Wall and floor tile used for interior and exterior decoration belongs to a class of ceramics known as whitewares. The production of tile dates back to ancient times and peoples, including the Egyptians, the Babylonians, and the Assyrians. For instance, the Step Pyramid for the Pharoah Djoser, built in ancient Egypt around 2600 B.C. , contained colorful glazed tile. Later, ceramic tile was manufactured in virtually every major European country and in the United States. By the beginning of the twentieth century, tile was manufactured on an industrial scale. The invention of the tunnel kiln around 1910 increased the automation of tile manufacture. Today, tile manufacture is highly automated.
The American National Standards Institute separates tiles into several classifications. Ceramic mosaic tile may be either porcelain or of natural clay composition of size less than 39 cm2 (6 in.2). Decorative wall tile is glazed tile with a thin body used for interior decoration of residential walls. Paver tile is glazed or unglazed porcelain or natural clay tile of size 39 cm2 (6 in.2) or more. Porcelain tile is ceramic mosaic tile or paver tile that is made by a certain method called dry pressing. Quarry tile is glazed or unglazed tile of the same size as paver tile, but made by a different forming method.
Europe, Latin America, and the Far East are the largest producers of tile, with Italy the leader at 16.6 million ft.2/day as of 1989. Following Italy (at 24.6 percent of the world market) are Spain (12.6 percent), Brazil and Germany (both at 11.2 percent), and the United States (4.5 percent). The total market for floor and wall tile in 1990 according to one estimate was $2.4 billion.
The United States has approximately 100 plants that manufacture ceramic tile, which shipped about 507 million ft.2 in 1990 according to the U.S. Department of Commerce. U.S. imports, by volume, accounted for approximately 60 percent of consumption in 1990, valued at around $500 million. Italy accounts for almost half of all imports, with Mexico and Spain following. U.S. exports have seen some growth, from $12 million in 1988 to about $20 million in 1990.
Because the tile industry is a relatively mature market and dependent on the building industry, growth will be slow. The United States Department of Commerce estimates a three to four percent increase in tile consumption over the next five years. Another economic analysis predicts that 494 million ft.2 will be shipped in 1992, a growth of about 4 percent from the previous year. Some tile manufacturers are a bit more optimistic; an American Ceramic Society survey showed an average growth of around 36 percent per manufacturer over the next five years.
The raw materials used to form tile consist of clay minerals mined from the earth's crust, natural minerals such as feldspar that are used to lower the firing temperature, and chemical additives required for the shaping process. The minerals are often refined or beneficiated near the mine before shipment to the ceramic plant.
The raw materials must be pulverized and classified according to particle size. Primary crushers are used to reduce large lumps of material. Either a jaw crusher or gyratory crusher is used, which operate using a horizontal
The initial step in ceramic tile manufacture involves mixing the ingredients. Sometimes, water is then added and the ingredients are wet milled or ground in a ball mill. If wet milling is used, the excess water is removed using filter pressing followed by spray drying. The resulting powder is then pressed into the desired tile body shape.
Secondary crushing reduces smaller lumps to particles. Hammer or muller mills are often used. A muller mill uses steel wheels in a shallow rotating pan, while a hammer mill uses rapidly moving steel hammers to crush the material. Roller or cone type crushers can also be used.
A third particle size reduction step may be necessary. Tumbling types of mills are used in combination with grinding media. One of the most common types of such mills is the ball mill, which consists of large rotating cylinders partially filled with spherical grinding media.
Screens are used to separate out particles in a specific size range. They operate in a sloped position and are vibrated mechanically or electromechanically to improve material flow. Screens are classified according to mesh number, which is the number of openings per lineal inch of screen surface. The higher the mesh number, the smaller the opening size.
A glaze is a glass material designed to melt onto the surface of the tile during firing, and which then adheres to the tile surface during cooling. Glazes are used to provide moisture resistance and decoration, as they can be colored or can produce special textures.
Once the raw materials are processed, a number of steps take place to obtain the finished product. These steps include batching, mixing and grinding, spray-drying, forming, drying, glazing, and firing. Many of these steps are now accomplished using automated equipment.
Mixing and grinding
Sometimes it is necessary to add water to improve the mixing of a multiple-ingredient batch as well as to achieve fine grinding. This process is called wet milling and is often performed using a ball mill. The resulting water-filled mixture is called a slurry or slip. The water is then removed from the slurry by filter pressing (which removes 40-50 percent of the moisture), followed by dry milling.
Tile bodies can also be prepared by dry grinding followed by granulation. Granulation uses a machine in which the mixture of previously dry-ground material is mixed with water in order to form the particles into granules, which again form a powder ready for forming.
