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In an interview, they explain why, despite – or perhaps because of – its minor imperfections, they regard the material as key to a sustainable building culture.

You have been conducting research into cast glass for architectural applications for years. What fascinates you about this material, and how relevant do you consider it to be in the building sector?

Faidra: Cast glass offers us a fascinating freedom of form, materiality and expression! Unlike float glass, which is confined to flat sheets, cast glass can take almost any shape and size. Its flexible and artisanal production process allows for countless compositions, transforming glass from a purely transparent, planar material into one that is colourful, sculptural and expressive. Cast glass introduces entirely new possibilities for architecture and redefines our conception of glass in buildings.

Telesilla: Within this narrative, we question whether architectural glass needs to be perfect or whether we can accommodate elements such as air bubbles, cord and colouration, for example through the use of waste glass resources, and therefore embrace small flaws as part of the unique identity of cast glass. At a time of growing urgency for sustainable building practices and a renewed interest in craftsmanship and material authenticity, cast glass offers an opportunity of merging material performance, circularity, and expressive form in unique ways.

Cast glass has very different structural behaviour to float glass. Why is this, and what does it mean for your research?

Telesilla: Cast glass is not simply a thicker version of float glass; it is an entirely different structural material, shaped by its unique manufacturing process and volumetric geometry. While float glass is produced under highly controlled industrial conditions, cast glass production remains largely artisanal, leading to greater variability in composition and defects. Moreover, given the volumetric nature of glass, not only surface flaws but also internal features can influence its structural performance. Yet, we still know very little about if and when bulk defects, for example inclusions or air bubbles, become critical. This becomes especially relevant if we aim to manufacture reliable structural components from recycled glass. At TU Delft, we are therefore developing new test methods that deliberately activate bulk defects, helping us understand and quantify the effect of imperfections on the mechanical properties.

Could you give us a few examples of the tests for cast glass that you are currently developing?

Faidra: Our research focuses on developing test methods that can characterise the strength, failure mode and impact of bulk flaws in volumetric cast glass. Conventional mechanical tests, such as four-point bending, predominantly activate surface flaws and are therefore not fully suitable for cast glass, where bulk defects may also govern failure. We focus, instead, on developing alternative methods that generate a uniform tensile stress field in the bulk, enabling both surface and internal flaws to be activated. This will allow us to quantify the influence of different defect types and establish when bulk defects become structurally critical. Our new methods include a modified theta test and a new tensile test, but we are also exploring several more unconventional approaches; these are still under development, so you will have to stay tuned!

Cast glass is still produced using traditional craft methods. To what extent does this further complicate the development of standardised testing procedures?

Telesilla: Craft-based production makes standardisation one of the biggest challenges for structural cast glass. Unlike the highly controlled float glass manufacturing, small-scale casting production introduces far greater variability in glass composition, finish surface quality and internal defects. At the same time, there are no established strength databases, quality-control procedures or design standards tailored to cast glass to set a widely-acceptable standard.

Faidra: Fundamental questions remain unanswered – for example, when do internal defects, such as air bubbles, become critical, how large can they be, and how many are acceptable? Developing reliable testing methods tailored to cast glass is essential for understanding and quantifying the effect of such variables on the strength and set the basis for quality control protocols and standards, so that craftsmanship can be combined with the predictability required for architectural and structural applications.

Let’s move on from basic research to practical application. What methods are available to give cast glass additional redundancy like lamination and reinforcement, and which do you consider the most promising?

Faidra:  A real challenge for structural cast glass beyond strength is resilience. Glass is inherently brittle, so the key to wider architectural application is ensuring safe behaviour after fracture. Lamination is a plausible solution, but for large or freeform cast components it is far from ideal. Unlike float glass, large cast glass panels often have flatness deviations of several millimetres, or even centimetres, requiring extensive post-processing (grinding and polishing) before they can be laminated. This adds cost, energy and material use, while interlayers reduce the recyclability of the component at the end-of-life.

Telesilla: We need to rethink redundancy altogether! That is why we are developing reinforced cast glass by embedding metal rods directly into the casting, similar to reinforced concrete. By pairing glass and metal of a compatible thermal expansion, we create a true composite without adhesives. The result is a relatively ductile cast glass component designed to combine safety, performance and recyclability!

In theory, cast glass has the potential to incorporate many different waste streams. What do manufacturers have to take into account in this regard?

Faidra: Indeed, glass casting offers a more flexible method for glass recycling: it can accommodate a much wider range of compositions than conventional float-glass production and yield volumetric glass products that can tolerate higher contamination rates. At TU Delft we have been addressing the key technical barriers of glass recycling: We are studying composition compatibility, for example through our research on glasses with thermal expansion mismatch, and the effect of various contaminants on the strength of glass, through our research on novel testing methods.

Telesilla: The greatest challenges that manufacturers face are shifting from technical to logistical: securing consistent waste streams, ensuring reliable supply and creating viable markets for upcycled products. The real opportunity for the manufacturers lies in redefining value and seeing recycled glass not as an inferior material, but as a high-performance architectural product whose unique character reflects its circular origin.

Are there any manufacturers producing cast glass from waste glass? What obstacles do they face?

Telesilla: A few manufacturers have explored industrial-scale kiln-casting with recycled glass, but almost exclusively using glass bottles. Bottle glass is already a well-established waste stream: it is relatively clean, compositionally consistent and easy to collect, making it the lowest-risk option. More diverse waste streams are being explored primarily by glass artists, who can work with smaller, less consistent batches. This highlights that the main barriers go beyond technical uncertainties, and involve logistical and often legislative obstacles. Upscaling glass recycling via casting requires reliable supply chains, quality control and testing methods that can accommodate greater material variability.

What does the future hold? Will there be an industry standard with defined test methods for cast glass in a few years’ time?

Faidra: Absolutely. Structural cast glass is reaching a turning point. We are experiencing a shift in architecture towards expressive, circular materials and a renewed appreciation for craftsmanship, which stimulates a rising demand for structural and bespoke cast glass solutions. At TU Delft, we are uniquely positioned to accelerate this transition through our distinctive multidisciplinary, full-spectrum research approach that integrates glass science, structural design and architectural thinking with real-world application.

Telesilla: Our research, spanning from fundamental material behaviour to the R&D of landmark cast glass projects, is already shaping industrial innovation and collaborations. The next step is to translate this knowledge into robust testing protocols and design guidelines that can form the foundation of future international standards. Our ambition is to shape cast glass‘s architectural agenda by establishing it as a safe, scalable and widely used structural and circular material.

About Telesilla Bristogianni and Faidra Oikonomopoulou

Dr Faidra Oikonomopoulou and Dr Telesilla Bristogianni are Associate Professor and Assistant Professor, respectively, at the Architectural Engineering + Technology Department, Delft University of Technology. Together, they have pioneered research into structural and circular cast glass, and lead an internationally recognised research group at the forefront of the field. Bridging glass science, architectural design and engineering, and real-world implementation, their work focuses on advancing next-generation structural glass applications, accelerating glass circularity, and shaping the future of design practice and international standards.