Pottery

Unfired "green ware" pottery on a traditional drying rack at Conner Prairie living history museum.

Pottery is a form of ceramic technology, where the clay is formed into vessels, generally with utilitarian purposes in mind. The production of pottery is a process where wet clay is shaped and allowed to dry. The formed clay, or piece, may be "bisque fired" in a kiln to harden it, and then fired a second time after adding a glaze or a piece may be once fired by applying appropriate glaze to the dry unfired clay and firing in one cycle.

Types of pottery

Aesthetic and artistic considerations have often been part of the formation of the pottery vessels, however modern mass production techniques have replaced the traditional role of pottery with mechanized reproduction, which has in turn caused the potter to be more focused on the aesthetic than the utilitarian in industrialized nations.

Traditionally, different world regions have produced different types of clay, also called bodies, with the potter digging clay out of natural banks in his own 'back yard.' In modern times, potters will often combine different clays and minerals to produce clay bodies suited to their specific purposes. Pottery that is fired at temperatures in the 800 to 1200 °C range, which does not vitrify in the kiln but remains slightly porous is often called earthenware or terra cotta. Clay formulated to be fired at higher temperatures, which is partially vitrified is called stoneware. Fine earthenware with a white tin glaze is known as faience. Porcelain is a very refined, smooth, white body that, when fired to vitrification, can have translucent qualities. Complex extremely high-fired ceramics, where the glaze and body fuse completely, are generally referred to as "products of ceramic technology." Ceramic technology is used for items such as electronic parts and Space Shuttle tiles.

Techniques

A man shapes pottery as it turns on a wheel. (Cappadocia, Turkey).

A person who makes pottery is traditionally known as a potter. The potter's most basic tool is his or her hands, however many of their tools have been created over the long history of pottery, including the potter's wheel, various paddles, shaping tools (or ribs), slab rollers, and cutting tools.

Forming techniques

There are three basic categories of forming techniques used in pottery—handwork, wheel work, and slipcasting. It's very common for wheel-worked pieces to be finished by handwork techniques. Slipcast pieces tend not to be, as that negates one of the prime advantages of casting.

Handwork methods are the most primitive and individual techniques, where pieces are constructed from hand-rolled coils, slabs, ropes, and balls of clay, often joined with a liquid clay slurry. No two pieces of handwork will be exactly the same, so it is not suitable for making precisely matched sets of items such as dinnerware. Doing handwork enables the potters to use their imagination to create one-of-a-kind works of art. These methods are often referred to as "handbuilding".

Classic potter's wheel in Erfurt, Germany.

The potter's wheel can be used for mass production, although often it is employed to make individual pieces. The process of making ceramic ware on the potter's wheel is called "throwing" or "turning." A ball of clay is placed in the center of a turntable, called the wheel head, which is turned chiefly using foot power (a kick wheel or treadle wheel) or a variable speed electric motor. Oftentimes, a disk of plastic, wood, or plaster is affixed to the wheel head, and the ball of clay is attached to the disk rather than the wheel head so that the finished piece can be removed easily. This disk is referred to as a bat. The wheel revolves rapidly while the clay is pressed, squeezed, and pulled gently into shape. The process of pressuring the clay into a radial symmetry, so that it does not move from side to side as the wheel head rotates is referred to as "centering" the clay—usually the most difficult skill to master for beginning potters.

Wheel work takes a lot of technical ability, but a skilled potter can produce many virtually identical plates, vases, or bowls in a day. Because of its nature, wheel work can only be used to initially create items with radial symmetry on a vertical axis. These pieces can then be altered by impressing, bulging, carving, fluting, faceting, incising, and other methods to make them more visually interesting. Often, thrown pieces are further modified by having handles, lids, feet, spouts, and other functional aspects added using the techniques of handworking. Pottery that is thrown on the wheel is often finished in a process known as trimming. The thrown piece is first allowed to dry to the leather-hard state then it is returned to the potter's wheel, usually with the rim down. The piece must be re-centered to allow trimming of the foot of the pot to create a smooth and well-defined surface.

There are two related techniques that improve repeatability of wheelwork. A jigger is a mould that is slowly brought down onto the outside of an object, while it is being turned on a wheel. A solid mould is used to form the inside of the piece. Similarly, a jogger is used to shape the inside of a piece, pressing the outside against a solid mould. Although these techniques have been in use since the 18th century, they are usually considered minor "industrial" methods by modern studio potters. There is contention among potters over whether a "jigged" piece can be considered "hand-produced."

Slipcasting is probably the easiest technique for mass-production, especially for shapes not easily made on a wheel. A liquid clay slip is poured into plaster moulds and allowed to harden slightly. This slip can be formulated to mature at a variety of temperatures. Once the plaster has absorbed most of the liquid from the outside layer of clay the remaining slip is poured back into the storage tub, and the piece is left to dry. Finally, the finished piece is removed from the mould, "fettled" (trimmed neatly), and allowed to air-dry. This method is commonly used for smaller decorative pieces such as figurines, which have many intricate details. In the United States, moulds and their slipcast pieces are primarily an industrial product, and are usually called "ceramics" to distinguish them from other pottery.

