Clay, which is a natural resource of the earth, originally came from a rock known as feldspar. These rocks underwent many physical changes from millions of years of weathering, interacting with sunlight, rain and ice. As the rocks were broken down into smaller particles, clay was formed. How clay weathers results in the various clay types used by potters. The type of clay used in creating pottery affects the quality of a finished piece. Often potters don't use only one clay; they sometimes use several types to obtain the desired results for their pottery pieces.
The three main types of clay pottery include earthenware, stoneware and kaolin. Each one is characterized by the clay mixture and temperature at which it's fired or baked. While earthenware and kaolin are fired using lower temperatures, stoneware is fired at much higher temperatures.
Earthenware clay contains high amounts of iron. Because it's a low-firing clay, its mineral ingredients don't turn into glass. This type of clay is opaque and porous, keeping a rich clay color. It is somewhat fragile and has a brighter color range than stoneware clay. It's typically used in creating Mexican pottery, Japanese raku and terra cotta.
Kaolin, the purest type of clay, is a fine white clay that's exceptionally shiny and smooth. Kaolin is used in making porcelain pottery, high-fire white ware and fine china. This clay requires a high temperature for firing and is able to tolerate temperature ranges from 2300 degrees Fahrenheit and higher without slumping or melting, according to the Ken George Pottery website.
Stoneware clay is made up of alumina and silica. This clay needs rather high temperatures for firing and can be either vitreous (hard, dense and absorbent) or semi-vitreous. It's porous and less fragile than earthenware, as well as being more opaque than kaolin clay. Stoneware's impurities cause the clay type be mostly gray or brown in color. Before being manufactured, it is usually glazed. Dinner plates are examples of pottery made from stoneware clay.
Ball Clay and Fire Clay
Ball clay, which is similar to porcelain, is made up of even finer particles. Most deposits of ball clay are found in Tennessee and Kentucky. Fire clay, which needs high-firing temperatures, is used in making kiln furniture and hard firebrick. Found almost anywhere, fire clays have high amounts of alumina and silica, in addition to usually containing some iron.
Slip Clays and Bentonite
Slip clays can be used as glazes and are known for adding a bright finish to stoneware pottery. Various slip clays are available. For example, while porcelain slip clays are composed of porcelain, clay and water, stoneware slip clays consist of stoneware clay. Albany is the most common slip clay. Bentonite, which is also used for glazing, has the smallest particles of any known clay and is used as a plasticizer. It's mostly found in western mountain states such as the Dakotas and in several Gulf States.