Ceramic glaze
Overview
 
Glaze is a layer or coating of a vitreous
Vitreous enamel
Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

 substance which has been fired to fuse to a ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 object to color, decorate, strengthen or waterproof it.
Glazing is functionally important for earthenware
Earthenware
Earthenware is a common ceramic material, which is used extensively for pottery tableware and decorative objects.-Types of earthenware:Although body formulations vary between countries and even between individual makers, a generic composition is 25% ball clay, 28% kaolin, 32% quartz, and 15%...

 vessels, which would otherwise be unsuitable for holding liquids due to porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

. Glaze is also used on functional and decorative stoneware
Stoneware
Stoneware is a vitreous or semi-vitreous ceramic ware with a fine texture. Stoneware is made from clay that is then fired in a kiln, whether by an artisan to make homeware, or in an industrial kiln for mass-produced or specialty products...

 and porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

. In addition to the functional aspect of glazes, aesthetic concerns include a variety of surface finishes, including degrees of gloss and matte, variegation and finished color.
Encyclopedia
Glaze is a layer or coating of a vitreous
Vitreous enamel
Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

 substance which has been fired to fuse to a ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 object to color, decorate, strengthen or waterproof it.

Use

Glazing is functionally important for earthenware
Earthenware
Earthenware is a common ceramic material, which is used extensively for pottery tableware and decorative objects.-Types of earthenware:Although body formulations vary between countries and even between individual makers, a generic composition is 25% ball clay, 28% kaolin, 32% quartz, and 15%...

 vessels, which would otherwise be unsuitable for holding liquids due to porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

. Glaze is also used on functional and decorative stoneware
Stoneware
Stoneware is a vitreous or semi-vitreous ceramic ware with a fine texture. Stoneware is made from clay that is then fired in a kiln, whether by an artisan to make homeware, or in an industrial kiln for mass-produced or specialty products...

 and porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

. In addition to the functional aspect of glazes, aesthetic concerns include a variety of surface finishes, including degrees of gloss and matte, variegation and finished color. Glazes may also enhance an underlying design or texture which may be either the "natural" texture of the clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 or an inscribed, carved or painted design.
Glaze is used on building materials. The Iron Pagoda
Iron Pagoda
The Iron Pagoda of Youguo Temple , Kaifeng City, Henan province, is a Buddhist Chinese pagoda built in 1049 C.E. during the Song Dynasty of China. The pagoda is so-named not because it is made of iron, but because its colour resembles that of iron...

, built in 1049 in Kaifeng
Kaifeng
Kaifeng , known previously by several names , is a prefecture-level city in east-central Henan province, Central China. Nearly 5 million people live in the metropolitan area...

, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, of glazed bricks is an example.

Composition

Ceramic glazes generally contain silica to form glass, in combination with a mixture of metal oxides such as sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

, potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 and calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 which act as a flux
Ceramic flux
A ceramic flux functions by promoting glass formation in clay bodies and glazes. Fluxes are used in glazes to lower the high melting point of silica. The most commonly used fluxes in a ceramic glaze are lead, boric, soda, potassium, lithium, calcium, magnesium, barium, zinc, and strontium...

 and allow the glaze to melt at a particular temperature, alumina (usually from added clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

) to stiffen the glaze and prevent it from running off the piece, colorants such as iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...

, copper carbonate or cobalt carbonate
Cobalt carbonate
Cobalt carbonate is the inorganic compound with the formula CoCO3. This reddish paramagnetic solid is an intermediate in the hydrometallurgical purification of cobalt from its ores, as an inorganic pigment, and as a precursor to catalysts. Commercially available pale violet basic cobalt...

, and sometimes opacifiers such as tin oxide
Tin oxide
Tin oxide may refer to:* Tin oxide , SnO* Tin dioxide , SnO2...

 or zirconium oxide.

Process

Glaze may be applied by dry dusting a dry mixture over the surface of the clay body or by inserting salt or soda into the kiln at high temperatures to create a sea of sodium vapor that interacts with the aluminium and silica oxides in the body to form and deposit glass (see Salt glaze pottery
Salt glaze pottery
Salt glaze pottery is stoneware with a glaze of glossy, translucent and slightly orange-peel-like texture which was formed by throwing common salt into the kiln during the higher temperature part of the firing process. Sodium from the salt reacts with silica in the clay body to form a glassy...

