While Ceramic products play huge role in improving the quality of life in the built environment, their connection to Clay is more often than not ,lost on many people .So let me clear the air and say here and now that ceramics are made from Clay – a material that is not particularly prized for building foundations on , but as it happens is the very foundation of this hard ,durable and appealing material . Clay is type of fine grained heavy soil that is elastic when it’s wet (see image below).
Clay is formed from the wear and tear of igneous rocks like granite which have feldspar-an alumina silicate mineral. There are generally five types of clay as follows:
- Kaolin:-It’s also called china clay and is the purest clay . It comes from the decomposition of the feldspar in granite rocks.
- Illite:-it’s a micaceous clay. For your information, Mica is a transparent stone used to make electrical equipment.
- Montmorillonite:-its aplastic clay of variable composition.
- Ball clay:-It contains organic matter and often blended with other clay types like kaolin.
- Terracotta clay:-This clay contains iron oxide that cause it to turn red when fired.
How ceramics are made
Ceramics are made from either wet or dry clay that is put through baking process at very high temperatures. Wet clay which contains both chemical bonded and physical bonded water is first of all moulded into required items .The wet clay items are then dried slowly to allow them to shrink without cracking. This is followed by firing, during which time, all physically bonded water is lost at a temperature of about 100 degrees Celsius.
On the other side of the coin, dry clay powder is used when items to be made require higher accuracy for example floor tiles. The dry clay powder is compressed into required shape and baked at high temperature.
As the temperature increases beyond 500 degrees Celsius , all chemically bonded water is lost as steam and vapours. At 800 degrees Celsius, Carbon in clay is burnt off as carbon dioxide and the clay material begin to undergo a sintering process that produces a porous material (has many small holes that allow air to pass through).
As the temperature increases toward 1200 degrees Celsius, the Alumina and Silica in clay recrystallize into material called Mullite.With additional increase in temperature ceramic material is formed because of increased re crystallization. At temperature beyond 1300 degrees Celsius all remaining Silica is recrystallized. In short, a brand new Ceramic materials is formed because of the re crystallization of clay (alumina and silica) at very high temperatures.
At this very high temperature potassium or sodium may be added to the mix which causes vitrification to occur producing an opaque ceramic product!
Ceramic products are widely used in building construction as floor, wall, roof ,sanitary materials as well as many other uses as expounded below..
The Clay used here is a blend of Silica Alumina and iron oxide (high silica content of 40-80%, and iron oxide 2-3 %).Fire-clay is used to make refractory products which are not affected by high temperature of fires. In particular dense products are used in fire resistance applications while on the other hand lower density products are often used for example in flue linings(metal tubes use to take smoke and heat from a fire to the outside of house).White glazed fireplace is also used to make urinals and laboratory sinks.
2.Brick and roofing tiles
Whereas Bricks are made from clay composed of 40-65% Silica, 1-25% Alumina and 3-9% iron oxide, Roofing tiles manufacture is more demanding –it requires same clay type used in brick making but must be of a finer quality . Also certain floor tile can be manufactured with this clay but then it will require firing up to 1130 degrees Celsius to produce sufficient vitrification(Image below).
Terracotta is a brown red clay used to make building materials. The clay use must be of finer grade than that used in making roofing tiles. Terracotta clay is use in manufacture of Terracotta floor tiles and decorative tiles.
Stoneware is made from secondary or impure clays. It’s used to make vitrified clay pipes, wall cladding and floor tiles.
A mix of various clays is used to make earthenware. This is often used to make drainage products.
Made from a mix of various clays and magnesium silicate or limestone that is turned into dry powder and pressed into tiles. The tiles are the glazed to improve their aesthetic appeal and make them water proof.
Vitreous china is made from blend of kaolin clay, ball clay, and quartz. It’s used to manufacture water closets, wash basins, floor tiles and swimming pools.
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Sources: Materials for Architects and Builders by Arthur Lyons, PhD.