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.