Ceramic glaze, or simply glaze, is a glassy coating on ceramics. It is used for decoration, to ensure the item is impermeable to liquids and to minimise the adherence of pollutants. [1]
Glazing renders earthenware impermeable to water, sealing the inherent porosity of earthenware. It also gives a tougher surface. Glaze is also used on stoneware and porcelain. In addition to their functionality, glazes can form a variety of surface finishes, including degrees of glossy or matte finish and color. Glazes may also enhance the underlying design or texture either unmodified or inscribed, carved or painted.
Most pottery produced in recent centuries has been glazed, other than pieces in bisque porcelain, terracotta, and some other types. Tiles are often glazed on the surface face, and modern architectural terracotta is often glazed. Glazed brick is also common. Sanitaryware is invariably glazed, as are many ceramics used in industry, for example ceramic insulators for overhead power lines.
The most important groups of traditional glazes, each named after its main ceramic fluxing agent, are:
Glaze may be applied by spraying, dipping, trailing or brushing on an aqueous suspension of the unfired glaze. The colour of a glaze after it has been fired 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, alternatively, special refractory "spurs" are used as supports. These are removed and discarded after the firing.
Historically, glazing of ceramics developed rather slowly, as appropriate materials needed to be discovered, and also firing technology able to reliably reach the necessary temperatures was needed. Glazes first appeared on stone materials in the 4th millennium BC, and Ancient Egyptian faience (fritware rather than a clay-based material) was self-glazing, as the material naturally formed a glaze-like layer during firing. Glazing of pottery followed the invention of glass around 1500 BC, in the Middle East and Egypt with alkali glazes including ash glaze, and in China, using ground feldspar. By around 100 BC lead-glazing was widespread in the Old World. [3]
Glazed brick goes back to the Elamite Temple at Chogha Zanbil, dated to the 13th century BC. The Iron Pagoda, built in 1049 in Kaifeng, China, of glazed bricks is a well-known later example. [4]
Lead glazed earthenware was probably made in China during the Warring States period (475 – 221 BC), and its production increased during the Han dynasty. High temperature proto-celadon glazed stoneware was made earlier than glazed earthenware, since the Shang dynasty (1600 – 1046 BCE). [5]
During the Kofun period of Japan, Sue ware was decorated with greenish natural ash glazes. From 552 to 794 AD, differently colored glazes were introduced. The three colored glazes of the Tang dynasty were frequently used for a period, but were gradually phased out; the precise colors and compositions of the glazes have not been recovered. Natural ash glaze, however, was commonly used throughout the country.
In the 13th century, flower designs were painted with red, blue, green, yellow and black overglazes. Overglazes became very popular because of the particular look they gave ceramics.
From the eighth century, the use of glazed ceramics was prevalent in Islamic art and Islamic pottery, usually in the form of elaborate pottery.[ citation needed ] Tin-opacified glazing was one of the earliest new technologies developed by the Islamic potters. The first Islamic opaque glazes can be found as blue-painted ware in Basra, dating to around the 8th century. Another significant contribution was the development of stoneware, originating from 9th century Iraq. [6] [ full citation needed ] Other places for innovative pottery in the Islamic world included Fustat (from 975 to 1075), Damascus (from 1100 to around 1600) and Tabriz (from 1470 to 1550).[ citation needed ]
Glazes need to include a ceramic flux which functions by promoting partial liquefaction in the clay bodies and the other glaze materials. Fluxes lower the high melting point of the glass forms silica, and sometimes boron trioxide.
Raw materials for ceramic glazes generally include silica, which will be the main glass former. Various metal oxides, such as those of sodium, potassium and calcium, act as flux and therefore lower the melting temperature. Alumina, often derived from clay, stiffens the molten glaze to prevent it from running off the piece. [7] Colorants, such as iron oxide, copper carbonate or cobalt carbonate, [7] and sometimes opacifiers including tin oxide and zirconium oxide, are used to modify the visual appearance of the fired glaze.
