Ceramic glaze

Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a ceramic body through firing. Glaze can serve to color, decorate or waterproof an item.[1] Glazing renders earthenware vessels suitable for holding liquids, sealing the inherent porosity of unglazed biscuit 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 unglazed biscuit porcelain, terracotta, or some other types. Tiles are almost always glazed on the surface face, and modern architectural terracotta is very often glazed. Glazed brick is also common. Domestic sanitary ware 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:

Modern materials technology has invented new vitreous glazes that do not fall into these traditional categories.

Bottle Iran 16
Composite body, painted, and glazed bottle. Iran, 16th century (Metropolitan Museum of Art)
Eiraku Wazen - Flared Vase with Dripping Glaze - Walters 491582 - Mark A
Detail of dripping rice-straw ash glaze (top), Japan, 1852


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 formers silica, and sometimes boron trioxide. These glass formers may be included in the glaze materials, or may be drawn from the clay beneath.

Raw materials of ceramic glazes generally include silica, which will be the main glass former. Various metal oxides, such as 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.[3] Colorants, such as iron oxide, copper carbonate, or cobalt carbonate,[3] and sometimes opacifiers like tin oxide or zirconium oxide, are used to modify the visual appearance of the fired glaze.


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 an atmosphere rich in sodium vapor that interacts with the aluminium and silica oxides in the body to form and deposit glass, producing what is known as salt glaze pottery. Most commonly, glazes in aqueous suspension of various powdered minerals and metal oxides 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 or similar tool, or applying it directly with a brush or other tool.

To prevent the glazed article from sticking to the kiln during firing, either a small part of the item is left unglazed, or it's supported on small refractory supports such as kiln spurs and Stilts that are removed and discarded after the firing. Small marks left by these spurs are sometimes visible on finished ware.

Decoration applied under the glaze on pottery is generally referred to as underglaze. Underglazes are applied to the surface of the pottery, which can be either raw, "greenware", or "biscuit"-fired (an initial firing of some articles before the glazing and re-firing).[4][5][6] 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 famously produced in Germany, England, the Netherlands, China, and Japan. The striking blue color uses cobalt as cobalt oxide or cobalt carbonate.[7]

Polychrome glazed pottery statue of heavenly guardian
Sancai lead-glazed figure of heavenly guardian, Tang dynasty

Decoration applied on top of a layer of glaze is referred to as overglaze. Overglaze methods include applying one or more layers or coats of glaze on a piece of pottery or by applying a non-glaze substance such as enamel or metals (e.g., gold leaf) over the glaze.

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.


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.

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.[8]

Lead glazed earthenware was probably made in China during the Warring States Period (475 – 221 BCE), 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).[9]

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. 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.[10] Other centers for innovative ceramic pottery in the Islamic world included Fustat (from 975 to 1075), Damascus (from 1100 to around 1600) and Tabriz (from 1470 to 1550).

Environmental impact

As of 2012, over 650 ceramic manufacturing establishments were reported in the United States, with likely many more across the developed and developing world.[1] Floor tile, wall tile, sanitary-ware, bathroom accessories, kitchenware, and tableware are all potential ceramic-containing products that are available for consumers.[11] Heavy metals are dense metals used in glazes to produce a particular color or texture.[5] Glaze components are more likely to be leached into the environment when non-recycled ceramic products are exposed to warm or acidic water.[12] Leaching of heavy metals occurs when ceramic products are glazed incorrectly or damaged.[12] Lead and chromium are two heavy metals commonly used in ceramic glazes that are heavily monitored by government agencies due to their toxicity and ability to bioaccumulate.[12][13]

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 (PbO) as a flux for its low melting range, wide firing range, low surface tension, high index of refraction, and resistance to devitrification.[14]

In polluted environments, nitrogen dioxide reacts with water (H
) to produce nitrous acid (HNO
) and nitric acid (HNO

+ 2NO

Soluble Lead(II) nitrate (Pb(NO
) forms when lead(II) oxide (PbO) of leaded glazes is exposed to nitric acid (HNO

PbO + 2HNO
+ H

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.