Decorative and finishing techniques

Contemporary pottery from Okinawa, Japan.

Additives can be worked into moist clay, prior to forming, to produce desired characteristics to the finished ware. Various coarse additives, such as sand and grog (fired clay which has been finely ground) give the final product strength and texture, and contrasting colored clays and grogs result in patterns. Colorants, usually metal oxides and carbonates, are added singly or in combinations to achieve a desired colour. Combustible particles can be mixed with clay or pressed into the surface to produce texture. Shredded fiberglass can be used as an additive to improve tensile strength in the finished piece. However, the resulting clay contains sharp fibers, is hard to work with and must be carefully handled.

Throughout history, potters have used a mixture of coloured clays as a distinctive decorating technique. In traditional studio pottery in Great Britain, these techniques were known as agateware. The name is derived from the agate stone, which shows bands of colours. In Japan, various techniques for combining coloured clay on the potter's wheel are jointly known as "neriage." An analogue of marquetry can also be made, by pressing small blocks of coloured clays together, and using the resulting mosaic to create distinctive patterns. The Japanese term for this technique is nerikome. Agateware and the other varieties of 'mottled' ware are made by combining two or more colours or varieties of clay into one completed piece. Different colours of clay are lightly kneaded or slapped together before being formed into a vessal or decorative item. This method is most commonly used for handbuilt pieces. Coloured clay can also be added to a base clay after it is centered on the wheel. Although in principle any clays can be combined, differing rates of drying/shrinkage and expansion in firing create structural difficulties. It is best to select a light neutral clay body, and then add a colourant to separate portions of the same body. The different coloured clays can then be joined without significant structural problems. Members of commercial clay 'families' often have a similar chemical composition and a similar shrinkage rate, and can be used together.

Burnishing, like the metalwork technique of the same name, involves rubbing the surface of the piece with a polished surface (typically wood, steel, or stone), to smooth and polish the clay. Finer clays give a smoother and shinier surface than coarser clays, as will allowing the pot to dry more before burnishing, although that risks breakage.

An ancient Armenian urn.

To give a finer surface, or a coloured surface, slip can be coated onto the leather-dry clay. Slip produced to a specific recipe is sometimes called an engobe. Slips or engobes can be applied by painting techniques, or the piece can be dipped for a uniform coating. Many pre-historic and historic cultures used slip as the primary decorating material on their ware. Sgraffito involves scratching through a layer of coloured slip to reveal a different colour or the base clay underneath. If done carefully, one colour of slip can be fired before a second is applied prior to the scratching or incising decoration. Often slips/engobes used in this process have a higher silica content, sometimes approaching a glaze recipe. This is particularly useful if the base clay is not of the desired colour or texture.

Glazing and firing techniques

Glazing is the process of coating the piece with a thin layer of a glassy material (often a mix of dolomite, frit, silica/flint, feldspar, sodium borate, clay and whiting plus metal oxides or carbonates). This is important for functional earthenware vessels, which would otherwise be unsuitable for holding liquids due to porosity. Glaze may be applied by dusting it over the clay, spraying, dipping, trailing or brushing on a thin slurry of glaze and water. Brushing tends not to give very even covering, but can be effective with a second coating of a coloured glaze as a decorative technique. With all glazed items, a small part of the item (usually on the base of the piece) must be left unglazed, else it will stick to the kiln during firing.

Glazes can be formulated to melt within the kiln at various temperatures called cones and denoted by a small triangle and a number, which run upwards from cone 1 at 1154 °C and backwards with a preceding 0. Cone 06, for example, is a lower temperature than cone 1 at approximately 999 °C. Glazes formulated to melt between cone 09 (~923 °C) and cone 01 (~1137 °C) are often referred to as "low fire", while glazes which melt between around cone 6 (~1222 °C) and cone 12 (~1326 °C) are called "high fire". Those which melt in the intermediate range are called "mid fire". The temperature within the kiln is often identified using small triangular Pyrometric cones of carefully formulated chemical mixtures which melt within a specific temperature range and begin to bend slightly—hence the term "cones" being used to denote temperature.

Some clays and glazes are oxygen-sensitive, most notably those containing iron and copper, and will change colour depending on the presence of oxygen during the firing. Kilns can either be "oxidized" by opening a port to allow oxygen into the interior or "reduced" by closing off the kiln from outside air to attain colors as desired.

A number of various firing techniques can be used in addition to normal glaze-firing. Most of these involve heating the kiln to a high temperature and then delivering an amount of dry chemical into the kiln's interior. Sulfur is commonly used, as are various salts or ashes. Such substances will stick to pieces within the kiln and melt onto their surfaces, often resulting in a mottled texture which has a distinctive "orange peel" feel. Colors generally depend on what chemical is added to the kiln. These techniques can have very unusual and frequently unexpected results whether used on an unglazed piece or in combination with normal glazing.

