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. 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:

Ash glaze, important in East Asia, simply made from wood or plant ash, which contains potash and lime.
Feldspathic glazes of porcelain.
Lead glazes, plain or coloured, are shiny and transparent after firing, which need only about 800 °C (1,470 °F). They have been used for about 2,000 years in China e.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. Known in the Ancient Near East and then important in Islamic pottery, from which it passed to Europe. Includes Hispano-Moresque ware, maiolica (also called majolica), faience, and Delftware.

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

From a firing temperature of 1250 ° C, stoneware is fired from the pieces. Porcelain is fired at temperatures up to 1400 ° C. Intercrystalline glass-like phases result, which provide a closed porosity and possibly a self-glaze. However, the surface is often rough and has the color of the corresponding base material. The glaze is made with additional materials that can be used to create a hard, closed surface layer and various colors. The components of the glaze form with each other and with the base material a glass layer made from a mixture of different oxides.

Glazes are applied to improve the aesthetic effect (color and effect glazes) or serve to improve mechanical and electrical properties.

For dishes, the glaze reduces the surface roughness, so they are easier to clean, and the scratch hardness is increased, which improves the properties of use, since there is less scratching.

High-voltage insulators made of electrical porcelain are glazed in order to increase the strength of the insulator by means of an inherent compressive stress. At the same time, a suitable chemical composition of the surface is achieved, which reduces the leakage current by reducing the conductivity (no water absorption). The reduced roughness also prevents faster soiling.

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. Colorants, such as iron oxide, copper carbonate, or cobalt carbonate, and sometimes opacifiers like tin oxide or zirconium oxide, are used to modify the visual appearance of the fired glaze.

Chemically, glazes (like other glasses) consist of a mixture of mineral flours. Occasionally, metals such as lead or gold are added as determining elements.

The minerals are, on the one hand, network formers such as silica (in the form of quartz powder), fluxes or melting point depressants such as alkali and alkaline earth oxides, mostly sodium and calcium oxide, which are often added in the form of feldspar or chalk, or boron and lead compounds, which are common can be used as frits, as well as aluminum oxide as a consistency enhancer and viscosity enhancer.

Lead glazes are particularly resistant to corrosion, whereas the low-melting components sodium and potassium are more easily removed.

In the salt glaze, which has been known since the late Middle Ages, rock salt (sodium chloride) is added to the fire, the flue gases of which flow around the kiln. The sodium oxide released at high temperature combines with the cullet and lowers the melting temperature of the surface layer so that a glass layer is formed.

The higher the firing temperature and the attainable resistance, the more limited the color palette. While the color white is created by dispersion (addition of tin oxide or zirconium oxide), other colors can only be achieved by adding coloring metal oxides. The blue cobalt glaze is well known. Green is created by chrome oxide, brown tones by manganese or the iron that is often already contained. Under a reducing burning atmosphere, an iron content leads to gray-blue shades.

Low-fired colorful ceramic glazes often still contain soluble components that release so much substance during use that they are still toxic. Often this applies to ornaments with applied engobes that are not completely “glazed” and are more crystalline compared to glazes and less closed on the surface.

Porcelain objects, which are burned smooth at 1450 ° C, are considered harmless – even if they contain toxic coloring substances. The heavy metals in the silicates are firmly glazed and bound with them.

The painting of porcelain and faience can be used as underglaze painting with sniper fire colors at high temperature, or temperature-sensitive glaze colors done, reduce heat to the glazed ware.

Certain oxides such as cobalt were long reserved for luxury productions. Indeed the purest cobalt came at great cost from the Middle East via Spain. That of Central Europe gave less deep and more gray blues.

Blue: cobalt + titanium (rutile)
Brown: iron + manganese
Bluish gray: iron + cobalt
Yellow: cobalt + vanadium
Black: copper + manganese
Ocher: iron + vanadium
Green: copper + iron or copper + chrome

The colors and textures of ceramic enamels also depend on the atmosphere of the firing in which they were formed:

Oxidizing (enough oxygen to burn all the fuel)
Reductive (there is not enough oxygen during cooking for all the fuel to be consumed and the flame will seek this oxygen in the very material of the enamel, thus changing its chemical properties and therefore its appearance).

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). 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.

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.

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).

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. 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).

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The green (unfired) ceramics are first subjected to a spray firing, among other things, in the manufacture of porcelain. The firing temperature is lower, not as high as with smooth firing after the glaze components have been applied. After scouring, the ceramics are poured, dipped or brushed with suspensions of the glaze components in water (frits, powder dissolved in water). The contact surfaces remain free to prevent them from merging with the built-in ovens.

