Page 14 - Curriculum Visions Dynamic Book

Giving metal the properties of glass
Some of the properties of glass are extremely useful and can be added to the properties of another material. That happens with metal and glass.
Porcelain enamelling, also called vitreous enamelling,
is a process of fusing a thin layer of glass to the surface of a metal object. It seals in the metal, prevents corrosion, and at the same time gives the metal an attractive glaze. Iron pots have been enameled for centuries. Enameled pots, pans, and bathtubs were developed in the 19th century and are still made today.
The problem with enamelware is that being made of glass, it can crack if the metal is dented. Metal has to be scrupulously clean if it is to be enameled. The enamel is applied (as in glazing pots) as a mixture of ground glass, clay, and water. The items are then fired in a furnace.
Glass ceramics
Glass is a material made by fully melting sand. A ceramic is a material made from clay that is heated in a furnace called a kiln to a lower temperature than is needed to make it fully molten. At this temperature some of the clay reacts, and
it fuses together to produce a fired piece of pottery that is waterproof and hard.
But it is possible to make a material that has the properties of both glass and ceramic. It is called a glass ceramic.
A glass ceramic is made by heating glass to a much higher temperature than normal. It becomes an opaque solid that is extremely strong and much less liable to shatter. What happens is that microscopic crystals form, making the glass opaque but leaving it like a glass nonetheless. Most glass ceramics are made by encouraging the growth of microscopic crystals, which is achieved
by adding a chemical called a nucleating agent. A glass ceramic is therefore a crystallized glass with perhaps 98% crystals stuck together by the remaining glass phase at the crystal boundaries.
Glass ceramics are finding an increasing range of uses where high temperature resistance and great strength are needed – from special cookware to the warheads of missiles.
(Below) This special glass is darkened and lightened by applying a small electric current.
See Vol. 4: Ceramics to find out more about glass ceramics.
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