Most commonly, glazes in aqueous suspension of various powdered minerals and metal oxides are applied by dipping pieces directly into the glaze [8] . Other techniques include pouring the glaze over the piece, spraying it onto the piece with an airbrush or similar tool, or applying it directly with a tool such as a brush. Though mostly obsolete, salt glaze pottery is another form of glazing. Dry-dusting a mixture over the surface of the clay body or inserting salt or soda into the kiln at high temperatures creates an atmosphere rich in sodium vapor. This interacts with the aluminium and silica oxides in the body to form and deposit glass [9] .
To prevent the glazed article from sticking to the kiln during firing, either a small part of the item is left unglazed, or it is supported on small refractory supports such as kiln spurs and stilts. The supports are then removed and discarded after the firing. Small marks left by these spurs are sometimes visible on finished ware.
Underglaze decoration is applied before the glaze, usually to unfired pottery ("raw" or "greenware") but sometimes to "biscuit"-fired (an initial firing of some articles before the glazing and re-firing). [10] [11] [12] A wet glaze—usually transparent—is applied over the decoration. The pigment fuses with the glaze, and appears to be underneath a layer of clear glaze; generally the body material used fires to a whitish colour. The best known type of underglaze decoration is the blue and white porcelain first produced in China, and then copied in other countries. The striking blue color uses cobalt as cobalt oxide or cobalt carbonate. [13] However many of the imitative types, such as Delftware, have off-white or even brown earthenware bodies, which are given a white tin-glaze and either inglaze or overglaze decoration. With the English invention of creamware and other white-bodied earthenwares in the 18th century, underglaze decoration became widely used on earthenware as well as porcelain.
Overglaze decoration is applied on top of a fired layer of glaze, and generally uses colours in "enamel", essentially glass, which require a second firing at a relatively low temperature to fuse them with the glaze. Because it is only fired at a relatively low temperature, a wider range of pigments could be used in historic periods. Overglaze colors are low-temperature glazes that give ceramics a more decorative, glassy look. A piece is fired first, this initial firing being called the glost firing, then the overglaze decoration is applied, and it is fired again. Once the piece is fired and comes out of the kiln, its texture is smoother due to the glaze.
Other methods are firstly inglaze, where the paints are applied onto the glaze before firing, and then become incorporated within the glaze layer during firing. This works well with tin-glazed pottery, such as maiolica, but the range of colours was limited to those that could withstand a glost firing, as with underglaze. Coloured glazes, where the pigments are mixed into the liquid glaze before it is applied to the pottery, are mostly used to give a single colour to a whole piece, as in most celadons, but can also be used to create designs in contrasting colours, as in Chinese sancai ("three-colour") wares, or even painted scenes.
Many historical styles, for example Japanese Imari ware, Chinese doucai and wucai, combine the different types of decoration. In such cases the first firing for the body, any underglaze decoration and glaze is typically followed by a second firing after the overglaze enamels have been applied.
Heavy metals are dense metals used in glazes to produce a particular color or texture. [11] Glaze components are more likely to be leached into the environment when non-recycled ceramic products are exposed to warm or acidic water. [14] Leaching of heavy metals occurs when ceramic products are glazed incorrectly or damaged. [14] Lead and chromium are two heavy metals which can be used in ceramic glazes that are heavily monitored by government agencies due to their toxicity and ability to bioaccumulate. [14] [15]
Metals used in ceramic glazes are typically in the form of metal oxides.
Ceramic manufacturers primarily use lead(II) oxide (PbO) as a flux for its low melting range, wide firing range, low surface tension, high index of refraction, and resistance to devitrification. [16] Lead used in the manufacture of commercial glazes are molecularly bound to silica in a 1:1 ratio, or included in frit form, to ensure stabilization and reduce the risk of leaching. [17]
In polluted environments, nitrogen dioxide reacts with water (H
2O) to produce nitrous acid (HNO
2) and nitric acid (HNO
3). [15]
H
2O + 2NO
2 → HNO
2 + HNO
3
Soluble Lead(II) nitrate (Pb(NO
3)
2) forms when lead(II) oxide (PbO) of leaded glazes is exposed to nitric acid (HNO
3)
PbO + 2HNO
3 → Pb(NO
3)
2 + H
2O
Because lead exposure is strongly linked to a variety of health problems, collectively referred to as lead poisoning, the disposal of leaded glass (chiefly in the form of discarded CRT displays) and lead-glazed ceramics is subject to toxic waste regulations.
