Glass-ceramic
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Glass-ceramic is a mixture of glass and ceramic materials (mainly lithium-, silicon-, or aluminium-oxides) yielding a material that is impervious to even extreme temperature shocks. Originally developed for the use in the mirrors and mirror mounts of astronomical telescopes, it has become known and entered the domestic market through its use in glass-ceramic cooktops.
The manufacturing process is very similar to that of glass, but the melt is then made to crystalize partly. The crystalline component of the resulting material has a regular crystal structure and a negative coefficient of thermal expansion, balancing the positive coefficient of the glass, such that glass-ceramic as a whole has a thermal expansion coefficient that is very close to zero.
Glass-ceramic is a mechanically very strong material and can sustain repeated and quick temperature changes up to 800 - 1000°C. At the same time, it has a very low heat conduction coefficient and can be made nearly transparent (15-20% loss in a typical cooktop) for radiation in the infrared wavelengths.
Today, there are two major types of electrical stoves with cooktops made of glass-ceramic:
- A glass-ceramic stove uses electrical heating coils or infrared halogen lamps as the heating elements. The surface of the glass-ceramic cooktop above the burner heats up, but the adjacent surface remains cool because of the low heat conduction coefficient of the material.
- An induction stove has induction hobs that heat a pot's bottom directly through electromagnetic induction (this works only with pots that have a ferromagnetic bottom).
Some well-known brands of glass-ceramics are Ceran (cooktops), Zerodur (telescope mirrors), or Macor. German manufacturer Schott has introduced Zerodur in 1968, Ceran followed in 1971.