PRODUCTION OF CERAMICS
Clay for ceramics is a stiff, viscous earth found in many variations throughout the world. It consists mainly of hydrated aluminum silicates and is derived mostly from the decomposition of feldspathic rocks. Most clays contain iron, giving them their red color, but in varying amounts. Clays also differ in plasticity. Red-burning clays that contain iron are generally more plastic than the white-burning pipe clays. Clays may be fusible from the presence of an alkaline flux such as lime or some other kind of material such as iron, which acts in the same manner. Clays can be made less fusible by increasing silica, such as sand, flint, or quartz. Clay compositions containing more water are more likely to shrink and become deformed in the kiln. This tendency is lessened by adding grog, pulverized fragments of pottery or fired clay from which the water has already been removed. Kaolin, the white clay used in porcelain, comes from a decomposition substance of the feldspar of granite (Boger, 1971).
A potter selects the clays and combines them through wedging or walking the clay to remove air bubbles. This process is similar to kneading dough for bread and gives the clay a better uniformity and consistency. For tiles, clay is pressed into molds and left to dry. When dry, they are fired in the kiln to produce the bisque tile. Glazes are applied to the bisque tile to create designs. Afterward, the tile is fired again in the kiln. The temperature and time of firing vary according to the type of clay used.
Types of Ceramics
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Porcelain tiles are made with kaolin clay and china stone. These two silicates of aluminum (fusible and nonfusible) are fired at a temperature that exceeds 2,200 degrees F, resulting in a very dense, white, translucent material. During the cooling stage, the materials fuse together and solidify again to increase in strength and hardness. Because porcelain tiles are dense, they have a low absorption rate. They are vitrified, translucent, and emit a musical note when struck.
Stoneware refers to clay pottery having a body fired to a state of vitrification that is non-porous. Stoneware is always fired in a high-temperature kiln, but it does not use kaolin. The colors of clay for stoneware are normally grey or red, and occasionally brown.
Earthenware is low-fired clay that is usually red, orange, or brown color. Earthenware was used for many of the ceramics made in the world. They often have glazes on top and require two firings.
Quarry tiles are strong, utilitarian tiles made from graded shale and fine clays with the color throughout the body. Normally they are not glazed and retain their clay color of brownish orange or reddish brown. They are the same colors as bricks, but they are much denser with a smoother surface.
Terra-cotta, sometimes called Saltillo, is formed from clay, but fired at a much lower temperature than quarry tiles. The firing process leaves tiny air pockets in the clay, resulting in a porous surface. The porousness helps the tiles retain heat, but also allows them to absorb moisture. Terra-cotta tiles come in a range of earth colors, from brick-red to soft pink, and include fiery oranges, yellows, and dusky browns. Terra-cotta tiles are handmade, which means they will vary slightly in thickness.
Mosaic tiles are fired clay already glazed and cut into small shapes. If they were cut before firing, they would not fit together due to the shrinkage from the firing.