Barium carbonate (BaCO3) is used to create a unique glaze color known as barium blue. However, the ethical nature of using barium carbonate for glazes on food contact surfaces has come into question. Barium poisoning by ingestion can result in convulsions, paralysis, digestive discomfort, and death. [18] It is also somewhat soluble in acid, [19] and can contaminate water and soil for long periods of time. These concerns have led to attempts to substitute Strontium carbonate (SrCO3) in glazes that require barium carbonate. [20] Unlike Barium carbonate, Strontium carbonate is not considered a safety hazard by the NIH. [21] [19] Experiments in strontium substitution tend to be successful in gloss type glazes, although there are some effects and colors produced in matte type glazes that can only be obtained through use of barium. [20]
To reduce the likelihood of leaching, barium carbonate is used in frit form and bound to silica in a 1:1 ratio. It is also recommended that barium glazes not be used on food contact surfaces or outdoor items. [22]
Chromium(III) oxide (Cr
2O
3) is used as a colorant in ceramic glazes. Chromium(III) oxide can undergo a reaction with calcium oxide (CaO) and atmospheric oxygen in temperatures reached by a kiln to produce calcium chromate (CaCrO
4). The oxidation reaction changes chromium from its +3 oxidation state to its +6 oxidation state. [23] Chromium(VI) is very soluble and the most mobile out of all the other stable forms of chromium. [24]
Cr
2O
3 + 2CaO + 3⁄2O
2 → CaCrO
4 [23]
Chromium may enter water systems via industrial discharge. Chromium(VI) can enter the environment directly or oxidants present in soils can react with chromium(III) to produce chromium(VI). Plants have reduced amounts of chlorophyll when grown in the presence of chromium(VI). [24]
Uranium(IV) oxide (U O 2)
Urania-based ceramic glazes are dark green or black when fired in a reduction or when UO2 is used; more commonly it is used in oxidation to produce bright yellow, orange and red glazes [25] Uranium glazes were used in the 1920s and 1930s for making uranium tile, watch, clock and aircraft dials. [26]
Uranium dioxide is produced by reducing uranium trioxide with hydrogen.
Chromium oxidation during manufacturing processes can be reduced with the introduction of compounds that bind to calcium. [23] Ceramic industries are reluctant to use lead alternatives since leaded glazes provide products with a brilliant shine and smooth surface. The United States Environmental Protection Agency has experimented with a dual glaze, barium alternative to lead, but they were unsuccessful in achieving the same optical effect as leaded glazes.[ citation needed ]
Pottery is the process and the products of forming vessels and other objects with clay and other raw materials, which are fired at high temperatures to give them a hard and durable form. The place where such wares are made by a potter is also called a pottery. The definition of pottery, used by the ASTM International, is "all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products". End applications include tableware, decorative ware, sanitary ware, and in technology and industry such as electrical insulators and laboratory ware. In art history and archaeology, especially of ancient and prehistoric periods, pottery often means only vessels, and sculpted figurines of the same material are called terracottas.
Delftware or Delft pottery, also known as Delft Blue or as delf, is a general term now used for Dutch tin-glazed earthenware, a form of faience. Most of it is blue and white pottery, and the city of Delft in the Netherlands was the major centre of production, but the term covers wares with other colours, and made elsewhere. It is also used for similar pottery, English delftware.
