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Pottery

2007 Schools Wikipedia Selection. Related subjects: Art; Business; Everyday
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   Unfired "green ware" pottery on a traditional drying rack at Conner
   Prairie living history museum.
   Enlarge
   Unfired "green ware" pottery on a traditional drying rack at Conner
   Prairie living history museum.

   Pottery is a type of ceramic material, which the American Society for
   Testing and Materials (ASTM) has defined as "(a)ll fired ceramic wares
   that contain clay when formed, except technical, structural, and
   refractory products." The term pottery is also used for a technique
   involving ceramic, where clay is mixed with other minerals and is
   formed into objects, including vessels generally designed for
   utilitarian purposes.

   A Pottery is a facility of any size, from a modest studio to an
   industrialized factory, where pottery is made. Where resources are
   available - raw materials, workers, transportation - groups of
   potteries may exist. Due to the large number of pottery factories, or
   colloquially 'Pot Banks', the City of Stoke-on-Trent in England became
   known as The Potteries; one of the first industrial cities of the
   modern era where as early as 1785 200 pottery manufacturers employed
   20,000 workers. The Potters is the nickname of the local football club,
   Stoke City F.C.. The same name is used for sports teams in the one-time
   "Pottery Capital of the World," East Liverpool, Ohio.

   Pottery production is a process by which a clay body, clay mixed with
   other minerals, is shaped and allowed to dry. The shaped clay body, or
   piece, ware or article, may be "bisque or biscuit fired" in a kiln to
   induce permanent changes that result in increased mechanical strength,
   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 body and
   firing in one cycle.

   With mass production techniques having replaced the traditional role
   studio potters have focused more on the aesthetic than the utilitarian

   Traditionally, different regions of the world have used produced
   different types of clay, sometimes mixed with other minerals, to
   produce regionally distinctive pottery. It is common for different
   clays and minerals to be mixed to produce clay bodies suited to
   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 bodies
   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

Techniques

   A man shapes pottery as it turns on a wheel. (Cappadocia, Turkey).
   Enlarge
   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 additional
   tools have been created over the long history of pottery manufacture,
   including the potter's wheel, various paddles, shaping tools (or ribs),
   slab rollers, and cutting tools.

Forming techniques

   Pottery can be shaped by a range techniques, including:
   Handwork pottery in Kathmandu, Nepal.
   Enlarge
   Handwork pottery in Kathmandu, Nepal.

   Handwork can be considered both the most primitive and the most
   individualized techniques, where pieces are constructed from
   hand-rolled coils, slabs, ropes, and balls of clay body, often joined
   with a slurry of clay body, known as slip. 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.
   Enlarge
   Classic potter's wheel in Erfurt, Germany.

   The potter's wheel. A ball of clay body is placed in the centre of a
   turntable, called the wheel head, which is turned either using foot
   power (a kick wheel or treadle wheel) or a variable speed electric
   motor. Oftentimes, a bat (a disk of plastic, wood, or plaster) is
   affixed to the wheel head, and the ball of clay body is attached to the
   bat rather than the wheel head so that the finished piece can be
   removed easily without distorting the piece. The wheel is made to
   revolve rapidly while the body is pressed, squeezed, and pulled gently
   into shape. The process of pressuring the body into a rotational
   symmetry, so that it does not move from side to side as the wheel head
   rotates is referred to as "centering" the body — a most important skill
   to master before the next steps, "throwing" (forming and raising the
   walls of the piece) and "trimming" or "turning" (removing excess clay
   to refine the shape or create a bottom or foot can be taken.

   Potter's wheels can be used for mass production, although often it is
   employed to make individual pieces. 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.

   Jiggering & jolleying These can be seen to be an extension of using a
   Potters wheel, and in comparison allow increased speed and improved
   repeatability. Jiggering is the technique where a shaped tool is slowly
   brought down onto plastic clay body that has been placed on top of a
   rotating plaster mould. The jigger tool shapes one face whilst the
   mould the other. Jiggering is the term specific to flatware such as
   plates whilst a similar technique, jolleying, refers to the production
   of holloware like cups. These techniques have been in use since at
   least the 18th century Partial automation of the jiggering and
   jolleying processes have long been used to increased production rates
   and de-skill the operation

   Roller-head Machine A machine for the shaping of pottery flatware on a
   rotating mould, as in a jigger, but with a rotary shaping tool instead
   of a fixed profile. The rotary tool is in the form of a shallow cone of
   the same diameter as the ware and shaped to produce the back of the
   article being made. The ware is completely shaped, by relatively
   unskilled labour, in one operation at a rate depending on the size of
   the ware of about 12 pieces per minute. The machine, developed from
   earlier attempts to improve on the use of a fixed tool, was patented by
   T. G. Green & Co. and H. J. Smith (Brit. Pat., 621 712, 14/4/49) with
   subsequent improvements by J. A. Johnson (Brit. Pat., 765 097, 2/1/57;
   895 988, 9/5/62). It is now widely used across the world

