Saturday, January 22, 2011

Clay, Pottery, and Ceramics

Pottery from Azerbaijan

History of Pottery and Ceramics

All Pottery is made from a very common and naturally occurring material - clay.  It can be be found in significant quantities virtually everywhere on the earth's crust.  Since the beginning of time, humans have made votive objects from clay which were used for worship, and functional pottery or earliest domestic vessels were used for storage, containers & funerary artifacts.  Earth, air, water and fire comprise the essential elements of pottery making.  Clay emerges from the earth, dries when exposed to air, and softens when blended with the right amount of water. 

Clay becomes Ceramic when it is exposed to extreme heat, which fuses the clay particles together and hardens the body to produce a stable material often more permanent than rock itself.  The greater the heat, the denser, more durable the clay becomes.   Although ceramics date back to 24,000 BCE, several pieces originate from 6000 BCE in Azerbaijan & Southern Hungary, 5000 BCE China and Japan, and 3000 BCE in Mesopotamia (Iran).  Today ceramics are used for both functional and sculptural objects. 

Two Geological categories of clay:  (Primary & Secondary)

Primary - or residual clays which have remained in their forming grounds
Secondary - sedimentary clays are those that have been eroded and carried away by water and earth movements. 

Clay varies in colour from white to reddish-brown, depending on the amount of iron and other impurities present. 

Three Main Categories of Clay:
Earthenware  (red terra-cotta clay or white) -  a secondary clay (traveled by erosion from its original location) and is found in and beside riverbeds.  It has a smooth, fine texture.  Earthenware has been widely used historically since it is by far the most abundant type of clay and because it has a low firing range (1830 - 2160 F).

Stoneware  (grey or brown) -  is a secondary clay but it is found in higher locations (further up the streams or hillsides) than earthenware.  It has a medium particle size and a medium to high temperature firing range (2190 - 2370 F).  Stoneware clay has a coarser, gritty texture but can be quite plastic.

Porcelain  (white) -  is derived from kaolin clay, a primary clay that is in limited supply worldwide.  Kaolins have a large particle size and low plasticity.  True porcelain is only fired at a high temperature (2340 - 2460 F) and because of its high silica content (glass) can be translucent. 

Plastic Clay
Clay is a highly malleable substance and its most important quality, plasticity, allows it to retain its shape when molded.  It is only by handling the clay - seeing how it responds to bending, rolling, pulling and pressing as well as shrinkage, will allow you to make an informed choice about which clay to use.

Stages of Clay:
Slip  -  watered down clay in a muddy form
Plastic  -  workable stage; molding stage; can recycle; can join to other pieces
Leather-hard  -  stiff and will hold its shape; join to other pieces; carve into; recycle
Bone-dry/ Greenware   -  can be carve into; very fragile; can recycle; work must be bone-dry for next firing stage (or it will explode in kiln)
Bisqueware  -  fired once in the kiln; can not be recycled; permanently shaped; if desired, ready for glazing
Stoneware (Glazeware)  -  second high temperature fire; can not be recycled; glossy, vitrified

Mattison, Steve.  The Complete Potter. (New York: Barron's Educational Series), 2003

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