Page 9 - Curriculum Visions Dynamic Book

See Vol. 5: Glass for more on glass.
fact, what allows a metal to bend; bending is internal breaking that only affects a few crystals at a time. As the break begins in a metal and the atoms slip, there are many more places along the fracture where the atoms can bond back again. Breaks are thus both little and temporary, and so the metal can be bent time and time again.
Ceramics have much more perfect crystals than metals; imperfections of the kind common in metals are rare in ceramics. That is why their bonds are stronger and why it is difficult to make a ceramic bend. Also, once the fracture starts, there are no imperfections to keep the fracture from spreading right across the material, and no other places where a slipped atom can bond back again. In fact, slipping brings particles of the same charges side by side,
and since like charges repel, helps push the broken parts away.
Glasses, which do not have a crystal structure and so are not regarded as ceramics, are even more likely to fracture under tension than ceramics.
Making ceramics
Glass and metals can both be melted and then be poured into moulds. Ceramics cannot be melted and re-formed. If a ceramic is melted, it turns into a glass.
To make a ceramic fit into a mould, it has to be ground into a powder and the powder added to a liquid so that it can be shaped and then heated. This process is called sintering, in which the tiny particles bond under heat.
This may seem like a very high-tech affair, but in fact people were doing it in ancient times. When wet clay is moulded, it is a powder of tiny clay particles in water. The clay is shaped into cups, bowls, plates, and so on, and then heated in a kiln. Sintering then occurs, the water is driven off, and the clay powder fuses together to make a hard, brittle ceramic. And the result: common cups, bowls, and all of the other materials that we use from day to day.
(Above, left, and below) The stages in making a durable ceramic.
First, the ceramic, in this case clay, is wetted and moulded. Then,
it is dried and heated. Finally,
it is coated with a glaze (a glass solution) and fired in a kiln. It is now hard, resists abrasion, and is watertight. But it is far more brittle than it was when simply soft clay.
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