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Ceramic glaze, or simply glaze, is a
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
y coating on ceramics. It is used for decoration, to ensure the item is impermeable to liquids and to minimize the adherence of pollutants. Glazing renders
earthenware Earthenware is glazed or unglazed Vitrification#Ceramics, nonvitreous pottery that has normally been fired below . Basic earthenware, often called terracotta, absorbs liquids such as water. However, earthenware can be made impervious to liquids ...
impermeable to water, sealing the inherent
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
of earthenware. It also gives a tougher surface. Glaze is also used on
stoneware Stoneware is a broad class of pottery fired at a relatively high temperature, to be impervious to water. A modern definition is a Vitrification#Ceramics, vitreous or semi-vitreous ceramic made primarily from stoneware clay or non-refractory fire ...
and
porcelain Porcelain (), also called china, is a ceramic material made by heating Industrial mineral, raw materials, generally including kaolinite, in a kiln to temperatures between . The greater strength and translucence of porcelain, relative to oth ...
. 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 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 al ...
produced in recent centuries has been glazed, other than pieces in bisque porcelain,
terracotta Terracotta, also known as terra cotta or terra-cotta (; ; ), is a clay-based non-vitreous ceramic OED, "Terracotta""Terracotta" MFA Boston, "Cameo" database fired at relatively low temperatures. It is therefore a term used for earthenware obj ...
, and some other types.
Tile Tiles are usually thin, square or rectangular coverings manufactured from hard-wearing material such as ceramic, Rock (geology), stone, metal, baked clay, or even glass. They are generally fixed in place in an array to cover roofs, floors, wal ...
s are often glazed on the surface face, and modern
architectural terracotta Architectural terracotta refers to a fired mixture of clay and water that can be used in a non-structural, semi-structural, or structural capacity on the exterior or interior of a building. Terracotta is an ancient building material that transla ...
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 line An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy along large distances. It consists of one or more conductors (commonly multiples of three) suspended by towers or poles. ...
s. The most important groups of traditional glazes, each named after its main ceramic fluxing agent, are: * Ash glaze, traditionally important in East Asia, simply made from wood or plant ash, which contains
potash Potash ( ) includes various mined and manufactured salts that contain potassium in water- soluble form.
and lime. * Feldspathic glazes of porcelain. * Lead glazes, plain or coloured, are glossy and transparent after firing, which need only about . They have been used for about 2,000 years in China ''e.g. sancai'', around the Mediterranean, and in Europe e.g.
Victorian majolica Victorian majolica properly refers to two types of majolica made in the second half of the 19th century in Europe and America. Firstly, and best known, there is the mass-produced majolica decorated with coloured lead glazes, made in Britain, Eur ...
. * 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. Known in the
Ancient Near East The ancient Near East was home to many cradles of civilization, spanning Mesopotamia, Egypt, Iran (or Persia), Anatolia and the Armenian highlands, the Levant, and the Arabian Peninsula. As such, the fields of ancient Near East studies and Nea ...
and then important in
Islamic pottery Islamic pottery occupied a geographical position between Chinese ceramics, and the pottery of the Byzantine Empire and Europe. For most of the period, it made great aesthetic achievements and influence as well, influencing Byzantium and Europe ...
, from which it passed to Europe. Includes
Hispano-Moresque ware Hispano-Moresque ware is a style of initially Islamic pottery created in Al-Andalus (Muslim Iberia), which continued to be produced under Christian rule in styles blending Islamic and European elements. It was the most elaborate and luxurious ...
, Italian Renaissance
maiolica 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 ("painted with stories") when depicting historical and ...
(also called
majolica In different periods of time and in different countries, the term ''majolica'' has been used for two distinct types of pottery. Firstly, from the mid-15th century onwards, ''maiolica'' was a type of pottery reaching Italy from Spain, Majorca a ...
),
faience Faience or faïence (; ) is the general English language term for fine tin-glazed pottery. The invention of a white Ceramic glaze, pottery glaze suitable for painted decoration, by the addition of an stannous oxide, oxide of tin to the Slip (c ...
and
Delftware 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 cen ...
. 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.


