Pottery is the ceramic ware made by potters.^[1] It can also refer to the material of which the potteryware is made,^[2] of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery, with the plural being potteries.^{[3] [4]}

The definition of pottery used by ASTM is "all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products."^[5] Some archaeologists use a different understanding by excluding ceramic objects such as figurines which are made by similar processes, materials and the same people but are not vessels.^[6]

Contents 1 Background 2 Methods of shaping 3 Decorating and glazing 3.1 Glazing 4 Firing 5 History 6 Archaeology 7 Environmental issues in production 8 Other usages 9 See also 10 References 11 External links

[edit] Background

Pottery is made by forming a clay body into objects of a required shape and heating them to high temperatures in a kiln to induce reactions that lead to permanent changes including increasing their strength and hardening and setting their shape. There are wide regional variations in the properties of raw materials used for the production of pottery, and this can lead to wares that are unique in character to a locality. It is common for clays and other materials to be mixed to produce clay bodies suited to specific purposes. Prior to some shaping processes, air trapped within the clay body needs to be removed. This is called de-airing and can be accomplished by a machine called a vacuum pug or manually by wedging. Wedging can also help to ensure an even moisture content throughout the body. Once a clay body has been de-aired or wedged, it is shaped by a variety of techniques. After shaping it is dried before firing. There are a number of stages in the drying process. Leather-hard refers to the stage when the clay object is approximately 15% moisture content. Clay bodies at this stage are very firm and only slightly pliable. Trimming and handle attachment often occurs at the leather-hard state. Clay bodies are said to be "bone-dry" when they reach a moisture content at or near 0%. Unfired objects are often termed greenware. Clay bodies at this stage are very fragile and hence can be easily broken.

[edit] Methods of shaping

Pottery can be shaped by a range of methods that include:

Hand building. This is the earliest forming method. Wares can be constructed by hand from coils of clay, flat slabs of clay, solid balls of clay or some combination of these. Parts of hand-built vessels are often joined together with the aid of slip, an aqueous suspension of clay body and cum. Hand-building is slower than wheel-throwing, but it offers the potter a high degree of control over the size and shape of wares. The speed and repetitiveness of other techniques is more suitable for making precisely matched sets of wares such as tablewares although some studio potters find hand-building more conducive to create one-of-a-kind works of art.

The potter's wheel. In a process called "throwing" (coming from the Old English word thrawan which means to twist or turn,^[7] a ball of clay is placed in the center of a turntable, called the wheel-head, which the potter rotates with a stick, with foot power 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--a most important skill to master before the next steps: opening (making a centered hollow into the solid ball of clay), flooring (making the flat or rounded bottom inside the pot), throwing or pulling (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).

Considerable skill and experience are required to throw pots of an acceptable standard and, while the ware may have high artistic merit, the reproducibility of the method is poor.^[8] Because of its inherent limitations, throwing can only be used to create wares with radial symmetry on a vertical axis. These can then be altered by impressing, bulging, carving, fluting, and incising. In addition to the potter's hands these techniques can use tools, including paddles, anvils & ribs, and those specifically for cutting or piercing such as knives, fluting tools and wires. Thrown pieces can be further modified by the attachment of handles, lids, feet and spouts.

Granulate pressing: As the name suggests, this is the operation of shaping pottery by pressing clay in a semi-dry and granulated condition in a mould. The clay is pressed into the mould by a porous die through which water is pumped at high pressure. The granulated clay is prepared by spray-drying to produce a fine and free-flowing material having a moisture content of between about 5 and 6 per cent. Granulate pressing, also known as dust pressing, is widely used in the manufacture of ceramic tiles and, increasingly, of plates.

