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The primary component of porcelain are clay, feldspar and silica. These materials are weighed, crushed and purified. They are then combined until they are in the appropriate form and then the clay is fired. Since porcelain is fired at a high temperature, it becomes vitrified (glassy). Unlike glass, this clay mixture holds its shape when fired at high temperatures making it easy to form into a sink shape.
After being formed, the sink is bisque fired, which means the sink is heated at a relatively low temperature to vaporize contaminants and minimize shrinkage during the firing. A second firing is done in a kiln which consists of a refractory lined sealed chamber with burner ports. It can fire only one batch of sinks at a time. Typically the sinks will enter a preheating period and move through a higher temperature zone and then a cooling zone.
During the firing process, carbon based impurities burn out as well as moisture. Sulfates begin to decompose and turn into gas which is all exhausted. Some of the minerals break down to form a liquid glass. This phase is required for shrinking and bonding the grains of other materials. After the desired density is achieved, the sink is cooled which cause the liquid glass to solidify, thereby forming a strong bond between the remaining crystalline grains.