History

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 Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...
. By around 100 BC lead-glazing was widespread in the
Old World The "Old World" () is a term for Afro-Eurasia coined by Europeans after 1493, when they became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia in the Eastern Hemisphere, previously ...
. Glazed brick goes back to the
Elam Elam () was an ancient civilization centered in the far west and southwest of Iran, stretching from the lowlands of what is now Khuzestan and Ilam Province as well as a small part of modern-day southern Iraq. The modern name ''Elam'' stems fr ...
ite Temple at Chogha Zanbil, dated to the 13th century BC. The Iron Pagoda, built in 1049 in
Kaifeng Kaifeng ( zh, s=开封, p=Kāifēng) is a prefecture-level city in east-Zhongyuan, central Henan province, China. It is one of the Historical capitals of China, Eight Ancient Capitals of China, having been the capital eight times in history, and ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
, of glazed bricks is a well-known later example. 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). During the
Kofun period The is an era in the history of Japan from about 300 to 538 AD (the date of the introduction of Buddhism), following the Yayoi period. The Kofun and the subsequent Asuka periods are sometimes collectively called the Yamato period. This period is ...
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 The Tang dynasty (, ; zh, c=唐朝), or the Tang Empire, was an Dynasties of China, imperial dynasty of China that ruled from 618 to 907, with an Wu Zhou, interregnum between 690 and 705. It was preceded by the Sui dynasty and followed ...
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 A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porce ...
. From the eighth century, the use of glazed ceramics was prevalent in
Islamic art Islamic art is a part of Islamic culture and encompasses the visual arts produced since the 7th century CE by people who lived within territories inhabited or ruled by Muslims, Muslim populations. Referring to characteristic traditions across ...
and
Islamic pottery Islamic pottery occupied a geographical position between Chinese ceramics, and the pottery of the Byzantine Empire and Europe. For most of the period, it made great aesthetic achievements and influence as well, influencing Byzantium and Europe ...
, usually in the form of elaborate
pottery 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 al ...
. 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 Basra () is a port city in Iraq, southern Iraq. It is the capital of the eponymous Basra Governorate, as well as the List of largest cities of Iraq, third largest city in Iraq overall, behind Baghdad and Mosul. Located near the Iran–Iraq bor ...
, dating to around the 8th century. Another significant contribution was the development of
stoneware Stoneware is a broad class of pottery fired at a relatively high temperature, to be impervious to water. A modern definition is a Vitrification#Ceramics, vitreous or semi-vitreous ceramic made primarily from stoneware clay or non-refractory fire ...
, originating from 9th century Iraq. Other places for innovative pottery in the Islamic world included Fustat (from 975 to 1075), Damascus (from 1100 to around 1600) and
Tabriz Tabriz (; ) is a city in the Central District (Tabriz County), Central District of Tabriz County, in the East Azerbaijan province, East Azerbaijan province of northwestern Iran. It serves as capital of the province, the county, and the distric ...
(from 1470 to 1550).


Composition

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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
, and sometimes
boron trioxide Boron trioxide or diboron trioxide is the oxide of boron with the formula . It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide or boria. It h ...
. Raw materials for ceramic glazes generally include silica, which will be the main glass former. Various metal oxides, such as those of
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
,
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
and
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
, act as
flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications in physics. For transport phe ...
and therefore lower the melting temperature.
Alumina Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...
, often derived from
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
, stiffens the molten glaze to prevent it from running off the piece. Colorants, such as
iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...
, copper carbonate or cobalt carbonate, and sometimes opacifiers including tin oxide and zirconium oxide, are used to modify the visual appearance of the fired glaze.


Process

Most commonly, glazes in aqueous suspension of various powdered
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
s and metal
oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
s are applied by dipping pieces directly into the glaze. Other techniques include pouring the glaze over the piece, spraying it onto the piece with an
airbrush An airbrush is a small, air-operated tool that atomizes and sprays various media, most often paint, but also ink, dye, and make-up. Spray painting developed from the airbrush and is considered to employ a type of airbrush. History Up unt ...
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 Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
and silica oxides in the body to form and deposit glass.''Dictionary Of Ceramics''. Arthur Dodd & David Murfin. 3rd edition. The Institute Of Minerals. 1994. To prevent the glazed article from sticking to the
kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or Chemical Changes, chemical changes. Kilns have been used for millennia to turn objects m ...
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
stilt Stilt is a common name for several species of birds in the family Recurvirostridae, which also includes those known as avocets. They are found in brackish or saline wetlands in warm or hot climates. They have extremely long legs, hence the grou ...
s. The supports are then removed and discarded after the firing. Small marks left by these spurs are sometimes visible on finished ware.


