A body that has been formed and dried but not fired is called a green body. A green body is made up of millions of tiny particles that are held together by weak Van Der Waals forces. When the body is fired these small particles fuse together through movement of atoms and become chemically bonded. Larger particles grow at the expense of smaller ones until an equilibrium size is reached.
Some ceramic products contain certain constituents that turn into a glass phase upon heating. When the ceramic is fired a process known as vitrification takes place whereby the glass phase liquefies and fills the open spaces between the particles. When the body cools the glass solidifies and acts to bond the unmelted particles together.