Glass collection research

The Collection & Research department , in collaboration with glass and ceramics restorer Mandy Slager, is conducting research into the museum's glass collection. After unusual drops began to appear on the surface of some glass objects, work began in 2008 to assess the condition of the entire glass collection, totalling almost 5,000 pieces.

The first few years of the project focused on the objects showing signs of degradation and chemical instability. This is primarily a consideration of external characteristics as factors indicating unstable glass can be seen with the naked eye, such as a dull appearance, drop formation (called ‘weeping glass’) and cracking. Unstable glass can also feel damp or even exhibit visible streaks of moisture, hence the term 'weeping glass'. With cracking glass, small hairline fractures appear alongside a fine network of smaller cracks (called ‘crizzling ) which can cause the glass to disintegrate into shards once if left unchecked.

Teamwork

With a team of museum staff and volunteers, the entire collection was examined, cleaned and documented between 2008 and 2011 under the guidance of glass restorers Mandy Slager and Frederike Burghout. The objects that were deemed unstable or potentially unstable were placed in separate storage cabinets, making it easier to track their conditions over time. Some 40 glass objects were named as so-called ‘sample objects' to be professionally photographed and monitored for changes in condition over the course of the project.

Chemical instability investigation

During the project, several objects have had their condition reassessed. Museum Boijmans Van Beuningen has also contributed to large-scale chemical engineering studies into degraded glass in 2017 and 2020. Guus Verhaar, technical physicist and postdoctoral researcher at TU Delft, is devoting his PhD research to the chemical properties of unstable glass, aiming to develop a way to identify instability at an early stage. Using ion chromatography (or IC), Verhaar was able to determine the materials that are deposited onto glass surfaces as a result of instability, the concentration of which can tell us something about the stage of the object’s so-called ‘glass sickness’. IC helps us to understand the composition of the glass as well as the environmental factors affecting it. The combination of unstable glass with an unfit atmosphere in storage can determine the course of glass degradation.

It is hoped that this technique can help us gain more insight into the relationships of chemical elements, both of glass and its environment, so that degradation can be detected at an early stage. With advancing insight, we hope that the condition of the glass collection can be stabilised using treatments or even, for example, by adjusting storage conditions to have lower humidity which seems to benefit degraded glass. Fluctuations in temperature, humidity and acids in the environment from, for example, wooden shelving, can exacerbate the process of degradation. Affected glass can never be restored – the process of deterioration is irreversible but can be slowed, which is why it is so important to intervene in time, as well as to avoid degraded glass contaminating other glass objects.

In 2022 and 2023, Museum Boijmans Van Beuningen’s glass project continued, with restorer Mandy Slager again examining a number of objects from the glass collection. This included the guide objects from 2008, the objects analysed using the IC process, a group of glass objects from a large 2017 donation, and a selection that had been returned after being on long-term loan at another museum.

Slager examined the objects under a small digital microscope and with different types of light before combining the results with the IC research, as well as information on the object’s previous storage conditions. This data yields a lot of information and provides a broader picture of the object's condition at this moment.

Mini microscope showing network of fine cracks. Photograph: Mandy Slager

Mini microscope showing drop formation. Photograph: Mandy Slager

Together with Museum Boijmans Van Beuningen, and in consultation with other museums such as the Rijksmuseum and Corning Museum of Glass, restorer Mandy Slager has developed a clear method for recording the conditions of glass and points of attention for preventive conservation. The new method is integrated into Conservation Studio – the collection registration and documentation system used by the museum to record conditions, restorations and treatments. The system is also used by other museums, and new developments – especially for glass collections – are followed with interest in the Netherlands and internationally.

There is not one specific type of glass in which instability occurs – it can be seen in glass of all ages and shapes, both industrially and hand-made. Distinct groups with characteristics of degradation include Venetian (more specifically Murano) glass from the 16th and 17th centuries, Dutch Leerdam glass from the first half of the 20th century, and more recent glass such as objects designed by Ettore Sottsass. These groups of unstable glass also stand out in other museum collections, and more research is needed into whether their chemical compositions show similarities, as manufacturing and working conditions may also have influenced stability. To gain more insight into production methods, more historical and practical research is necessary, and glass-blowers themselves are important sources of knowledge.

Museum Boijmans Van Beuningen had an initial session with contemporary makers specialising in Leerdam glass but also in historical glass. Their knowledge gives us more insight into the making process, but also into identifying weaker spots in the final object that may be susceptible to degradation. This can even help clarify which markings have been mistaken for degradation which were actually caused by tools during the making process. Mandy Slager joined Marc Barreda, Durk Valkema and Anna Carlgren to look at a selection of objects with specific conservation problems, providing many interesting new insights.

Mandy Slager looks at a glass object from the Museum Boijmans Van Beuningen collection with glass-blower Marc Barreda

Bringing together different sources of knowledge means we are learning more and more about degraded glass, but this process also raises more questions. The research into glass degradation must therefore continue, and Museum Boijmans Van Beuningen is keen to encourage knowledge-sharing with other museums, investigating the possibility of carrying out additional technical analysis with the help of the Dutch Cultural Heritage Agency. Through collaboration between conservators, analysts, restorers and glass-makers, we are always taking the next step forward.