Clay is a fine-grained aggregate of hydrous silicate particles. It is usually formed by chemical weathering or hydrothermal activity and is defined by geologists by the size of the grain, as well as the content. Clay is plastic when wet, but firm when dry.
Pottery clay is the clay used to make the three categories of pottery: earthenware, stoneware, and porcelain. If baked by intense heat, a process known as firing that is undertaken in a special oven called a kiln, it becomes permanently solid. Firing is used both to harden all three types of pottery and to adhere glaze to it or otherwise permanently color it.
Earthenware — including faience, delft, and majolica — is pottery made from porous pottery clay fired at low temperatures. Because it is fired at low heat, the clay does not lose its porous nature and does not become translucent.
Stoneware is fired at high temperature, and so becomes nonporous. Jasper ware and basalt ware – both developed by English potter Josiah Wedgwood in the eighteenth century, are examples of stoneware. Stoneware tends towards a grayish color when fired, and though vitrified, is more opaque than porcelain. Porcelain, also called china, is fired using very high heat, resulting in a white, nonporous, translucent pottery.
Different types of pottery are made from different types of pottery clay, including kaolin, ball clay, stoneware clay, glacial clay, slip clay, and bentonite. These types of pottery clay differ in color, plasticity, and firing properties, and they are found in different parts of the country. After firing, clay pottery can be used for many kinds of items. The type of clay chosen will depend on the type of object being made. There are so many types of clay products, requiring different clay properties, that choosing the component pottery clay is an important part of a potter’s work.