pottery

Main types

Earthenware

The earliest forms of pottery were made from clays that were fired at low temperatures, initially in pit-fires or in open bonfires. They were hand-formed and undecorated. Earthenware can be fired as low as 600 °C (1,112 °F), and is normally fired below 1,200 °C (2,190 °F).

Because unglazed earthenware is porous, it has limited utility for storing liquids or as tableware. However, earthenware has had a continuous history from the Neolithic period to today. It can be made from a wide variety of clays, some of which fire to a buff, brown, or black colour, with iron in the constituent minerals resulting in a reddish-brown. Reddish coloured varieties are called terracotta, especially when unglazed or used for sculpture. The development of ceramic glaze made impermeable pottery possible, improving the popularity and practicality of pottery vessels. Decoration has evolved.

Stoneware

Stoneware is pottery that has been fired in a kiln at a relatively high temperature, from about 1,100 °C (2,010 °F) to 1,200 °C (2,190 °F), and is stronger and non-porous to liquids. The Chinese, who developed stoneware very early on, classify this together with porcelain as high-fired wares. In contrast, stoneware could only be produced in Europe from the late Middle Ages, as European kilns were less efficient, and the right type of clay was less common. It remained a speciality of Germany until the Renaissance.

Stoneware is very tough and practical, and much of it has always been utilitarian, for the kitchen or storage rather than the table. But "fine" stoneware has been important in China, Japan, and the West, and continues to be made. Many utilitarian types have also come to be appreciated as art.

Porcelain

Porcelain is made by heating materials, generally including kaolin, in a kiln to temperatures between 1,200 and 1,400 °C (2,200 and 2,600 °F). This is higher than that used for the other types, and achieving these temperatures was a long struggle, as was determining which materials were needed. The toughness, strength, and translucence of porcelain, relative to other types of pottery, arise mainly from vitrification and the formation of the mineral mullite within the body at these high temperatures.

Although porcelain was first made in China, the Chinese traditionally do not recognise it as a distinct category, grouping it with stoneware as "high-fired" ware, opposed to "low-fired" earthenware. This confuses the issue of when it was first made. A degree of translucency and whiteness was achieved by the Tang dynasty (AD 618–906), and considerable quantities were being exported. The modern level of whiteness was not reached until much later, in the 14th century. Porcelain was also made in Korea and in Japan from the end of the 16th century, after suitable kaolin was located in those countries. It was not made effectively outside East Asia until the 18th century.

Archaeology

The study of pottery can help provide insight into past cultures. Fabric analysis (see section below), used to analyse the fabric of pottery, is an important part of archaeology for understanding the archaeological culture of the excavated site by studying the fabric of artifacts, such as their usage, source material composition, decorative pattern, color of patterns, etc. This helps to understand characteristics, sophistication, habits, technology, tools, trade, etc., of the people who made and used the pottery. Carbon dating reveals the age. Sites with similar pottery characteristics share the same culture; those with distinct cultural characteristics but some overlap indicate cultural exchange, such as trade, living in the vicinity, or continuity of habitation. Examples are black and red ware, redware, Sothi-Siswal culture and Painted Grey Ware culture. The six fabrics of Kalibangan are a good example of how fabric analysis can identify a differentiated culture that was previously thought to be typical of Indus Valley civilisation (IVC) culture.

Due to its inherent durability, pottery serves as a primary source of archaeological data, often surviving as diagnostic fragments long after artifacts made of organic materials have decayed. When synthesised with broader archaeological evidence, the analysis of ceramic assemblages is instrumental in constructing theories regarding a society's socioeconomic organisation, economic prosperity, and cultural evolution. Furthermore, ceramic studies allow for significant inferences concerning a culture's daily life, religious practices, and social hierarchies. The decorative and functional choices reflected in pottery can also reveal a society's complex attitudes toward neighbouring groups, its internal worldview, and its conceptualisation of the universe.

It is valuable to examine pottery as an archaeological record of potential interactions between peoples. When pottery is placed within the context of linguistic and migratory patterns, it becomes an even more prevalent category of social artifact. As proposed by Olivier P. Gosselain, it is possible to understand ranges of cross-cultural interaction by looking closely at the chaîne opératoire of ceramic production.