). Liquid glazes—suspensions of various powdered mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s, and metal oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

s—can be applied by dipping pieces directly into the glaze, pouring the glaze over the piece, spraying it onto the piece with an airbrush
Airbrush
An airbrush is a small, air-operated tool that sprays various media including ink and dye, but most often paint by a process of nebulization. Spray guns developed from the airbrush and are still considered a type of airbrush.-History:...

 or similar tool, with a brush, or with any tool that will achieve the desired effect.

To prevent the glazed article sticking to the kiln
Kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...

 during firing either a small part of the item is left unglazed or special refractory supports, kiln spurs
Kiln spurs
Kiln spurs are supports, often in the shape of a tripod, used to maintain the shape and separate pieces of ceramic during the firing process....

, are used as supports which are removed and discarded after the firing.
Small marks left by these spurs can sometimes be visible on finished ware.

Decoration applied under the glaze on pottery is generally referred to as underglaze
Underglaze
Underglaze is a method of decorating ceramic articles, the decoration is applied to the surface before it is glazed. Because the glaze will subsequently cover it such decoration is completely durable, but because the subsequent glost firing is at a higher temperature than used in on-glaze...

. Underglazes are applied to the surface of the pottery, which can be either raw, "greenware", or "bisque" fired (an initial firing of some articles before the glazing and re-firing). 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. An example of underglaze decoration is the well-known "Blue and white porcelain" porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

 famously produced in England
Spode
Spode is a well-known English brand of pottery and homewares based in Stoke-on-Trent.- The overview :Spode is a Stoke-on-Trent based pottery company that was founded by Josiah Spode in 1770...

, The Netherlands
Delftware
Delftware, or Delft pottery, denotes blue and white pottery made in and around Delft in the Netherlands and the tin-glazed pottery made in the Netherlands from the 16th century....

, China and Japan
Imari porcelain
Imari porcelain is the name for Japanese porcelain wares made in the town of Arita, in the former Hizen Province, northwestern Kyūshū. They were exported to Europe extensively from the port of Imari, Saga between latter half of 17th century and former half of 18 th century, Japanese as well as the...

. The striking blue color is achieved by using the powerful colorant cobalt
Cobalt blue
Cobalt blue is a cool, slightly desaturated blue color, historically made using cobalt salts of alumina. It is used in certain ceramics and painting; the different cobalt pigment smalt, based on silica, is more often used directly in tinted transparent glasses...

 in the form of either cobalt oxide or cobalt carbonate, both of which are still commonly used in glaze decoration today.

Decoration applied on top of a layer of glaze is referred to as overglaze
Overglaze
Is more usually called on-glaze and refers to:* The outer layer or coat of glaze on a piece of ceramics.* Also, a decoration, usually enamel, applied over a glaze....

. Overglaze methods include applying one or more layers or coats of glaze on a piece of pottery
Pottery
Pottery is the material from which the potteryware is made, of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery . Pottery also refers to the art or craft of the potter or the manufacture of pottery...

 or by applying a non-glaze substance such as enamel
Vitreous enamel
Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

 or metals (i.e., gold leaf
Gold leaf
right|thumb|250px|[[Burnishing]] gold leaf with an [[agate]] stone tool, during the water gilding processGold leaf is gold that has been hammered into extremely thin sheets and is often used for gilding. Gold leaf is available in a wide variety of karats and shades...

) over the glaze.

Overglaze colors are low-temperature glazes that give ceramics a more decorative, glassy look. A piece is fired first, overglaze is applied, and it is fired again. Once the piece is fired and comes out of the kiln, its texture becomes smoother because of the glaze.

History

During the Kofun period
Kofun period
The is an era in the history of Japan from around 250 to 538. It follows the Yayoi period. The word kofun is Japanese for the type of burial mounds dating from this era. The Kofun and the subsequent Asuka periods are sometimes referred to collectively as the Yamato period...

 of Japan, Sue ware
Sue ware
was a blue-gray form of high-fired pottery which was produced in Japan and southern Korea during the Kofun, Nara, and Heian periods of Japanese history. It was initially used for funerary and ritual objects, and originated from a Korean stoneware style known as “Kaya ware”...

 was decorated with greenish natural ash glaze
Ash glaze
Ash glazes are ceramic glazes which were formulated from wood-ash .-Appearance:The glaze has glasslike and pooling characteristics which puts emphasis on the surface texture of the piece being glazed. When the glaze is mostly made up of ash, the final result is mostly dark brown to green. The pots...

s. From 552 to 794 AD, differently colored glazes were introduced. The three colored glazes of the Tang Dynasty
Tang Dynasty
The Tang Dynasty was an imperial dynasty of China preceded by the Sui Dynasty and followed by the Five Dynasties and Ten Kingdoms Period. It was founded by the Li family, who seized power during the decline and collapse of the Sui Empire...

