Page 25 - Curriculum Visions Dynamic Book

(Below) Bone china, a form of porcelain that is strong, hard, and slightly translucent. It can be fashioned into ware that is far more delicate than any other kind of whiteware.
Porcelain
When kaolin is fired to
make it glassy and naturally watertight, it is called
porcelain. However, there
is a difference in quality
between true porcelain (which
is used to hold chemicals in laboratories and when a very high quality is needed, such as in spark- plugs) and the porcelain used for the objects we generally know as “china.”
All porcelains can be made without glazing because their pores are filled with glass in vitrification. They may, however, still be glazed a particular colour or finish.
(Above) Porcelain tableware.
china has great strength and can resist blows much better than other kinds of whiteware. This extra
strength is caused by the more thorough internal bonding of the fused clay.
There are many kinds of china, ranging from the best table china, including bone china
(which is made with six parts bone ash, four parts china stone, and three and a half parts china clay), through a lower-grade “hotel” china, to some sinks.
Processing whiteware
Each type of whiteware is processed differently. Tiles, plates, cups, and bowls are pressed into shape, while most plumbing fittings, such as basins and toilet bowls, are cast using a clay slurry. Much tableware is also machine formed
on a mass-production version of a potter’s wheel. This process is called “jiggering.”
Because of the huge amount of whiteware made, it
is not often batch processed in industrialized countries
but rather is sent through tunnel kilns on conveyor belts. Earthenware and stoneware are fired at about 1,200°C and china and other porcelains at 1,300°C.
Kaolin begins to turn glassy over a wide range of temperatures, and so the exact temperature is not critical. For example, the first change to glass begins at about
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