Glass-ceramic
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'''Glass-ceramic''' materials share many properties with both [[glass]] and more traditional crystalline [[ceramic]]s. It is formed as a glass, and then made to [[crystal]]ize partly by [[heat treatment]]. Unlike [[sintering|sintered]] ceramics, glass-ceramics have no pores between crystals.
While materials such as [[uranium glass|"vaseline" glass]] are also glass-ceramics, the term mainly refers to a mix of [[lithium]]-, [[silicon]]-, and [[aluminium]]-[[oxide]]s 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 [[mirror]]s and mirror mounts of astronomical [[telescope]]s, these materials have become known and entered the domestic market through its use in glass-ceramic [[stove|cooktops]], as well as [[cookware and bakeware]].
[[Image:Glass ceramic cooktop.jpg|right|thumb|240px|A glass-ceramic [[kitchen stove]] ([[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 (material)|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 [[Celsius|°C]]. At the same time, it has a very low [[Thermal conductivity|heat conduction coefficient]] and can be made nearly transparent (15–20% loss in a typical cooktop) for radiation in the [[infrared]] [[wavelength]]s.
[[As of 2004|Today]], there are two major types of electrical [[stove]]s 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 cooker|induction stove]] heats a [[metal]] pot's bottom directly through [[electromagnetic induction]].
Some well-known brands of glass-ceramics are ''[http://www.us.schott.com/hometech/english/products/ceran/index.html Ceran]'' (cooktops), ''[http://www.eurokera.com Eurokera]'' (cooktop, stoves and fireplaces), ''[[Zerodur]]'' (telescope mirrors), or ''[[Macor]]''. [[Germany|German]] manufacturer [[Schott Glass|Schott]] introduced Zerodur in [[1968]], Ceran followed in [[1971]]. [http://www.neg.co.jp/eng/index.html Nippon Electric Glass of Japan] is another worldwide manufacturer of glass ceramics, whose related products in this area include Firelite and Neoceram fire-rated glass.
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.
== Care and cleaning ==
Glass-ceramic cooktops can be scratched very easily, so care must be taken not to slide the cooking pans over the surface. Food with a high sugar content (such as jam) should never be allowed to dry on the surface if it spills, otherwise damage will occur. Cleaning is best carried out by using a soft cloth along with a special glass-ceramic cleaner that applies a thin protective film on the glass.[http://www.eurokera.com/products/cooking/cleaning.html]
{{Glass science}}
[[Category:Glass-ceramics]]
[[Category:Ceramic materials]]
[[Category:Glass engineering and science]]
[[de:Glaskeramik]]
[[es:Cocina vitrocerámica]]
[[fa:شیشه-سرامیک]]
[[fr:Plaque vitrocéramique]]
[[pl:Tworzywo szklano-ceramiczne]]