 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.

From between the eighth century, the use of glazed ceramics was prevalent in Islamic art
Islamic art
Islamic art encompasses the visual arts produced from the 7th century onwards by people who lived within the territory that was inhabited by or ruled by culturally Islamic populations...

 and Islamic pottery
Islamic pottery
Medieval Islamic pottery occupied a geographical position between Chinese ceramics and the pottery of the Byzantine Empire and Europe. For most of the period it can fairly be said to have been between the two in terms of aesthetic achievement and influence as well, borrowing from China and...

, usually assuming the form of elaborate pottery
Pottery
Pottery is the material from which the potteryware is made, of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery . Pottery also refers to the art or craft of the potter or the manufacture of pottery...

. Tin-opacified glazing
Tin-glazing
Tin-glazing is the process of giving ceramic items a tin-based glaze which is white, glossy and opaque, normally applied to red or buff earthenware. The opacity and whiteness of tin glaze make it valued by its ability to decorate with colour....

 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
Basra
Basra is the capital of Basra Governorate, in southern Iraq near Kuwait and Iran. It had an estimated population of two million as of 2009...

, dating to around the 8th century. Another significant contribution was the development of stoneware
Stoneware
Stoneware is a vitreous or semi-vitreous ceramic ware with a fine texture. Stoneware is made from clay that is then fired in a kiln, whether by an artisan to make homeware, or in an industrial kiln for mass-produced or specialty products...

, originating from 9th century Iraq. Other centers for innovative ceramic pottery in the Islamic world included Fustat (from 975 to 1075), Damascus (from 1100 to around 1600) and Tabriz
Tabriz
Tabriz is the fourth largest city and one of the historical capitals of Iran and the capital of East Azerbaijan Province. Situated at an altitude of 1,350 meters at the junction of the Quri River and Aji River, it was the second largest city in Iran until the late 1960s, one of its former...

 (from 1470 to 1550).

In the 13th century, flower designs were painted with red, blue, green, yellow, and black overglazes. Overglazes became very popular because of the polished look they gave ceramics
Ceramic art
In art history, ceramics and ceramic art mean art objects such as figures, tiles, and tableware made from clay and other raw materials by the process of pottery. Some ceramic products are regarded as fine art, while others are regarded as decorative, industrial or applied art objects, or as...

.

See also

  • Pottery
  • Glaze defects
    Glaze Defects
    Glaze defects are any flaws in the surface quality of a ceramic glaze, its physical structure or its interaction with the body.-Body/glaze interaction problems:...

  • Vitreous Enamel
    Vitreous enamel
    Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

  • Frit
    Frit
    Frit is a ceramic composition that has been fused in a special fusing oven, quenched to form a glass, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble...

  • Porcelain
    Porcelain
    Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

  • Swatow ware
    Swatow ware
    Swatow ware or Swatow is a common name for a group of mainly late Ming Dynasty export porcelain from China intended for the South East Asian market....

  • Ash glaze
    Ash glaze
    Ash glazes are ceramic glazes which were formulated from wood-ash .-Appearance:The glaze has glasslike and pooling characteristics which puts emphasis on the surface texture of the piece being glazed. When the glaze is mostly made up of ash, the final result is mostly dark brown to green. The pots...

  • Shino (glaze)
    Shino (glaze)
    is a generic term for a family of pottery glazes. They tend to range in color from milky white to a light orange, sometimes with charcoal grey spotting, known as "carbon trap" which is the trapping of carbon in the glaze during the firing process...

  • Celadon
    Celadon
    Celadon is a term for ceramics denoting both a type of glaze and a ware of a specific color, also called celadon. This type of ware was invented in ancient China, such as in the Zhejiang province...

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