Injection Moulding: is a shape-forming process adapted for the tableware industry from the method long established for the forming of thermoplastic and some metal components.^[9] It has been called Porcelain Injection Moulding, or PIM.^[10] Suited to the mass production of complex-shaped articles, one significant advantage of the technique is that it allows the production of a cup, including the handle, in a single process, and thereby eliminates the handle-fixing operation and produces a stronger bond between cup and handle.^[11] The feed to the mould die is a mix of approximately 50 to 60 percent unfired body in powder form, together with 40 to 50 percent organic additives composed of binders, lubricants and plasticisers.^[10] The technique is not as widely used as other shaping methods.^[12]

Jiggering and jolleying: These operations are carried out on the potter's wheel and allow the time taken to bring wares to a standardized form to be reduced. Jiggering is the operation of bringing a shaped tool into contact with the plastic clay of a piece under construction, the piece itself being set on a rotating plaster mould on the wheel. The jigger tool shapes one face while the mould shapes the other. Jiggering is used only in the production of flat wares, such as plates, but a similar operation, jolleying, is used in the production of hollow-wares such as cups. Jiggering and jolleying have been used in the production of pottery since at least the 18th century. In large-scale factory production, jiggering and jolleying are usually automated, which allows the operations to be carried out by semi-skilled labor.

Roller-head machine: This machine is for shaping wares on a rotating mould, as in jiggering and jolleying, but with a rotary shaping tool replacing the fixed profile. The rotary shaping tool is a shallow cone having the same diameter as the ware being formed and shaped to the desired form of the back of the article being made. Wares may in this way be shaped, using relatively unskilled labor, in one operation at a rate of about twelve pieces per minute, though this varies with the size of the articles being produced. Developed in the U.K. just after World War II by the company Service Engineers, roller-heads were quickly adopted by manufacturers around the world; they remain the dominant method for producing flatware.^[13]

Pressure casting – specially developed polymeric materials allow a mould to be subject to application external pressures of up to 4.0 MPa–so much higher than slip casting in plaster moulds where the capillary forces correspond to a pressure of around 0.1 - 0.2 MPa. The high pressure leads to much faster casting rates and, hence, faster production cycles. Furthermore, the application of high pressure air through the polymeric moulds upon demoulding the cast means a new casting cycle can be started immediately in the same mould, unlike plaster moulds which require lengthy drying times. The polymeric materials have much greater durability than plaster and, therefore, it is possible to achieve shaped products with better dimensional tolerances and much longer mould life. Pressure casting was developed in the 1970s for the production of sanitaryware although, more recently, it has been applied to tableware.^{[14][15][16][17]}

RAM pressing: A factory process for shaping table wares and decorative ware by pressing a bat of prepared clay body into a required shape between two porous molding plates. After pressing, compressed air is blown through the porous mould plates to release the shaped wares.

Slipcasting: 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.

[edit] Decorating and glazing

Pottery may be decorated in a number of ways including:

• In the clay body, for example, by incising patterns on its surface.
• Underglaze decoration (in the manner of many blue and white wares).
• In-glaze decoration
• On-glaze decoration
• Enamel

Additives can be worked into the clay body prior to forming, to produce desired effects in the fired wares. Coarse additives such as sand and grog (fired clay which has been finely ground) are sometimes used to give the final product a required texture. Contrasting coloured clays and grogs are sometimes used to produce patterns in the finished wares. Colorants, usually metal oxides and carbonates, are added singly or in combination to achieve a desired colour. Combustible particles can be mixed with the body or pressed into the surface to produce texture and don't for get shaping.

Agateware: Named after its resemblance to the quartz mineral agate which has bands or layers of colour that are blended together, agatewares are made by blending clays of differing colours together but not mixing them to the extent that they lose their individual identities. The wares have a distinctive veined or mottled appearance. The term "agateware" is used to describe such wares in the United Kingdom; in Japan the term "neriage" is used and in China, where such things have been made since at least the Tang Dynasty, they are called "marbled" wares. Great care is required in the selection of clays to be used for making agatewares as the clays used must have matching thermal movement characteristics.

Banding: This is the application by hand or by machine of a band of colour to the edge of a plate or cup. Also known as "lining", this operation is often carried out on a potter's wheel.

Burnishing: The surface of pottery wares may be burnished prior to firing by rubbing with a suitable instrument of wood, steel or stone to produce a polished finish that survives firing. It is possible to produce very highly polished wares when fine clays are used or when the polishing is carried out on wares that have been partially dried and contain little water, though wares in this condition are extremely fragile and the risk of breakage is high.

