The term clay refers to a number of earthy materials that are composed of minerals rich in alumina, silica and water. Clay is not a single mineral, but a number of minerals. When most clays are wet, they become "plastic" meaning they can be formed and molded into shapes. When they are "fired" (exposed to very high temperatures), the water is driven off and they become as hard as stone. Clay is easily found all over the world. As a result, nearly all civilizations have used some form of clay for everything from bricks to pottery to tablets for recording business transactions.
The minerals that make up clay are so fine that until the invention of X-ray diffraction analysis, these minerals were not specifically known. Under extremely high magnification, one can see that clay minerals can be shaped like flakes, fibers, and even hollow tubes. Clays can also contain other materials such as iron oxide (rust), silica, and rock fragments. These impurities can change the characteristics of the clay. For example, iron oxide colors clay red. The presence of silica increases the plasticity of the clay (that is, makes it easier to mold and form into shapes).
Clays are categorized into six categories in industry. These categories are ball clay, bentonite, common clay, fire clay, fuller's earth, and kaolin.
Clays are common all over the world. Some regions, as might be expected, produce large quantities of specific types of clay. It is estimated that the state of Georgia has kaolin clay reserves of 5 to 10 billion tons. The United States is self-sufficient so it imports only small amounts of clay from Mexico, Brazil, United Kingdom, Canada, and assorted other nations. The United States exports nearly half of its production worldwide.
The nations producing the most significant amounts of the various clays are as follows:
The United States both imports and exports clays and clay products. It is estimated that the United States consumes about 37.6 million tons of clays each year.
Ball clays are good quality clays used mostly in pottery but are also added to other clays to improve their plasticity. Ball clays are not as common as other clay varieties. One third of the ball clay used annually is used to make floor and wall tiles. It is also used to make sanitary ware, pottery, and other uses.
Bentonite is formed from the alteration of volcanic ash. Bentonite is used in pet litter to absorb liquids. It is used as a mud in drilling applications. It is also used in other industrial applications such as the "pelletizing" of iron ore.
Common clay is used to make construction materials such as bricks, cement, and lightweight aggregates.
Fire clays are all clays (excluding bentonite and ball clays) that are used to make items resistant to extreme heat. These products are called refractory products. Nearly all (81%) of fire clays are used to make refractory products.
Fuller's earth is composed of the mineral palygorskite (at one time this mineral was called "attapulgite"). Fuller's earth is used mostly as an absorbent material (74%), but also for pesticides and pesticide-related products (6%).
Kaolinite is a clay composed of the mineral kaolin. It is an essential ingredient in the production of high quality paper and some refractory porcelains.
When necessary, calcium carbonate and talc can be used in place of clay as filler in some applications. However, clay is so abundant in all its forms that such substitutions may only be necessary if the alternative materials are less expensive than clay (which is not very likely).