Porcelain
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Background
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, reprenting the most popular unglazed variety.
Although porcelain is frequently ud as a synonym for china, the two are not identical. They remble 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 u, 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 Chine porcelain consisted of kaolin (china clay) and pegmatite, a coar type of granite. Porcelain was unknown to European potters prior to the importation of Chine wares during the Middle Ages. Europeans tried to duplicate Chine 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. The alternatives became k
nown as soft-paste, glassy, or artificial porcelains. However, becau 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 ud.
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. Becau 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).
占星术Raw Materials
黑发晶的功效与作用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 the 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
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 process—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.
is treated, all clays vitrify (develop glassy qualities), only at extremely high temperatures unless they are mixed with materials who 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 u. The principal clays ud 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 pha 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 remblance to glass is visible in quartz (its crystalline form), opal (its amorphous form), and sand (its impure form). Silica is the most common filler ud 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.
The Manufacturing
Process
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After the raw materials are lected and the desired amounts weighed, they go through a ries of preparation steps. First, they are crushed and purified. Next, they are mixed together before being s
ubjected to one of four forming process—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 pha is firing, a heating step that takes place in a type of oven called a kiln.
Crushing the raw materials
∙ 1 First, the raw material particles are reduced to the desired size, which involves using a variety of equipment during veral crushing and grinding
steps. Primary crushing is done in jaw crushers which u swinging metal jaws.
Secondary crushing reduces particles to 0.1 inch (.25 centimeter) or less in
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diameter by using mullers (steel-tired wheels) or hammer mills, rapidly
moving steel hammers. For fine grinding, craftspeople u ball mills that
consist of large rotating cylinders partially filled with steel or ceramic grinding media of spherical shape.
Cleaning and mixing
∙ 2 The ingredients are pasd through a ries of screens to remove any under- or over-sized materials. Screens, usually operated in a sloped position, are
vibrated mechanically or electromechanically to improve flow. If the body is
to be formed wet, the ingredients are then combined with water to produce the desired consistency. Magnetic filtration is then ud to remove iron from the
slurries, as the watery mixtures of insoluble material are called. Becau
iron occurs so pervasively in most clays and will impart
After bisque firing, the porcelain wares are put through a glazing operation,回老家作文
which applies the proper coating. The glaze can be applied by painting,
dipping, pouring, or spraying. Finally, the ware undergoes a firing step in an
oven or kiln. After cooling, the porcelain ware is complete.
an undesirable reddish hue to the body if it oxidizes, removing it prior to firing is esntial. If the body is to be formed dry, shell mixers, ribbon mixers, or
intensive mixers are typically ud.
Forming the body
3 Next, the body of the porcelain is formed. This can be done using one of
填写的英文four methods, depending on the type of ware being produced:
o soft plastic forming, where the clay is shaped by manual molding, wheel throwing, jiggering, or ram pressing. In wheel throwing, a potter
places the desired amount of body on a wheel and shapes it while the
wheel turns. In jiggering, the clay is put on a horizontal plaster mold of
家庭wifithe desired shape; that mold shapes one side of the clay, while a heated
die is brought down from above to shape the other side. In ram
pressing, the clay is put between two plaster molds, which shape it
while forcing the water out. The mold is then parated by applying
vacuum to the upper half of the mold and pressure to the lower half of
the mold. Pressure is then applied to the upper half to free the formed
老人简笔画body.
o stiff plastic forming, which is ud to shape less plastic bodies. The body is forced through a steel die to produce a column of uniform girth.
This is either cut into the desired length or ud as a blank for other
forming operations.
o pressing, which is ud to compact and shape dry bodies in a rigid die or flexible mold. There are veral types of pressing, bad on the
direction of pressure. Uniaxial pressing describes the process of
applying pressure from only one direction, whereas isostatic pressing
entails applying pressure equally from all sides.
o slip casting, in which a slurry is poured into a porous mold. The liquid is filtered out through the mold, leaving a layer of solid porcelain body.
Water continues to drain out of the cast layer, until the layer becomes
rigid and can be removed from the mold. If the excess fluid is not
drained from the mold and the entire material is allowed to solidify, the
process is known as solid casting.
Bisque-firing
∙ 4 After being formed, the porcelain parts are generally bisque-fired, which entails heating them at a relatively low temperature to vaporize volatile
contaminants and minimize shrinkage during firing.
Glazing
∙ 5 After the raw materials for the glaze have been ground they are mixed with water. Like the body slurry, the glaze slurry is screened and pasd through
magnetic filters to remove contaminants. It is then applied to the ware by
means of painting, pouring, dipping, or spraying. Different types of glazes can be produced by varying the proportions of the constituent ingredients, such as
alumina, silica, and calcia. For example, increasing the alumina and
decreasing the silica produces a matte glaze.
Firing
∙ 6 Firing is a further heating step that can be done in one of two types of oven, or kiln. A periodic kiln consists of a single, refractory-lined, aled chamber
with burner ports and flues (or electric heating elements). It can fire only one
batch of ware at a time, but it is more flexible since the firing cycle can be
adjusted for each product. A tunnel kiln is a refractory chamber veral
hundred feet or more in length. It maintains certain temperature zones
continuously, with the ware being pushed from one zone to another. Typically, the ware will enter a preheating zone and move through a central firing zone
before leaving the kiln via a cooling zone. This type of kiln is usually more
economical and energy efficient than a periodic kiln.
∙7 During the firing process, a variety of reactions take place. First, carbon-bad impurities burn out,
chemical water evolves (at 215 to 395 degrees
Fahrenheit or 100 to 200 degrees Celsius), and carbonates and sulfates begin
to decompo (at 755 to 1,295 degrees Fahrenheit or 400 to 700 degrees
Celsius). Gas are produced that must escape from the ware. On further
heating, some of the minerals break down into other phas, and the fluxes
prent (feldspar and flint) react with the decomposing minerals to form liquid glass (at 1,295 to 2,015 degrees Fahrenheit or 700 to 1,100 degrees Celsius).
The glass phas are necessary for shrinking and bonding the grains. After
the desired density is achieved (greater than 2,195 degrees Fahrenheit or 1,200 degrees Celsius), the ware is cooled, which caus the liquid glass to solidify,
thereby forming a strong bond between the remaining crystalline grains. After cooling, the porcelain is complete.
