Vitreous enamel, also called porcelain enamel, is a glass-like substance made by fusing powdered glass to a metal, ceramic or glass surface (substrate) by firing. If the enamel covers the entire body of a ceramic object, it is usually also called a glaze while the distinction should be made that it is a "low fired lead glaze". If you look for example at a two thousand years old "green glazed" Han dynasty dog or jar, the surface would be called a "glaze" while this green glaze - based on copper and lead - is the same substance that was used for green leaves and grass in for example famille verte decorations of the late 17th century, but then called "enamel". This distinction made we'll stick to enamels from here on.
In Chinese ceramics enamels may be applied either to a pre-fired unglazed body (biscuit) or the surface of a high-fired glaze, after which the object is fired a second time at a lower temperature usually between 750 and 850 °C (1,380 and 1,560 °F).
The powder melts, flows, and then hardens to a smooth, durable vitreous coating. The word comes from the Latin vitreum, meaning "glass". The defining key point is the melting temperature and if there is lead present or not.
Lead glazes occurs from the Han dynasty and appears again during the Tang dynasty. As on-glaze painted enamel decoration in China they seems to have first appeared on stoneware made at the Cizhou kilns in the Song or Jin dynasties.
By the mid 15th century most of what Jingdezhen knew about enamels had been developed in the north.
Oxides
All ceramic colors are somehow based on oxides of metals. For those chemically challenged, a metallic oxide is a metal which have become chemically bound to oxygen typically coming from the air. A good example of this is rust, which is iron bound to oxygen.
A Chinese lead enamel or lead glaze is generally made by mixing of three parts lead oxide with one part powdered quarts to which a small amount of metallic oxides are added as colorant. When heated up to 700 to 800 centigrade in a special enamel (muffle) kiln this mixture will melt into a liquid glass-like substance.
To apply this dry powder substance for porcelain decoration, the powder is mixed with a media like water, glue, turpentine or oil to produce a paste which is then applied thinly or thickly to the porcelain surface, usually with a brush. If the oxide powder is just stirred into water you will get a suspension where the powder will slowly settle to the bottom. The glaze, being technically the same as melted glass, is fragile, prone to scratching and can leak lead into drinks and food stuff, but looks fantastic, when in good condition.
The following metal oxides colors could be used to create the following colors:
Antimony (yellow, in combination with lead)
Chromium (yellow)
Cobalt (blue)
Copper (green, red, depending on firing)
Gold (purple)
Iron (yellow, red and black depending on concentration and firing)
Manganese (purples and browns)
Tin (white)
A red lead-based enamel with the red pigment coming from iron oxide was invented during the Song dynasty. The lead was over saturated with red iron oxide in a suspension rather than a solution, which produced a dry matte color better suited for outlines than for washes. One peculiarity with this enamel was that it comes out brighter red the more finely the iron oxide is ground. Depending on saturation, concentration and how finely ground the oxide is this same enamel can come out as yellow, orange, bright red, all the way to a full dark red. Any impurities even with iron itself can spoil the result and particularly the yellow is likely to misfire. From its first appearance on Cizhou pottery during the Northern Song dynasty it then appeared on Shufu export wares together with green and turquoise. From the Hongwu (1368-1398) period we have one single dish finely decorated with overglaze red dragons, found recently in the early Ming capital of Nanjing. During the Yongle (1402-1425) period less than 5 percent of all porcelains are decorated, with red as the most important enamel, probably introduced as a replacement for the difficult underglaze copper red. Later, when used for outlines together with enamels, the iron red lines was always painted just beyond the enamel limits as it was easily dissolved by the transparent colors.
Green enamel was first used on Cizhou pottery during the Northern Song dynasty together with red and yellow. It then appears on Shufu export wares together with red and turquoise. It could be created by mixing copper oxide with lead and quarts sand. With a high lead content the green color turns a dark emerald color. Light or dark green is the same enamel but with different amounts of iron added.
Yellow enamel was first used on Cizhou pottery during the Northern Song dynasty together with red and green. The enamel was probably created by a low concentration of iron oxide in a lead solution i.e. the same as was used to create the yellow to amber colors in the Tang sancai glazes. Iron yellow becomes a staple enamel from early Ming and onwards. Iron yellow occurs during Yongle but so far only known as on-bisque.
Instead of lead, saltpeter could be used to make the quartz sand and copper powder melt into a turquoise-blue alkali-glaze. Its earliest appearance seems to have been on Tang pottery. It is then found on northern Cizhou wares during Song, on the Yuan dynasty Shufu wares of Jingdezhen and also on some very rare Imperial Mongolian porcelains of the 1330's. Turquoise alkali-glaze continued to be used on porcelain both as a monochrome as well as an extra color in combinations with lead-based overglaze enamels from the early Ming dynasty up until today. On Swatow "split pagoda" dishes from the late Ming turquoise was sometimes the dominating color of decoration together with black and iron red.
During the mid 15th century a "black" enamel makes its appearance. It was initially used for flat washes but becomes later indispensable for providing thin black outlines for overglaze enamel painting. It was made from the same cobalt pigments used for underglaze decoration, but possibly an impure and low grade cobalt not suitable to be used as underglaze blue. During the Chenghua (1465-1487) period and 200 years later under the Kangxi period brownish cobalt containing manganese and iron seems to have been used for this purpose, which then needed a transparent copper green glaze coat to turn a glossy black. A good grade cobalt could also be mixed with some lead to give a solid black. These cobalt enamels was later used extensively to provide thin black outlines for overglaze enamel painting with or without an extra coating of green or transparent glaze.
Different shades of Aubergine from intense 'brinjal' purple to brown and light watery purple are all made by the Chinese low-grade cobalt-iron-manganese ore mixed with lead and quartz sand.
Brown based on manganese occurs as a transparent enamel or as a dry oxide suspension glaze for outlines. It was possibly made from the same low-grade Chinese cobalt-iron-manganese ore that was used for underglaze blue but from the 'impure end' deemed not suitable for blue, then mixed with lead and quartz sand. A transparent Manganese brown enamel made its first appearance on Xuande (1426-1435) period experimental doucai wares to become more established during the Chenghua (1465-1487) period. High-fired brown (not enamels) occurs as early as Yongle.
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