Another process, called pressure glazing, has recently been developed. This process combines glazing and shaping simultaneously by pressing the glaze (in spray-dried powder form) directly in the die filled with the tile body powder. Advantages include the elimination of glazing lines, as well as the glazing waste material (called sludge) that is produced with the conventional method.
Dry glazing is also being used. This involves the application of powders, crushed frits (glass materials), and granulated glazes onto a wet-glazed tile surface. After firing, the glaze particles melt into each other to produce a surface like granite.
For tile that only requires a single firing—usually tile that is prepared by wet milling—roller kilns are generally used. These kilns move the wares on a roller conveyor and do not require kiln furnitures such as batts or saggers. Firing times in roller kilns can be as low as 60 minutes, with firing temperatures around 2,102 degrees Fahrenheit (1,150 degrees Celsius) or more.
A variety of pollutants are generated during the various manufacturing steps; these emissions must be controlled to meet air control standards. Among the pollutants produced in tile manufacture are fluorine and lead compounds, which are produced during firing and glazing. Lead compounds have been significantly reduced with the recent development of no-lead or low-lead glazes. Fluorine emissions can be controlled with scrubbers, devices that basically spray the gases with water to remove harmful pollutants. They can also be controlled with dry processes, such as fabric filters coated with lime. This lime can then be recycled as a raw material for future tile.
The tile industry is also developing processes to recycle wastewater and sludge produced during milling, glazing, and spray-drying. Already some plants recycle the excess powder generated during dry-pressing as well as the overspray produced during glazing. Waste glaze and rejected tile are also returned to the body preparation process for reuse.
Most tile manufacturers now use statistical process control (SPC) for each step of the manufacturing process. Many also work closely with their raw material suppliers to ensure that specifications are met before the material is used. Statistical process control consists of charts that are used to monitor various processing parameters, such as particle size, milling time, drying temperature and time, compaction pressure, dimensions after pressing, density, firing temperature and time, and the like. These charts identify problems with equipment, out of spec conditions, and help to improve yields before the final product is finished.
The final product must meet certain specifications regarding physical and chemical properties. These properties are determined by standard tests established by the American Society of Testing and Materials (ASTM). Properties measured include mechanical strength, abrasion resistance, chemical resistance, water absorption, dimensional stability, frost resistance, and linear coefficient of thermal expansion. More recently, the slip resistance, which can be determined by measuring the coefficient of friction, has become a concern. However, no standard has yet been established because other factors (such as proper floor design and care) can make results meaningless.
In order to maintain market growth, tile manufacturers will concentrate on developing and promoting new tile products, including modular or cladding tile, larger-sized tile, slip- and abrasion-resistant tile, and tile with a polished, granite or marble finish. This is being accomplished through the development of different body formulations, new glazes, and glaze applications, and by new and improved processing equipment and techniques. Automation will continue to play an important role in an effort to increase production, lower costs, and improve quality. In addition, changes in production technology due to environmental and energy resource issues will continue.
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The term porcelain refers to a wide range of ceramic products that have been baked at high temperatures to achieve vitreous, or glassy, qualities such as translucence and low porosity. Among the most familiar porcelain goods are table and decorative china, chemical ware, dental crowns, and electrical insulators. Usually white or off-white, porcelain comes in both glazed and unglazed varieties, with bisque, fired at a high temperature, representing the most popular unglazed variety.
Although porcelain is frequently used as a synonym for china, the two are not identical. They resemble one another in that both are vitreous wares of extremely low porosity, and both can be glazed or unglazed. However, china, also known as soft-paste or tender porcelain, is softer: it can be cut with a file, while porcelain cannot. This difference is due to the higher temperatures at which true porcelain is fired, 2,650 degrees Fahrenheit (1,454 degrees Celsius) compared to 2,200 degrees Fahrenheit (1,204 degrees Celsius) for china. Due to its greater hardness, porcelain has some medical and industrial applications which china, limited to domestic and artistic use, does not. Moreover, whereas porcelain is always translucent, china is opaque.
Hard-paste or "true" porcelain originated in China during the T'ang dynasty (618-907 A.D.); however, high quality porcelain comparable to modern wares did not develop until the Yuan dynasty (1279-1368 A.D.). Early Chinese porcelain consisted of kaolin (china clay) and pegmatite, a coarse type of granite. Porcelain was unknown to European potters prior to the importation of Chinese wares during the Middle Ages. Europeans tried to duplicate Chinese porcelain, but, unable to analyze its chemical composition, they could imitate only its appearance. After mixing glass with tin oxide to render it opaque, European craftspeople tried combining clay and ground glass. These alternatives became known as soft-paste, glassy, or artificial porcelains. However, because they were softer than genuine porcelain, as well as expensive to produce, efforts to develop true porcelain continued. In 1707 two Germans named Ehrenfried Walter von Tschimhaus and Johann Friedrich Bottger succeeded by combining clay with ground feldspar instead of the ground glass previously used.