Chromium(III) oxide

Chromium(III) oxide (Cr
) 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
). The oxidation reaction changes chromium from its +3 oxidation state to its +6 oxidation state.[15] Chromium(VI) is very soluble and the most mobile out of all the other stable forms of chromium.[16]

+ 2CaO + ​32O

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).[16]


Chromium oxidation during manufacturing processes can be reduced with the introduction of compounds that bind to calcium.[15] 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.


Periodo nara, giara invetriata a tre a colori, VIII sec

Pottery, Nara period

Meissen Zwiebelmuster

Meissen porcelain, with blue underglaze decoration on porcelain


Mug with blue underglaze decoration on porcelain.


Coloured lead glazes majolica circa 1870

Vzorkovník glazur

Test slabs of different glazes

Minton tin-glaze Majolica oval plate decorated by Thomas Kirkby in Renaissance style after Mantegna original

Tin-glazed majolica decorated with metallic oxide colours, Mintons, circa 1870.

20110111- MG 9803-Edit

20th century glazing technique

See also


  1. ^ a b Division, Company Statistics. "Statistics of U.S. Businesses Main Page". www.census.gov. Archived from the original on 2015-11-26. Retrieved 2015-11-27.
  2. ^ C D Fortnum, 1875, MAIOLICA, Chapter II on Enamelled or Stanniferous Glazed Wares "It was found that by the addition of a certain portion of the oxide of tin to the composition of glass and oxide of lead the character of the glaze entirely alters. Instead of being translucent it becomes, on fusion, an opaque and beautifully white enamel…"
  3. ^ a b Madan, Gaurav (2005). S.Chands Success Guide (Q&A) Inorganic Chemistry. S. Chand Publishing. ISBN 9788121918572.
  4. ^ "Cleaning Biscuit Fired Ceramic Ware" Hulse D.K, Barnett W.C. UK Pat.Appl.GB2287643A
  5. ^ a b Denio, Allen A. (1980-04-01). "Chemistry for potters". Journal of Chemical Education. 57 (4): 272. doi:10.1021/ed057p272.
  6. ^ "Roller Kilns For The Fast Biscuit And Glost Firing Of Porcelain" Rodriguez Mamolar M.J., De La Fuente Revuelta J. Ceram. Inf.(Spain) 20, No.202. 1994. Pg. 25–27
  7. ^ 'Ceramics Glaze Technology.' J.R.Taylor & A.C.Bull. The Institute Of Ceramics & Pergamon Press. Oxford. 1986
  8. ^ Daiheng, Gao (2002). Chinese Architecture – The Lia, Song, Xi Xia and Jin Dynasties (English ed.). Yale University Press. pp. 166, 183. ISBN 978-0-300-09559-3.
  9. ^ Zhiyan, Li (2002). Chinese Ceramics -- From the Paleolithic Period through the Qing Dynasty (English ed.). New York & London, Beijing: Yale University Press, Foreign Languages Press. pp. 144, 145, 152. ISBN 978-0-300-11278-8.
  10. ^ Mason (1995), p. 5
  11. ^ Environmental Protection Agency. (February 2003) Economic Impact Analysis of the Clay Ceramics Manufacturing NESHAP: Final Rule (EPA Publication No. EPA-452/R-03-007). Research Triangle Park, NC: Environmental Protection Agency.
  12. ^ a b c 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 Oct. 2015
  13. ^ a b Baltrusaitis, Jonas; Chen, Haihan; Rubasinghege, Gayan; Grassian, Vicki H. (2012-12-04). "Heterogeneous Atmospheric Chemistry of Lead Oxide Particles with Nitrogen Dioxide Increases Lead Solubility: Environmental and Health Implications". Environmental Science & Technology. 46 (23): 12806–12813. doi:10.1021/es3019572. ISSN 0013-936X. PMC 3518381. PMID 23057678.
  14. ^ Lehman, Richard. Lead Glazes for Ceramic Foodware. 1st ed. Research Triangle Park: International Lead Management Center, 2002. International Lead Management Center
  15. ^ a b c Verbinnen, Bram; Billen, Pieter; Van Coninckxloo, Michiel; Vandecasteele, Carlo (2013-06-04). "Heating Temperature Dependence of Cr(III) Oxidation in the Presence of Alkali and Alkaline Earth Salts and Subsequent Cr(VI) Leaching Behavior". Environmental Science & Technology. 47 (11): 5858–5863. doi:10.1021/es4001455. ISSN 0013-936X. PMID 23635007.
  16. ^ a b Oliveira, Helena (2012-05-20). "Chromium as an Environmental Pollutant: Insights on Induced Plant Toxicity". Journal of Botany. 2012: 1–8. doi:10.1155/2012/375843.


