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UNDER CONSTRUCTION
 Pottery
Image: Kiln in Hanley, The Potteries, The Midlands, England. - source unknown.
Pottery is the ceramic ware made by potters. Major types of pottery include earthenware, stoneware, and porcelain.
The places where such wares are made are called potteries. Pottery is
one of the oldest human technologies and art-forms, and remains a major
industry today. Ceramic art covers the art of pottery, whether in items made for use or purely for decoration.
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 clays used by potters
and this often helps to produce wares that are unique in character to a
locality. It is common for clays and other minerals 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 75-85% dry. 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.
Methods of shaping
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 and turntable, shaping tools (paddles, anvils, ribs), rolling tools (roulettes, slab rollers, rolling pins), cutting/piercing tools (knives, fluting tools, wires) and finishing tools (burnishing stones, rasps, chamois).
Pottery can be shaped by a range of methods that include:
Handwork or hand building.
This is the earliest and the most individualized and direct 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, a runny mixture of clay 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 wheel-throwing is
more suitable for making precisely matched sets of wares such as table wares. Some studio
potters find hand building more conducive to fully using the imagination to create one-of-a-kind works of art, while others find this with the wheel.
A potter shapes a piece of pottery on an electric-powered potter's wheel
The potter's wheel. In the process that is called "throwing" (coming from the Old English word thrawan,
which means to twist or turn) , 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 treadle wheel) or with a variable speed electric motor. (Often, a disk of plastic, wood or plaster — called a bat
— is first set on the wheel-head, and the ball of clay is thrown on the
bat rather than the wheel-head so that the finished piece can be
removed intact with its bat, without distortion.)
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 (and often most difficult) 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).
From around 7th century BC until the introduction of slip casting in the 18th century AD, the potter's wheel was the most effective method of mass producing
pottery, although it is also often employed to make individual pieces.
Wheel-work makes great demands on the skill of the potter, but an
accomplished operator can make many near-identical plates, vases, or
bowls in the course of a day's work. Because of its inherent
limitations, wheel-work can only be used to create wares with radial symmetry on a vertical axis. These can then be altered by impressing, bulging, carving, fluting, faceting, incising,
and by other methods making the wares 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.
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 whilst 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.
Shaping on a potter's kick wheel; Gülşehir, Turkey
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. The roller-head machine is now used in factories
worldwide.
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.
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 five and six per
cent. Granulate pressing, also known as dust pressing, is widely used in the manufacture of ceramic tiles and, increasingly, of plates.
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.
Decorating and glazing
Pottery may be decorated in a number of ways, including:
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 colored 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 color. Combustible particles can be
mixed with the body or pressed into the surface to produce texture.
Agateware: So-named after its resemblance to the quartz mineral agate
which has bands or layers of color that are blended together.
Agatewares are made by blending clays of differing colors 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 color 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 color 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
color of the underlying clay. With care it is possible to apply a
second coat of engobe of a different color to the first and to incise
decoration through the second coat to expose the color 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 color, 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 color
- 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
Glazing
Glaze
is a glassy coating applied to pottery, the primary purposes of which
include decoration and protection. Glazes are highly variable in
composition but usually comprise a mixture of ingredients that
generally, but not always, mature at kiln temperatures lower than that
of the pottery that it coats.
One
important use of glaze is in rendering pottery vessels impermeable to
water and other liquids. Glaze may be applied by dusting it over the
clay, spraying, dipping, trailing or brushing on a thin slurry
composed of glaze minerals and water. Brushing tends not to give an
even covering but can be effective as a decorative technique.
The
color of a glaze before it has been fired may be significantly
different than afterwards. 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.
Special methods of glazing are sometimes carried out in the kiln. One example is salt-glazing, where common salt is introduced to the kiln to produce a glaze of mottled, orange peel
texture. Materials other than salt are also used to glaze wares in the
kiln, including sulfur. In wood-fired kilns fly-ash from the fuel can
produce ash-glazing on the surface of wares, and the use of an ash and clay mix can result in alkaline glazes, as used in Catawba Valley Pottery in the eastern United States.
Firing
Firing
produces irreversible changes in the body. It is only after firing that
the article can be called 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 degrees Celsius; stonewares at between
about 1100 to 1300 degrees Celsius; and porcelains at between about
1200 to 1400 degrees Celsius.
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.
History
Earliest known ceramics are the Gravettian figurines that date to 29,000 to 25,000 BC
It is
believed 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 degrees Celsius,
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.
Pottery vessels made by the Incipient Jōmon people of Japan from around 10,500 BCE have also been found. The term "Jōmon"
means "cord-marked" in Japanese. This refers to the markings made on
clay vessels and figures using sticks with cords wrapped around them.
Pottery which dates back to 10,000 BCE have also been excavated in China.
It appears that pottery was independently developed in North Africa during the tenth millennium b.p. and in South America during the seventh millennium b.p.
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 during the Mehrgarh Period II (5500 - 4800 BCE) and Merhgarh Period III (4800 - 3500 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 has been found in a number of sites in
the Indus valley
civilization.
In the Mediterranean, during the Greek Dark Ages (1100–800 BCE), artists used geometric designs such as squares, circles and lines to decorate amphoras and other pottery. The period between 1500-300 BCE in ancient Korea is known as the Mumun Pottery Period.
The quality of pottery has varied historically, in part dependent upon the repute in which the potter's craft was held by the community. For example, 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. The distinctive Red Samian ware of the Early Roman Empire was copied by regional potters throughout the Empire. The Dark Age period saw a collapse in the quality of European pottery which did not recover in status and quality until the European Renaissance.
In 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 neighbours, 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.
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.
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.
Whilst the risk of 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. Another, more recent study at Laney College, Oakland, California suggests that all these factors can be controlled in a well designed workshop environment.
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]
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". For the same reason the largest football club in the city are known as "The Potters".
More info...
List of pottery terms
Anagama kiln
Arts and Crafts movement
Asbestos-Ceramic
Bone china
Celadon
Ceramic
Ceramics (art)
Chinese porcelain
Delftware
Dipped ware
Earthenware
Faience
Fiestaware
Glaze defects
History of pottery in Palestine
Iranian pottery
Jasperware
Kakiemon pottery
Longquan celadon
Maiolica of Renaissance Italy
Native American pottery
Pit fired pottery
Poole Pottery
Porcelain
Pottery of Ancient Greece
Raku ware
Rockingham Pottery
Royal Doulton — Henry Doulton, John Doulton
Sancai
Saggar fired pottery
Salt glaze pottery
Sea pottery
Slipware
Stoneware
Staffordshire Potteries
Studio pottery
Talavera, Mexican maiolica
Wedgwood — Enoch Wedgwood, Josiah Wedgwood
Victorian majolica
Vietnamese pottery
Yixing pottery
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