The methods used to produce pottery in early Sub-Saharan Africa fall into three categories: techniques visible (decoration, firing, and post-firing techniques), techniques related to the materials (selection or processing of clay, etc.), and techniques for molding or fashioning the clay. These three categories can be used to consider the implications of the recurrence of a particular sort of pottery in different areas. Generally, the techniques that are easily visible (the first category mentioned above) are readily imitated and may indicate a more distant connection between groups, such as trade in the same market or even relatively close settlements. Techniques that require more studied replication (i.e., the selection of clay and the fashioning of clay) may indicate a closer connection between people, as these methods are usually only transmissible between potters and those otherwise directly involved in production. Such a relationship requires the ability of the involved parties to communicate effectively, implying pre-existing norms of contact or a shared language between the two. Thus, the patterns of technical diffusion in pot-making visible in archaeological findings also reveal patterns of societal interaction.

Fabric analysis

The "clay body" is also called the "paste" or the "fabric", which consists of 2 things, the "clay matrix" – composed of grains of less than 0.02 mm grains which can be seen using the high-powered microscopes or a scanning electron microscope, and the "clay inclusions" – which are larger grains of clay and could be seen with the naked eye or a low-power binocular microscope. For geologists, fabric analysis means the spatial arrangement of minerals in a rock. For Archaeologists, the "fabric analysis" of pottery entails the study of clay matrix and inclusions in the clay body, as well as the firing temperature and conditions. Analysis is done to examine the following 3 in detail:

How pottery was made, e.g., material, design, such as shape and style, etc.

Its decorations, such as patterns, colors of patterns, slipped (glazing), or unslipped decoration

Evidence of the type of use.

The Six fabrics of Kalibangan is a good example of fabric analysis.

Clay bodies and raw materials

Body, or clay body, is the material used to form pottery. Thus, a potter might prepare or order from a supplier such an amount of earthenware, stoneware, or porcelain body. The compositions of clay bodies vary considerably, and include both prepared and 'as dug'; the former being by far the dominant type for studio and industry. The properties also vary considerably, and include plasticity and mechanical strength before firing; the firing temperature needed to mature them; properties after firing, such as permeability, mechanical strength, and colour.

There can be regional variations in the properties of raw materials used for pottery, and these can lead to wares that are unique in character to a locality.

The main ingredient of the body is clay. Some different types used for pottery include:

Kaolin, sometimes referred to as china clay, is a key ingredient in porcelain, which was first used in China around the 7th and 8th centuries.

Ball clay: An extremely plastic, fine grained sedimentary clay, which may contain some organic matter.

Fire clay: A clay having a slightly lower percentage of fluxes than kaolin, but usually quite plastic. It is a highly heat-resistant form of clay that can be combined with other clays to increase the firing temperature and may be used as an ingredient to make stoneware-type bodies.

Stoneware clay: Suitable for creating stoneware. Has many characteristics of both fire clay and ball clay, with a finer grain like ball clay, but is more heat-resistant like fire clay.

Common red clay and shale clay have vegetable and ferric oxide impurities, which make them useful for bricks, but are generally unsatisfactory for pottery except under special conditions of a particular deposit.

Production

The production of pottery includes the following stages:

Preparing the clay body.

Shaping

Drying

Firing

Glazing and decorating. (This can be undertaken before firing. Also, additional firing stages after decoration may be needed.)

Shaping

Before being shaped, clay must be prepared. This may include kneading to ensure an even moisture content throughout the body. Air trapped within the clay body needs to be removed, or de-aired, and can be accomplished either by a machine called a vacuum pug or manually by wedging. Wedging can also help produce an even moisture content. Once a clay body has been kneaded and de-aired or wedged, it is shaped by a variety of techniques, which include:

Hand-building: This is the earliest forming method. Wares can be constructed by hand from coils of clay, combining flat slabs of clay, or pinching solid balls of clay, or some combination of these. Parts of hand-built vessels are often joined with the aid of slip. Some studio potters find hand-building more conducive for 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) a ball of clay is placed in the centre 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 throwing, the wheel rotates while the solid ball of soft clay is pressed, squeezed, and gently pulled upwards and outwards into a hollow shape. Skill and experience are required to throw pots to an acceptable standard, and while the ware may have high artistic merit, the method's reproducibility is poor. Because of its inherent limitations, throwing can only be used to create wares with radial symmetry on a vertical axis.

