When is glass not quite glass? When it’s glass-ceramic
Like two remarkably similar cousins who eventually go their separate ways, glass and glass-ceramics have distinctly different properties and attributes – but still bear a striking family resemblance.
Glass-ceramics became a kitchen staple in the mid-20th century with glossy good looks, easy-to-clean surface, and ability to go from freezer to oven to table in style.
That same wholesome material is a favorite in the aerospace and defense industry. It forms heat-resistant missile nosecones and huge radomes protecting microwave antenna.
It’s often opaque, always non-porous, and typically very tough and thermally stable. But glass-ceramic material, no matter what form it takes, starts life as a glass whose composition formula includes specific amounts of an element like phosphorus, silver, or titanium. Once formed, it looks just like conventional transparent glass.
After formation, though, an intense application of carefully controlled heat makes those added elements serve as nucleating agents. In the process, they form “seeds” around which crystals can grow. And that’s where this close cousin of glass develops its own unique properties.
Those regular patterns of crystals – something optical glassmakers try strenuously to avoid -- can scatter light, rather than letting it pass smoothly through. The phenomenon usually results in an opaque material, although lower levels of nucleating agents can make it translucent instead.
The glass-ceramic formula can be manipulated to create other attributes that are highly attractive to designers. It can have low dielectric loss, resistance to heat shock, and great insulating properties. Some formulations can also be machined into intricate, precise shapes – something not easily done with glass – thus giving designers an attractive, durable alternative to injection-molded parts.
This gives glass-ceramics a place in components for watches, headphones, appliances, and many other products where strength and cool beauty are important factors. There’s significant potential for luxury packaging out of glass ceramics, especially for high-end cosmetics or spirits with brands that radiate a sleek, contemporary feel.
Some new translucent glass-ceramics may also prove their worth in display applications. Designers are envisioning glass-ceramic refrigerator doors or countertops, for example, that are perfectly glossy and black. But with the right sort of backplane and internal light source, the opaque-appearing material would become transparent and a vivid display would appear as if from nowhere. After use, it would then disappear completely, with no sign of a frame or bezel like conventional displays need.
While it has many distinctive properties, many other aspects of glass-ceramics remain identical to those of glass. The smooth feel, the stain resistance, the ability to chemically strengthen through ion exchange – those are all properties that the close glass cousins continue to share.
And, like many cousins, glass and glass-ceramics work well together. Electronics manufacturers are beginning to experiment with combination glass and glass-ceramic encasements for a wide variety of devices.
For a cellphone or a tablet, that means one smooth, cool, familiar feel from two different but highly compatible materials.