem is particularly appropriate. The technique can yield detailed information on the spatial distribution of both impurity and matrix atoms within a solid with a lateral distribution of 100 nm and a depth resolution of 10 nm combined with a very high sensitivity (parts per million). SIMS is being used to measure the hydrogen, sodium, potassium and silicon profiles within the surface region. This allows information on the kinetics of cation movement to be obtained. In order to improve the quality of the hydrogen profile, deuterated (heavy) water is being used for the accelerated ageing experiments because the heavier isotope has a better detection limit. The analyses are made quantitative by means of ion-implanted standards of deuterium, sodium and potassium.

Fig 2(b). Scanning electron micrographs of a surface of a Venetian glass goblet: Detail of junction between two fissures. Field of view 11.5 um. (click image for larger version)

The scanning electron microscope can give information about the nature of the glass surface and the features which have developed on it. Energy-dispersive analysis of x-rays generated by interaction of the electron beam with the glass surface allows the mapping of elements across the surface in combination with observations of microstructural features. The results obtained from the SIMS and SEM techniques are supported by wet chemical analysis of the glasses under investigation and polarised light optical microscopy of polished sections.

Early results

The research programme began in July 1991 and the work in the early stages has been concentrated on an object from the Museum's collection of Venetian glass. This was a broken and deaccessioned goblet approximately 23cm high with a base 6.5cm in diameter. The surface of the object was severely crizzled and debris which was assumed to have been produced by the decay reactions was present on the surface.

Fig 3. Optical micrograph of polished section taken normal to the surface showing surface layer with bifurcated crack and crack formation behind near-surface layer. Field of view 220 um. (click image for larger version)

Fragments from the base of the goblet were rinsed with water and acetone, mounted on aluminium stubs and gold-coated. They were examined in the scanning electron microscope: some micrographs are shown in Fig 2. The first micrograph (a) shows a typical area of the crizzled surface with intersecting fissures and an area where a flake has spalled away from the surface can also be seen. Micrograph (b) shows a fissure at a higher magnification: the sharp upper edges have corresponding irregularities on opposite faces, confirming that the cracks are formed by contraction of the surface. Surface debris is visible in all the micrographs.

Fig 3 is an optical micrograph taken in reflected polarised light. The cracks running inwards from the surface often show bifurcation, suggesting a possible mechanism for the initiation of spalling. Under the microscope the difference in reflectance between the surface layer and the bulk glass can be detected by an experienced operator.

Fig 4. Depth profile of near-surface layer obtained as intensity/time plot by the dynamic SIMS technique (click image for larger version)

Quantitative chemical analysis of selected points on the surface observed in the SEM was carried out by energy-dispersive x-ray analysis, EDXA. This information is being used in conjunction with classical analysis to provide batch compositions for the preparation of synthetic analogues of the Venetian glass. It is worth noting that the spot analyses obtained from the crizzle fissures showed lower silica contents than the remainder of the surface, with associated increase in network-modifying oxides.

Examination of the glass surface by SIMS has been carried out using two modes of operation. In the 'dynamic' mode a high flux of oxygen ions is directed at the surface in ultra-high vacuum and the secondary ions produced are analysed by the mass spectrometer as a function of time. Fig 4 shows a plot of the intensity (and thus composition) profile as a function of time (and thus depth). In the example shown there appears to be a build-up of alkali and alkaline earth elements at the surface relative to the material remote from the surface, and an intermediate region for which the silicon content is uniform but where the cationic species are low relative to those within the bulk glass; this band corresponds to the surface layer observed by optical microscopy. In the second SIMS mode, that of 'imaging', the primary oxygen ion beam is scanned across the surface and generates a chemical map of the surface. The images obtained showed the fissures produced by crizzling to be associated with increased levels of potassium.

Looking ahead

The experiments which have been performed so far, and the results which have been obtained in the study of glass from the Venetian goblet, have confirmed that SEM and SIMS techniques used in combination can certainly improve our understanding of the processes of deterioration. The observations made can for the most part be explained by the chemical interactions with atmospheric water outlined earlier in this paper, although the exact mechanism by which the advancing cracks propagate through and behind the reacted surface layer are not so easily explained. It is clear that the work must continue as a study not only of museum artefacts but also of synthetic glasses, so that materials which have already suffered deterioration over long periods can be compared with glasses which have been subjected to accelerated decay under imposed conditions. The sensitivity of the SIMS technique will be particularly useful for this purpose. By making quantitative measurements of the changing concentration profiles in the near-surface layer during the early stages of deterioration, it would be possible to gain a much better understanding of the nature and mechanisms of the surface decay.

The opportunity for collaboration across disciplines and between institutions has been very rewarding and has already yielded useful quantitative information. The intention now is to continue the work by means of a postgraduate studentship sponsored by the Science and Engineering Research Council with the V&A under the CASE scheme. In the continuing programme the aim is to understand the fundamental science of the deterioration phenomena so that the best procedures can be recommended for storage, display and handling of precious glass objects.

Acknowledgements

We are grateful to the Victoria and Albert Museum and Imperial College for providing the opportunity for this work, to the Nuffield Foundation for the provision of an Under-graduate Research Bursary for one of us (Afi Amaku), and to the University of London for financial support. We are also grateful to Dr Chris Halls for his advice and for his help in obtaining the optical micrograph shown in Fig 3.

References

1  Oakley, V L,'Vessel glass deterioration at the Victoria and Albert Museum: Surveying the Collection', The Conservator, 14, 1990, pp30-36
2  Newton, R G and Daivson, S, 'Conservation of glass', Butterworths, London, 1989
3  Brill, R H, 'Conservation in archaeology and the applied arts', IIC Proceedings, Stockholm, 1975, pp121-131