Press moulding: a simple technique of shaping by manually pressing a lump of clay body into a porous mould.

Granulate pressing: a highly automated technique of shaping by pressing clay body in a semi-dry and granulated form in a mould. The body is pressed into the mould by a porous die through which water is pumped at high pressure. The fine, free-flowing granulated body is prepared by spray drying a high-solids content slip. Granulate pressing, also known as dust pressing, is widely used in the manufacture of ceramic tiles and, increasingly, of plates.

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 flatware, such as plates, but a similar operation, jolleying, is used in the production of hollowware, such as cups. Jiggering and jolleying have been used in pottery production since at least the 18th century. In large-scale factory production, jiggering and jolleying are usually automated, allowing these operations to be carried out by semi-skilled labour.

Drying

Before firing, the water in an article needs to be removed. Several different stages, or conditions of the article, can be identified:

Greenware refers to unfired objects at any stage of dryness, but is most often used to refer to objects ready to be fired.

Plastic is condition of a clay body that has sufficient moisture, around 20%, to be shaped without cracking.

Leather-hard refers to a clay body that has been dried partially. At this stage, the clay object has approximately 15% moisture content. Clay bodies at this stage are very firm and only slightly pliable. Trimming and handle attachment often occur at the leather-hard state.

Bone-dry refers to clay bodies when they reach a moisture content at or near 0%. At that moisture content, the item is ready to be fired. Additionally, the piece is extremely brittle at this stage and must be handled with care.

Chamber dryers are rooms with controlled environments used for the uniform removal of moisture from ceramic ware before firing.. By regulating temperature, humidity, and airflow, chamber dryers prevent the uneven shrinkage and cracking that often plague ambient-air drying. Modern systems often utilise "waste heat" from kilns to increase energy efficiency, maintaining precise "drying curves" tailored to specific clay bodies.. This stage is critical for ensuring structural integrity, as any residual deep-seated moisture can lead to catastrophic steam explosions during the high-heat firing process.

Firing

Firing produces permanent and irreversible chemical and physical 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 higher firing temperatures are used, the physical, chemical, and mineralogical properties of the constituents in the body are greatly altered. In all cases, the reason for firing is to harden the wares permanently, and the firing regime must be appropriate to the materials used.

As a rough guide, modern earthenwares are normally fired at temperatures in the range of about 1,000–1,200 °C (1,830–2,190 °F); stonewares between about 1,100–1,300 °C (2,010–2,370 °F); and porcelains between about 1,200–1,400 °C (2,190–2,550 °F). Historically, reaching high temperatures was a long-lasting challenge, and earthenware can be fired effectively as low as 600 °C (1,112 °F), achievable in primitive pit firing. The time spent at any particular temperature is also important; the combination of heat and time is known as heatwork.

Kilns can be monitored by pyrometers, thermocouples and pyrometric devices.

The atmosphere within a kiln during firing can affect the appearance of the body and glaze. Key to this is the differing colours of the various oxides of iron, such as iron(III) oxide (also known as ferric oxide or Fe2O3) which is associated with brown-red colours, whilst iron(II) oxide (also known as ferrous oxide or FeO) is associated with much darker colours, including black. The oxygen concentration in the kiln influences the type, and relative proportions, of these iron oxides in fired the body and glaze: for example, where there is a lack of oxygen during firing the associated carbon monoxide (CO) will readily react with oxygen in Fe2O3 in the raw materials and cause it to be reduced to FeO.

An oxygen-deficient condition, called a reducing atmosphere, is generated by preventing the complete combustion of the kiln fuel; this is achieved by deliberately restricting the air supply or by supplying an excess of fuel.

Pottery can be fired using a variety of methods, with a kiln being the most common. Both the maximum temperature and the firing duration influence the final characteristics of the ceramic. 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.

Decorating

Pottery may be decorated in many different ways. Some decoration can be done before or after the firing, and may be undertaken before or after glazing.

Painting has been used since early prehistoric times, and can be very elaborate. The painting is often applied to pottery that has been fired once and may then be overlaid with a glaze. Many pigments change colour when fired, and the painter must allow for this.

