When you think of glass, what do you think about? Car windshields? Eyeglasses? The test tubes and beakers used in science class? What about space telescopes, or the sleek cooktop panels that are appearing in kitchens everywhere. While it might seem like all of these things are made with glass, that isn’t necessarily true. Space telescopes and cooktop panels aren’t usually made with glass at all. Rather, they are made with a closely related material, glass-ceramic. While glass-ceramics often look and feel like glass, they have different properties, and there is a different manufacturing process involved. Engineers and product designers select glass or glass-ceramics based on those properties.

With over 130 years of innovation, our experts love to talk about being pioneers in glass technology, but they’re also experts in glass-ceramic too. And with Ceramics Expo 2019 on the horizon, what better time to talk about the basics, like what exactly is the difference between glass and glass-ceramic?

Making glass and glass-ceramics

The basic ingredient of glass is silica or sand. Depending on how the product will be used, chemists and glass scientists add secondary chemicals like boron oxides, soda-lime, or aluminum oxides to produce glass with certain properties, e.g., clarity, resistance to scratches, resistance to thermal shock, or chemical resistance. Measured precisely and refined in a melting tank, the raw materials are heated to a temperature of about 2,400 F. At that point, the molten glass is about the consistency of honey, and is ready to be shaped into tubes, blocks, flat panels, fibers, or any of the dozens of other forms that glass is supplied in.

The process of making glass-ceramics is a lot like making glass. The basic ingredients are sand, lithium, and aluminum oxides, but one special ingredient is added to make glass ceramic: a special nucleating agent is added to the raw materials. After melting raw materials, the mixture acts exactly like glass – it is cooled very slowly to about 1,100 F. At that temperature, the nucleating agent enables the formation of crystal nuclei — the building blocks of crystals. Raising the temperature again encourages those crystal nuclei to grow and to rearrange themselves into a crystal structure. The material is then cooled to room temperature. The result is a material that looks and feels like glass, but is more durable and better able to handle extreme temperatures and temperature changes. Chemists can enhance these properties by changing the chemical mixture of the glass-ceramic, or by modifying the production process, such as the length of time the material is heated.

How we use glass and glass-ceramics

Designers and product engineers select glass and glass-ceramics possessing properties that meet the needs of their product or project. The diversity of glass formulations and the sophisticated material and process know-how of glass experts present many options. Soda-lime glass, for example, is one standard glass consumers come into contact with the most. It’s found in home windows and drinking glasses. Replacing soda and lime with boron oxide yields a specialty borosilicate glass, prized for its chemical resistance, that’s often used for labware, such as in containers for highly sensitive pharmaceuticals. Aluminosilicate glasses are selected for high heat resistance and hardness — they might be found in high temperature thermometers and in high-heat halogen light bulbs.

Glass-ceramic is often chosen for products calling for a different set of properties, one of which is its extremely low coefficient of thermal expansion. Meaning it doesn’t change shape when it is heated or cooled. In space, a telescope that passes through the shadow of the moon might see a significant temperature fluctuation. That fluctuation is something an ordinary glass mirror could not withstand, leading it to warp. When focusing light over galaxies and light-years, even the tiniest shift in a mirror could lead to blurry images potentially impacting the results of a space mission decades in the making. That’s why a special kind of glass-ceramic, SCHOTT ZERODUR®, is often selected as a mirror substrate for use in space-based telescopes. It’s because of these properties that the European Southern Observatory (ESO) selected ZERODUR^® for the Extremely Large Telescope (ELT) project. To ensure this and future projects have access to the melting capacities and post-processing options for a variety of applications, SCHOTT made significant investments in its glass-ceramic competence center in Mainz, Germany.

Back on earth, glass-ceramics are found throughout the home. The glass-ceramic cooktops that underpin the design of sleek modern kitchens are formulated to resist scratches. Resistance to scratching is important because accidents happen in a kitchen. We drop salt, sugar and cans of soup onto the surface of the cooktop and move pots and pans back and forth. An easily-scratched cooktop would look unattractive after just a couple months. For safety reasons, cooktops are also formulated to limit the dispersion of heat when a burner is turned on. Manufacturers would not want high temperatures on one burner ring to make the rest of the cooktop so hot you couldn’t touch it.

You can also find glass-ceramic offering a view inside your oven or barbecue grill, or as a cover to an outside infrared heater. NEXTREMA® was designed for extreme conditions providing high temperature resistance, near zero thermal expansion, a wide transmittance spectrum, chemical resistance, and more. It’s a material with nearly limitless potential.

How we got here

The first spectacles for corrective vision were invented in the 13th century. Galileo used a telescope to upend all of the common wisdom of the 17th century. Despite those huge impacts, glass production was, until fairly recently, a hit-or-miss endeavor. Glass was useful, but it was very hard to make glass of consistent quality that was suitable for detailed scientific research. It wasn’t until the late 19th century that scientists formulated a way to systematically make glass of consistent quality – free of bubbles and striations – with the defined precise properties – refractive index, optical homogeneity, and light transmission. Otto Schott, the man for whom the technology company SCHOTT is named, was a chemist whose family had a background in glassmaking. He invented a method for melting glass in small quantities, paving the way for experimentation with new glass materials. He invented borosilicate glass, and some of his formulations, like SCHOTT FIOLAX®, continue to be relevant today.

What followed has been more than a century of innovation through the addition of additives into the glass melt and experiments with heat, including the invention of glass-ceramic occurring around half a century ago. The development of glass science and technical glass has enabled much of modern life, from the fingerprint readers and camera lenses on our smartphones, to the cooktop panels that are a feature of modern kitchens. Without glasses of specific technical properties, many of the things we take for granted wouldn’t exist or perform as needed.

• Glass tubing and its properties
• SCHOTT NEXTREMA® glass-ceramic for extreme conditions
• ZERODUR® extremely low expansion glass-ceramic