An hua

An hua (Chinese: 暗花; pinyin: ànhuā) is a term used in Chinese ceramics meaning secret or veiled decoration; the designs being visible through transmitted light, produced either by incising the design into the porcelain before glazing and firing or by delicate slip-trailing in white slip on the porcelain body. It is also called "secret" or "hidden decoration".

Incised, carved or mould-impressed monochrome decoration under a ceramic glaze was the dominant type of decoration during the Song dynasty, in types of white wares such as Ding ware and Qingbai. But the term is mainly used of the still more discreet version of the technique popularized under the Yongle Emperor (1360-1424) of the Ming dynasty (1368-1644), and continued later, particularly during the reign of the Yongzheng Emperor (1723-1735) in the Qing dynasty. Very often dragons were concealed in the interior of bowls.

Some an hua dishes were found inside a storage jar in the mid-15th century (1450-1487?) Belankan shipwreck off the coast of Indonesia, found circa 1999.

Of a teapot in the Victoria and Albert Museum (see gallery), the museum says: "One side is very subtly incised with a design of rocks and plants, and the other with a poetic couplet that reads: "At night in springtime one moment is precious like a thousand pieces of gold, the orchid sends forth its fragrance in the shade of the moon." These are the first two lines from a poem by the famous eleventh-century writer Su Shi". This suggests it was made for a scholarly gentleman.

Biscuit (pottery)

Biscuit (also known as bisque) refers to pottery that has been fired but not yet glazed. Biscuit is any pottery that has been fired in a kiln without a ceramic glaze. This can be a final product such as biscuit porcelain, or unglazed earthenware, often called terracotta, or, most commonly, an intermediary stage in a glazed final product.

The porous nature of biscuit earthenware means that it readily absorbs water, while vitreous wares such as porcelain, bone china and most stoneware are non-porous even without glazing. The temperature of biscuit firing is today usually at least 1000°C, although higher temperatures are common. The firing of the ware that results in the biscuit article causes permanent chemical and physical changes to occur. These result in a much harder and more resilient article which can still be porous, and this can ease the application of glazes.

The first firing is called the biscuit firing (or "bisque firing"), and the second the glaze or glost firing.

Ceramic flux

Fluxes are substances, usually oxides, used in glasses, glazes and ceramic bodies to lower the high melting point of the main glass forming constituents, usually silica and alumina. A ceramic flux functions by promoting partial or complete liquefaction. The most commonly used fluxing oxides in a ceramic glaze contain lead, sodium, potassium, lithium, calcium, magnesium, barium, zinc, strontium, and manganese. These are introduced to the raw glaze as compounds, for example lead as lead oxide. Boron is considered by many to be a glass former rather than a flux.

Some oxides such as calcium flux significantly only at high temperature. Lead is the traditional low temperature flux, but it is now avoided because it is toxic even in small quantities. It is being replaced by other substances, especially boron and zinc.In clay bodies a flux creates a limited and controlled amount of glass, which works to cement crystalline components together. Fluxes play a key role in the vitrification of clay bodies by reducing the overall melting point. The most common fluxes used in clay bodies are potassium oxide and

sodium oxide which are found in feldspars. A predominant flux in glazes is calcium oxide which is usually obtained from limestone. The two most common feldspars in the ceramic industry are potash feldspar (orthoclase) and soda feldspar (albite).


Deglazing refers to the removal of a shiny or smooth surface.

