An incurable disease is sweeping through museum glass collections:glass disease. The symptoms are a greasy appearance followed by a dull appearance. There is no cure yet, but the cause is known. The water in the air causes the glass to deteriorate slowly.
Glassware in your kitchen, especially if it's close to the stove, gets a greasy haze. No reason to panic of course. Clean it up a bit and maybe replace the filters in the extractor hood again. But when precious glass objects from a carefully preserved museum collection look like they've been standing next to a smoking deep-fat fryer, it's a different story. There is more going on here than an innocent layer of dirt. The dull haze could mean that the glass is slowly degrading. In museum circles this is known as glass disease. Guus Verhaar, PhD student at the University of Amsterdam, is working at the Rijksmuseum on a method to predict which glass is susceptible to this condition.
Crying and sweating
Glass is one of those materials that you think is untouchable. Yes, it is fragile, but at the same time it can take a lot. Archaeological excavations have provided us with glass beads and shards thousands of years old. Chemists use glassware for experiments with the most aggressive substances. Windows are exposed to large temperature differences and air pollution for years without any problems.
However, glass can also degrade. Simply through contact with water from the air. “When the humidity is high, certain components in the glass can react with water from the air. As a result, salts are formed on the glass surface. This creates droplets on the surface of the glass. Gel-like droplets that make it look like there's a greasy layer on the glass. This is sometimes called 'sweating' or 'crying' glass," says Verhaar.
“If the glass is then placed in a much drier environment, those droplets dry out and salt crystals form on the surface. Another problem arises when very small droplets of water are also absorbed into the outer layer of the glass. In a drier environment, that water can evaporate and that causes very fine cracks in the top layer. We call this crizzling. The glass will then look dull.”
First colorless, now pink
Verhaar takes a crate with the label 'sick glasses' on it. He has some trouble with that designation. “When you say 'disease', it implies that treatment or recovery is possible, but that is not the case. The damage is permanent and at best we can delay the process.” From the crate he conjures up an elegant jug made of frosted pink glass. The crizzling is obvious, but it is so evenly distributed that it hardly disturbs. “Not only was this jug once transparent, it was also colorless,” says Verhaar almost casually as he searches further into the crate.
Colorless? That is hard to imagine with this obviously pink object. Is color change also a symptom of glass disease? “Here it is an indirect consequence. This glass contains manganese, which was added to make the glass colorless and beautifully clear. When the degradation process started due to contact with water from the air, the manganese was oxidized. And manganese oxide is pink.”
The humidity and especially changes in it play an important role in the development of glass disease, but that is not the only thing. “We know that certain types of glass have a much higher risk of degradation due to their specific composition.” The raw material for glass is sand, which largely consists of silica (silicon dioxide, SiO2 ) consists. Silica is the so-called network former in the glass. The additives reduce the melting temperature of the silica, making the material easier to work with.
Traditionally, substances have been used that contain many salts with sodium (Na) or potassium (K), such as soda or the ashes of burnt plants. “The problem lies in these salts,” explains Verhaar. “The sodium and potassium ions are relatively mobile within the silica network. On the surface, they can react with water from the atmosphere.”
Stabilizing lime
What glass processors have known for a long time is that the additives make the glass more unstable and that is why stabilizers are added, such as lime. “When lime is heated, calcium is released, which ensures stability in the network.” Unlike Na + and K + is calcium (Ca) divalent, you write Ca 2+ . The positive charges attract relatively negatively charged atoms, in the silica network it is oxygen (O). Ca 2+ can attract two oxygen atoms and thus provides more structure in the network, so that Na and K can roam less easily.
Verhaar investigates the relationship between the components of a glass, the degradation products on the surface and the susceptibility to degradation. “The amount of sodium/potassium salts is important, but perhaps the ratio to the calcium content is a good indicator of the stability of a glass.” By measuring which degradation products contain contaminated glass, he hopes to develop a predictive method.
“If we can show that certain degradation products are characteristic of the glass disease and we can show those products on a glass that is not yet visually affected, we can determine which glass is unstable and at risk.” Conservators and restorers can then take measures to limit the risks, says Verhaar. “That can be by storing and displaying a glass under certain conditions. Humidity is an important environmental factor, but it can also mean that a glass is not lent or may not be placed near other materials.” Wood, for example, is notorious for the acids it releases. This can cause major problems for a fragile glass.
Venetian glass
Glassblowers knew very well how to influence the properties of their glass, but their pursuit of ever more beautiful glass has, ironically, had disastrous results. Verhaar:“The famous Venetian glass is one of the most important problem groups for museums. The glassblowers in Venice wanted to make very clear glass, so they purified the different raw materials before adding them. This resulted in beautifully clear glass, but it also became much less stable. Stabilizing ingredients have probably also disappeared as a result of that purification.”
Incidentally, the glass disease can also affect contemporary glass. Verhaar grabs a modern Irish Coffee glass there. There is a greasy layer with droplets on it. “It also feels greasy,” Verhaar confirms. “The acids from the environment migrate into the glass, making the surface relatively basic. That gives a soapy layer.” For Verhaar, these glasses are valuable research material. You cannot just come across museum glass, even if you are conducting your research for the Rijksmuseum.
Cotton swab
In order to arrive at a workable method for testing (museum) glass, it must be a technique that does not damage the glass. But you do need something to be able to analyze what exactly is on the glass surface. Verhaar is consciously looking for a method that is easy to apply. His tools are refreshingly low-tech :a cotton swab! With this he samples the glass, which means that he gently wipes a little bit of the greasy layer or the small crystals on the surface. “It could just be a cotton swab from the drugstore, nothing special.”
Then the swab is placed in deionized water to loosen the sample. The final step is to analyze the sample in an ion chromatograph containing two types of separation columns:one for the positively charged cations (such as Na + , K + , Ca 2+ ) and one for the negatively charged anions (especially acetate CH3 COO - and format HCOO - ). “I chose ion chromatography because this is a very sensitive technique. You only need a tiny bit of sample. Only seven microliters. That is essential for this research, because there is very little material to work with.”
Dummies
Verhaar has since carried out many model measurements on damaged, but not museum pieces, including on the Irish Coffee glass. He shows the result and a huge Na peak catches the eye, while for the anions formate clearly dominates. "With this technique you can therefore say something about the composition and the concentrations of the degradation products on the glass surface, concludes Verhaar. "The presence of sodium formate on a glass may indicate a risk factor." But much more work is needed to arrive at a reliable measurement method.The next step in his research is to measure against a series of dummies.
“I'm going to make different salt solutions in different concentrations and apply them to model slides. Then I will sample each slide and analyze it with ion chromatography. This way I can determine whether you can reliably determine which salts a sample contains and in what proportion to each other with this technique.” If that works, it's time to work with real samples of museum pieces in various stages of degradation. “That would be a first step in establishing a relationship between the concentrations of the risky salts and the damage to the glass.” It remains to be seen which objects he can use his measurements on. Because even a gentle stroke with a cotton swab is a risky operation for fragile glass.