When the clay has been moulded into shape it is heated fiercely in a kiln. This process is called firing. During firing the edges of the clay crystals melt and a glassy substance forms. As the clay cools, the glass hardens and holds the clay together so that it becomes hard. However, this also makes the pottery brittle.

Earthenware is a type of pottery has many tiny holes in it after it has been fired and it cannot hold water. To make it watertight it is coated with a mixture of ground glass and water called a glaze and fired again at an even hotter temperature. The glaze fills the holes and makes the pottery water tight. It also gives the pottery a smooth shiny surface. The white crockery used in many places every day is earthenware.

Stoneware is fired at a higher temperature than earthenware and this makes the surface clay crystals melt and form a natural glaze. This makes the pottery watertight. Storage jars and jugs are examples of stoneware.

Porcelain is made from a mixture of China clay and granite. The granite must contain a large amount of the mineral called feldspar. This type of clay is heated most strongly of all. The feldspar melts and forms a glassy material which holds the clay particles together. Bone china is a form of porcelain.