Deglazing (cooking), using a liquid to remove cooked-on residue from a pan

Deglazing (engine mechanics), abrading the polished surface of a cylinder

Removal of the ceramic glaze from pottery


Flambé is also a type of ceramic glaze.Flambé (, French: [flɑ̃be]; also spelled flambe) is a cooking procedure in which alcohol is added to a hot pan to create a burst of flames. The word means "flamed" in French.Flambéing is often associated with tableside presentation of certain liqueur-drenched dishes set aflame, such as Bananas Foster or Cherries Jubilee, when the alcohol is ignited and results in a flare of blue-tinged flame. However, flambéing is also a step in making coq au vin, and other dishes and sauces, using spirits, before they are brought to the table. By partially burning off the volatile alcohol, flambéing reduces the alcoholic content of the dish while keeping the flavors of the liquor.


Glaze or glazing may refer to:

Glaze (metallurgy), a layer of compacted sintered oxide formed on some metals

Glaze (cooking technique), a coating of a glossy, often sweet, mixture applied to food

Glaze (ice), a layer of ice caused by freezing rain

Glaze (painting technique), a layer of paint, thinned with a medium, so as to become somewhat transparent

Glaze (surname)

Glazing (window), a transparent part of a wall

Ceramic glaze, a vitreous coating to a ceramic material whose primary purposes are decoration or protection

Glazed (album), a 1993 album by the Canadian rock band Mystery Machine

Glaze defects

Glaze defects are any flaws in the surface quality of a ceramic glaze, its physical structure or its interaction with the body.

Jacob Vollrath

Jacob Johann Vollrath (September 19, 1824 – May 15, 1898) was an industrialist in the city of Sheboygan, Wisconsin in the United States. He founded The Vollrath Company.Vollrath was born on September 19, 1824 in Dörrebach in the Prussian Rhineland, where he learned the trade of molding (casting of wrought iron).

He migrated to the United States in the 1840s and settled in Sheboygan in 1853. In 1874 he began to manufacture porcelain enamelware made of cast iron coated with ceramic glaze. In 1884 founded the Jacob J. Vollrath Manufacturing Company, which grew steadily under his leadership and which he headed until his death in 1898.Vollrath invented "gray enameling" (which describes a particular method of manufacture, not a color).Vollrath married Elizabeth Margaret Fuchs in 1847 and had six children. He was the father-in-law (twice) of Kohler Company founder John Michael Kohler and helped him get started in business. He was succeeded as president of The Vollrath Company by his son, Carl August Vollrath, grandson Jean C Vollrath (1894–1976), and great-grandson Walter Jodok Kohler JrTwo other sons, Jacob Vollrath Jr. (1894–1964) and Walter J. Vollrath, Sr, (–1964) had served as officers of the Polar Ware Co., of Kiel, WI.The Vollrath family which he founded was long prosperous and prominent in Sheboygan affairs.


Oxblood or ox-blood is a color that is a dark shade of red. It resembles burgundy, but has more purple and dark brown hues. The French term sang-de-boeuf, or sang de boeuf, with the same meaning (but also "cow blood") is used in various contexts in English, but especially in pottery, where sang de boeuf glaze in the color is a classic ceramic glaze in Chinese ceramics.The name is often used in fashion, especially for shoes. The term oxblood can be used to describe a range of colors from red to reddish-purple to nearly black with red, brown and blue undertones.