Engobe: This is a clay slip, often white or cream in colour, that is used to coat the surface of pottery, usually before firing. Its purpose is often decorative though it can also be used to mask undesirable features in the clay to which it is applied. Engobe slip may be applied by painting or by dipping to provide a uniform, smooth, coating. Engobe has been used by potters from pre-historic times until the present day and is sometimes combined with sgraffito decoration, where a layer of engobe is scratched through to reveal the colour of the underlying clay. With care it is possible to apply a second coat of engobe of a different colour to the first and to incise decoration through the second coat to expose the colour of the underlying coat. Engobes used in this way often contain substantial amounts of silica, sometimes approaching the composition of a glaze.

Litho: This is a commonly-used abbreviation for lithography, although the alternative names of transfer print or "decal" are also common. These are used to apply designs to articles. The litho comprises three layers: the colour, or image, layer which comprises the decorative design; the cover coat, a clear protective layer, which may incorporate a low-melting glass; and the backing paper on which the design is printed by screen printing or lithography. There are various methods of transferring the design while removing the backing-paper, some of which are suited to machine application.

Gold: Decoration with gold is used on some high quality ware. Different methods exist for its application, including:

• Best gold - a suspension of gold powder in essential oils mixed with a flux and a mercury salt extended. This can be applied by a painting technique. From the kiln, the decoration is dull and requires burnishing to reveal the full colour
• Acid Gold – a form of gold decoration developed in the early 1860s at the English factory of Mintons Ltd, Stoke-on-Trent. The glazed surface is etched with diluted hydrofluoric acid prior to application of the gold. The process demands great skill and is used for the decoration only of ware of the highest class.
• Bright Gold – consists of a solution of gold sulphoresinate together with other metal resonates and a flux. The name derives from the appearance of the decoration immediately after removal from the kiln as it requires no burnishing
• Mussel Gold – an old method of gold decoration. It was made by rubbing together gold leaf, sugar and salt, followed by washing to remove solubles

[edit] Glazing

Glaze is a glassy coating on pottery, the primary purposes of which are decoration and protection. One important use of glaze is to render porous pottery vessels impermeable to water and other liquids. Glaze may be applied by dusting the unfired composition over the ware or by spraying, dipping, trailing or brushing on a thin slurry composed of the unfired glaze and water. The colour of a glaze before it has been fired may be significantly different than afterward. To prevent glazed wares sticking to kiln furniture during firing, either a small part of the object being fired (for example, the foot) is left unglazed or, alternatively, special refractory "spurs" are used as supports. These are removed and discarded after the firing.

Some specialised glazing techniques include:

• Salt-glazing, where common salt is introduced to the kiln during the firing process. The high temperatures cause the salt to volatize, depositing it on the surface of the ware to react with the body to form a sodium aluminosilicate glaze. In the 17th and 18th centuries, salt-glazing was used in the manufacture of domestic pottery. Now, except for use by some studio potters, the process is obsolete. The last large-scale application before its demise in the face of environmental clean air restrictions was in the production of salt-glazed sewer-pipes.^[18][19]
• Ash glazing - ash from the combustion of plant matter has been used as the flux component of glazes. The source of the ash was generally the combustion waste from the fuelling of kilns although the potential of ash derived from arable crop wastes has been investigated.^[20] Ash glazes are of historical interest in the Far East although there are reports of small-scale use in other locations such as the Catawba Valley Pottery in the United States. They are now limited to small numbers of studio potters who value the unpredictability arising from the variable nature of the raw material.^[21]

[edit] Firing

Firing produces irreversible changes in the body. It is only after firing that the article or material is pottery. In lower-fired pottery, the changes include sintering, the fusing together of coarser particles in the body at their points of contact with each other. In the case of porcelain, where different materials and higher firing-temperatures are used, the physical, chemical and mineralogical properties of the constituents in the body are greatly altered. In all cases, the object of firing is to permanently harden the wares and the firing regime must be appropriate to the materials used to make them. As a rough guide, earthenwares are normally fired at temperatures in the range of about 1000 to 1200 °C; stonewares at between about 1100 to 1300 °C; and porcelains at between about 1200 to 1400 °C. However, the way that ceramics mature in the kiln is influenced not only by the peak temperature achieved but also by the duration of the period of firing. Thus, the maximum temperature within a kiln is often held constant for a period of time to soak the wares to produce the maturity required in the body of the wares.

