Glass-ceramic

Glass-ceramic materials shares many properties with both glass and more traditional crystalline ceramics. It is formed as a glass, and then made to crystalize partly by heat treatment. Unlike sintered ceramics, glass-ceramics have no pores between crystals.

While materials such as "vaseline" glass are also glass-ceramics, the term mainly refers to a mix of lithium-, silicon-, and aluminium-oxides which yields an array of materials with interesting thermomechanical properties. The most commercially important of these have the distinction of being impervious to thermal shock. Originally developed for use in the mirrors and mirror mounts of astronomical telescopes, these materials have become known and entered the domestic market through its use in glass-ceramic cooktops, as well as cookware and bakeware.

A glass-ceramic cooktop (2004)

The crystalline component of thermal glass-ceramics, beta spodumene, has a negative coefficient of thermal expansion, which contrasts with the positive coefficient of the glass. Adjusting the proportion of these two materials offers a wide range of possible coefficients in the finished composite.

When an interface between materials will be subject to thermal fatigue, glass-ceramics can be adjusted to match the coefficient of the material they will be bonded to. At a certain point, generally between 70% and 78% crystallinity, the two coefficients balance such that the 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 heats 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.

The same class of material is also used in Corningware dishes, which can be taken from the freezer directly to the oven with no risk of thermal shock.