Glaze: Perhaps the most common form of decoration, which also serves as protection to the pottery, by being tougher and keeping liquid from penetrating the pottery. Glaze may be colourless, especially over painting, or coloured and opaque.

Crystalline glaze: characterised by crystalline clusters of various shapes and colours embedded in a more uniform and opaque glaze, produced by the slow cooling of the glost fire.

Carving: Pottery vessels may be decorated by shallow carving of the clay body, typically with a knife or similar instrument used on the wheel. This is common in Chinese porcelain of the classic period.

Burnishing: The surface of pottery wares may be burnished before 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.

Terra Sigillata is an ancient form of decorating ceramics that was first developed in Ancient Greece.

Lithography, also called litho, 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 contains the decorative design; the cover coat, a clear protective layer that 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 for transferring the design while removing the backing paper, some of which are suited to machine application.

Glazing

Glaze is a glassy coating on pottery, and reasons to use it include decoration, ensuring the item is impermeable to liquids, and minimizing the adherence of pollutants.

Glaze may be applied by spraying, dipping, trailing, or brushing on an aqueous suspension of the unfired glaze. The colour of a glaze after firing may be significantly different from before firing. 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 special refractory "spurs" are used as supports. These are removed and discarded after the firing.

Some specialised glazing techniques include:

Salt-glazing – common salt is introduced to the kiln during the firing process. The high temperatures cause the salt to volatilise, depositing it on the surface of the ware, where it reacts 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.

Ash glazing – Ash from the combustion of plant matter has been used as the flux component of glazes. The ash generally came from combustion waste from kiln fuelling, although the potential of ash derived from arable crop wastes has been investigated. 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 a small number of studio potters who value the unpredictability that arises from the raw material's variability.

Health and environmental issues

Although many of the environmental effects of pottery production have existed for millennia, some have been amplified by modern technology and production scales. The principal factors for consideration fall into two categories:

Effects on workers: Notable risks include silicosis, heavy metal poisoning, poor indoor air quality, dangerous sound levels and possible over-illumination.

Effects on the general environment.

Historically, lead poisoning (plumbism) was a significant health concern to those glazing pottery. This was recognised at least as early as the nineteenth century. The first legislation in the UK to limit pottery workers' exposure to lead was included in the Factories Act Extension Act in 1864, with further amendments introduced in 1899.

Silicosis is an occupational lung disease caused by inhaling large amounts of crystalline silica dust, usually over many years. Workers in the ceramic industry can develop it due to exposure to silica dust in the raw materials; colloquially, it has been known as 'Potter's rot'. Less than 10 years after its introduction in 1720 as a raw material for the British ceramics industry, the negative effects of calcined flint on workers' lungs had been noted. In one study reported in 2022, of 106 UK pottery workers, 55 per cent had at least some stage of silicosis. Exposure to siliceous dusts is reduced by either processing and using the source materials as aqueous suspension or as damp solids, or by the use of dust control measures such as local exhaust ventilation. These have been mandated by legislation, such as The Pottery (Health and Welfare) Special Regulations 1950 in the UK. The Health and Safety Executive in the UK has produced guidelines on controlling exposure to respirable crystalline silica in potteries, and the British Ceramics Federation provides a guidance booklet.

Environmental concerns include off-site water pollution, air pollution, disposal of hazardous materials, disposal of rejected ware and fuel consumption.

History

Part of the history of pottery is prehistoric, dating to pre-literate cultures. Therefore, much of this history can only be found among the artifacts of archaeology. Because pottery is so durable, pottery and shards of pottery survive for millennia at archaeological sites and are typically the most common and important artifacts to survive. Many prehistoric cultures are named after the pottery that is the easiest way to identify their sites, and archaeologists have developed the ability to recognise different types from the chemistry of small shards.

Before pottery becomes part of a culture, several conditions must generally be met.

First, there must be usable clay available. Archaeological sites where the earliest pottery was found were near deposits of readily available clay that could be properly shaped and fired. China has large deposits of a variety of clay, which gave it an advantage in the early development of fine pottery. Many countries have large deposits of various clays.