The atmosphere within a kiln during firing can affect the appearance of the finished wares. An oxidising atmosphere, produced by allowing air to enter the kiln, can cause the oxidation of clays and glazes. A reducing atmosphere, produced by limiting the flow of air into the kiln, can strip oxygen from the surface of clays and glazes. This can affect the appearance of the wares being fired and, for example, some glazes containing iron fire brown in an oxidising atmosphere, but green in a reducing atmosphere. The atmosphere within a kiln can be adjusted to produce complex effects in glaze.

Kilns may be heated by burning wood, coal and gas or by electricity. When used as fuels, coal and wood can introduce smoke, soot and ash into the kiln which can affect the appearance of unprotected wares. For this reason, wares fired in wood- or coal-fired kilns are often placed in the kiln in "saggars", lidded ceramic boxes, to protect them. Modern kilns powered by gas or electricity are cleaner and more easily controlled than older wood- or coal-fired kilns and often allow shorter firing times to be used. In a Western adaptation of traditional Japanese Raku ware firing, wares are removed from the kiln while hot and smothered in ashes, paper or woodchips which produces a distinctive carbonised appearance. This technique is also used in Malaysia in creating traditional labu sayung.^[22][23]

[edit] History

It is believed^{[by whom?]} that the earliest pottery wares were hand-built and fired in bonfires. Firing times were short but the peak-temperatures achieved in the fire could be high, perhaps in the region of 900 °C, and were reached very quickly. Clays tempered with sand, grit, crushed shell or crushed pottery were often used to make bonfire-fired ceramics because they provided an open-body texture that allows water and other volatile components of the clay to escape freely. The coarser particles in the clay also acted to restrain shrinkage within the bodies of the wares during cooling which was carried out slowly to reduce the risk of thermal stress and cracking. In the main, early bonfire-fired wares were made with rounded bottoms to avoid sharp angles that might be susceptible to cracking. The earliest intentionally-constructed kilns were pit-kilns or trench-kilns--holes dug in the ground and covered with fuel. Holes in the ground provided insulation and resulted in better control over firing.^{[citation needed]}

The earliest-known ceramic objects are Gravettian figurines such as those discovered at Dolni Vestonice in the modern-day Czech Republic. The Venus of Dolní Věstonice (Věstonická Venuše in Czech) is a Venus figurine, a statuette of a nude female figure dated to 29,000–25,000 BCE (Gravettian industry).^[24] The earliest pottery vessels found include those excavated from the Yuchanyan Cave in southern China, dated from 16,000 BCE,^[25] and those found in the Amur River basin in the Russian Far East, dated from 14,000 BCE.^[26][27]

Other earlier pottery vessels include those made by the Incipient Jōmon people of Japan from around 10,500 BCE have also been found.^[28][29] The term "Jōmon" means "cord-marked" in Japanese. This refers to the markings made on the vessels and figures using sticks with cords during their production. It appears that pottery was independently developed in North Africa during the 10,000 BCE^[30] and in South America during the 10,000 BCE^[31] In several cultures, the earliest vessels were made either by hand-shaping or by rolling the clay into a thin round cord which was then coiled round on itself to form the vessel. The earliest history of pottery production in the Near East can be divided into four periods, namely: the Hassuna period (5,000-4,500 BCE), the Halaf period (4,500-4,000 BCE), the Ubaid period (4,000-3,000 BCE), and the Uruk period (3,500-2,000 BCE).

The invention of the potter's wheel in Mesopotamia sometime between 6,000 and 4,000 BCE (Ubaid period) revolutionized pottery production. Specialized potters were then able to meet the expanding needs of the world's first cities. Pottery was in use in ancient India, including areas now forming Pakistan and northwest India, during the Mehrgarh Period II (5,500-4,800 BCE) and Merhgarh Period III (4,800-3,500 BCE), known as the ceramic Neolithic and chalcolithic. Pottery, including items known as the ed-Dur vessels, originated in regions of the Indus Valley and have been found in a number of sites in the Indus Valley Civilization.^[32][33]

In the Mediterranean, during the Greek Dark Ages (1,100–800 BCE), amphoras and other pottery were decorated with geometric designs such as squares, circles and lines. The period between 1,500-300 BCE in ancient Korea is known as the Mumun Pottery Period.^[34] In the Chalcolithic period in Mesopotamia, Halafian pottery achieved a level of technical competence and sophistication, not seen until the later developments of Greek pottery with Corinthian and Attic ware.^{[citation needed]} The distinctive Red Samian ware of the Early Roman Empire was copied by regional potters throughout the Empire.