Earthenware is glazed or unglazed nonvitreous pottery that has normally been fired below 1,200 °C (2,190 °F). Basic earthenware, often called terracotta, absorbs liquids such as water. However, earthenware can be made impervious to liquids by coating it with a ceramic glaze, and such a process is used for the great majority of modern domestic earthenware. The main other important types of pottery are porcelain, bone china, and stoneware, all fired at high enough temperatures to vitrify. End applications include tableware and decorative ware such as figurines.
Pottery and porcelain is one of the oldest Japanese crafts and art forms, dating back to the Neolithic period. Types have included earthenware, pottery, stoneware, porcelain, and blue-and-white ware. Japan has an exceptionally long and successful history of ceramic production. Earthenwares were made as early as the Jōmon period, giving Japan one of the oldest ceramic traditions in the world. Japan is further distinguished by the unusual esteem that ceramics hold within its artistic tradition, owing to the enduring popularity of the tea ceremony. During the Azuchi-Momoyama period (1573–1603), kilns throughout Japan produced ceramics with unconventional designs. In the early Edo period, the production of porcelain commenced in the Hizen-Arita region of Kyushu, employing techniques imported from Korea. These porcelain works became known as Imari wares, named after the port of Imari from which they were exported to various markets, including Europe.
Creamware is a cream-coloured refined earthenware with a lead glaze over a pale body, known in France as faïence fine, in the Netherlands as Engels porselein, and in Italy as terraglia inglese. It was created about 1750 by the potters of Staffordshire, England, who refined the materials and techniques of salt-glazed earthenware towards a finer, thinner, whiter body with a brilliant glassy lead glaze, which proved so ideal for domestic ware that it supplanted white salt-glaze wares by about 1780. It was popular until the 1840s.
In different periods of time and in different countries, the term majolica has been used for two distinct types of pottery.
"Blue and white pottery" covers a wide range of white pottery and porcelain decorated under the glaze with a blue pigment, generally cobalt oxide. The decoration was commonly applied by hand, originally by brush painting, but nowadays by stencilling or by transfer-printing, though other methods of application have also been used. The cobalt pigment is one of the very few that can withstand the highest firing temperatures that are required, in particular for porcelain, which partly accounts for its long-lasting popularity. Historically, many other colours required overglaze decoration and then a second firing at a lower temperature to fix that.
Maiolica is tin-glazed pottery decorated in colours on a white background. The most renowned Italian maiolica is from the Renaissance period. These works were known as istoriato wares when depicting historical and mythical scenes. By the late 15th century, multiple locations, mainly in northern and central Italy, were producing sophisticated pieces for a luxury market in Italy and beyond. In France, maiolica developed as faience, in the Netherlands and England as delftware, and in Spain as talavera. In English, the spelling was anglicised to majolica, but the pronunciation usually preserved the vowel with an i as in kite.
Transfer printing is a method of decorating pottery or other materials using an engraved copper or steel plate from which a monochrome print on paper is taken which is then transferred by pressing onto the ceramic piece. Pottery decorated using this technique is known as transferware or transfer ware.
Underglaze is a method of decorating pottery in which painted decoration is applied to the surface before it is covered with a transparent ceramic glaze and fired in a kiln. Because the glaze subsequently covers it, such decoration is completely durable, and it also allows the production of pottery with a surface that has a uniform sheen. Underglaze decoration uses pigments derived from oxides which fuse with the glaze when the piece is fired in a kiln. It is also a cheaper method, as only a single firing is needed, whereas overglaze decoration requires a second firing at a lower temperature.
Chinese ceramics are one of the most significant forms of Chinese art and ceramics globally. They range from construction materials such as bricks and tiles, to hand-built pottery vessels fired in bonfires or kilns, to the sophisticated Chinese porcelain wares made for the imperial court and for export.
Tin-glazing is the process of giving tin-glazed pottery items a ceramic glaze that is white, glossy and opaque, which is normally applied to red or buff earthenware. Tin-glaze is plain lead glaze with a small amount of tin oxide added. The opacity and whiteness of tin glaze encourage its frequent decoration. Historically this has mostly been done before the single firing, when the colours blend into the glaze, but since the 17th century also using overglaze enamels, with a light second firing, allowing a wider range of colours. Majolica, maiolica, delftware and faience are among the terms used for common types of tin-glazed pottery.
