Connecting sheet glass using heat bonding can theoretically result in perfectly transparent structures. But are they also manufacturable and are they strong enough? After countless fracture tests, PhD student Lisa Rammig concludes that welding and lamination can provide very high transparency and strength. But application in architecture is challenging.

Transparency crucial in architectural applications

In her research, she investigated a manual heat bonding technique in which glass is connected using a burner and lathe, which requires heating of the entire glass element to avoid thermal shock breakage. Various connections were tested, which were heat bonded from 5mm thick borosilicate sheets. ‘If you butt weld two glass edges against each other, very little can be seen of the connection. The reflection that makes edges visible as color when joining glass with traditional means is omitted. That really improves the transparency.’

Rammig works in California with Eckersley O'Callaghan, a specialist engineering firm for glass and facade design. They are known for their work on many Apple retail stores and high end enclosures everywhere in the world. Driven by the retail concept, in which products are displayed and supposed to be visible from outside the store, transparency is crucial. That’s why for glass structures the focus is always on the most visible part: the connections. This applies to glass facades but also to glass stairs and other glass structures.

Welding glass

In the “technical” architecture of a few decades ago, prominent metal fittings and cables were popular, but today even a dark silicone connection is deemed not good enough. The desire for optimal transparency requires invisible connections. Innovative techniques such as welding or bending glass using heat are the alternative. It is the only way to avoid reflections that would otherwise appear on the edges of the glass where it is connected. ‘If three dimensional elements are made, glass becomes a sculptural element instead of a flat surface. Due to its geometry, it can then also be stiffer than flat glass.’

Rammig investigated in a laboratory at the University of Cambridge how stress develops in the glass during welding. She then loaded the workpieces according to a fixed protocol until they broke. This gave an idea of the strength of the heat bonded glass connections. The research showed that the heat bonded joints had comparable strength to the original material. ‘With the disclaimer that they must be manufactured without inclusions and geometric imperfections.’ And that is not easy with manually produced glass connections. Automated, laser-guided welding technology could prevent such imperfections, but that technology is still in its infancy.

Sculptural elements

Is the use of heat bonded connections the future for facade technology? That is by no means certain, because glass elements that are formed this way have to go into the oven to prevent them from breaking due to thermal stress. That requires a very large oven. Laser-assisted welding can potentially remove this barrier, as it allows for highly localized heating reducing the thermal stress induced. But then the transport of fully glazed facade parts is still difficult. "All-glass sculptural elements, such as stairs, may be a more obvious application," says Rammig. "You can make them completely transparent and monolithic in this manner, without any metal fittings- and they can be isolated from building movements."

But such structures would have to be built partly from laminated glass, in order to exclude risks. Gorilla glass – as used in modern mobile phone screens – may also offer opportunities.

In short, heat bonding technology makes more transparent glass constructions possible, but its use has its limits. Rammig: “New architectural applications in glass are always perceived risky. I think they have to take their place in architecture step by step, in a continuous evolutionary process. But we have to be persistent to keep moving forward.”

Published: May 2022

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