Glass doesn't have to be perfect

You have been working with load-bearing structures made of cast glass for around 10 years. What was the impetus for this?

Faidra Oikonomopoulou: It all began in 2014 with the Crystal Houses in Amsterdam. The architectural firm MVRDV had designed a façade made of bonded cast glass blocks for the flagship store in the city centre as an accurate, yet completely transparent reproduction of the previous historic façade. The structural engineers ABT approached our research group to assist them in the development of the façade construction. At the time, there was little knowledge on how to produce a 10 x 12-metre self-supporting façade made of adhesively-bonded cast glass components. There were no known adhesives for such an application, and the strength values of bonded glass blocks were also unknown. We therefore had to carry out all the research and development work for the project – both in terms of structural verification and the on-site implementation.

Telesilla Bristogianni: Subsequently, we also supervised the construction of the façade – every day from six in the morning until six in the evening. We even worked on site with the craftsmen to build the first square metre of the glass façade to teach them the assembly method we had developed in the laboratory.

Where do you see the potential of cast glass?

Faidra Oikonomopoulou: Cast glass can take on more than just the shape of traditional bricks. It has an enormous shaping potential and a great potential from a structural point of view. This motivated us to continue our research – also in the direction of circular constructions. This is how we came up with the idea of interlocking glass blocks that can be dry-stacked with an intermediate layer, thus avoiding the use of permanent bonding. And we asked ourselves: Could we also use recycled glass for these blocks? We started experimenting with everyday glass objects first: Old TV screens, oven doors, waste from glass blowing and much more.

Where does this glass waste end up at the moment?

Telesilla Bristogianni: Currently, only container glass is being recycled in Europe on a large scale; the rest usually ends up downcycled and eventually in landfill. There are logistical and technical reasons for this. Transport costs are high, there are hardly any established collection systems and it is difficult to separate glass from other materials, for example in the case of old insulating glass units. In addition, there are different optical and structural properties and standards depending on the type of glass product. For example, the float glass industry places extremely high optical demands on its products. The raw materials must therefore be completely free of contamination and colour variations. This practically eliminates the use of post-consumer recycled glass.

What about the structural properties of cast glass made from recycled glass?

Faidra Oikonomopoulou: We have carried out numerous bending and breaking tests in our laboratory. We want to find out which type of contamination and glass cullet composition leads to which optical and structural properties of the cast glass. Many glass coatings are not problematic in this respect, but there are some materials, such as glass ceramics, that cause major issues.

Telesilla Bristogianni: Overall, however, three-dimensional cast structures are much more fault-tolerant than flat glass in terms of load-bearing behaviour. When glass panes fail, this is often due to tiny cracks on their surface, which grow quickly and then lead to breakage. We can minimise this risk with cast waste glass by encapsulating it in a layer of high-quality glass. If the flaws are in the bulk, they are rarely triggered.

You will also be exhibiting some of your cast glass experiments at glasstec 2024. They have a very expressive aesthetic.

Telesilla Bristogianni: For us as architects, this is perhaps the most important point of all. Glass doesn't have to be perfect. It can have flaws and these can be very beautiful. Think of the oven door in your kitchen, for example: It has a black frit containing chromium oxide. If you melt it, the frit turns into a kind of green, transparent haze in the glass.

Faidra Oikonomopoulou: Melting glass is a bit like cooking. High and low melting temperatures produce completely different effects. The size of the cullet and the speed of the cooling process also have an influence on the end result. This also applies in structural terms: If you slowly cool down glass so that it fully crystallises, it ultimately becomes stronger.

Telesilla Bristogianni: In the end, we want to incorporate all our findings into a database that will tell you the optical and technical properties you can expect if you use this or that waste raw material and process it in one way or another – just like a cookbook!

Have you already been able to apply your research in practice?

Faidra Oikonomopoulou: We were able to apply our findings on glass with impurities to the sculpture "The Mirage" at Apple's headquarters in Cupertino (USA) in 2022. It was designed by architects Zeller & Moye and artist Katie Paterson and consists of around 480 cast glass columns made from sand collected from approximately70 different desert regions around the world.

Telesilla Bristogianni: The design team and Apple wanted to know how glass properties would be influenced by the various sands, each having a different chemical composition. The challenge was not so much the resulting load-bearing behaviour in this case but achieving a seamless colour gradient between the columns. We had to develop a prediction model to attain this. Thus, we analysed the composition of the raw material using X-ray fluorescence and X-ray diffraction techniques and also carried out multiple melting tests with around 20 types of sand. The different colours are created entirely without added pigments, just from the material of the desert. The blue columns, for example, are made primarily from black volcanic sand, primarily from Hawaii. The iron in this sand is still in a reduced state. In yellow or reddish sands, the iron has already been oxidised and produces a greenish glass.

Faidra Oikonomopoulou: Each desert sand would produce six to seven columns. We also had to consider the final production sequence in the glass studio. For example, very light-coloured columns should not be cast directly after a dark blue batch because there may still be blue glass residue in the furnace. Furthermore, we had to consider the available stock of sands at any given time. Not all sands were available when the final production started. It was like solving a giant jigsaw!

In another research project, you are working on the topological optimisation of cast glass structures. What exactly are you trying to optimise?

Faidra Oikonomopoulou: The most important limiting factor in cast glass is the annealing or cooling time after the casting process. This time span increases exponentially with the glass cross-section. This effect is particularly extreme with the largest objects ever made of glass, the mirrors of large space telescopes that require months of annealing. They are now manufactured with a kind of honeycomb structure in order to optimise the ratio of mass to stiffness and to reduce the annealing time.

These mirrors were our source of inspiration. I thought to myself: Why not use topology optimisation for large cast glass components in architecture? By optimising the stiffness to mass ratio we can achieve large, monolithic structural glass elements with reduced cooling times, yet a high load-bearing capacity. We are now working with Dr Charalampos Andriotis, an Assistant Professor at our university, and a team at MIT to develop appropriate optimisation algorithms. These must, of course, be tailored to the properties of glass – such as the material's different tensile and compressive strength and annealing schedule.

What specific applications are you hoping for from the technology?

Faidra Oikonomopoulou: Glass beams or entire vaulted structures made of cast glass are one example, or small full-glass pedestrian bridges as well as glass floors. These would probably have to be protected from above with a float glass pane. A certain amount of redundancy would have to be built into them so that the structure does not fail completely if a glass support breaks.

Telesilla Bristogianni: We have also experimented with cast glass elements, which we have reinforced with metal in a similar way to concrete. This is definitely a topic that we would like to explore further in the coming year.