Advanced Equipment and Imaginative Design Expand Curved Glass Possibilities

By Scott Sowers

Incorporating curved glass into industrial design was a new thing in 1934 when Chrysler rolled out a car with a curved, one-piece windshield. Things have progressed a lot since then. Architects continue pushing the limits on what a building can look like by baking in rounded corners and flowing shapes. Glass manufacturers are doing their best to make those visions come to life.

“It’s more than a renewed interest in curved glass,” says Ferran Figuerola, CEO of Spain-based Cricursa, which Tvitec, another Spanish glass fabricator, recently purchased. “We believe there’s always been an interest in curved glass. Today there are more possibilities to technically produce curved glass, which was not possible years ago, either by size, shape or performance. The … use of curved glass is richer than before.”

Double Bend

Refining oven design and advances in the presses used to push the glass into shape are leading to the ongoing evolution of glass bending. At the most recent glasstec conference held in Dusseldorf, Germany, Lisa Rammig, senior associate with Eckersley O’Callaghan talked with USGlass magazine about a project with NorthGlass that showcased a panel using a unique double bend.

“The panels at the tallest point are about 13 feet tall, doubly curved, and laminated using two layers. They have two different frits on them to show an increase in the impression of the double curvature,” she says, explaining this aesthetic was possible because of a new oven developed by NorthGlass, which also fabricated the glass.

Added Strength

Curving glass changes how building facades are engineered. “By bending glass, we reduce its deflection rate, and it becomes stiffer,” says Figuerola. “It’s the same thing we can do with a piece of paper. By curving the paper, we can make it stand on the table, and it will stay stiff. We could even use a curved paper to hold a pencil on top of it.”

Going Skinny

Thinner glass responds best to bending, but it also challenges the process. “Thinner glass is generally more malleable, but going too thin can yield distorted glass,” says Yago Martinez, director of sales for the Mexican fabricator Cristacurva. “There is a sweet spot that varies depending on the desired size and radius. But even if you had a machine that could do everything, there is a point in which there is something else that limits what you can do. Be it the glass itself, the low-E coating, a ceramic frit, or the need to laminate after bending.”

Feel the Heat

Advanced glass bending requires ovens that use stainless steel molds and more than 1100 degrees Fahrenheit heat. The most popular types are heat-strengthened and fully tempered glass. This process usually requires a high capital investment, which can act as a barrier to entry, depending on how in-depth they wish to go with their products and capabilities.

“We are equipped with 21 different oven configurations to produce curved glass,” says Figuerola. “We have 11 ovens to curve annealed glass in different sizes with different technologies. For tempered or heat-strengthened glass, we have a choice of ten different oven configurations. We can produce extremely tight radii, as well as large girths or heights up to 18 meters.”

Newer ovens use convection with heating elements on the top and bottom to distribute heat more evenly. “The [directional] focus of heat has been improved significantly,” says Kimmo Kuusela, vice president of strategic accounts and innovation for Glaston Corp.’s architectural business.

Going Cold

Cold bending is another process done on tempered glass. It doesn’t require an oven and often happens in the field. Martinez says there seems to be some confusion in the industry about the cold bending process.

“Cold bending consists of taking flat glass and forcing it into shape by either having a pre-bent frame and bonding the glass to it, or by bonding the flat glass to a flat frame and then bending both together. In either case, it starts with flat glass, so I would not consider cold bent glass to be curved glass, as it does not involve curving by a glass fabricator. It’s all on the glazing contractor’s side; the glass was fabricated flat.”

Martinez also points out that annealed glass is bent by heating it to its softening point and then letting it cool down at a slow pace. The slow cooling method keeps it in the annealed form as opposed to heat-strengthened, where it is cooled rapidly, or tempered, where it’s cooled even faster.

The Carbon Question

Curved glass can create a product with less embodied carbon. However, the subject also leads to a debate within the industry.

“Curved glass is stiffer than flat glass. From a deflection standpoint, there is a chance that deflection specs can be met with thinner glass, provided that it also meets all other project requirements,” says Martinez. “If proper calculations allow the use of thinner glass, this would also mean lower embodied carbon, due to the less mass of glass used overall.”

The reduction can also come from large-scale design choices, if double-bent panels are employed. “As soon as we add this double curvature, we actually need less glass to be able to achieve the same stiffness, and that is something that has a lot of potential,” says Rammig. “[The industry has] a problem in that glass is produced in a process that is very inefficient and uses a lot of energy, and a lot of glass goes to waste; therefore, the embodied carbon is very high. If we think smartly about the way we design with glass, then we can use less material to achieve the same result.”

Figuerola, however, isn’t entirely convinced. “We can probably find cases in both directions,” he says. “If we could reduce the thickness of glass by curving it, then we could easily say that we are reducing the amount of embodied carbon. If we can reduce the amount of material in a building, we would be reducing the amount of embodied carbon in that building. But, in most cases, the thickness of the glass will be the same, whether it’s
flat or curved. If both are tempered, then the embodied carbon will be the same, as the energy needed to produce them will also be the same.”

Limitations

For all its potential, curved glass also comes with its limitations and challenges. The first is quality, says Figuerola. “[The industry wants] a curved glass with excellent optical quality, without quenching marks, scratches on the coating and anisotropies. The second will be tolerance, making it as perfect as possible compared to the design.”

He says the next steps in the process, such as laminating and insulating, can also pose challenges. “If the shape is complicated, sometimes the main difficulty is laminating curved glass or assembling the insulating glass unit.”

Getting the right mix of equipment, process and personnel can also put the brakes on executing new curves. “Curved glass takes incredible know-how to get right,” says Martinez. “It is a very complex process with many variables. Not even the best equipment will be able to solve [certain issues] without an experienced and knowledgeable team behind it. The biggest challenge is fabricating curved glass within project specifications and tolerances and with good optical quality.”

Despite the challenges, the product’s allure cannot be denied. “The greatest potential of curved glass is that it provides an added feature to the design and form. We can still work in the rest of the parameters when talking about flat glass, including performance level, resistance, color, sound control, image and transparency level,” says Figuerola.

The concept of curving the glass, instead of the framing that holds it in place, continues to infiltrate the design world. “Every time I see a curved façade that uses segmented glass, I see wasted potential,” says Martinez. “It [segmented glass] is often substituted for budgetary reasons, but when a building is designed to follow an organic shape, switching to segmented glass often completely changes that design and alters the building’s character. In this industry, there aren’t many things more beautiful than seeing a well-executed curved façade with real curved glass. As curved glass becomes more affordable, easier to procure and involves fewer restrictions on what architects can design, I am hopeful that more and more buildings will incorporate it.”

Scott Sowers is a contributing writer for USGlass magazine.

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