Later in the eighteenth century the English further improved upon the recipe for porcelain when they invented bone china by adding ash from cattle bones to clay, feldspar, and quartz. Although bone china is fired at lower temperatures than true porcelain, the bone ash enables it to become translucent nonetheless. Because it is also easier to make, harder to chip, and stronger than hard porcelain, bone china has become the most popular type of porcelain in the United States and Britain (European consumers continue to favor hard porcelain).
The primary components of porcelain are clays, feldspar or flint, and silica, all characterized by small particle size. To create different types of porcelain, craftspeople combine these raw materials in varying proportions until they obtain the desired green (unfired) and fired properties.
Although the composition of clay varies depending upon where it is extracted and how it is treated, all clays vitrify (develop glassy qualities), only at extremely high temperatures unless they are mixed with materials whose vitrification threshold is lower. Unlike glass, however, clay is refractory, meaning that it holds its shape when it is heated. In effect, porcelain combines glass's low porosity with clay's ability to retain its shape when heated, making it both easy to form and ideal for domestic use. The principal clays used to make porcelain are china clay and ball clay, which consist mostly of kaolinate, a hydrous aluminum silicate.
Feldspar, a mineral comprising mostly aluminum silicate, and flint, a type of hard quartz, function as fluxes in the porcelain body or mixture. Fluxes reduce the temperature at which liquid glass forms during firing to between 1,835 and 2,375 degrees Fahrenheit (1,000 and 1,300 degrees Celsius). This liquid phase binds the grains of the body together.
Silica is a compound of oxygen and silicon, the two most abundant elements in the earth's crust. Its resemblance to glass is visible in quartz (its crystalline form), opal (its amorphous form), and sand (its impure form). Silica is the most common filler used to facilitate forming and firing of the body, as well as to improve the properties of the finished product. Porcelain may also contain alumina, a compound of aluminum and oxygen, or low-alkali containing bodies, such as steatite, better known as soapstone.
To make porcelain, the raw materials—such as clay, felspar, and silica—are first crushed using jaw crushers, hammer mills, and ball mills. After cleaning to remove improperly sized materials, the mixture is subjected to one of four forming processes—soft plastic forming, stiff plastic forming, pressing, or casting—depending on the type of ware being produced. The ware then undergoes a preliminary firing step, bisque-firing.
ProcessAfter the raw materials are selected and the desired amounts weighed, they go through a series of preparation steps. First, they are crushed and purified. Next, they are mixed together before being subjected to one of four forming processes—soft plastic forming, stiff plastic forming, pressing, or casting; the choice depends upon the type of ware being produced. After the porcelain has been formed, it is subjected to a final purification process, bisque-firing, before being glazed. Glaze is a layer of decorative glass applied to and fired onto a ceramic body. The final manufacturing phase is firing, a heating step that takes place in a type of oven called a kiln.
Crushing the raw materials
Forming the body
The character of the raw materials is important in maintaining quality during the manufacturing process. The chemical composition, mineral phase, particle size distribution, and colloidal surface area affect the fired and unfired properties of the porcelain. With unfired body, the properties evaluated include viscosity, plasticity, shrinkage, and strength. With fired porcelain, strength, porosity, color, and thermal expansion are measured. Many of these properties are monitored and controlled during manufacturing using statistical methods. Both the raw materials and the process parameters (milling time and forming pressure, for example) can be adjusted to achieve desired quality.
The FutureHigh-quality porcelain art and dinnerware will continue to enhance the culture. Improvements in manufacturing will continue to increase both productivity and energy efficiency. For instance, a German kiln manufacturer has developed a prefabricated tunnel kiln for fast firing high-quality porcelain in less than 5 hours. Firing is achieved by partly reducing atmosphere at a maximum firing temperature of 2,555 degrees Fahrenheit (1,400 degrees Celsius). The kiln uses high-velocity burners and an automatic control system, producing 23,000 pounds (11,500 kilograms) of porcelain in 24 hours.
Manufacturers of porcelain products may also have to increase their recycling efforts, due to the increase in environmental regulations. Though unfired scrap is easily recycled, fired scrap poses a problem: mechanically strong and therefore hard to break down, it is usually dumped into landfills. However, preliminary research has shown that fired scrap can be reused after thermal quenching (where the scrap is reheated and then quickly cooled), which makes it weaker and easier to break down. The scrap can then be used as a raw material.
Porcelain appears to be playing a more important role in technical applications. Recent patents have been issued to Japanese and American companies in the area of electrical insulators and dental prostheses. NGK Insulators, Ltd., a Japanese manufacturer, has developed high-strength porcelain for electrical insulators, whereas Murata Manufacturing Co. has developed low-temperature-sintering porcelain components for electronic applications.
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