Wood firing is another type of firing which involves using wood, rather than gas or electricity as in most modern kilns, to heat the kiln's interior. An example of a wood fired kiln is the Chinese Anagama, also adopted and used by Korean and Japanese potters. Wood firing is frequently time-consuming, as the kiln must be stoked for days, but the pieces which emerge often have characteristic patches of orange color on the clay itself, known as "blushing".

The Western adaptation of Raku firing, a traditional Japanese technique, has enjoyed a deal of popularity due to its relative ease. The kiln is heated to a low temperature, usually no higher than cone 06, and then ware is pulled out of the kiln while still hot (using tongs, of course) and smothered in ashes, paper, or woodchips. This can be done in an enclosed container, which allows the supply of oxygen to be cut off and reduction to take place. The finished products of this process are not suitable for functional use, as the clay remains porous and may have some toxic chemicals held within it as a result of burning the surrounding woodchips or paper used to smother it. However, because of the low temperature, it is an extremely quick and easy technique to do, and the clay has a distinctive black color.

Production stages

All pottery items go through a series of stages during construction.

1. The raw clay is wedged to make its moisture and other particle distribution homogeneous and to remove air bubbles. It is then shaped either by hand or using tools such as a potter's wheel, an extruder, or a slab roller. Water is used to keep the clay flexible during construction and to keep it from cracking.
2. Work that is thrown on the wheel often needs to be trimmed or turned to make its thickness uniform and/or to form a foot on the piece. This process is done when the piece has stiffened enough to survive manipulation. This condition is called leather hard.
3. The piece is allowed to air dry until it is hard and dry to the touch. At this stage it is known as greenware. Items of greenware are very brittle but they can be handled with care. Greenware items are often sanded with fine grade sandpaper to ensure a smooth finish in the completed item.
4. Sometimes the greenware is given a coating of a liquid clay slip. This is most often done to give a coloured base for decoration, other than the colour of the main clay.
5. The greenware is often given a preliminary lower range firing in a kiln. Once it has been fired, the clay is known as biscuit ware or bisque.
6. Biscuit ware is normally a plain red, white, or brown colour depending on which type of clay is used. This is decorated with glaze and then fired again to a higher temperature.
7. Some pieces are not bisque-fired before being glazed. These pieces are called once-fired.

History

Incipient Jomon pottery (10,000-8,000 BC), Tokyo National Museum, Japan.

Pottery is an ancient technology, and is one of the key technologies in the formation of civilization. The creation of pottery has been advanced as new tools became available to the potter, such as the electric potter's wheel and the electric kiln. Potters also take advantage of more modern innovations in the fields of chemistry and plastics.

Broken pottery in archaeological sites, called potsherds, help identify the resident culture and date the stratum, by the formation style and decoration. The relative chronologies based on pottery are essential for dating the remains of non-literate cultures and help in the dating of some historic cultures as well.

Palaeolithic pottery

Pottery found in the Japanese islands has been dated, by uncalibrated radiocarbon dating, to around the 11th millennium BC, in the Japanese Palaeolithic at the beginning of the Jomon period. This is the oldest known pottery.

In Europe, burnt clay was already known in the late Palaeolithic (Magdalenian) and was used for female figurines, like the "Venus" of Dolni Vestonice, as well as figures of animals.

Neolithic pottery

In Palestine, Syria, and south-eastern Turkey, the earliest finds of clay pots date from Neolithic times, around the 8th millennium BC (black burnished ware). Before that, clay had been used to make statuettes of humans and animals that were sometimes burned as well. In the preceding pre-pottery Neolithic, vessels made of stone, gypsum, and burnt lime (vaiselles blanches or white ware) had been used. Sometimes a mixture of clay and lime was used—not very successfully—in the earliest pottery.

See also

• History of pottery in Palestine
• Venus of Dolní Věstonice
• Nevala Cori figurines
• Ancient Greek Pottery Gallery

Reference

• Hamer, Frank and Janet, The Potter's Dictionary of Materials and Techniques, A & C Black Publishers, Limited, London, England, Third Edition 1991. ISBN 0-8122-3112-0.
• Rice, Prudence M. Pottery Analysis – A Sourcebook. London and Chicago: University of Chicago Press, 1987. ISBN 0226711188.

See also

• Anagama kiln
• Arts and Crafts Movement
• Bone china
• Celadon
• China (pottery)
• Contemporary ceramic studio
• Delftware
• Earthenware
• Edward Bingham (artist)
• Greek pottery
• History of pottery in Palestine
• Ilobasco
• Jasperware
• Kakiemon pottery
• Longquan celadon
• Mata Ortiz Pottery
• Native American pottery
• Mayan pottery
• Pewabic Pottery
• Pit fired pottery
• Porcelain
• Pottery of Ancient Greece
• Raku
• Saggar fired pottery
• Salt glaze pottery
• Slipware
• Stoneware
• Josiah Wedgwood – Wedgwood