Ceramic glaze, or simplyglaze, is aglassy coating on ceramics. It is used for decoration, to ensure the item is impermeable to liquids and to minimize the adherence of pollutants.^[1]

Glazing rendersearthenware impermeable to water, sealing the inherentporosity of earthenware. It also gives a tougher surface. Glaze is also used onstoneware andporcelain. 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.

Mostpottery produced in recent centuries has been glazed, other than pieces inbisque porcelain,terracotta, and some other types.Tiles are often glazed on the surface face, and modernarchitectural terracotta is oftenglazed. Glazed brick is also common. Sanitaryware is invariably glazed, as are many ceramics used in industry, for example ceramic insulators foroverhead power lines.

The most important groups of traditional glazes, each named after its mainceramic fluxing agent, are:

• Ash glaze, traditionally important in East Asia, simply made from wood or plant ash, which containspotash andlime.
• Feldspathic glazes of porcelain.
• Lead glazes, plain or coloured, are glossy and transparent after firing, which need only about 800 °C (1,470 °F). They have been used for about 2,000 years in Chinae.g.sancai, around the Mediterranean, and in Europe e.g.Victorian majolica.
• Salt-glaze, mostly European stoneware. It uses ordinary salt.
• Tin-glaze, which coats the ware with lead glaze made opaque white by the addition of tin.^[2] Known in theAncient Near East and then important inIslamic pottery, from which it passed to Europe. IncludesHispano-Moresque ware, Italian Renaissancemaiolica (also calledmajolica),faience andDelftware.

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 tokiln 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, andAncient 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 includingash glaze, and in China, using groundfeldspar. By around 100 BC lead-glazing was widespread in theOld World.^[3]

Glazed brick goes back to theElamite Temple atChogha Zanbil, dated to the 13th century BC. TheIron Pagoda, built in 1049 inKaifeng,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 theKofun period of Japan,Sue ware was decorated with greenish naturalash glazes. From 552 to 794 AD, differently colored glazes were introduced. The three colored glazes of theTang 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 gaveceramics.

From the eighth century, the use of glazed ceramics was prevalent inIslamic art andIslamic pottery, usually in the form of elaboratepottery.^{[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 inBasra, dating to around the 8th century. Another significant contribution was the development ofstoneware, originating from 9th century Iraq.^{[6][full citation needed]} Other places for innovative pottery in the Islamic world includedFustat (from 975 to 1075), Damascus (from 1100 to around 1600) andTabriz (from 1470 to 1550).^{[citation needed]}

Glazes need to include aceramic 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 formssilica, and sometimesboron trioxide.

Raw materials for ceramic glazes generally include silica, which will be the main glass former. Various metal oxides, such as those ofsodium,potassium andcalcium, act asflux and therefore lower the melting temperature.Alumina, often derived fromclay, stiffens the molten glaze to prevent it from running off the piece.^[7] Colorants, such asiron oxide,copper carbonate orcobalt carbonate,^[7] and sometimes opacifiers includingtin oxide andzirconium oxide, are used to modify the visual appearance of the fired glaze.

Most commonly, glazes in aqueous suspension of various powderedminerals and metaloxides 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 anairbrush 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 thealuminium and silica oxides in the body to form and deposit glass.^[9]

To prevent the glazed article from sticking to thekiln during firing, either a small part of the item is left unglazed, or it is supported on small refractory supports such askiln spurs andstilts. 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 theblue and white porcelain first produced in China, and then copied in other countries. The striking blue color usescobalt ascobalt oxide orcobalt carbonate.^[13] However many of the imitative types, such asDelftware, have off-white or even brownearthenware bodies, which are given a white tin-glaze and either inglaze or overglaze decoration. With the English invention ofcreamware 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 theglost 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 firstlyinglaze, where the paints are applied onto the glaze before firing, and then become incorporated within the glaze layer during firing. This works well withtin-glazed pottery, such asmaiolica, 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 mostceladons, but can also be used to create designs in contrasting colours, as in Chinesesancai ("three-colour") wares, or even painted scenes.

Many historical styles, for example JapaneseImari ware, Chinesedoucai andwucai, 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 beleached 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 tobioaccumulate.^[14][15]

Metals used in ceramic glazes are typically in the form of metal oxides.

Ceramic manufacturers primarily uselead(II) oxide (PbO) as a flux for its low melting range, wide firing range, low surface tension, high index of refraction, and resistance todevitrification.^[16] Lead used in the manufacture of commercial glazes are molecularly bound to silica in a 1:1 ratio, or included infrit form, to ensure stabilization and reduce the risk of leaching.^[17]

In polluted environments,nitrogen dioxide reacts with water (H
₂O) to producenitrous acid (HNO
₂) andnitric acid (HNO
₃).^[15]

H
₂O + 2NO
₂ →HNO
₂ +HNO
₃

SolubleLead(II) nitrate (Pb(NO
₃)
₂) forms when lead(II) oxide (PbO) of leaded glazes is exposed tonitric 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 aslead poisoning, the disposal of leaded glass (chiefly in the form of discarded CRT displays) and lead-glazed ceramics is subject totoxic 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 substituteStrontium carbonate (SrCO₃) in glazes that require barium carbonate.^[20] Unlike Barium carbonate, Strontium carbonate is not considered a safety hazard by theNIH.^[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 withcalcium oxide (CaO) and atmospheric oxygen in temperatures reached by a kiln to producecalcium chromate (CaCrO
₄). The oxidation reaction changes chromium from its +3oxidation 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 (UO₂)

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 makinguranium tile, watch, clock and aircraft dials.^[26]

Uranium dioxide is produced byreducinguranium trioxide withhydrogen.

UO₃ + H₂ → UO₂ + H₂O at 700 °C (973 K)

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. TheUnited 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]}

• Ceramic glazes
•  
  Pottery,Nara period
•  
  Meissen porcelain, with blue underglaze decoration on porcelain
•  
  Mug with blue underglaze decoration on porcelain.
•  
  Coloured lead glazes majolica circa 1870
•  
  Test slabs of different glazes
•  
  Tin-glazed majolica decorated with metallic oxide colours, Mintons, circa 1870.
•  
  20th century glazing technique

• Hamer, Frank; Hamer, Janet (1991).The Potter's Dictionary of Materials and Techniques (Third ed.). London, England: A & C Black Publishers, Limited.ISBN 0-8122-3112-0.