Page 19 - Curriculum Visions Dynamic Book

(Below) Cooker tops are made of glass ceramic and take advantage
of its thermal and expansion properties. Glass ceramic is used because its thermal expansion coefficient is close to zero (it was originally developed for telescope mirrors in which distortion due
to uneven temperature is a major issue). Because the expansion is close to zero, it resists thermal shocks such as spilling cold liquid on a hot plate. It is also hard (it
has been used in body armor)
and therefore resists scratching
by abrasive cleaners. Finally, it is relatively transparent to infrared rays. Halogen bulbs or burner elements below the glass ceramic can heat the pan on top via infrared light transmitted through the burner and thus at the same time minimise heating of the burner itself.
Expansion properties
Ordinary soda lime glass expands considerably when heated and shrinks when cooled. Because of this and because of the poor thermal conductivity of glass, if glass is suddenly heated or cooled (as, for example, plunging hot glass into cold water), the surface will try to expand or shrink much sooner than the middle. That will set up great stresses and lead to shattering.
Borosilicate glasses are normally used to get over this problem because they expand only a third as much as soda lime glass. However, it is possible to reduce swelling and shrinking almost to zero by adding 7.5% titanium oxide to pure silica glass.
It appears in mirrors used in outer space, where temperature changes can be sudden and dramatic.
Heat is conducted through a solid material
by conduction and radiation. Clear glass does not conduct heat better as it gets hot. Some types of glass, for example, glass ceramics, are transparent to infrared (heating) radiation and so can be used as hob burners.
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