Ceramics / Pottery
The earliest ceramics were pottery objects made from clay, either by itself or mixed with other materials. Pottery is one of the oldest human technologies and art-forms, and remains a major industry today. The clay is most often fired in a kiln and then glazed and re-fired to create a colored, smooth surface. Glazing is functionally important for earthenware vessels, which would otherwise be unsuitable for holding liquids due to their porous nature. Glaze is also used on functional and decorative stoneware and porcelain. In addition to the functional aspect of glazes, aesthetic concerns include a variety of surface finishes, including degrees of gloss and matte, variegation and finished color. Glazes may also enhance an underlying design or texture which may be either the "natural" texture of the clay or an inscribed, carved or painted design.
The potter's most basic tool is the hand. However, many additional tools have been developed over the long history of pottery manufacturing, including the potter's wheel, shaping tools (paddles, anvils, ribs), rolling tools (roulettes, slab rollers,rolling pin), cutting/piercing tools (knives, fluting tools, wires) and finishing tools (burnishing stones, rasps, chamois).
Hand building is the earliest forming method. Wares can be constructed by hand from coils of clay, from flat slabs of clay, from solid balls of clay or some combination of these. Parts of hand-built vessels are often joined together with the aid of slurry or slip, an aqueous suspension of clay body and water. Hand building is slower and more gradual than wheel-throwing, but it offers the potter a high degree of control over the size and shape of wares. While it isn't difficult for an experienced potter to make identical pieces of hand-built pottery, the speed and repetitiveness of other techniques is more suitable for making precisely matched sets of wares such as table ware.
In the process that is called "throwing", a ball of clay is placed in the center of a turntable, called the wheel-head, which the potter rotates with a stick, or with foot power (a kick wheel ) or with a variable speed electric motor. During the process of throwing the wheel rotates rapidly while the solid ball of soft clay is pressed, squeezed, and pulled gently upwards and outwards into a hollow shape. The first step of pressing the rough ball of clay downward and inward into perfect rotational symmetry, is called centering the clay, before the next steps: opening (making a centered hollow into the solid ball of clay), throwing (drawing up and shaping the walls to an even thickness), and trimming or turning (removing excess clay to refine the shape or to create a foot). Slip casting is often used in the mass-production of ceramics and is ideally suited to the making of wares that cannot be formed by other methods of shaping. A slip, made by mixing clay body with water, is poured into a highly absorbent plaster mold. Water from the slip is absorbed into the mould leaving a layer of clay body covering its internal surfaces and taking its internal shape. Excess slip is poured out of the mold, which is then split open and the molded object removed. Slipcasting is widely used in the production of sanitary wares and is also used for making smaller articles, such as intricately-detailed figurines.
Earthenware is a common ceramic material, which is used extensively for pottery tableware and decorative objects. Although body formulations vary between countries and even between individual makers, a generic composition is 25% ball, 28% kaolin, 32% quartz, and 15% feldspar. Earthenware is one of the oldest materials used in pottery. While red earthenware made from red clays is very familiar and recognizable, white and buff colored earthenware clays are also commercially available and commonly used.
Earthenware is commonly bisque or biscuit, fired to temperatures between 1000 and 1,150 °C (1800 and 2100 °F and glaze- fired from 950 to 1,050 °C (1,742 to 1,922 °F). However examples of the reverse — low biscuit and high glost firing — can also be found: this can be popular with some studio potters where bisque temperatures may be 900 to 1,050 °C (1,652 to 1,922 °F) with glost temperatures in the range of 1,040 to 1,150 °C (1,904 to 2,102 °F). The exact temperature will be influenced by the raw materials used and the desired characteristics of the finished ware. The higher firing temperatures are likely to cause earthenware to bloat. After firing, the body is porous and opaque with colours ranging from white to red depending on the raw materials used.
Earthenware may sometimes be as thin as bone china and other porcelains, though it is not translucent and is more easily chipped. It is also less strong, less tough, and more porous than stoneware, but is less expensive and easier to work. Due to its higher porosity, earthenware must usually be glazed in order to be watertight.
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between 1,200°C (2,192 °F) and 1,400 °C (2,552 °F). The toughness, strength, and translucence of porcelain arise mainly from the formation of glass and the mineral mullite within the fired body at these high temperatures. Porcelain can informally be referred to as "china" in some English-speaking countries, as China was the birth place of porcelain making. Properties associated with porcelain include low permeability and elasticity; considerable strength, hardness, glassiness, brittleness, whiteness, translucence, and resonance; and a high resistance to chemical attack and thermal shock.
Raku ware is a type of pottery that was traditionally used in the Japanese tea ceremony in Japan, most often in the form of tea bowls. It is traditionally characterized by the removal of pieces from the kiln while still glowing hot. Typically, pieces removed from the hot kiln are placed in masses of combustible material (e.g., straw, sawdust, or newspaper) to provide a reducing atmosphere for the glaze and to stain the exposed body surface with carbon. It is important for a kiln to have a door that is easily opened and closed, because when the artwork in the kiln has reached the right temperature (over 1000 degrees Celsius) it must be taken out quickly using tongs and put in a metal or tin container with combustible material, which reduces the pot, leaving certain colors and patterns. Once the lid of the container is closed the reduction oxidation process begins. The temperature change from the kiln to the container (usually a trashcan of some sort) is where the combustible material is and where the magic of raku happens. Raku techniques have been adopted and modified by contemporary potters worldwide.
Stoneware is a vitreous or semi-vitreous ceramic ware of fine texture made primarily from non-refractory fire clay. Stoneware's maturation temperature ranges from about 1200 °C to 1315 °C (2192 °F to 2399 °F). In essence, it is man-made stone. Stoneware, which, though dense, impermeable and hard enough to resist scratching by a steel point, differs from porcelain because it is more opaque, and normally only partially vitrified. It may be vitreous or semi-vitreous. It is usually coloured grey or brownish because of impurities in the clay used for its manufacture, and is normally glazed.