[edit] Archaeology

For archaeologists, anthropologists and historians the study of pottery can help to provide an insight into past cultures. Pottery is durable, and fragments, at least, often survive long after artifacts made from less-durable materials have decayed past recognition. Combined with other evidence, the study of pottery artifacts is helpful in the development of theories on the organisation, economic condition and the cultural development of the societies that produced or acquired pottery. The study of pottery may also allow inferences to be drawn about a culture's daily life, religion, social relationships, attitudes towards neighbors, attitudes to their own world and even the way the culture understood the universe.

Chronologies based on pottery are often essential for dating non-literate cultures and are often of help in the dating of historic cultures as well. Trace-element analysis, mostly by neutron activation, allows the sources of clay to be accurately identified and the thermoluminescence test can be used to provide an estimate of the date of last firing. Examining fired pottery shards from prehistory, scientists learned that during high-temperature firing, iron materials in clay record the exact state of Earth's magnetic field at that exact moment.

[edit] Environmental issues in production

Although many of the environmental effects of pottery production have existed for millennia, some of these have been amplified with modern technology and scales of production. The principal factors for consideration fall into two categories: (a) effects on workers, and (b) effects on the general environment. Within the effects on workers, chief impacts are indoor air quality, sound levels and possible over-illumination. Regarding the general environment, factors of interest are fuel consumption, off-site water pollution, air pollution and disposal of hazardous materials.

Historically, "plumbism" (lead poisoning) was a significant health concern to those glazing pottery. This was recognised at least as early as the nineteenth century, and the first legislation in the United Kingdom to limit pottery workers’ exposure was introduced in 1899.^[35] While the risk to those working in ceramics is now much reduced, it can still not be ignored. With respect to indoor air quality, workers can be exposed to fine particulate matter, carbon monoxide and certain heavy metals. The greatest health risk is the potential to develop silicosis from the long-term exposure to crystalline silica. Proper ventilation can reduce the risks, and the first legislation in the United Kingdom to govern ventilation was introduced in 1899.^[35] Another, more recent, study at Laney College, Oakland, California suggests that all these factors can be controlled in a well-designed workshop environment.^[36]

The use of energy and pollutants in the production of ceramics is a growing concern. Electric firing is arguably more environmentally friendly than combustion firing although the source of the electricity varies in environmental impact.^{[citation needed]}

[edit] Other usages

Due to the large number of pottery factories or, colloquially, "Pot Banks", the English city of Stoke-on-Trent, one of the first industrial cities of the modern era where, as early as 1785, two hundred pottery manufacturers employed 20,000 workers, is often called "The Potteries".^[37] For the same reason, the largest football club in the city, is known as "The Potters".^[38]

[edit] See also

[edit] References

• ASTM Standard C 242-01 Standard Terminology of Ceramic Whitewares and Related Products
• Ashmore, Wendy & Sharer, Robert J., (2000). Discovering Our Past: A Brief Introduction to Archaeology Third Edition. Mountain View, California: Mayfield Publishing Company. ISBN 978-0072978827
• Barnett, William & Hoopes, John (Eds.) (1995). The Emergence of Pottery. Washington: Smithsonian Institution Press. ISBN 1-56098-517-8
• Childe, V. G., (1951). Man Makes Himself. London: Watts & Co.
• Rice, Prudence M. (1987). Pottery Analysis – A Sourcebook. Chicago: University of Chicago Press. ISBN 0-226-71118-8.
• Historynet.com
• Tschegg, C., Hein, I., Ntaflos, Th., 2008. State of the art multi-analytical geoscientific approach to identify Cypriot Bichrome Wheelmade Ware reproduction in the Eastern Nile delta (Egypt). Journal of Archaeological Science 35, 1134-1147.

[edit] External links

• How pottery is made
• Neolithic Pottery Manufacture
• Pottery manufacture in recent past
• Pottery-related objects and photographs to explore online
• Ceramic Collection at the Royal Military College of Canada Museum
• Customs and Working Practices of a Victorian Pottery
• Ancient pottery in Canada

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