In the case of smooth firing, the glaze melts and its components combine with one another and with the broken glass. Glassy mixed oxides are formed.

If the expansion coefficient of the glaze layer is greater than that of the base material, cracks can form. These cracks are sometimes recognized and used as design elements (craquelé). In the opposite case, that the tension of the glaze layer is higher, i.e. the glaze layer is under permanent compressive stress, the strength is increased, which can also be desired depending on the application.

As the plumbiferous varnish has a coefficient of expansion greater than the terracotta itself (cooked mud), small cracks may appear that could filter the liquids contained by the container, which in many cases causes the food introduced into the glazed vessels to start forming lead salts very poisonous. In the nineteenth century it was discovered that glazing could be done without lead and without the consequent danger, being replaced by feldspathic glazing.

Toxicity, ecotoxicity, certification
If the glazes (in the sense of any “substance applied to the surface of the tiles between the shaping and the final stage of the firing of the tile”) contain lead, cadmium or antimony (or one of their compounds), for obtain the European Ecolabel, the glazes must not contain more than:

0.5% of their lead mass
0.1% of their mass in cadmium
0.25% of their mass in antimony

Types of glazes
There are several types of glazes depending on the fluxes used:

alkaline glazes – with sodium, potassium or lithium salts;
boron glazes – boric acid (melting temperature 600 ° C);
lead glazes – lead oxide. The alquifoux, a lead sulfide glaze used in the south of France until its partial ban in the 1950s, gave green or yellow varnished colors typical of Provencal productions. Lead glazes are almost no longer used because of their toxicity;
“Bristol” glazes – with zinc oxide. Less toxic than the previous ones, they gradually replaced them.
Many glazing recipes are available to obtain different textures (matte, shiny, rough), or a more or less dense covering (opaque, translucent).

The celadon refers to both a color and a type of ceramic unique to China (Chinese: qingci青瓷, literally “green porcelain”) and the Far East. This enamel has a bluish to olive green hue and is characteristic of a particularly sought after production of ancient Chinese ceramic.

An example of this high temperature enamel is obtained, in reduction, with this type of recipe:

Feldspar: 40%
Silica: 30%
Chalk (Calcium carbonate): 20%
Kaolin: 10%

Optionally, you can add 5% (in addition) of talc and 1% of ocher or iron oxide.

The tenmoku
Black Japanese enamel spotted with brown says “chamois”, this enamel is obtained with the following recipe:

Feldspar: 45%
Chalk: 12%
Ball Clay: 5%
Silica: 36%
Bentonite: 2%
Red iron oxide (hematite): + 8%

The shino
There are many different shinoes. They generally resemble a thick, opaque, mat glass, from white to orange or brown. Two Shino recipes:

Syphite nepheline: 70%
Kaolin: 30%
Salt: + 3%

Nepheline Syenite: 80%
Kaolin: 20%
Salt: + 3%

Ash enamels

“Cream” ash enamel:
Feldspar: 38%
Wood ash: 31%
Chalk: 23%
Silica: 8%

Ash green enamel:
Feldspar: 18%
Wood ash: 46%
Ball clay: 27%
Kaolin: 9%
Copper carbonate: + 3%

Ash blue enamel:
Feldspar: 38%
Wood ash: 31%
Chalk: 25%
Silica: 6%
Cobalt oxide: +1%

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. Floor tile, wall tile, sanitary-ware, bathroom accessories, kitchenware, and tableware are all potential ceramic-containing products that are available for consumers. 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. Leaching of heavy metals occurs when ceramic products are glazed incorrectly or damaged. 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.

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.

In polluted environments, nitrogen dioxide reacts with water (H2O) to produce nitrous acid (HNO2) and nitric acid (HNO3).

H2O + 2NO2 → HNO2 + HNO3

Soluble Lead(II) nitrate (Pb(NO3)2) forms when lead(II) oxide (PbO) of leaded glazes is exposed to nitric acid (HNO3)

PbO + 2HNO3 → Pb(NO3)2 + H2O

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 (Cr2O3) 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 (CaCrO4). The oxidation reaction changes chromium from its +3 oxidation state to its +6 oxidation state. Chromium(VI) is very soluble and the most mobile out of all the other stable forms of chromium.

Cr2O3 + 2CaO + 3⁄2O2 → CaCrO4

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).

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 has experimented with a dual glaze, barium alternative to lead, but they were unsuccessful in achieving the same optical effect as leaded glazes.