Second, it must be possible to heat the pottery to temperatures that will achieve the transformation from raw clay to ceramic. Methods to reliably create fires hot enough to fire pottery did not develop until late in the development of cultures.

Third, the potter must have time available to prepare, shape, and fire the clay into pottery. Even after control of fire was achieved, humans did not seem to develop pottery until a sedentary life was achieved. It has been hypothesized that pottery was developed only after humans established agriculture, which led to permanent settlements. However, the oldest known pottery is from the Czech Republic and dates to 28,000 BC, at the height of the most recent ice age, long before the beginnings of agriculture.

Fourth, there must be a sufficient need for pottery to justify the resources required for its production.

Early pottery

Methods of forming: Hand-shaping was the earliest method used to form vessels. This included the combination of pinching and coiling.

Firing: The earliest method for firing pottery wares was the use of bonfires pit-fired pottery. Firing times might be short, but the peak temperatures achieved in the fire could be high, perhaps in the region of 900 °C (1,650 °F), and were reached very quickly.

Clay: Early potters used whatever clay was available to them in their geographic vicinity. However, the lowest-quality common red clay was adequate for low-temperature fires used to make the earliest pots. Clay tempered with sand, grit, crushed shell, or crushed pottery was often used to make bonfire-fired ceramics because it provided an open-body texture that allowed water and the clay's volatile components to escape freely. The coarser particles in the clay also acted to restrain shrinkage during drying, thereby reducing the risk of cracking.

Form: In the main, early bonfire-fired wares were made with rounded bottoms to avoid sharp angles that might be susceptible to cracking.

Glazing: the earliest pots were not glazed.

The potter's wheel was invented in Europe in the 5th millennium BC, and revolutionised pottery production. The earliest potter's wheel dates to the middle of the 5th millennium BC from the Cucuteni–Trypillia culture in western Ukraine.

Moulds were used to a limited extent as early as the 5th and 6th century BC by the Etruscans and more extensively by the Romans.

Slipcasting, a popular method for shaping irregularly shaped articles. It was first practised, to a limited extent, in China as early as the Tang dynasty.

Transition to kilns: The earliest intentionally constructed were pit-kilns or trench-kilns, holes dug in the ground and covered with fuel. Holes in the ground provided insulation and improved control over firing.

History by region

Pottery may well have been discovered independently in various places, probably by accidentally creating it at the bottom of fires on a clay soil. The earliest known ceramic objects are Gravettian figurines, such as those discovered at Dolní Věstonice in the modern-day Czech Republic. The Venus of Dolní Věstonice is a Venus figurine, a statuette of a nude female figure dated to 29,000–25,000 BC (Gravettian industry). But there is no evidence of pottery vessels from this period. Weights for looms or fishing nets are a very common use for the earliest pottery. Sherds have been found in China and Japan from a period between 12,000 and perhaps as long as 18,000 years ago. As of 2012, the earliest pottery vessels found anywhere in the world, dating to 20,000 to 19,000 years before the present, was found at Xianren Cave in the Jiangxi province of China.

Other early pottery vessels include those excavated from the Yuchanyan Cave in southern China, dated from 16,000 BC, and those found in the Amur River basin in the Russian Far East, dated from 14,000 BC.

The Odai Yamamoto I site, belonging to the Jōmon period, currently has the oldest pottery in Japan. Excavations in 1998 uncovered earthenware fragments which have been dated as early as 14,500 BC.

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. Recent research has elucidated how Jōmon pottery was used by its creators.

It appears that pottery was independently developed in Sub-Saharan Africa during the 10th millennium BC, with findings dating to at least 9,400 BC from central Mali, and in South America during the 9,000s–7,000s BC. The Malian finds date to the same period as similar finds from East Asia – the triangle between Siberia, China and Japan – and are associated in both regions to the same climatic changes (at the end of the ice age new grassland develops, enabling hunter-gatherers to expand their habitat), met independently by both cultures with similar developments: the creation of pottery for the storage of wild cereals (pearl millet), and that of small arrowheads for hunting small game typical of grassland. Alternatively, the creation of pottery in the case of the Incipient Jōmon civilisation could be due to the intensive exploitation of freshwater and marine organisms by late glacial foragers, who started developing ceramic containers for their catch.