Is glass a ceramic?

Is glass ceramic?

No, glass is not ceramic. Ceramics are made from clay baked at high temperatures. Whereas glass is a non-crystalline material that is formed by melting silica sand and other ingredients. 

Clay is fired to the point of vitrification, whereas glass is melted and cooled to hardness. You create glass by heating liquid sand (sand with silicon dioxide). It must reach a temperature of 1700°C before melting can occur.

Devitrification is the process by which a glassy substance may crystallize. So, if a transparent piece of glass is devitrified it will no longer be clear because the many boundaries between the crystals will scatter light.

So, although glass is not ceramic, the two materials have some similarities and can be used for similar purposes. Each has unique properties that make them suitable for different applications.

Glasses are often used in windows and optical instruments because they are transparent and relatively lightweight. And ceramics are used in pottery and other household items because they can withstand higher temperatures than glass. 

Glass is brittle and can break easily but has a lower thermal conductivity than most ceramics, meaning that it does not transfer heat as well as ceramics do. This makes it a better insulator and ideal for applications such as windows and bottles. 

What is Glass-Ceramic then?

Polycrystalline materials produced through controlled crystallization of base glass are known as “glass-ceramics.” 

Glass-ceramic materials share many characteristics with both glasses and ceramics. In contrast to spontaneous crystallization, which is generally not desired in glass production, glass-ceramics have an amorphous phase and one or more crystalline phases.

Glass-ceramics are similar to glass but lack inherent brittleness. Because they have properties of both ceramics and glass, they can be produced more quickly and efficiently than their ceramic counterparts. Some glass-ceramic seals do not require brazing and can withstand temperatures up to 700°C when used for sealing.

Glass-ceramics have a variety of materials with unique properties, including:

  • zero porosity
  • high strength
  • toughness
  • translucency of opaqueness
  • pigmentation
  • opalescence
  • low or negative thermal expansion
  •  high-temperature stability
  • fluorescence, machinability
  • ferromagnetism
  • resorbability or biocompatibility

In manufacturing, glass-ceramics are valued for having the strength of ceramic with the hermetic sealing capabilities of glass.

Glass-ceramics are made in two steps. The first step is to form glass by a glass-manufacturing process. The cooled glass is then reheated in a second stage. The glass crystallizes partially during the heat treatment. Then nucleation agents are added to the base composition of the glass-ceramic, which aids and regulates the crystallization process. Because there is rarely any pressing and sintering, glass-ceramics have no pores, unlike sintered ceramics.

The discovery of Glass-Ceramics

A man named Donald Stookey, a renowned glass scientist is credited with the invention of glass ceramics while working at Corning Inc.

The prototype originated from a glass substance, Fotoform, which Stookey had been looking for a photo-etchable material to put in television displays.

He heated a Fotoform plate in a furnace at 900 degrees Celsius and discovered an opaque, milky-white plate within the furnace instead of the molten mess he was expecting.

He studied the new material, dubbed Fotoceram, and discovered that it was considerably more durable than Fotoform.

And his research into glass-ceramic materials would result in the creation of two more, one being used as the radome on missiles’ nose cones and the other becoming Corningware, a line of consumer kitchenware.

Corning executives announced that Stookey had developed a “new basic material” called Pyroceram, which was said to be light, durable, capable of being an electrical insulator while also being thermally shock resistant. 

There were just a few materials available at the time that offered the precise set of characteristics that Pyroceram did, and it was launched as Corningware kitchenware on August 7, 1958.

Pyroceram’s success prompted Corning to redouble its efforts to strengthen glass, which became Project Muscle by the technical directors of Corning.

Corning’s glass team developed Chemcor, now known as Gorilla Glass, in 1962

Corning’s glass team developed Chemcor (now known as Gorilla Glass) in 1962, a “lesser-known” glass-ceramic material nicknamed “ultrastrong.”

In 1961, Corning launched Centura Ware, a new line of Pyroceram that was lined with a glass laminate (invented by John MacDowell) and treated with the Chemcor process. Stookey went on to investigate the qualities of glass-ceramics in 1966 when he figured out how to make it transparent.

While Corning refused to commercialize his new invention until the late 1970s, Nippon Electric Glass released FireLite, a transparent glass-ceramic material designed for use with fire-rated doors and other safety materials. In 1988, they debuted.

This fire-rated glass-ceramic, which is 5 mm thick and made of a glass-ceramic blend, can sustain the pressure of a fire hose for 20–90 minutes (depending on the grade of ceramic utilized) in a furnace while still allowing 88% of visible light to pass through its surface. 

It is currently produced today by firms like TGP (Technical Glass Products), which belongs to the Allegion safety group.

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