Colour and decoration

Underglaze 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 ...
decoration is applied before the glaze, usually to unfired pottery ("raw" or "greenware") but sometimes to "
biscuit A biscuit is a flour-based baked food item. Biscuits are typically hard, flat, and unleavened. They are usually sweet and may be made with sugar, chocolate, icing, jam, ginger, or cinnamon. They can also be savoury, similar to crackers. ...
"-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; generally the body material used fires to a whitish colour. The best known type of underglaze decoration is the
blue and white porcelain "Blue and white pottery" () covers a wide range of white pottery and porcelain decorated underglaze, under the glaze with a blue pigment, generally cobalt(II) oxide, cobalt oxide. The decoration was commonly applied by hand, originally by brush p ...
first produced in China, and then copied in other countries. The striking blue color uses
cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...
as
cobalt oxide Cobalt oxide is a family of chemical compounds consisting of cobalt and oxygen atoms. Compounds in the cobalt oxide family include: * Cobalt(II) oxide (cobaltous oxide), CoO * Cobalt(III) oxide (cobaltic oxide), Co2O3 *Cobalt(II,III) oxide Co ...
or cobalt carbonate. However many of the imitative types, such as
Delftware 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 cen ...
, have off-white or even brown
earthenware Earthenware is glazed or unglazed Vitrification#Ceramics, nonvitreous pottery that has normally been fired below . Basic earthenware, often called terracotta, absorbs liquids such as water. However, earthenware can be made impervious to liquids ...
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 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-glazed earthenware, lead glaze with a small amount of Tin( ...
pottery, such as
maiolica 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 ("painted with stories") when depicting historical and ...
, 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
celadon Celadon () is a term for pottery denoting both wares ceramic glaze, glazed in the jade green Shades of green#Celadon, celadon color, also known as greenware or "green ware" (the term specialists now tend to use), and a type of transparent glaze, ...
s, 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 is a Western term for a brightly-coloured style of Japanese export porcelain made in the area of Arita, in the former Hizen Province, northwestern Kyūshū. They were exported to Europe in large quantities, especially between the second half ...
, 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.


Environmental impact

Heavy metals are dense metals used in glazes to produce a particular color or texture. Glaze components are more likely to be leached into the environment when non-recycled ceramic products are exposed to warm or acidic water.Omolaoye, J.A,, A. Uzairu, and C.E. Gimba. "Heavy Metal Assessment of Some Ceramic Products Imported into Nigeria from China." ''Archives of Applied Science Research'' 2.5 (2010): 120-25. Web. 15 October 2015 Leaching of heavy metals occurs when ceramic products are glazed incorrectly or damaged. 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.


Metal oxide chemistry

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


Lead(II) oxide

Ceramic manufacturers primarily use
lead(II) oxide Lead(II) oxide, also called lead monoxide, is the inorganic compound with the molecular formula Pb O. It occurs in two polymorphs: litharge having a tetragonal crystal structure, and massicot having an orthorhombic crystal structure. Modern ...
(PbO) as a flux for its low melting range, wide firing range, low surface tension, high index of refraction, and resistance to
devitrification Devitrification is the process of crystallization in a formerly crystal-free (amorphous) glass. The term is derived from the Latin ''vitreus'', meaning '' glassy'' and '' transparent''. Devitrification in glass art Devitrification occurs in glass ...
.Lehman, Richard. ''Lead Glazes for Ceramic Foodware''. 1st ed. Research Triangle Park: International Lead Management Center, 2002
International Lead Management Center
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. In polluted environments,
nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, is an intermediate in the s ...
reacts with water () to produce
nitrous acid Nitrous acid (molecular formula ) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite () salts. It was discovered by Carl Wilhelm Scheele, who called it " phlogisticated acid of niter". Nitrous ac ...
() and
nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...
(). + 2 → + Soluble Lead(II) nitrate () forms when lead(II) oxide (PbO) of leaded glazes is exposed to
nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...
() PbO + 2 → + Because lead exposure is strongly linked to a variety of health problems, collectively referred to as
lead poisoning Lead poisoning, also known as plumbism and saturnism, is a type of metal poisoning caused by lead in the body. Symptoms may include abdominal pain, constipation, headaches, irritability, memory problems, infertility, numbness and paresthesia, t ...
, 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 and strontium carbonate