Overglaze decoration, overglaze enamelling, or on-glaze decoration, is a method of decorating pottery, most often porcelain, where the coloured decoration is applied on top of the already fired and glazed surface, and then fixed in a second firing at a relatively low temperature, often in a muffle kiln. It is often described as producing "enamelled" decoration. The colours fuse on to the glaze, so the decoration becomes durable. This decorative firing is usually done at a lower temperature which allows for a more varied and vivid palette of colours, using pigments which will not colour correctly at the high temperature necessary to fire the porcelain body. Historically, a relatively narrow range of colours could be achieved with underglaze decoration, where the coloured pattern is applied before glazing, notably the cobalt blue of blue and white porcelain.
This is a list of pottery and ceramic terms.
Fritware, also known as stone-paste, is a type of pottery in which frit is added to clay to reduce its fusion temperature. The mixture may include quartz or other siliceous material. An organic compound such as gum or glue may be added for binding. The resulting mixture can be fired at a lower temperature than clay alone. A glaze is then applied on the surface.
Tin-glazed pottery is earthenware covered in lead glaze with added tin oxide which is white, shiny and opaque ; usually this provides a background for brightly painted decoration. It has been important in Islamic and European pottery, but very little used in East Asia. The pottery body is usually made of red or buff-colored earthenware and the white glaze imitated Chinese porcelain. The decoration on tin-glazed pottery is usually applied to the unfired glaze surface by brush with metallic oxides, commonly cobalt oxide, copper oxide, iron oxide, manganese dioxide and antimony oxide. The makers of Italian tin-glazed pottery from the late Renaissance blended oxides to produce detailed and realistic polychrome paintings.
Cizhou ware or Tz'u-chou ware is a wide range of Chinese ceramics from between the late Tang dynasty and the early Ming dynasty, but especially associated with the Northern Song to Yuan period in the 11–14th century. It has been increasingly realized that a very large number of sites in northern China produced these wares, and their decoration is very variable, but most characteristically uses black and white, in a variety of techniques. For this reason Cizhou-type is often preferred as a general term. All are stoneware in Western terms, and "high-fired" or porcelain in Chinese terms. They were less high-status than other types such as celadons and Jun ware, and are regarded as "popular", though many are finely and carefully decorated.
China painting, or porcelain painting, is the decoration of glazed porcelain objects such as plates, bowls, vases or statues. The body of the object may be hard-paste porcelain, developed in China in the 7th or 8th century, or soft-paste porcelain, developed in 18th-century Europe. The broader term ceramic painting includes painted decoration on lead-glazed earthenware such as creamware or tin-glazed pottery such as maiolica or faience.
Lead-glazed earthenware is one of the traditional types of earthenware with a ceramic glaze, which coats the ceramic bisque body and renders it impervious to liquids, as terracotta itself is not. Plain lead glaze is shiny and transparent after firing. Coloured lead glazes are shiny and either translucent or opaque after firing. Three other traditional techniques are tin-glazed, which coats the ware with an opaque white glaze suited for overglaze brush-painted colored enamel designs; salt glaze pottery, also often stoneware; and the feldspathic glazes of Asian porcelain. Modern materials technology has invented new glazes that do not fall into these traditional categories.
Ceramic art is art made from ceramic materials, including clay. It may take varied forms, including artistic pottery, including tableware, tiles, figurines and other sculpture. As one of the plastic arts, ceramic art is a visual art. While some ceramics are considered fine art, such as pottery or sculpture, most are considered to be decorative, industrial or applied art objects. Ceramic art can be created by one person or by a group, in a pottery or a ceramic factory with a group designing and manufacturing the artware.
Pottery only began to be glazed from the mid second millennium BC, coincident with the first production of glass.