   Ram Pressing A process for the plastic shaping of tableware and
   sanitaryware by pressing a bat of the prepared body between two porous
   plates or mould units; after the pressing operation, air is blown
   through the porous mould parts to release the shaped ware. Although the
   process was patented in 1952 by Ram Incorporated, US Pats, 2 584 109 &
   10, the term is often used for similar type shaping processes

   Granulate pressing This involves the shaping of ware by compressing
   semi-dry and granulated body in a die, one side of which is a solid
   surface and the other a flexible membrane through which fluid pressure,
   of up to 100 MN.m-2, is transferred. The granulate is produced by spray
   drying to give a fine and free flowing material with a low moisture
   content of around 5-6%. Also known as dust pressing the technique is
   widely used in manufacture of wall and floor tiles, and increasingly of
   plates

   Turning is similar to that used for the shaping of metal and wood
   articles. A blank of clay body, with low water content in a condition
   known as leatherhard, is mounted on a lathe whilst a cutting tool
   follows a profile to removes excess material and hence leave a shaped
   article

   Slipcasting is a technique often used for mass-production, and ideally
   suits shapes that can not be made by other methods. Liquid clay body
   slip is poured into plaster moulds, the permeability of the mould
   drawing water from the slip to leave a layer the clay body of the
   internal shape of the mould. This method is almost universally used for
   sanitaryware, toilets and wash basins, as well being widely used for
   smaller pieces such as figurines which have many intricate details

Decorative and finishing techniques

   Contemporary pottery from Okinawa, Japan.
   Enlarge
   Contemporary pottery from Okinawa, Japan.

   For aesthetic reasons articles can be decorated by a large variety of
   techniques , including the application of colours and designs.
   Decoration can be:
     * In the body
     * On the surface of the body but beneath the glaze, underglaze
       decoration
     * In the glaze
     * On the surface of the glaze, on-glaze decoration

   Additives can be worked into moist clay body, 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) to give the final product 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 the body or pressed
   into the surface to produce texture.

   Agateware A long established technqiue is to partially mix different
   coloured clay bodies to give a distinctive 'mottled' decoration. In
   Great Britain, these techniques are known as agateware, after agate
   stone which shows similar colour banding, whilst in Japan a similar
   technqiue is called "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. To ensure the
   final articles are unfaulted, care is needed with selection to choice
   different bodies of compatible shrinkage and thermal expansion

   Burnishing, like the metalwork technique of the same name, involves
   rubbing the surface of the unfired article with a suitable tool,
   typically of wood, steel or stone, to give a smooth, polished surface
   which remains even after firing. Finer clays give a smoother, shinier
   surface than coarser clays, as will allowing the pot to dry more before
   burnishing although that risks breakage
   An ancient Armenian urn.
   Enlarge
   An ancient Armenian urn.

   Engobe, to give a finer surface, or a coloured surface, slip can be
   coated onto the leather-dry body. 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.

   Banding, 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

   Litho, is a commonly used abbreviation for lithography although
   alternatives names of transfer or decal or used. These are used to
   apply designs to artciles. The litho comprises three layers: the
   colour, or image, layer which comprises the decorative design; the
   covercoat, a clear protective layer, which may incorporate a
   low-melting glass; 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 applications, 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 is reveal the full colour
     * Acid Gold – a form of gold decoration developed in the early 1860s
       at Mintons Ltd., Stoke-on-Trent, UK. The glazed surface is etched
       with dilute 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 resinates 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

Glazing

   A glaze is a vitreous coating to a ceramic material whose primary
   purposes include decoration or protection. Glazes can be considered
   specialised forms of glass. Glazing is the process of coating the piece
   with a thin layer of material that during firing forms the glass
   coating. Compositions are varied but are usually a mixture of minerals
   which generally, but not always, mature at temperatures lowers than the
   body itself. Glaze can be important for functional earthenware vessels,
   which would otherwise be unsuitable for holding liquids due to
   permeability. 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 a very even covering but can be
   effective as a decorative technique. To prevent the glazed article
   sticking to the kiln during firing either a small part of the item is
   left unglazed or special refractory supports, spurs, are used as
   supports which are removed and discarded after the firing.

Evolution of Glazing technique

   The glazing technique was greatly developed by Muslim potters who first
   used and improved this old tradition. In their early days (8th
   century), potters of Samarra and Baghdad employed lead glazes with
   relief designs, a technique which came down from the Romans who spread
   it in countries of the Mediterranean basin, including Egypt. The nature
   of these wares consisted mostly of red earthenware on which the shiny
   green or yellowish brown glazes were applied.

   As they became more experienced, these potters introduced another
   glazing method, which they developed from attempting to imitate the
   popular white Chinese ceramics. In doing so, the Muslim potters
   introduced tin oxides to lead glaze, giving it a white finish. Lane
   explained how and why these potters achieved this: "It was impossible
   to reproduce the beautiful surface texture of these (Chinese ware) by
   using an ordinary clear lead glaze over a white slip, and the
   Mesopotamian potters therefore hit on a device used long before them by
   the Egyptians. Tin oxide was mixed with a modified form of lead glaze,
   which by the suspended particles was rendered perfectly opaque and
   white. When applied to a well purified yellow or pinkish clay, this
   glaze achieved a most deceptive similarity to the T'ang porcelain. But
   fortunately the Baghdad potters did not rest content as imitators. They
   were tempted to paint on the fine white surface, using cobalt blue,
   copper green, manganese purple, and sometimes antimony yellow". (Lane,
   A. (1947) `Early Islamic Pottery', Faber and Faber, London, p.13).

