Ceramic 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. 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. By around 100 BC lead-glazing was widespread in the Old World.^[3]

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.^[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 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.^{[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 in Basra, dating to around the 8th century. Another significant contribution was the development of stoneware, originating from 9th century Iraq.^{[6][full citation needed]} Other places for innovative pottery in the Islamic world included Fustat (from 975 to 1075), Damascus (from 1100 to around 1600) and Tabriz (from 1470 to 1550).^{[citation needed]}

Raw materials for ceramic glazes generally include silica, which will be the main glass former. Various metal oxides, such as those of 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.^[7] Colorants, such as iron oxide, copper carbonate or cobalt carbonate,^[7] and sometimes opacifiers including tin oxide and zirconium oxide, are used to modify the visual appearance of the fired glaze.

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, and sometimes boron trioxide.^{[clarification needed]}

Salt glazing is a vapor-glazing process where salt or soda ash is introduced into a high-temperature kiln. The resulting sodium vapor reacts with the alumina and silica in the clay body, forming a durable sodium-silicate glass coating.^[8] Though historically significant, the method is now largely obsolete due to the release of toxic hydrogen chloride gas and other environmental concerns.^[9]

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).^[24][25][26] 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 first produced in China, and then copied in other countries. The striking blue color uses cobalt as cobalt oxide or cobalt carbonate.^[27] However many of the imitative types, such as Delftware, have off-white or even brown earthenware 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 pottery, such as maiolica, 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 celadons, 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, 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.

​The leaching of lead (Pb) and cadmium (Cd) from glazes can be a public health concern. These heavy metals have historically been used as fluxes to create smooth, high-gloss finishes at low temperatures or to provide specific chemical stability.^[28] However, if a glaze is improperly formulated or fired, its chemical structure may remain unstable.^[29] When such ceramics come into contact with acidic substances—such as citrus juice or coffee—the acid can attack the glaze matrix, releasing metal ions into the food or drink.^[30]

​Ingestion risks are cumulative and severe. Lead is a potent neurotoxin linked to developmental delays and cardiovascular issues, while cadmium is a carcinogen that can cause renal failure over prolonged exposure.^[31] Consequently, international bodies such as the ISO enforce strict permissible limits for metal release.^[32] Modern scientific developments in ceramic chemistry, including the use of fritted materials and computer-aided oxide balancing, have significantly improved glaze stability.^[33] As a result, products from reputable manufacturers that adhere to contemporary safety standards are safe for functional use. Despite these advancements, risks persist in artisanal pottery with poor oversight and antique ceramics where the chemical bond is insufficient to prevent leaching.^[34]

Beyond the health concerns of heavy metal leaching, ceramic glazes present significant environmental challenges during the manufacturing and disposal stages. The extraction of raw materials, such as lithium, cobalt, and barium, often involves energy-intensive mining processes that can lead to habitat destruction and soil erosion.^[35]

Furthermore, during the firing process in kilns, volatile compounds and greenhouse gases—including fluorine and sulphur oxides—can be released into the atmosphere if proper filtration systems are not utilised.^[36]

The management of waste glaze is equally critical; liquid glaze runoff containing heavy metals can contaminate groundwater and local ecosystems if discarded into standard drainage systems without prior wastewater treatment.^[37] As a result, modern studios and factories are increasingly adopting "closed-loop" systems to reclaim glaze sediment and reduce the ecological footprint of ceramic production.^[38]

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

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