THE ART OF TRADITIONAL INDUSTRY POTTERY

Pottery is the material from which the pottery ware is made, of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery (plural "potteries"). Pottery also refers to the art or craft of the potter or the manufacture of pottery.

The definition of pottery used by ASTM is "all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products."  Some archaeologists use a different understanding by excluding ceramic objects such as figurines which are made by similar processes, materials and the same people but are not vessels.

BACKGROUND

Pottery is made by forming the clay body into objects of a required shape and heating them to high temperatures in a kiln which removes all water from the clay, which induces reactions that lead to permanent changes including increasing their strength and hardening and setting their shape. A clay body can be decorated before or after firing. Prior to some shaping processes, clay must be prepared. kneading helps to ensure an even moisture content throughout the body. Air trapped within the clay body needs to be removed. This is called de-airing and can be accomplished by a machine called a vacuum pug or manually by wedging. Wedging can also help produce even moisture content. Once a clay body has been kneaded and de-aired or wedged, it is shaped by a variety of techniques. After shaping it is dried and then fired.

PHYSICAL STAGES OF CLAY

Ø  Green ware refers to unfired objects. Clay bodies at this stage are in their most plastic form. They are soft and malleable. Hence they can be easily deformed by handling.

Ø  Leather-hard refers to a clay body that has been dried by exposing it to the air for a period of time. At this stage the clay object has approximately 15% moisture content. Clay bodies at this stage are very firm and only slightly pliable. Trimming and handle attachment often occurs at the leather-hard state.

Ø  Bisque refers to the clay after the object is shaped to the desired form and fired in the kiln for the first time, known as "bisque fired". This firing changes the clay in the object in several ways. The clay hardens to a form that is no longer plastic. Mineral components of the clay will undergo chemical changes that will change the color of the clay.

Ø  Glaze fired is the final stage of some pottery making. A glaze may be applied to the bisque form and the object can be decorated in several ways. After this the object is "Glaze fired" at a very high temperature. This causes the glaze material to harden and causes the glaze and decoration to adhere to the object. The glaze firing may also harden the body still more as chemical processes continue to occur in the body.

Ø  Quartz inversion when pottery bodies are heated during the firing process, silica changes its crystalline structure several times causing a rapid expansion in size. During the cooling cycle of the firing, silica may revert to crystalline structures that occur at the lower temperatures. This can cause the pot to crack as it contracts. Slow firing cycles can minimize or eliminate this problem.

In the making of earthenware, the object may be only "Once-fired" to create a glazed pot.

Bone-dry refers to Clay bodies when they reach a moisture content at or near 0%. This will occur after glaze firing, when that is done, or after bisque firing in the case of once-fired pottery.

 SOURCE   :    http://en.wikipedia.org/wiki/Pottery