La Pipe d'Argile
Types of natural clays
There is a great number of very different clays in nature. This variety is explained by the various geological conditions at the origin of the formation of clay beds. Different clays can be mixed one with another. From this wide range, the ceramist distinguishes certain categories, based on the similarity of plasticity, texture, composition and use.
Kaolin is of particular interest to the potter. It is essential to manufacture real porcelain. Kaolin beds exist in Asia, Africa, North America as well as in Europe, but they are much rarer than layers of other types of clays. In China, white burning kaolin is more common than everywhere else on Earth. It is more plastic and easier to work with than those of other countries.
The manufacture of white pottery began in China at the beginning of the Han dynasty, 200 years before our era. Originally, Chinese potters made soft and white clay bodies from kaolin. The control of firing up to approximately 2300 oF and the first white vitrified pottery using clay bodies made mainly from kaolin exist in China since at least the 6th century of our era. Slowly they learned how to reach higher firing temperatures and how to modify their clay bodies to obtain the hardness, whiteness and translucidity of genuine porcelain. The discovery of porcelain was a real technical triumph in the field of ceramics.
Kaolins are primary clays formed by local decomposition of feldspar. They are mainly found in small depressions rather than in vast stratified layers and they are relatively free from mineral impurities like iron. Their grain is coarse and they are not plastic, compared to most sedimentary clays. Pure kaolins are very refractory and their melting point exceed 3200 oF. Alone they are very difficult to use because of their low plasticity and high melting point. Consequently kaolin is rarely employed alone, other materials are added to make it more plastic and to lower its melting point to obtain vitrified pottery. Kaolin modified in this way is called porcelain.
Ball clays (very plastic clays)
Ball clays have, so to speak, contrary propreties than kaolins. They are part of secondary or alluvial clays. They contain more iron, are more fusible, much more plastic and their particles are smaller. Ball clays and kaolins are really complementary, and are often mixed together to obtain clay bodies which are easier to work. Although less pure than kaolins, they contain relatively little iron and other impurities, and fired at approximately 2400 oF, they take on a gray or brown hue and are dense and tight.
It is impossible to use them alone in pottery because of their excessive shrinkage which can reach up to 20 % when fired to their maturation temperature. They are used in addition with other clays to increase their plasticity. In preparing porcelain bodies, their addition to the paste is essential to correct the lack of plasticity of kaolin. However, in higher quantity than 15 % there is the risk of appearance of gray or brown.
Unfired, these clays are generally dark gray because of the presence of carbonic matter. This carbon, entirely burned in the kiln does not affect the color of the fired body. Such a clay is more plastic since it contains more carbon. However, certain ball clays contain little carbon and are completely white in their natural state.
Ball clays are used to manufacture a large variety of products. In the United States, they are extracted from important layers in Tennessee and Kentucky.
"Fire clays" are not a kind of clay so clearly defined as kaolin or ball clay. The name "fire clay" indicates that they are refractory or resistant to heat. Some are very plastic, others are not, and their color after firing is variable. A clay which does not melt or become deformed at 2700 oF is called "fire clay". The fact that one clay is refractory or resistant to heat indicates mainly that it is relatively pure and free from iron, although many fire clays burn brown or chestnut due to small concentrations of iron minerals.
Fire clays are used to manufacture varied products, especially refractory brick and other products for furnaces, boilers and crucibles. Metallurgical industries of copper, iron and steel depend on the use of blast furnaces, which are covered with refractory brick that can support the melting point of these metals.
Some use these clays in addition to stoneware or other clays intended to manufacture cassettes and other kiln hardware to increase their resistance to heat. Fire clays can give more roughness or the desired grain to pastes of stoneware. They are also employed to fill up fissures around the gates of furnaces, to manufacture pyrometric cones and to steady the charging plates in kilns.
Stoneware clays are plastic, secondary or sedimentary clays whose vitrification is obtained between 2200 oF and 2400 oF. Fired, their color goes from chamois or clear gray to darker gray or to brown. They vary much in plasticity and firing temperatures and there is not a clear distinction between fire clays and stoneware. In fact, the classification of the types of clay depends more on its ceramic use than on its true chemical or physical nature or on its geological origin. The same clay can very well be used as fire clay, to make some refractory brick, and to manufacture stoneware fired at high temperature.
Many clays are usable to manufacture stoneware without any modification. They can have the exact necessary plasticity to be turned and present the specific characters of stoneware in drying and firing. The small workshops of the last century, which produced utility stoneware, usually used a clay body extracted in the area and prepared without adding any other material. This kind of natural stoneware bodies can give very pleasant colors and textures and can well accept slips and glazes for stoneware fired at high temperatures.
After genuine porcelain, stoneware is second in sophistication and refinement of traditional artisanal potteriy. These clays are strong, dense and tight, and because of quite similar pysical properties of glazes and bodies, their glazes adheres well to their potteries.
Most of natural clays can be called earthenware or common clays. These clays contain enough iron and other mineral impurities to become tight and hard, fired between 1750 oF and 2000 oF. They are the easiest of all clays to fire, even rudimentary wood or straw fueled kilns can reach these relatively low temperatures. So all primitive pottery was and is allways made from this type of clay. In their natural state, they are gray, greenish, red or brown because of the iron oxide that they contain.
Fired, their color goes from pink to black while passing by all varieties of yellow, red and brown, following the particular quality of each clay and the conditions of firing. The majority of ceramic made in the world uses this kind of clay; bricks, tiles, ceramic pipes and other similar products are made from this clay. Common red clays can be very plastic, even too plastic and sticky to be employed alone, sometimes its the opposite because of the presence of sand or small rocks. Potters will search of soft and plastic earthenware, which they can possibly modify by adding a little sand or nonplastic clay. Brick-makers will look for less fine bodies containing sand and other nonplastic debris which they will be able to press, dry and fire without hazard of warping, cracking or excessive shrinking.
Earthenware is the most common of all traditional artisanal pottery. This clay body is much softer and porous then other potteries, and absorbs more liquid than all other ceramics. Some glazes used on earthenware contain toxic lead as a flux and because of significant differences of body and glaze properties, will peel of more easely.
Great quantities of common red clays are present on the Earth's surface. Although a great part is unusable in ceramic because of the presence of calcite debris (limestone) or some other alkaline and soluble salts, enormous reserves of good earthenware remains for the manufacture common goods.
Potter's porcelain is never a natural clay body. It is mainly composed of kaolins mixed with small amounts of ball-clay to increase its plasticity and ground feldspar to lower it's maturation temperature. Genuine translucent porcelains are fired between 2300 oF and 2550 oF. Some special very vitrious porcelain, containing more kaolin and less flux, is fired at much higher temperatures.
Real white and translucent porcelain is the finest of all potteries. It is verry dense, hard and absorbs much less liquid then any other ceramic body. Specifically formulated glazes for a particular porcelain adheres very well to it's body when it is fired to maturity at the same time than the clay body.
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