Barium carbonate Barium carbonate is the inorganic compound with the formula BaCO3. Like most alkaline earth metal carbonates, it is a white salt that is poorly soluble in water. It occurs as the mineral known as witherite. In a commercial sense, it is one of ...
(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. It is also somewhat soluble in acid, and can contaminate water and soil for long periods of time. These concerns have led to attempts to substitute
strontium carbonate Strontium carbonate (SrCO3) is the carbonate salt of strontium that has the appearance of a white or grey powder. It occurs in nature as the mineral strontianite. Chemical properties Strontium carbonate is a white, odorless, tasteless powder ...
(SrCO3) in glazes that require barium carbonate. Unlike barium carbonate, strontium carbonate is not considered a safety hazard by the NIH. 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. 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.


Chromium(III) oxide

Chromium(III) oxide () is used as a colorant in ceramic glazes. Chromium(III) oxide can undergo a reaction with
calcium oxide Calcium oxide (formula: Ca O), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term '' lime'' connotes calcium-containing ...
(CaO) and atmospheric oxygen in temperatures reached by a kiln to produce calcium chromate (). The oxidation reaction changes chromium from its +3
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
to its +6 oxidation state. Chromium(VI) is very soluble and the most mobile out of all the other stable forms of chromium. + 2CaO + → 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). Uranium(IV) oxide ( U O2) 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 Uranium glazes were used in the 1920s and 1930s for making uranium tile, watch, clock and aircraft dials. Uranium tile Uranium dioxide is produced by reducing uranium trioxide with
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
. : UO3 + H2 → UO2 + H2O at 700 °C (973 K)


Prevention

Chromium oxidation during manufacturing processes can be reduced with the introduction of compounds that bind to calcium. 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 The Environmental Protection Agency (EPA) is an independent agency of the United States government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it began operation on De ...
has experimented with a dual glaze, barium alternative to lead, but they were unsuccessful in achieving the same optical effect as leaded glazes.


Gallery

File:Periodo nara, giara invetriata a tre a colori, VIII sec.JPG, Pottery,
Nara period The of the history of Japan covers the years from 710 to 794. Empress Genmei established the capital of Heijō-kyō (present-day Nara). Except for a five-year period (740–745), when the capital was briefly moved again, it remained the capita ...
File:Meissen Zwiebelmuster.jpg, Meissen porcelain, with blue underglaze decoration on porcelain File:Potterymugs.jpg, Mug with blue underglaze decoration on porcelain. File:GlazeLeadMintonSwansmajolica18637.jpg, Coloured lead glazes majolica circa 1870 File:Vzorkovník glazur.JPG, Test slabs of different glazes File:Minton_tin-glaze_Majolica_oval_plate_decorated_by_Thomas_Kirkby_in_Renaissance_style_after_Mantegna_original.jpg, Tin-glazed majolica decorated with metallic oxide colours, Mintons, circa 1870. File:Cup of coffee on black background.jpg, 20th century glazing technique


See also

*
Celadon Celadon () is a term for pottery denoting both wares ceramic glaze, glazed in the jade green Shades of green#Celadon, celadon color, also known as greenware or "green ware" (the term specialists now tend to use), and a type of transparent glaze, ...
* Frit * Glaze defects * Pottery#Glazing and firing techniques * Shino ware * Swatow ware * Uranium tile *
Vitreous enamel Vitreous enamel, also called porcelain enamel, is a material made by melting, fusing powdered glass to a substrate by firing, usually between . The powder melts, flows, and then hardens to a smooth, durable vitrification, vitreous coating. The wo ...


References


Bibliography

* {{Authority control Painting techniques Artistic techniques Pottery Glass applications Glass compositions Ceramic glazes Ceramic engineering