Qingbai ware

Qingbai ware (青白 qīngbái „green-white“, formerly "Ch'ing-pai" etc.) is a type of Chinese porcelain produced under the Song Dynasty and Yuan dynasty, defined by the ceramic glaze used. Qingbai ware is white with a blue-greenish tint, and is also referred to as Yingqing ("shadow green", although this name appears only to date from the 18th century). It was made in Jiangxi province in south-eastern China, in several locations including Jingdezhen, and is arguably the first type of porcelain to be produced on a very large scale. However, it was not at the time a prestigious ware, and was mostly used for burial wares and exports, or a middle-rank Chinese market. The quality is very variable, reflecting these different markets; the best pieces can be very thin-walled.Qingbai ware was made with a white porcelain body, fired with a glaze that produced a slight blue-green tint. The kilns used pine wood as fuel, producing a reducing atmosphere that produced the tint. Qingbai ware was used by commoners, and never seems to have been made for imperial use; its quality only came to be appreciated by collectors several centuries later. In the 14th century the same manufacturers turned to the new blue and white porcelain, using the same body, which saw the end of Qingbai ware.Many types of items were made: as well as the usual plates and bowls, there were teapots and small round lidded boxes, usually described as for cosmetics. Items made for burial included tall funerary urns with complicated, and rather crowded, sets of figures. There are also tomb figures, though less care is expended on these than on the famous sancai figures of the Tang dynasty. Small Buddha statues, often with highly detailed hair, clothes and accessories, come from late in the period.A variety of forming techniques were used, tending for basic shapes to move over the period from wheel-thrown vessels decorated by carving with a knife (incised) or impressed decoration, to moulded bodies. Shapes and decoration had much in common with Ding ware from northern China; indeed the Jingdezhen white wares preceding Qingbai are known as "Southern Ding".


Redware as a single word is a term for at least two types of pottery of the last few centuries, in Europe and North America. Red ware as two words is a term used for pottery, mostly by archaeologists, found in a very wide range of places. However, these distinct usages are not always adhered to, especially when referring to the many different types of pre-colonial red wares in the Americas, which may be called "redware".

In the great majority of cases the "red" concerned is the natural reddish-brown of the fired clay, and the same sort of colour as in terracotta (which most types of red ware could also be called) or red brick. The colour to which clay turns when fired varies considerably with its geological makeup and the conditions of firing, and as well as terracotta red, covers a wide range of blacks, browns, greys, whites and yellows.

Of the two "redware" types, both made in the 17th to 19th centuries (with modern revivals or imitations), the European was unglazed stoneware, mostly for teapots, jugs and mugs, and moderately, sometimes very, expensive. The American redware was cheap earthenware, very often with a ceramic glaze, used for a wide variety of kitchen and dining functions, as well as objects such as chamberpots.


A resist, used in many areas of manufacturing and art, is something that is added to parts of an object to create a pattern by protecting these parts from being affected by a subsequent stage in the process. Often the resist is then removed.

For example in the resist dyeing of textiles, wax or a similar substance is added to places where the dye is not wanted. The wax will "resist" the dye, and after it is removed there will be a pattern in two colours. Batik, shibori and tie-dye are among many styles of resist dyeing.Wax or grease can also be used as a resist in pottery, to keep some areas free from a ceramic glaze; the wax burns away when the piece is fired. Song dynasty Jizhou ware used paper cut-outs and leaves as resists or stencils under glaze to create patterns. Other uses of resists in pottery work with slip or paints, and a whole range of modern materials used as resists. A range of similar techniques can be used in watercolour and other forms of painting. While these artistic techniques stretch back centuries, a range of new applications of the resist principle have recently developed in microelectronics and nanotechnology. An example is resists in semiconductor fabrication, using photoresists (often just referred to as "resists") in photolithography.

Sang de boeuf glaze

Sang de boeuf glaze, or sang-de-boeuf, is a deep red colour of ceramic glaze, first appearing in Chinese porcelain at the start of the 18th century. The name is French, meaning "ox blood" (or cow blood), and the glaze and the colour are also called ox-blood or oxblood in English, in this and other contexts.

Sang de boeuf was one of a number of new "flambé" glazes, marked by "unpredictable but highly decorative and varying effects", developed in the Jingdezhen porcelain kilns during the Kangxi reign (1662–1722). According to one scholar: "In its finer examples, this spectacular glaze gives the impression that one is gazing through a limpid surface layer, which is slightly crazed and strewn with countless bubbles, to the color that lies underneath".As with most Chinese red glazes, the main colouring agent is copper oxide, fired in a reducing atmosphere (without oxygen); finishing them in an oxidizing atmosphere may have been part of the process. From the late 19th century onwards, usually after lengthy experiment, many Western potters produced versions of the Chinese glaze, which is technically very difficult to achieve and control.