   In a third stage, the Muslim potter attempted to produce a closer
   version of the golden and silver vessels of paradise described numerous
   times in the Quran. In Surah 76, for example, Allah promises His
   devotees: "Enter ye the Garden, ye and your wives, in (beauty and)
   rejoicing. To them will be passed round, dishes and goblets of gold:
   there will be there all that the souls could desire, all that their
   eyes could delight in: and ye shall abide therein (for eye). Such will
   be the Garden of which ye are made heirs for your (good) deeds (in
   life). Ye shall have therein abundance of fruit, from which ye shall
   have satisfaction ".(43:71-3). In Surah 43, the Exalted reminds the
   believers: "And amongst them (the Devotees of Allah) will be passed
   round vessels of silver and goblets of crystal. Crystal-clear, made of
   silver: they will determine the measure thereof (according to their
   wishes). And they will be given to drink there of a Cup (of Wine) mixed
   with Zanjabil, A fountain there, called Salsabil" (76:15-8). Lustre
   provided the right ingredients for producing such vessels in a cheaper
   and acceptable way as Islam prohibits the use of gold and silver
   vessels. This is not the first time we see Muslim craftsmen try to
   imitate paradise themes. Similar approach was adopted in architecture,
   gardening, and even carpet making. The technique involved preparing
   pigments by mixing silver or copper oxides with an earthy vehicle such
   as ochre, and then vinegar or grape juice were added as medium. This
   pigment is then painted onto the glassy surface of the vessel, which
   has been glazed and fired once. The vessel is then fired for a second
   time in a reducing kiln, then the ochre is rubbed away.

Firing

   Firing is an essential stage in the manufacture of pottery, during
   which chemical and physical reactions occur which causes the material
   to be permanently altered. The process is carried out in a kiln.

   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. Sulphur 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 colour
   on the clay itself, known as "blushing".

   The Western adaptation of Raku firing, a traditional Japanese
   technique, has enjoyed a great 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 colour.

Production stages

   All pottery items go through a series of stages during construction.
    1. Initially the clay body is mixed to homogenise the moisture
       content, and any entrapped air needs to be removed. This can be
       achieved either manually by a technique called wedging, or by using
       machines: dedicated mixers are used to homogenise the moisture
       whilst de-airing pugs, sometimes called pug mills, are used to
       remove the air. The body is then shaped either by hand or using a
       variety of tools and machines. Examples include the potter's wheel,
       extruders, rollers, jigger / jolley, RAM press and rollerhead
    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 occasionally sanded with fine grade sandpaper to ensure a
       smooth finish in the completed item.
    4. Sometimes the greenware is given a coating of a clay slip. This is
       most often done to give a coloured base for decoration, other than
       the colour of the main body.
    5. The greenware is often given a preliminary firing in a kiln. Once
       it has been fired the articles are known as biscuit ware or bisque.
       Depending on historic tradition the biscuit firing can be higher or
       lower in temperature than the final firing
    6. Biscuit ware is normally a plain red, white, or brown colour
       depending on which type of initial raw materials 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.
   Enlarge
   Incipient Jomon pottery (10,000-8,000 BC), Tokyo National Museum,
   Japan.

   The development of pottery was a milestone in human history. These
   durable and watertight containers enabled people to boil and steam food
   which allowed them to exploit new sources of food such as shellfish,
   acorns, and leafy vegetables. Soft boiled foods could be eaten by
   toothless children and the elderly, which permitted caregivers to spend
   more time producing food. In Japan, for instance, the introduction of
   pottery was followed by a population explosion. In the archaeology of
   the Eastern Woodlands of North America the introduction of pottery is
   referred to as the container revolution.

   Since pottery is a durable, man-made artifact which was utilized by
   various cultures around the world, it has proven to be a boon for
   archaeologists. Broken pottery in archaeological sites, called sherds
   or shards, 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. Trace element
   analysis, mostly by neutron activation, allows the sources of clay to
   be accurately identified.

   While ceramics had been developed in Europe also, pottery was first
   developed by the Jomon in Japan around 10,500 BCE. It appears that
   pottery was then independently developed in North Africa during the
   10th millennium b.p. and in South America during the 7th millennium
   b.p.

   The invention of the potter's wheel in Mesopotamia sometime between
   6,000 and 2,400 BCE revolutionized pottery production. Specialized
   potters were then able to meet the burgeoning needs of the world's
   first cities.

   While artistic value of Classical Greek and Roman pottery largely
   consisted of the surface decoration, the pottery itself was an
   important art form in China, where efficient kilns allowed high
   temperature ware to be fired with wood, long before the use of coal.
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