For Chinese ceramics, some museums and books prefer the term "sang de boeuf", some "oxblood", in both cases with varying use of hyphens, and capitals and italics for "sang de boeuf". The most common Chinese name for the glaze is lángyáohóng (郎窑红, "Lang kiln red"). Another Chinese name for this type of glaze is niúxiěhóng (牛血红, "ox-blood red/sang de boeuf").

Tellurite glass

Tellurite glasses contain tellurium oxide (TeO2) as the main component.


Tenmoku (天目, also spelled "temmoku" and "temoku") is a type of Japanese pottery and porcelain that originates in imitating Chinese stoneware Jian ware (建盏) of the southern Song dynasty (1127–1279), original examples of which are also called tenmoku in Japan.

Shapes are simple and bold, with tea bowls the most typical. The emphasis is on the ceramic glaze, where a number of distinct effects can be produced, some including an element of randomness that has a philosophical appeal to the Japanese. The tea-masters who developed the Japanese tea ceremony promoted the aesthetic underlying tenmoku pottery.

Torre de San Martín

The Torre de San Martín (English: St. Martin's Tower) is a medieval structure in Teruel, Aragon, northern Spain. Built in Aragonese Mudéjar style in 1316 and renovated in the 16th century, it was added to the UNESCO Heritage List in 1986 together with other Mudéjar structures in Teruel.

The tower was built between in 1315 and 1316. In 1550 its lower section was restored due to the erosion caused by humidity. Like other structures in Teruel, it is a gate-tower decorated with ceramic glaze. The road passes through an ogival arch. The tower takes its names from the annexed church of St. Martin, dating to the Baroque period.

The tower follows the scheme of the Almohad minarets, with two concentric square towers between which are the stairs. The inner tower has three floors covered with cross vaults.


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.

However, because the main or glost firing is at a higher temperature than used in overglaze decoration, the range of available colours is more limited, and was especially so for porcelain in historical times, as the firing temperature required for the porcelain body is especially high. Early porcelain was largely restricted to underglaze blue, and a range of browns and reds. Other colours turned black in a high-temperature firing.

Examples of oxides that do not lose their colour during a glost firing are the cobalt blue made famous by Chinese Ming dynasty blue and white porcelain and the cobalt and turquoise blues, pale purple, sage green, and bole red characteristic of İznik pottery. The painting styles used are covered at (among other articles): china painting, blue and white pottery, tin-glazed pottery, maiolica, Egyptian faience, Delftware. In modern times a wider range of underglaze colours are available.

An archaeological excavation at the Tongguan kiln Site proved that the technology of underglazed color figure arose in the Tang and Five Dynasties periods and originated from Tonguan, Changsha.

Uranium dioxide

Uranium dioxide or uranium(IV) oxide (UO2), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used as MOX fuel. Prior to 1960, it was used as yellow and black color in ceramic glazes and glass.

Uranium tile

Uranium tiles have been used in the ceramics industry for many centuries, as uranium oxide makes an excellent ceramic glaze, and is reasonably abundant. In addition to its medical usage, radium was used in the 1920s and 1930s for making watch, clock and aircraft dials. Because it takes approximately three metric tons of uranium to extract 1 gram of Ra-226, prodigious quantities of uranium were mined to sustain this new industry. The uranium ore itself was considered a waste product and taking advantage of this newly abundant resource, the tile and pottery industry had a relatively inexpensive and abundant source of glazing material. Vibrant colors of orange, yellow, red, green, blue, black, mauve, etc. were produced, and some 25% of all houses and apartments constructed during that period [circa 1920–1940] used bathroom or kitchen tiles that had been glazed with uranium. These can now be detected by a geiger counter that detects the beta radiation emitted by uranium's decay chain. In most situations, the radiation exposure is not excessive, but there may be exceptions for pure uranium oxide (which produces red-orange coloration as a glaze) on bathroom floors, which can pose a hazard for infants crawling around.

The use of uranium in ceramic glazes ceased during World War II and didn’t resume until 1959. In 1987, NCRP Report 95 indicated that no manufacturers were using uranium-glaze in dinnerware.

Glass science topics
Pottery and claywork
Base minerals, and glazes
Main types, by body
Forming techniques
Processes and decoration
History of pottery


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